Effect of drought stress on photosynthetic characteristics, growth and sugar accumulation of field-grown sweet sorghum.

We studied the effect of drought on photosynthesis, growth, and sugar accumulation in field-grown sweet sorghum (Sorghum bicolor (L.) Moench). Stomata closed when drought stress developed during the day. As a consequence, photosynthesis was reduced. The water use efficiency of drought-stressed leaves was slightly lower than that of controls and this reduction was more pronounced early in the morning and in the evening. This suggested that not only stomata limit photosynthesis under drought: photosynthesis at high CO2 was lower in drought-stressed than in control leaves, suggesting that carbon metabolism was inhibited. Electron transport was also reduced, but an increased number of electrons appeared necessary to fix CO2 under drought. Before anthesis, growth was not affected by drought but, after anthesis, elongation of the last internodes was greater in controls than in drought-stressed plants. Both controls and drought-stressed plants accumulated sucrose and starch in the stem after anthesis. The accumulation was low and ceased early in the stem apex. It is likely that seeds in the panicle compete with the apex for carbon. During panicle maturation sucrose and starch content was higher in drought-stressed than in control stems. At the end of our experiments, however, the sucrose content per plant was similar in all samples. We conclude that sugar accumulation is less sensitive to drought than photosynthesis. Irrespective of drought conditions, (i) before anthesis growth is the predominant sink for photosynthates; (ii) after anthesis sugar accumulation in the stem becomes the preferential sink at the expense of growth of apical internodes.