Introduction

The geo-morphological complexity of vineyard terroirs has a dictating role in temporal and long term climatic and edaphic conditions, which should be taken into account when making decisions regarding variety and rootstock choices, vineyard trellising, row orientation and plant density for establishment. It has already been demonstrated that grapevine roots respond to soil nutrition and water as well as cultivation practices (Archer and Strauss, 1985; Archer and Strauss, 1989; Archer and Strauss, 1990; Conradie et al., 1996; Hunter, 1998; van Leeuwen et al., 2019). Soil also constitutes a source of warm and cold air and humidity dissipation. Its physical and chemical composition largely controls plant growth by determining water and nutrient release capacity and availability to plant roots, organic matter decomposition and mineralisation, and in particular biological activity (Steenwerth et al., 2008; Hunter et al., 2010; Renouf et al., 2010; Echeverría et al., 2017). The sensitivity of grapevine leaves to sunlight exposure and plant water status means that canopy structure dimensions, geometric shapes, and pruning and foliage management practices are critical if optimal functioning is to be reached (Kriedemann, 1968; Pandey and Farmahan, 1977; Hunter et al., 1988a, Hunter et al., 1988b; Smart et al., 1990; Hunter et al., 1995; Hunter et al., 2004; Hunter et al., 2014a). Together, terroir factors and vineyard practices largely determine the suitability of canopy and root system functioning for product-objective-orientated grape production and wine style.

The photosynthetic output of grapevine canopies has been frequently studied over the years includes different varieties, trellising systems and environmental conditions (Kriedemann, 1968; Pandy and Farmahan, 1977; Hunter and Visser, 1989; Düring, 1991; Chaumont et al., 1994; Hunter et al., 1994; Naor and Wample, 1994; Hunter, 1998; Zufferey et al., 1999). Data sets have mostly been obtained from single leaf measurements using small clip-on chambers to confined surfaces on different leaves distributed in the canopy (Hunter et al., 2014a, and references therein), but whole vine measurements on either single or multiple vines enclosed in large commercially-constructed gas analyser-connected chambers have also been conducted (Poni et al., 2009, and references therein). The latter is argued to present net canopy photosynthetic diurnal and seasonal output, accommodating leaf orientation and inclination, as well as direct, diffuse and reflected radiation, as dictated by growth structure and plasticity. Yet in such atmosphere-controlled conditions the detailed potential or function at individual leaf level is undefined and photosynthetic output merely represents a collective functional outcome. Efforts therefore continue in gaining a better understanding of the proportional differential contributions of micro-environmental factors (light, humidity, air flow, turbulence), phyllotaxy, leaf position, leaf size and leaf age to net photosynthetic potential/capacity; thus going beyond net output from artificially enclosed vines. In terms of the latter, it remains a challenge to create conditions in which field-grown or pot-grown vines function without growth or micro-climate constraints and interactive complications; similarly, it is notoriously difficult to reach ultimate optimal and representative conditions in the field when single leaf measurements are conducted. Furthermore, varying seasonal and diurnal/nocturnal climatic conditions, physiological and growth responses, and management practices that change source:sink relationships, seriously complicate efforts to upscale and relate values obtained at single leaf level to whole plant photosynthetic capacity, during the typically varying temporal and seasonal phenology, and canopy and grape development periods (Hunter, 2000). A grapevine canopy is often still considered as a single, static unit instead of a very dynamic composite foliage unit containing leaves of different ages, sizes, and origins, and which comprises a mixture of source and sink activities that respond to both nocturnal and diurnal conditions. Capturing the complexity of grapevine canopies quantitatively under field conditions with regards to microclimate, leaf morphology and physiological status, and physical constraints by applying quality-controlled methodology, which is precise, calibrated and consistent, remains unsatisfactory and an enormous current and future challenge.

The intricacy of the relationship between single leaf and whole plant carbon balance and water relations has been shown by Medrano et al. (2015) in a study focused on plant water-use efficiency. Reviewing various models on stomatal conductance, Damour et al. (2010) expressed the relevancy of continued attempts to integrate the complex impact of environmental factors on stomatal regulation and carbon fixation under field conditions, especially considering climate change-related factors, such as high air temperature, intense radiation and drought. Calibrating a model that used several semi-empirical equations with direct and diffuse radiation as the main factors, Haasbroek et al. (2000) found a very good regression when the output was compared with field measurements of photosynthesis of Cabernet Sauvignon on a slanting trellis. Using three-dimensional and radiative transfer modelling approaches to simulate canopy light microclimate efficiencies of two cultivars (Syrah, Grenache) trained to different trellising systems (Vertical shoot positioned, Gobelet, Bilateral free cordon) with accompanying shoot orientations (partially wired and free), genotypic growth and shoot architecture (along with vigour) emerged as strong impacting factors (Louarn et al., 2008). Results thus highlighted the difficulty in integrating genotypic behaviour and requirements with climate parameters and objectives related to viticultural and oenological performance.

It is plausible to attempt simulations of optimal radiative transfer through a grapevine canopy absorption, emission and scattering medium, which is extremely heterogeneous in architecture, soil-climate sensitivity, shoot growth, spatial definition, and other turbulent intra-microclimatic conditions (temperature, humidity, wind) and which is notably subjected to the interfering effects of neighbouring grapevine bodies, and practical manipulation. However, it involves a magnitude of assumptions and difficulties, despite there being some ostensibly successful simplified individual cases. Furthermore, the reproduction of modelled gas exchange output would be further compromised if the complex dynamics of morpho-physiological responses, source:sink activity and carbon partitioning of the various vegetative-reproductive entities (including the root system) inside the grapevine body are ignored or not fully elucidated (Haasbroek et al., 2000; Hunter, 2000; Fourcaud et al., 2008; Hunter et al., 2010, Hunter et al., 2016; Prieto et al., 2012; Prieto et al., 2019). Despite intensive efforts, structural-functional growth simulations seem far from elucidated to a point where common application and practice have become acceptable, especially for the grapevine of which growth, yield and product quality are dictated by a multitude of impacting factors, such as soil, climate, various cultivation practices and wine quality objectives. Under field conditions, these factors are barely separable; therefore, not only are they major research challenges, but they also provide the opportunity to understand and demonstrate integrative relationships at practical level, which can be applied to optimise the use of available resources.

