Analysis of "sink-photoassimilation" relationship in field-grown 'Cabernet Sauvignon' grapevines.

elationships between sink strength and leaf photoassimilation capability were evaluated on field-grown 'Cabernet Sauvignon' grapevines. Single shoots were subjected to cluster deprivation, girdling and defoliation treatments four weeks before harvest and compared to control untreated shoots. The capacity of grapevine leaves, irrespective of their type and age, to turn light into energy clearly depends on their ability to allocate the sugars produced via photosynthesis under daylight. While total defoliation of individual shoots obviously caused a remarkable reduction in stem starch accumulation, cluster deprivation as well as shoot girdling resulted in a notable accumulation of sugars as starch grains in the chlorenchyma cells, situated just below the epidermis of shoots. These two treatments elicited a severe drop in leaf photosynthesis imputable to an overall curtailment of sink strength. This inhibitory feedback mechanism of assimilate metabolism, which was also found in the leaves of laterals, highlighted a direct link between growth activity and sugar-storage sites on one hand and carbohydrate-formation sites (i.e., chloroplasts) on the other. Therefore, both primary and lateral leaves of grapevines fully activated the physiological processes involved in synthesis only if they can allocate their end products. If not, there is a marked down-regulation in CO2 fixation and water transpiration processes, which lead to a substantial decrease in water loss. The lack of active metabolic sinks in grapevine, a situation that could arise in response to excessive cluster thinning or low bud load, can depress the photosynthetic activity of the entire vine and nullify the expected advantages, especially as to grape quality, from a particularly high leaf-to-cluster ratio