Carbohydrate partitioning, photosynthesis and growth in Lemna gibba G3. II. Effects of phosphorus limitation

The relationship between CO₂ assimilation rate, growth and partitioning of carbon among starch, sucrose, glucose and fructose were studied in phosphorus (P_i)-limited Lemna gibba L. G3. Two experimental models were used: 1) Cultures were grown at various stable, suboptimal rates regulated by the supply of P_i; 2) cultures growing at optimal rates were transferred to P_i-free medium. The response to a P_i deficiency can be divided into two phases. Phase I is characterized by hyperactivity of the sucrose synthesis pathway, leading to high levels of glucose and fructose. Phase II is characterized by starch accumulation associated with a decrease in the cytoplasmic pools of soluble sugars owing to inhibition of carbon export from the chloroplast. A strong negative correlation was found between the CO₂ assimilation rate and starch levels. No significant correlation was found between assimilation and ATP levels and decrease in relative growth rate did not significantly affect the adenylate energy charge (EC). The regulatory aspects of the partitioning of carbon among soluble sugars and starch as well as the negative correlation between carbohydrate levels and CO₂ assimilation at P_i-limited growth are discussed.