In this study, it is hypothesised that the spatial radiation angle and radiation distribution of differently orientated, vertically trained grapevine rows affect vineyard soil conditions, (multi-level) canopy photosynthetically active radiation, leaf water potential and leaf photosynthetic activity. Measurements were temporally conducted under field conditions in the grape ripening period of the growth season. The assessments were supplemented by diurnal photographic images, schematic reconstructions and simple canopy 3D modelling to provide clear representations of the canopies and spatial radiation patterns in the field.

Materials and methods

1. Vineyard

Shiraz(clone SH 9C)/101-14 Mgt was planted during 2003 to four row orientations (treatments), i.e. North-South, East-West, North-East-South-West, and North-West-South-East, which were replicated on a flat site of approximately 3 ha with uniform clayey loam soil at the Robertson Experiment Farm of the Agricultural Research Council (ARC) Infruitec-Nietvoorbij Institute for Deciduous Fruit, Vines and Wine in the Breede River Valley, Robertson (33°49'35'' Latitude S/19°52'53'' Longitude E/159 m a.s.l.), South Africa (Southern Hemisphere). Vines were spaced at a fixed distance of 1.8 x 2.7 m, cordon trained, and pruned to two bud spurs, spaced approximately 14 cm apart. Vertical shoot-positioned canopies of an average height of 1.2 m (from cordon to top, and with cordon height of 0.7 m) (Hunter et al., 2017) had approximately four leaf layers (from side to side) and were uniformly managed (Figure 1). Vines were only shoot positioned and topped, both actions being performed on an average of three times a year. Primary and secondary shoots were positioned between four sets of movable wires. A cover crop (rye) was sown after harvest and killed before budding. Vines were irrigated on a weekly basis at a volume of 14 mm. Irrigation took place during the season’s period of highest evaporative demand, due to the low winter rainfall in the region, which averages 150-300 mm per annum (Hunter et al., 2016). Irrigation was done according to the regional crop factor and ET₀ values (obtained from the automatic weather station close to the vineyard). Overall, the annual management of the vines was done as similar as possible and close to commercial conditions in the field.

Figure 1. Example of a typical post-véraison canopy of Shiraz planted in four row orientations.

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3. Canopy light microclimate measurements and sun/shade digital and schematic patterns

The photosynthetic photon flux density (PPFD over a wavelength range of 400-700 nm) (µmol/m²/s) was determined during the grape ripening period by means of four PAR (photosynthetically active radiation) line quantum sensors (LI-191R, LI-COR, Lincoln, Nebraska, USA), each measuring 180° and fixed together in such a manner that light interception was measured along the canopy in four directions (upwards, downwards, sideways; Figure 2). It was measured immediately next to (at a distance of less than 5 cm away from the canopy wall) and in the centre (inside) of the basal, middle and apical zones, of the canopy; on top of the canopy; and at the bottom of the canopy below the cordon. Ambient PAR was measured in the row approximately 30 cm higher than the canopy. Each PAR value represents incoming light, directed by a 1 m (L) x 12.7 mm (W) quartz rod under a diffuser to a single filtered silicon photodiode (LI-COR Biosciences). An infinite number of spectral sensing points over the 1 m sensing area are therefore integrated and averaged into a single value. For each position, the integrated output of each line quantum sensor was digitally displayed on four channels of a MCSystems 120-04EX data logger and manually recorded. These measurements were taken under clear sky conditions at 08:00, 10:00, 13:00, 16:00, and 18:00. At the same time points, digital camera photographs of sun and shade patterns of the canopies were taken down the rows of the different row orientations. Furthermore, schematic patterns of sun/shade patterns of the differently orientated rows were constructed based on measured distances of sun/shade ground patterns at the time points mentioned above.

Figure 2. Schematic representation of the photosynthetically active radiation (PAR), photosynthesis (Pn) and leaf water potential (ΨL) measurements taken post-véraison in the Shiraz vineyard planted in four row orientations.

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Results

1. Temperature and wind speed

The ambient temperature and wind speed of the two days of measurements during two seasons are shown in Figure 3. The respective temperatures reached 30-35 °C and largely fluctuated according to the wind speed, which reached maxima of approximately 4-6 m/s. These conditions can be considered to be representative of variation on a typical summer’s day on the terroir where the study was carried out (Hunter and Bonnardot, 2011; Hunter et al., 2016).

Figure 3. Temperature and wind speed on the days of measuring in the vertically trellised Shiraz/101-14 Mgt vineyard planted to four different row orientations.

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2. Radiation patterns

Sun/shade photographs (showing depth and inclination of the incoming light) and photosynthetic active radiation (PAR) measurements showed complimentary trends. The sun/shade photographs, taken down the rows of the different row orientations during the late grape ripening period (on the day of microclimate and physiological measurements in the first season of the experiment), are shown in Figure 4; corresponding PAR values are shown in Table 1. These light patterns are complimented by schematic shade patterns (Figure 5).

Table 1. Photosynthetically active radiation, measured in different positions in the canopies of Shiraz/101-14 Mgt planted in four different row orientations (measurements taken on 04/03/2010).

