Effect of day length and night temperature on starch accumulation and degradation in soybean

The effect of the day length on the accumulation and the degradation of the starch in leaf, stem and root tissues of preflowering soybean plants was determined by growing plants under a 7 or 14 h light regime. As has been reported previously, the rate of starch accumulation by leaves was inversely related to day length. High sucrose content was associated with a high rate of starch accumulation. Stem tissue showed diurnal fluctuations in starch content and the rate of accumulation was also inversely related to day length. This starch resulted from photosynthesis within the stem itself. A negligible amount of starch was found in root tissue of both sets of plants. The rate of starch breakdown in leaves of 7 h plants was significantly less than that in 14 h plants. Nevertheless, leaf starch in short day length plants was depleted at least 4 h prior to the end of the dark period. In both sets of plants, degradation of stem starch started simultaneously with that in the leaves and continued throughout the dark period, although at a much lower rate than that of leaves. Thus, stem starch acted as a buffer once leaf starch was depleted, providing carbohydrates to the plant, although in small quantities. To determine if soybean leaves adjust their rate of starch accumulation during the light period to different dark period temperatures, plants were grown under temperature regimes of 30/20 °C and 30/30 °C. Plants did not differ in rate of starch accumulation or CO2 exchange rate, but did show large differences in growth characteristics. High temperature plants had significantly greater leaf area and tended to have greater leaf area ratio. Thus, despite similar rates of starch accumulation on a leaf area basis, high temperature plants accumulated greater amounts of starch on a per plant basis.