Regulating yield and wine quality of minimal prunning systems through the application of gibberellic acid
The use of gibberellic acid to increase berry size and to loosen up bunches in the production of seedless table grape varieties has a long history. The use of gibberellic acid on wine grapes, however, is very limited, mainly because some varieties can show substantial losses in bud fertility and thus in commercial yield. We tried to exploit the feature of decreases in bud fertility as a response to gibberellic acid applications (in the form of GA3) to regulate yield and quality aspects of minimally pruned (MP) White Riesling grapevines where other thinning methods have failed. Single applications of 50 mg L-1 GA3 at 500 L ha-1 were used during full bloom in each year from 2002-2004 (MP GA3-02/03/04) in one MP treatment, while the second MP treatment received no GA3 in 2003 (MP GA3-02/04). Treatments were compared with untreated MP vines and with a standard pruned, vertically shoot positioned system (VSP). GA3 applications reduced inflorescence number by about 30-50% the year following treatment, while shoot number remained unchanged. GA3 treated MP vines responded to this type of « thinning » by a 10-28 % increase in berry weight. Total yield of MP vines was reduced by 26-49% the year after first time application approaching VSP yield. Yield of the continuously treated vines (MP GA3-02/03/04) remained comparable to pruned VSP vines up to now (harvest 2005) (about 9-13 tons per ha). Discontinuing the GA3 treatment for one year caused yield to re-approach the one of untreated MP vines the next year. Despite of the increase in berry weight, bunch structure remained less compact as compared to VSP fruit. Sugar levels at harvest of vines treated with GA3 the previous year were similar to sugar levels of VSP vines and superior to untreated MP vines. We found no treatment effect on glycosyl-glucose (bound secondary metabolites, GG) concentration but a slight increase in phenols and titratable acidity the year after GA3 treatment. Sensory evaluation of the resulting wines revealed no or only minimal differences with a slight preference for the least yielding VSP and MP GA3-02/03/04 wines. The outlined strategy seems promising for the production of quality fruit in a highly economic viticultural system.
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