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

Morcuende, R., Perez, P., Martinez-Carrasco, R., Martin del Molino, I., Sanchez de la Puente, L.
Plant, cell and environment 1996 v.19 no.8 pp. 976-982
source-sink relationships, ribulose-bisphosphate carboxylase, carbon dioxide, Glycine max, buds, sugar phosphates, photosynthates, cutting, leaf conductance, chemical constituents of plants, enzyme activity, chlorophyll, cold, shoots
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 micromole 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 (GOP) 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 micromole 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.