Endogenous Rhythms in Photosynthesis, Sucrose Phosphate Synthase Activity, and Stomatal Resistance in Leaves of Soybean (Glycine max [L.] Merr.)

Abstract

Experiments were conducted with soybean (G. max [L.]. Merr. cv. Ransom) plants to determine if diurnal rhythms in net CO2 exchange rate (CER), stomatal resistance and sucrose-phosphate synthase (SPS) activity persisted in constant environmental conditions (constant light, LL; constant dark DD) and to assess the importance of these rhythms to the production of nonstructural carbohydrates (starch, sucrose and hexose). Rhythms in CER, stomatal resistance and SPS activity were observed in constant environmental conditions but the rhythms differed in period length, amplitude and phase. These photosynthetic parameters are not controlled in a coordinated manner. The activity of UDPG [UDP-glucose] pyrophosphorylase, another enzyme involved in sucrose formation, did not fluctuate rhythmically in constant conditions but increased with time in plants in LL. In LL, the rhythm in CER was correlated positively with fluctuations in total chlorophyll (r = 0.810) and chlorophyll a (r = 0.791) concentrations which suggested that changes in pigment concentration were associated with, but not necessarily the underlying mechanism of, the rhythm in photosynthetic rate. Assimilate export rate, net starch accumulation rate and leaf sucrose concentration also fluctuated in constant light. No single photosynthetic parameter was closely correlated with fluctuations in assimilate export during LL; assimilate export may have been controlled by interactions among the endogenous rhythms in CER, SPS activity or other metabolic factors which were not measured.