Long- and short-term responses of leaf carbohydrate levels and photosynthesis to decreased sink demand in soybean

Axillary buds and the apical portion of shoots of soybean [Glycine max (L.) Merr. cultivar Turchina] plants were trimmed to investigate long-term regulation of photosynthesis by sink demand at ambient CO2 and 22 degrees C, Also, in intact and trimmed shoots, the CO2 level was increased to 660 mu mol mol(-1) and temperature was lowered to 5 degrees C to examine the superimposed short-term responses of photosynthesis to low sink demand, Under growth conditions, trimming the shoots increased leaf photosynthesis and the levels of sucrose, glucose-6-phosphate (G6P) and 3-phosphoglycerate (PGA), as well as the G6P/fructose-6-phosphate (F6P) and sucrose/starch ratios, while it decreased the level of starch and the triose-phosphate (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, TP)/PGA ratio, Photosynthesis enhancement was accompanied by increased chlorophyll contents and ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) activity. Sink removal consistently increased photosynthesis measured under a variety of conditions (growth CO2 or a short-term change to 660 mu mol mol(-1) CO2; growth temperature or a short-term change to 5 degrees C), except when low temperature was combined with ambient CO2; the increase in photosynthesis was higher under short-term elevated CO2 than at ambient CO2, In contrast with its effect at ambient CO2, shoot trimming increased the levels of TP and ribulose-1,5-bisphosphate (RuBP) and the TP/PGA ratio under high-CO2 conditions.