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Row orien	Can side	Measure direction	Photosynthetically active radiation (µmol/m2/s)
			Basal zone of canopy							Middle zone of canopy									Apical zone of canopy
			08:00	10:00	13:00	16:00	18:00	Ave	08:00		10:00	13:00		16:00		18:00	Ave	08:00		10:00	13:00	16:00	18:00	Ave
NS	E	Up	81	733	36	3	7	172	246		375	73		8		51	106	366		345	125	12	51	180
		Down	3	54	12	21	2	18	35		28	14		15		2	19	34		34	18	29	6	24
		E	112	1190	91	6	24	285	1398		670	82		9		37	439	1152		1019	105	30	65	474
		W	2	4	2	41	2	10	2		5	5		61		63	27	5		5	10	68	9	19
	W	Up	33	16	15	326	12	80	39		41	22		687		95	177	61		50	181	938	100	266
		Down	2	9	20	51	3	17	2		6	9		42		20	16	78		9	11	47	29	35
		E	2	22	2	3	1	6	11		16	2		5		2	7	5		33	4	6	4	10
		W	35	40	97	1175	63	282	41		60	91		1102		1045	468	47		71	123	1276	754	454
	Cntr	Up	25	82	6	28	6	28	25		18	9		83		8	29	311		32	8	70	6	76
		Down	5	15	6	19	2	9	26		14	5		12		1	12	3		12	8	22	9	20
		E	60	191	9	8	4	54	414		112	15		8		8	111	478		28	24	18	8	216
		W	8	8	15	125	13	34	15		23	20		313		8	76	25		29	17	540	80	34
	Ave Ave		31	197	26	150	12	83	169		114	29	195		112		124	214		139	53	255	93	151
EW	N	Up	51	328	663	567	12	324	10		730	463		209		9	284	340		551	1254	825	30	600
		Down	10	28	51	40	5	27	6		32	39		18		5	20	29		34	42	28	6	28
		S	2	6	15	8	1	6	2		6	13		6		2	6	22		15	39	31	8	23
		N	57	170	515	346	46	227	48		542	590		184		43	281	122		507	747	230	68	335
	S	Up	25	24	25	24	13	22	8		50	74		22		25	36	35		145	424	47	39	138
		Down	8	14	12	12	6	10	6		15	15		12		10	12	9		29	21	21	9	18
		S	82	90	108	82	58	84	65		84	108		84		80	84	99		125	138	110	95	113
		N	2	3	3	2	2	2	6		29	24		8		2	14	9		37	32	12	3	19
	Cntr	Up	10	18	61	10	3	20	10		73	59		13		5	32	36		235	290	152	22	147
		Down	3	10	9	10	2	7	5		14	12		8		2	8	6		10	18	9	5	10
		S	4	22	13	8	9	11	8		24	33		16		18	20	22		43	52	39	22	36
		N	15	63	41	18	3	28	29		85	103		17		6	48	48		164	151	24	30	83
	Ave Ave		22	65	126	94	13	64	17		140	128	50		17		70	65		158	267	127	28	129
NE-SW	NW	Up	15	12	652	484	64	245	27		48	662		294		137	234	76		304	1520	724	194	564
		Down	9	18	64	42	6	28	5		6	40		43		25	24	9		25	29	58	23	29
		SE	6	4	9	6	2	5	26		15	13		13		4	14	48		20	33	35	6	28
		NW	30	78	583	816	134	328	55		82	195		734		568	327	71		119	699	1190	691	554
	SE	Up	223	19	30	41	12	65	137		188	53		58		38	95	311		192	417	148	47	223
		Down	12	32	10	6	2	12	36		26	20		8		3	19	29		40	21	21	6	23
		SE	336	125	84	54	28	125	871		176	95		84		48	255	674		153	120	103	60	222
		NW	3	0	2	8	2	3	5		8	13		44		21	18	18		24	69	99	63	55
	Cntr	Up	44	18	68	44	24	40	25		39	246		172		16	100	81		70	406	215	21	159
		Down	3	15	28	10	3	12	15		14	29		9		8	15	25		15	25	18	18	20
		SE	18	39	13	11	6	17	163		80	18		24		8	59	225		63	58	58	9	83
		NW	12	17	57	130	91	61	21		30	266		114		55	97	55		61	253	185	179	147
	Ave Ave		59	31	133	138	31	79	115		59	138	133		78		105	135		91	304	238	110	175
NW-SE	NE	Up	144	700	45	9	6	181	279		605	450		15		15	273	319		726	1484	117	42	538
		Down	32	40	45	17	2	27	54		42	28		6		2	26	53		58	29	10	47	39
		NE	544	538	225	5	20	266	1077		1041	458		69		37	536	1201		1470	548	105	56	676
		SW	3	9	3	3	2	4	5		9	8		3		6	6	10		21	48	29	8	23
	SW	Up	16	9	41	76	25	33	56		229	33		149		38	101	106		132	408	585	97	266
		Down	2	9	17	28	3	12	8		12	21		21		32	19	12		15	25	26	29	21
		NE	4	4	2	2	2	3	41		108	9		8		8	35	360		69	73	20	4	105
		SW	38	51	108	156	80	87	55		60	94		213		616	208	71		103	130	548	633	297
	Cntr	Up	68	79	16	25	22	42	102		259	24		51		68	101	464		195	308	77	94	228
		Down	5	10	18	1	2	7	18		20	12		8		3	12	25		17	23	14	10	18
		NE	107	125	35	16	6	58	189		924	45		26		9	239	675		292	82	35	28	222
		SW	9	15	5	12	61	20	27		17	10		32		91	35	35		40	61	96	255	97
	Ave Ave		81	132	47	29	19	62	159		277	99	50		77		133	278		262	268	139	109	211
			Top of canopy							Bottom of canopy									Outside/Ambient PAR
NS		Up	450	669	1244	315	152	566	450		71	105		81		12	144	-		1299	1899	1085	320	1151
		Down	14	9	10	36	8	15	14		97	56		55		3	45	-		65	141	56	62	81
		E	499	259	43	24	30	171	499		705	20		34		2	252	-		1447	90	71	52	415
		W	24	61	125	1281	415	381	24		52	49		535		46	141	-		97	212	1286	1113	677
		Ave	247	250	356	414	151	284	247		231	58		176		16	146	-		727	586	625	387	581
EW		Up	181	2136	1618	1039	262	1047	3		16	3		3		2	5	748		1490	1899	1045	230	1082
		Down	3	14	9	10	5	8	45		75	95		56		14	57	91		117	119	75	23	85
		S	30	101	118	78	129	91	22		50	48		37		13	34	114		116	114	97	76	103
		N	38	224	396	360	26	209	27		63	58		58		21	45	222		675	854	419	74	449
		Ave	63	619	535	372	105	339	24		51	51		39		12	35	294		600	747	409	101	430
NE-SW		Up	664	923	1777	1126	194	937	32		6.1	15		19		3	15	745		1488	1803	1074	279	1078
		Down	17	26	31	36	14	25	31		54	117		62		5	54	62		119	116	48	9	71
		SE	495	148	71	78	35	165	242		45	58		24		4	75	849		347	103	108	67	295
		NW	65	77	728	1144	659	535	29		26	255		43		24	75	86		92	944	1328	580	606
		Ave	310	294	652	596	226	415	84		33	111		37		9	55	436		512	742	640	234	512
NW-SE		Up	867	1129	1852	833	155	967	81		5	8		2		0	19	742		1354	1861	1144	212	1063
		Down	15	14	26	12	12	16	15		97	89		70		3	55	31		64	139	108	8	70
		NE	561	501	561	82	52	351	341		67	91		20		6	105	1393		1522	630	91	56	738
		SW	85	88	114	288	517	218	18		38	29		32		2	24	105		94	100	655	630	317
		Ave	382	433	638	304	184	388	114		52	54		31		3	51	568		759	683	500	226	547

