Long‐ and short‐term effects of decreased sink demand on carbohydrate levels and photosynthesis in wheat leaves

Abstract

ABSTRACTWheat (Triticum aestivum L.) ears were removed to investigate long‐term regulation of photosynthesis by sink demand at ambient CO2 and 22 °C. The CO2 level was also increased to 660 μmol mol−1 and temperature was lowered to 5 °C to examine short‐term responses of photosynthesis to low sink demand. Sink removal inhibited photosynthesis and increased leaf levels of glucose, fructose and ribulose‐1, 5‐bisphosphate (RuBP), and the glucose‐6‐phosphate (G6P)/fructose‐6‐phosphate (F6P) and RuBP/3‐phosphoglycerate (PGA) ratios under growth conditions, but had no effect on the activity and activation state of ribulose‐1, 5‐bisphosphate carboxylase oxygenase (Rubisco) either under growth or short‐term conditions, suggesting an inhibition of photosynthesis by decreased in vivo catalysis of Rubisco. Photosynthesis increased similarly in eared and earless shoots after a rise in CO2 concentration, and the ratio of triose‐phosphates (glyceraldehyde 3‐phosphate and dihydroxyacetone phosphate, TP) to PGA was similar or higher for removed than intact ears, suggesting that feedback inhibition of photosynthesis was not caused by a limitation of ATP synthesis in chloroplasts. Under short‐term conditions (660 μmol mol−1 CO2, 5 °C), TP and RuBP levels and the TP/PGA and TP/RuBP ratios were increased by sink removal, indicating an additional limitation of photosynthesis by the rate of RuBP regeneration.