Carbohydrate partitioning, photosynthesis and growth in Lemna gibba G3.I. Effects of nitrogen Limitation

Abstract

The growth rate of Lemna giba L. G3 was varied by limiting the supply of nitrogen (N) under otherwise constant condition. Two experimental approaches were used. 1) A series of suboptimally growing cultures were supplid daily with exponentially increasing doses on N. 2) Optimally growing cultures were transferred to a N‐fre medium and cultivated in it for 10 days. Lveles of starch, sucrose, glucose, fructose, and systems. At RGR ranging from optimal to 50% of optimal caused decreased levels of soluble sugars, but increased lvels of starch.Starch accumulation showed a strong negative correlation with the CO2 assimilation rate, indicating increased triose phosphate/inorganic phosphate (TP/Pi) ratio in the chloroplast causing end product inhibition of photosyntheisis. The data indicat the quantitative rlationship betwen the photosynthetic activity and the carbon utilization rate influnces the activity of the sucrose synthesis pathway and thus the rate of the triose hosphate/Pi exchange at the chloroplast membrane, which in turn regulates the activity of starch synthesis and the Calvin cycle.