Row orien = row orientation; Can side = canopy side; Cntr = centre of canopy; Ave = average

Figure 4. Photographs (Top) and simplified 3-D canopy modelled images (Bottom) of the sun/shade patterns of vertically trellised Shiraz/101-14 Mgt planted in four different row orientations at the ARC Robertson Experiment Farm on 4 March (photographs taken during grape ripening on 04/03/2010).

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Figure 5. Schematic representation (Top) and simplified perspective view of 3-D canopy modelled images (and the vineyard site in the bottom right-hand corner) (Bottom) of the sun/shade patterns of vertically trellised Shiraz/101-14 Mgt planted in four different row orientations at the ARC Robertson Experiment Farm on 4 March (schematic representations compiled based on distances of sun/shade ground patterns measured at ground level on 04/03/2010).

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Simplified 3-D modelled images of canopy (Figure 4) and soil exposure (radiation intensity and duration) (Figure 6) corresponded almost perfectly with the photographs and field-measured schematic representations, thus confirming that the sun/shade patterns of the vertically trellised vineyard are affected by row orientation. From early morning (08:00) to late afternoon (18:00), the light patterns of the NS orientated canopies clearly show movement of the sun position in the sky over the canopies and the partial sunray-blocking effects of adjacent vine rows. High direct radiation occurred in the basal canopy zone on the E canopy side only at 10:00 (received from above and E); in the middle zone at 08:00 (received from E) and 10:00 (received from above and E); and in the apical zone at 08:00 and 10:00 (both times received from above and E); in all cases, the major light component was received from the E. On the W side of the canopy, high radiation in the basal zone was only received at 16:00 from above and W; in the middle zone at 16:00 from above and W, and at 18:00 from the W; and in the apical zone at 16:00 from above and W, and at 18:00 from the W. In parallel to the above, the central part of the canopy was penetrated by higher levels of light during early morning from the E and during the afternoon from the W, whereas the canopy interior was largely shaded during midday as the sun moved over the canopy from E to W. From late morning to afternoon, the light patterns of the EW orientated canopies mainly showed a dominating radiation effect on the N canopy side, from above, and in the apical part of the canopy. The central part of the canopy was largely shaded, except for the apical zone that was penetrated by a reasonable amount of radiation from the N and from above. Except for early morning (up to 10:00), the NE-SW orientated canopies mainly received high radiation on the NW canopy wall (especially from 13:00) and from above. On the SE side, high radiation was also received during early morning (08:00), after which it faded towards late afternoon. Radiation was also evident from above, especially in the apical zone of the canopy. A similar pattern occurred in the centre of the canopy. The light pattern of the NW-SE orientated canopies was dominated by radiation on the NE canopy wall, especially up to 13:00, thus showing a diurnal and almost mirror image of that of the NE-SW orientation. Early to late afternoon radiation was dominant on the SW side, and radiation from above increased in the higher parts of the canopy. Except for late afternoon, when high levels of radiation were received from the SW, the central part of the canopy mainly received radiation from the NE during the morning (up to 10:00), together with reasonable levels of light from above. The outside (ambient) PAR was intense in the morning until13:00. For the NS orientation, the top, bottom and outside of the canopy mainly received radiation from directly above, as well from the E in the morning and the W in the afternoon; during midday, high radiation was only received from above the canopy. The EW rows received the highest radiation component from above in particular, as well as from the N. For the NE-SW orientation, high levels of radiation were also received from above, as well as from the SE in the morning (up to 10:00) and NW in the afternoon (from 13:00); whereas a similar pattern occurred for the NW-SE orientation, but with high levels of radiation received from the NE in the morning (up to 13:00) and the SW in the afternoon (from 16:00).

Figure 6. Simplified 3-D canopy modelled images of the soil radiation exposure intensity and duration (accumulated diurnal sun hours) in vertically trellised Shiraz/101-14 Mgt planted in four different row orientations at the ARC Robertson Experiment Farm on 04/03/2010.

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Discussion

The upper values of both ambient temperature and wind speed were mostly on the limit in terms of suitability of field climatic conditions for photosynthetic activity, restricting limits being higher than 35 °C and direct velocity of 4 m/s, respectively, onto the leaves (Kriedemann, 1968; Freeman et al., 1982; Hamilton, 1989; Hunter and Bonnardot, 2011; Hochberg et al., 2015). However, these conditions are generally affected by the vineyard row orientation and are therefore variable throughout the day (Hunter et al., 2016).

