Association between sucrose-phosphate synthase activity in leaves and plant growth rate in response to altered aerial temperature

Vegetative soybean plants (Glycine max (L.) Merrill, cv. 'Ransom') were grown at three aerial temperatures to examine the relationship between sucrose-phosphate synthase (SPS) activities of leaves and whole-plant growth rate. Plans were exposed for 17 days to day/night temperatures of 26/22°C, 22/18°C, or 18/14°C, or 18/14°C, which resulted in a progressive lowering of relative growth rates (0.129, 0.109 and 0.067 g g^-1 day^-1, respectively). Net CO₂ exchange rates (CER), SPS activities and calculated assimilate export rates of the most recently expanded trifoliolate leaf in the canopy were decreased at lower temperatures. The temperature effects were reversible, as CER and SPS activities of selected expanded leaves were increased within 3 days after plants were switched from 18/14°C to 30/26°C and decreased following transfer from 26/22°C to 18/14°C/. The results are interpreted as evidence that changes in the activity of SPS are an integral part of the coordinatedadjustments in the leaf photosynthetic system which limit formation and export of sucrose as the temperature in the growth environment declines.