Photosynthesis and Assimilate Partition in Fruiting and Non-Fruiting Grapevine Shoots

Abstract

The identification in a plant of limiting conditions due to the source or the sink is a vital step in any study of crop productivity. In fact dry matter accumulation in a fruit can be limited either by the supply of assimilates or by the storage capacity of the fruit. Unfavourable environmental conditions or strong competing sinks can be responsible for a poor carbon flow to the economically useful organ. Manipulation of the sink (e.g., through its complete or partial removal) is a technique used in order to alter the demand for assimilates. It is, therefore, possible to check whether photosynthesis is under the control of the sink activity. The relationship between the rate of assimilate consumption by the sink and photosynthetic activity has been the subject of great controversy (3; 11; 14). There is ample evidence (3; 14) that photosynthetic carbon fixation responds, at least in some physiological stages, to an increa sed sink strength. Not so evident, however, is the often suggested product-inhibition hypothesis (4; 11), i.e. that accumulation of assimilates in an illuminated leaf may be responsible for a decrease in net photosynthesis. Some recent data (10; 12) did not support this hypothesis. However, it seems plausible (3) that some fraction of the photoassimilate, namely starch, can have inhibition effects on photosynthesis. In shading ex periments with soybeans (14) starch decreased markedly while sucrose and photosynthesis increased in those leaves that exported carbon most intensely. Glucose and fructose concentrations remained constant. This supports Ho (5) and Geiger (3) hypothesis that sucrose concentration in the source leaf mesophyll controls export. It was also stated (6) that leaves with a higher carbon status export at a higher rate for a given net photosynthetic rate. Some of those conflicting observations may be related to the species studied (7). Fruit load reduced the starch content and the soluble carbohydrates in the leaves of the eggplant, while in the strawberry plant the leaf content of soluble sugars was equally higher whether fruits were present or absent.