Sink feedback regulation of photosynthesis in vines: measurements and a model

An experimental and modelling study of source-sink interactions in Vitis vi nifera L., cv. Cabernet Sauvignon, rooted cuttings under non-limiting envir onmental conditions with a 12 h photoperiod is presented here. After 4 h, m easured photosynthesis, stomatal conductance and leaf carbohydrate content reached maximum values. Over the remainder of the photoperiod, photosynthes is and stomatal conductance decreased continuously, whereas leaf carbohydra te content remained relatively constant. Because the experiment took place in a non-limiting environment, the results suggest that stomatal regulation of photosynthesis was mediated by an internal factor, possibly related to sink activity. A simple I-source, 2-sink model was developed to examine the extent to which the data could be explained by a hypothetical sink-to-sour ce feedback mechanism mediated by carbohydrate levels in either the mesophy ll, the source phloem or the phloem of one of the two sinks. Model simulati ons reproduced the data well under the hypothesis of a phloem-based feedbac k signal, although the data were insufficient to elucidate the detailed nat ure of such a signal. In a sensitivity analysis, the steady-state response of photosynthesis to sink activity was explored and predictions made for th e partitioning of photosynthate between the two sinks. The analysis highlig hts the effectiveness of a phloem-based feedback signal in regulating the b alance between source and sink activities. However, other mechanisms for th e observed decline in photosynthesis, such as photoinhibition, endogenous c ircadian rhythms or hydraulic signals in the leaf cannot be excluded. Never theless, it is concluded that the phloem-based feedback model developed her e may provide a useful working hypothesis for incorporation into plant grow th models and for further development and testing.