Low temperature effects on grapevine photosynthesis: the role of inorganic phosphate

The photosynthetic response of grapevine leaves (Vitis vinifera L. cv. Riesling) to low temperature was studied to determine the role of end-product limitation and orthophosphate (P_i) recycling to the chloroplast under these conditions. As reported previously, the response of photosynthesis in air to stomatal conductance declined at temperatures below 15°C, suggesting that at low temperatures inhibition of photosynthesis in grapevine has a strong non-stomatal component. Stimulation of carbon assimilation at ambient CO₂ by reducing O₂ from 21 to 2 kPa, O₂ declined to zero below 15°C, a phenomenon often associated with a restriction in photosynthesis due to end-product-synthesis limitation. This stimulation could be restored by feeding P_i. Photosynthesis in leaf disks at both high and low irradiances in non-photorespiratory conditions (1% CO₂) was highly sensitive to reductions in temperature. Below 15°C, feeding Pi caused a large stimulation of photosynthetic O₂ evolution. Metabolite measurements indicated that despite a decline in Rubisco carbamylation state, ribulose 1,5-bisphosphate (RuBP) levels dropped at low temperature and the ratio of 3-phosphoglycerate (3-PGA) to triose phosphate (TP) remained largely unchanged. These results suggest that grapevine-leaf photosynthesis is severely restricted at low temperature by non-stomatal mechanisms. The return of P_i to the chloroplast plays an important role in this limitation but a coordinated set of regulatory processes maintain a homeostasis of phosphorylated sugar levels.