A Quantification of the Significance of Assimilatory Starch for Growth of Arabidopsis thaliana L. Heynh.

These studies use starch synthesis mutants to quantify the contribution of assimilatory starch to whole plant growth and form. Arabidopsis thaliana (L.) Heynh plants were used with null plastid phosphoglucomutase (T Caspar, SC Huber, CR Sommerville, [1986] Plant Physiol 79; 1-7) or 7% of wild-type ADP-glucose pyrophosphorylase (T-P Lin, T Caspar, CR Sommerville, J Preiss [1988] Plant Physiol 88; 1175-1179). The daily turnover of starch and the rate of biomass increase in the mutants and the wild type were investigated during growth in a 14 hour light/10 hour dark cycle in high irradiance (600 micromoles per square meter per second) and nitrogen (6 millimolar NH(4)NO(3)), in high irradiance and low nitrogen (0.1 millimolar NH(4)NO(3)) or in low irradiance (80 micromoles per square meter per second) and high nitrogen. There is some variability in the data, but the following conclusions can be drawn. Growth was slow in the absence of starch turnover. In high nitrogen conditions, about 1 mole of carbon per gram dry weight per day was incorporated additionally into structural biomass for every one mole of carbon turned over as starch per gram dry weight per day. In low nitrogen, the gain was much lower. This indicates that temporary storage of photosynthate is important for rapid growth in high nitrogen, but not in low nitrogen when carbohydrate is in excess. Starch-deficient plants showed the usual decrease of the shoot/root ratio in low nitrogen and increase of the ratio in low light. This shows that adjustment of plant form to nitrogen nutrition and irradiance is not mediated via regulation of photosynthate partitioning in the leaf. Starch deficient plants had lower shoot/root ratios than the wild type and the nitrogen concentration in their leaves was increased. It is discussed how interactions between carbohydrate allocation, respiration and growth at the organ and whole plant level generate these changes. We conclude that mutants with a decreased capacity to carry out a particular partial process provide a powerful tool to disect complex mutually interacting systems, and define and quantify causal interactions at the level of whole plant growth.