The canopy radiation interception patterns of the respective row orientations were very different and are comparable to the general meso- and microclimate patterns reported by Hunter et al. (2016) and Zorer et al. (2017). Briefly, the photographic, schematic and modelled patterns and the PAR measurement observations showed that the NS vine rows received low radiation in the early morning and late afternoon and were sunlit on the E wall in the morning, straight on top of the canopy during mid-day, and on the W wall in the afternoon. Sunrays were therefore largely blocked by the adjacent vine rows early morning and late afternoon on the E and W sides respectively. Grape bunches were diffused sunlit or shaded, concomitant to these patterns. The EW orientated rows had a sunlit N side, whereas the S side was largely shaded; grape bunches of these rows were therefore predominantly illuminated from the N and received less sun than those of the NS row orientation. The sun/shade patterns of the NW-SE and NE-SW orientations were almost mirrored diurnally, canopies of both row orientations being illuminated during the morning; however, the former orientation switched sunlit walls later in the day than the latter orientation. The former was therefore exposed to a longer period of morning-incoming sunrays, whereas the latter was exposed to a longer period of afternoon-incoming sunrays.

The radiation that is reflected from the soil into and around the canopy may change according to soil type and colour (e.g., clay versus calcareous versus sandy soils) and in combination with different soil covers or mulches. These factors affect surface colour, as well as thermal storage in the soil and thermal conductivity from soil to atmosphere, canopy and grapes (Hunter, 1998), which in turn are affected by the cordon and canopy height, canopy density and inter-row spacing. Furthermore, all of these sunlight/shade patterns changed slightly with the ecliptic changes as the growth season progressed from budding to full grape ripeness, for example, the canopy walls of the EW orientated rows were more equally illuminated during the green berry period of the growth season, whereas the still developing canopies of all row orientations permitted higher penetration of sunlight during the same period.

Variations in the atmosphere that result in the scattering and transmission of solar radiation during the growth season were a constant factor determining the direct and diffuse radiation components reaching the canopies (Jones and Rotenberg, 2011). The composition of sunlight reaching the interior of the canopies (and proportional number or portions of bunches) also most likely changes from a ratio of preferred higher short:long wavelengths (Red:Far-red radiation) to the less preferred higher long:short wavelengths as the growth season progresses and the volume and density of the canopy continue to increase (Smart, 1987). This has implications for photo-morphogenesis and the activity of many enzymes involved in the photosynthetic process and secondary compound formation, via the effect on phytochrome (Mitrakos and Shropshire, 1972; Smart, 1987). The results only reflect the photosynthetically active region of the incoming solar short wave radiation (380–710 nm) that comprises 21–46 % of energy, 4 % of ultra-violet (290–380 nm) and 50–70 % of near infrared (710–4 000 nm); therefore, radiation may have further impacted the energy, radiation and temperature regulation (and heat storage) of the canopy to incoming solar energy (Ross, 1981; Jones and Rotenberg, 2011). The different canopy sides are fully or proportionally exposed at different times of the day. Therefore, the thermal (long wave or infra-red between 3000–4000 nm and 100000 nm) radiation portion (release of energy) from the canopy bodies also dynamically changed diurnally, which may have led to differences in energy balances (radiative transfer, sensible heat transfer, latent heat transfer, and transfer to or from storage) of the differently orientated canopies. Nocturnal energy exchange is affected. The energy balance fluxes of the canopies changed according to the soil-vineyard-atmosphere continuum, irrespective of row orientation, but were dominated by the radiative fluxes onto and through the canopies.

On a practical level, the height, homogeneousness and density of the canopy walls affected the extent of exposure/shading of the different canopy sections (from bottom to top; e.g., bunch exposure timing and duration) and the soil. This is qualified by the spatial distribution of the incoming light, which is affected by the orientation of the row. In a broader sense, different latitudes also affect the sun path and thus the inclination and energy level of the incoming radiation. The latitude is taken into account in the 3D modelling approach that was used in this study.

Al-Kayssi et al. (1990) found that solar energy absorption and the heat storage capacity of soil both increase as water content increases, but in the present study, such responses were not found to be consistent in the different soil layers. Water fluctuation in the different soil volumes most likely mainly resulted from the surface area between the rows being exposed to sun, a generally higher concentration of roots directly below the vines (Hunter, 1998), and a high evaporative demand on the canopy (affecting leaf vapour pressure deficits) on the day of measurement. The soil in the experimental vineyard was accepted as being uniform with similar bulk density (which was determined before establishing the vineyard, following which the same management actions were applied between the rows). The latter per se is therefore unlikely to have affected the thermal diffusivity of the soil (thermal diffusivity = ratio of thermal conductivity:volumetric heat capacity), and thus the results as found. The temperature fluxes in the top soil layers generally paralleled the increase in ambient temperature, whereas the temperature variation (amplitude) generally faded with soil depth. In the afternoon, all soil layers of the EW orientated rows reached the highest temperature values - most probably due to the larger and continuously exposed inter-row surfaces - in comparison to those of the other row orientations (as also confirmed by the canopy sun/shade photographs, schematic presentations and 3D modelling of canopy sun/shade patterns and the intensity and duration of soil surface exposure); this corresponded to lower soil water contents between the rows in the afternoon.

The results of this study fit the general positive relationship which has been found between air temperature and soil temperature, due to the effect of energy balances at ground surface level (Zheng et al., 1993; Wu and Nofziger, 1999). The larger and longer inter-row exposed area in vineyards with EW row orientation (Figure 6) may have led to higher thermal radiation and dampening from the soil, which may have affected the temperature of the canopy-body (Jones and Rotenberg, 2011). It is conceivable that the radiation climate, atmospheric conditions and soil conditions, along with the architecture, morphology and orientation of the different canopies, affected canopy heat fluxes and absorption, reflection, transmission and emittance of radiation. Under normal field conditions, this may never be a steady state condition, but rather a dynamic system, as impacting factors, such as wind fluxes and clouds, will always induce resistance (impediment) or stimulation in the morphologically and physiologically active canopy bodies.

