Plants are in general subject to rapid fluctuations in the environment, particularly as regards the interception of light and therefore water regime. It is important to know the duration and the amplitude of response of the water regime (first leaf water potential) when light interception changes suddenly under natural conditions. widely planted grape variety in the world) was chosen mainly because it offers convenient possibilities of canopy manipulation, ranging from an open and exposed type architecture, to a closed and shaded configurations simply by moving inside or outside a «foldable Lyre» type trellis. Leaf water potential was measured throughout particular days corresponding to stable or variable controlled conditions. The experimental design was an Ecotron in which it is possible to control water and mineral supply and the levels of light interception by plants which have normal development and are growing under natural environmental conditions. The most significant results that emerged from this study are that leaf water potential reacts quickly to any change of light interception, and above all, that the response is almost instantaneous - occurring within few minutes - when transpiration demand decreases, and is significantly longer when the transpiration demand increases, even if the water supply corresponds to maximum transpiration conditions. This shows that the grapevine has a general regulatory system which encourages the plant to economise water even if the supply is sufficient. This could indicate the existence of an overall plant regulatory system - mediated via the water potential - which is important for the coherence of the plant’s physiology and which is quite independent of the environment. From a general point of view, this would confirm the existence of biological triptych network with a superimposed regulatory element (CARBONNEAU, 1996 ; CARBONNEAU and DELOIRE, 2001).
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