The Ψ_L results show that radiative energy has induced changes at soil, canopy and physiological levels, which complimentarily affected plant water relations by driving a general decrease from the morning to the afternoon for all row orientations. The highest Ψ_PL and Ψ_SL values that were measured for the EW orientated canopies are in agreement with those found by Hunter et al. (2016), but seem to contrast with the apparent higher depletion of soil water contents and higher soil temperatures found in the EW orientated rows in the afternoon. Medrano et al. (2015) also showed the complexity of water relations in grapevine canopies and the difficulty in upscaling from leaf to canopy level under field conditions on a diurnal and seasonal basis, reflecting the effects of leaf position, light interception, canopy density, soil water conditions and additionally, night transpiration and respiratory losses.

Photosynthetic activity of all the leaves in the canopy was clearly affected by the inclination of the sun, as demonstrated by the much higher P_n of the N versus S canopy sides. This was also found by Hunter et al. (2016). The differences in P_n amongst canopy sides may be due to stomatal movements responding to intrinsic physiological inter-relationships and diurnal below- and above-ground environmental impact. Reduced P_n in the afternoon has also been previously found under different field conditions (Hunter et al., 1994; Haasbroek et al., 2000), and over-estimations with leaf gas exchange models are common under ample sunlight conditions in the afternoon, indicating photo-inhibition (Prieto et al., 2012). The lower afternoon P_n may have been driven by a combination of high radiation intensity, lower Ψ_L, and increasing ambient temperatures, peaking at around 15:00 to 17:00 at 30–35 °C on the days of measurement in this study. The preceding diurnal carbon assimilation, as clearly demonstrated by Hunter et al. (1994), may also have benefitted the canopy as a whole, irrespective of side. Feedback inhibition may well have been a restricting physiological response to further high P_n rates in the afternoon, but under field conditions, the existence of carbon partitioning into photosynthesising reproductive and perennial (including root) tissues renders this physiological mechanism more complex than just a simple regulation by starch and/or sucrose build-up (Hunter et al., 1994, Hunter et al., 1995). The preference for transitory starch accumulation instead of sucrose export from the leaves during the day changes in favour of the former, especially during late season. Furthermore, soil water content decreased from 10:00 until 16:00, whereas the temperature in the top soil layer (0–60 cm) increased. These conditions may have increased the impact of radiative (ultraviolet, visible and infra-red) energy and soil emittance on leaf temperature; the normal evaporation cooling mechanism of the leaves for optimum activity may thus have been inefficient for leaf temperature regulation and energy balance control (Jones and Rotenberg, 2011). Altogether, these conditions may also have triggered a restricting stomatal control signal from the roots and in the leaves in the afternoon, most likely involving abscisic acid (Lovisolo et al., 2002; Patakas et al., 2005; Hunter et al., 2014a).

The general decrease in photosynthetic output from the apical to the basal part of the canopy was also found by Hunter and Visser (1989) and Hunter et al. (1994). The low erratic values measured in the interior of the canopy in both basal and middle zones are to be expected, due to the commonly occurring lower light conditions in these zones that limit P_n. However, these leaves clearly responded positively to light exposure from the basal to the apical zone, although they were mostly sheltered from direct radiation. In addition, higher overall P_n in, for example, the NS and EW row orientations also corresponded to higher water retention in the canopies. These results confirm the importance of canopy age, light exposure and water potential in the driving of grapevine functioning.

Given the above, results indicate a cumulative physiological impact on temporal status. This also emerged from intensive WUE measurements on various genotypes, showing the significant effects of leaf position, light interception and water balances (Medrano et al., 2015). The results are in agreement with those found for the same variety under different (Novello and Hunter, 2004) and similar (Hunter et al., 2016) growth conditions. However, the WUE results are not in agreement with the findings of Medrano et al. (2015), who found the highest values at midday, relatively high values in the centre of the canopy, and no specific trend for different leaf positions; these discrepancies may have resulted from differences in growth conditions. The results clearly display an interaction between the trellising system, row orientation and growth (including vigour and accommodation of the shoots). This was also shown in modelling approaches with different cultivars and trellising system configurations (Louarn et al., 2008), confirming the difficulty in integrating climate, genotype and practices in order to reach the product objectives of yield and quality.

The WUE results in this study point to a complex whole plant internal regulation of output, despite the agreeable trends found between the different physiological parameters and radiation. Overall, the results reflect the positive impact of the azimuth and penetration of the sun into the canopy on P_n during the ripening period. The P_n performance of canopy sides during this period seem mostly driven by the presence and responsiveness of secondary leaves, confirmed in this study by their generally higher P_n output than that of primary leaves. The measurements were taken during the latter part of the ripening period when secondary leaves were generally younger and more responsive to environmental changes (see also Hunter et al., 2014). These leaves have an increasingly important role in supplying and transporting sucrose, as well as in supporting further formation of secondary compounds inside the bunches during the course of ripening (Allen and Lacey, 1993; Marais et al., 1999; Hunter and Ruffner, 2001; Ojeda et al., 2002). They may also be assumed to be less sensitive to abiotic influences on the plant, compared to primary leaves. Patakas et al. (1997) found that the capability for osmo-regulation was almost the same in mature and immature leaves, but the elasticity of cell walls, and thus the ability to maintain positive cell turgor, decreased with age. Younger secondary leaves may therefore have increased isohydric behaviour (active stomatal regulation of transpiration – Naor and Wample, 1994; Escalona et al., 2002), which may contribute to a better ability to buffer the impact of unfavourable environmental conditions (e.g., high temperatures and low soil water contents) on grape development and ripening (Hunter, 2000; Hunter et al., 2004; Novello and Hunter, 2004; Hunter et al., 2014a, 2014b). In addition to water management (and appropriate fertilisation programmes), it is important for the initiation and development of secondary leaves to be stimulated pre-véraison by judicious canopy management in order to maintain canopy photosynthetic capacity and to increase the potential of the canopy to support and protect the grapes when adverse environmental conditions are experienced during the ripening period (Hunter, 2000). In this experimental vineyard, the primary:secondary leaf area ratio during this period was calculated to be 0.85; the final canopy and leaf exposure dimensions measured over a 7-year period between treatments have been found to be similar (data presented in Hunter et al., 2017). The contribution of secondary leaves to total canopy photosynthetic output was thus substantially higher than that of primary leaves during ripening, even when excluding smaller (apical) leaves (on both primary and secondary shoots) that may have still been displaying sink-behaviour (and despite the fact that active growth had already halted).

Under any circumstances, the water status of soil and plant is critical for their maintenance, restriction or stimulation of growth and survival, and thus for attaining product objectives (Hunter et al., 2014a, Hunter et al., 2014b). In addition, as Ψ and P_n of the leaves are sensitive to sun exposure, the latitude (e.g., Southern versus Northern grape growing countries) and concomitant ecliptic changes during the growth season affect the response and output of the canopy, relative to the vineyard row orientation. For example, Zufferey et al. (1999) found that the net diurnal photosynthesis of Chasselas leaves on S exposed canopy sides of EW orientated rows in the Northern hemisphere was highest during the whole growth season, in comparison to the N side found in this study. Moutinho-Pereira et al. (2003) found a higher photosynthetic rate for Touriga Nacional leaves on the NE side of NW-SE orientated rows in the morning, while that of leaves on the SW side was reduced in the afternoon due to stomatal and non-stomatal restrictions.

In general, the integrated photosynthetic output of the canopy and the metabolic activity of the vine can be further improved by timely and properly executed canopy management in order to stimulate the development of source-behaving secondary leaf availability during the grape ripening period and accommodating all leaves in such a way that they satisfy the dual foliage functioning objective; i.e., to reach the maximum physiological potential for supplying to the grapes, and at the same time to fulfil the physical role of protecting the grape bunches against over-exposure to the sun (Kriedemann, 1968; Pandey and Farmahan, 1977; Hunter et al., 1988a; Hunter et al., 1988b; Smart et al., 1990; Hunter et al., 1994; Hunter et al., 1995; Hunter, 2000; Hunter et al., 2004; Poni et al., 2009; Chorti et al., 2010; Hulands et al., 2014; Hunter et al., 2016; Zorer et al., 2017). The terroir environmental conditions (specifically: orographic, edaphic and climatic) largely determine the balance between these two objectives. Such conditions may range from extreme cool conditions, in which thin canopies and almost full grape exposure may be required, to extreme hot conditions, in which partial protection of the grapes by the leaves would be favourable for yield and disease-free grape ripening to achieve optimum ripeness; for the latter, it is critical to understand and manage the interplay between grape exposure and ripeness level (between under- and over-ripe) in both environmental scenarios.

In the present study, direct and diffused radiation interception was measured in the canopy layers of the different row orientations. The diffused radiation component of the EW row orientation was hampered by the vertical shoots and their topping, as well as the sun path from E to W over the top of the canopy. However, a shift towards direct exposure of the N side occurred as the growth season progressed. Nonetheless, an EW row orientation would provide more protection to grapes in especially hot climatic conditions (particularly on the S side, in the Southern hemisphere), whereas it may most likely be too shaded in cool conditions (particularly for red grapes). In contrast, the NS orientation resulted in a larger direct radiation component because of the (almost) perpendicular angle of incidence of the sun in the morning on the E side and in the afternoon on the W side; however, grapes were diffusedly shaded over the (critical) midday period. The diffused radiation component was largest, and direct radiation component smallest, for the NW-SE and NE-SW orientations, the major difference being that the former received higher levels of incidence in the morning on the NE side and the latter received higher levels of incidence in the afternoon on the NW side; in general, slightly more radiation was intercepted by the NW-SE orientation. Despite the diffused radiation, the central canopy zones of both of these row orientations were better exposed than those of the NS and EW orientations. Several studies have already shown that it is best to be cautious when there is the possibility of direct radiation on the grapes (especially in hot climates), as this may have deleterious consequences (Smart et al., 1990; Allen and Lacey, 1993; Hunter et al., 2004; Tarara et al., 2008; Chorti et al., 2010; Hulands et al., 2014), such as high pulp temperatures, bunch rot, poor skin colouring, low titratable acidity, high pH and loss of flavour. Diffused radiation is thus generally preferred for grape and wine quality, although a shift towards a larger direct radiation portion may benefit the grapes in cool to cold conditions. Evidently, the heterogeneous ripening of grapes exposed on the two sides of vertical canopies of differently orientated rows can occur under any management conditions, and intra-bunch compositional differences can be expected. The level of impact of the differential conditions induced by a specific row orientation could naturally be determined by the terroir conditions, sensitivity of the variety-rootstock combination, and the objectives regarding yields as well as grape and wine quality. Although the effects of over- and under-exposure of grapes on composition and wine quality are relatively well-known, the specific implications of the intensity and duration of direct and diffused radiation at different times of the day and growth season have still not been elucidated and present a major challenge under field conditions.

A number of integrated factors drive the functional response of the grapevine canopy to radiation at any given time: continuous adaptation to or compensation for temporal changes in the root system (responding to the biotic and abiotic soil environment and aboveground growth and maintenance demands from vegetative and reproductive organs); water relation status (as affected by evaporative demands and internal physiological requirements and mechanisms); biochemical status and responsiveness, which may lead to an unsaturated, temporally saturated or fully saturated plant substrate or enzyme reaction status (according to the combined impact of diurnal/nocturnal environmental conditions and physiological processes in aboveground and belowground organs); general leaf age composition (source-sink status ratios and levels) and exposure of the canopy, as affected by the seasonal growth progression, physiological senescence, and physical leaf loss or abscission, either as a result of manipulation, environmental stress or because of the normal perennial cycle (Hunter, 2000). The energy balances and seasonal dynamics of physiological parameters (such as carbon assimilation and distribution, and water relations) can therefore be affected by the specific location, environmental conditions and long- and short-term cultivation practices to which the grapevines are exposed and which could impact aspects such as vigour potential and bud fertility. Bearing this in mind, the yield of the NS orientated vines was highest, followed by that of the NW-SE, EW and NE-SW orientated vines (Hunter et al., 2017), despite the results indicating higher plant water retention of the EW orientated vines and similar overall average photosynthetic activity to that of the NS orientated vines. The NS orientated vines also had the most stable yields over the seven consecutive years of monitoring (Hunter et al., 2017). The yields were differentially affected by canopy orientation and the ripeness level of the grapes when harvested. Total yield losses of 17 % occurred over the measuring period, from a ripeness level of approximately 23 °Balling (total soluble solids) to approximately 27 °B. In affecting spatial and temporal leaf and grape bunch exposure (Hunter et al., 2016; Zorer et al., 2017) and carbon allocation (Hunter et al., 2017), row orientation clearly has a critical role in the extent to which objectives related to physiological output and yield (as well as grape and wine quality) can be reached; it is important to be judicious when considering canopy (micro)climatic exposure and vineyard practices during both pre- and post-véraison periods to favour the maintenance and formation of compounds in the berries (Bois et al., 2008; Hunter et al., 2010; Hunter and Bonnardot, 2011; Hunter et al., 2014b; Hunter et al., 2016). High photosynthetic activity (sucrose availability) during the pre-véraison period contribute to primary and secondary compound pool availability in the berries when the ripening period starts, whereas it will largely buffer a decrease in organic acid (conversion to salt forms and metabolism) and an increase in pH during the post-véraison period. This favours further development and accumulation of the secondary compound pool required for quality grape composition and wine (Iland, 1987; Patrick, 1997; Riou, 1998; Hunter, 2000; Hunter and Ruffner, 2001; Ojeda et al., 2002; Hunter et al., 2004; Hunter and Bonnardot, 2011).

Choice of row orientation is therefore more complicated than merely selecting the highest or lowest water potential and photosynthetic output; it must also take into account terroir- and management-related factors, as well as other (viticultural) factors, such as bud fertility, yield, health and composition of grapes, and wine quality objectives. Furthermore, the radiation patterns of the differently orientated canopies change according to the ecliptic changes as the growth season progresses from bud break to full grape ripeness. Along with growth and source:sink dynamics in the canopy (Hunter, 2000) and canopy height, density and seasonal practices, the intensity and duration of exposure/shade can induce temporal and seasonal (including cumulative) differential leaf responses and have an effect on canopy functioning and grape development and ripening. This should all be considered at a commercial level, taking into account grape and wine product objectives. Judiciously chosen practices and careful management are therefore necessary to avoid risks and obtain the full value of a long-term cultivation practice, such as vineyard row orientation.

Results of this study provide evidence of the interaction between environmental impact factors (especially radiation), genotype growth characteristics and physiological response mechanisms and control, and cultivation practices. This detailed study on light microclimate and a variety of individual leaves under common field cultivation conditions may lay a better foundation for the up- and downscaling to, and understanding of, the light conditions, photosynthesis and leaf water potential of whole plants and differently orientated and managed canopies. A broad image of the horizontal and vertical functional layers in the canopies has been obtained, which can also be used to predict potential canopy physiological output under different terroir conditions, whether by means of quantitative observations or modelling. Such output is affected by grapevine growth and manipulation (by means of, e.g., pruning, shoot topping, leaf thinning), canopy geometric shape, space and composition (primary and secondary shoot number and lengths; shoot internode lengths and distribution; wired or free hanging shoots and their orientation; leaf size, distribution, age and morphology), inter-row spacing, and radiation inclination, penetration, efficiency, sufficiency and consistency. These are all in turn largely affected by the direction of the grapevine rows, terroir conditions and management practices (thus affecting vine-to-vine uniformity and continuity). Site specific conditions (orography; soil fertility, depth, water holding capacity and drainage; rainfall; macro- and meso-climate) are therefore critical to the establishment of practical and functional vineyard row orientations, defining the extent to which grapevine morphological development and physiological output related to yield, grape composition and wine quality will be favoured.

This study contributes to better understanding the complexity of interrelationships between the microclimate, water status and photosynthesis of leaves in canopies affected by vineyard row orientation within specific terroir conditions. Data on leaf water potential showed that EW vineyard row orientation resulted in the highest overall water retention in the canopy, despite lower afternoon soil water contents and higher soil temperatures. In general, the NE-SW orientation showed the lowest overall water potential in the canopy. The water retention of leaves in the centre of the canopies tended to be the highest, while photosynthesis was at its lowest, with output being clearly limited by the light blocking effect of the leaves on the outer rim of the canopy. Photosynthetic activity reflected the positive impact of the sun azimuth during the ripening period in the Southern Hemisphere; leaves on the E- and N-exposed canopy sides were evidently superior during this period, mostly been driven by the presence and responsiveness of secondary leaves. Results confirmed the need for management actions that will ensure the presence of secondary shoots with younger leaves in the canopy in order to maintain the carbohydrate pool, supply carbohydrate to bunches and reserve compartments, sustain grape ripening, and protect bunches from over-exposure. Concomitantly, the study clarified the differential importance of leaves in differently orientated canopies and the requirement for judicious exposure.

This comprehensive and novel study contributed to the hitherto demanding challenge of capturing the physiological complexity of grapevine canopies under field environmental conditions. The knowledge acquired will be of great value in facilitating the selection of grapevine row orientation, row and plant spacing, irrigation and fertilisation, as well as canopy management practices at commercial level for different soil and climatic conditions. The sun/shade patterns could assist in obtaining very clear visualisations and judgements of the intensity and duration of exposure of the grapes in grapevine rows, and the expected concomitant morphological-chemical implications for any product objective. This should be considered in existing and future vineyard planting per terroir, and according to grape and wine product objectives of the vineyard, to guarantee uncompromised and sustainable fertility and yield, healthy grapes, and manageable and predictable grape composition.

Acknowledgements

We would like to thank the Agricultural Research Council and the South African Wine Industry (through Winetech) for funding. We would also like to extend our gratitude to the personnel of the Viticulture Department and the farm personnel at the Robertson Experiment Farm of ARC Infruitec-Nietvoorbij for their assistance. Furthermore, we are sincerely grateful to many international and local collaborators and specialists for their diverse contributions.

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