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 degradationof the starch in leaf, stem and root tissues of prefloweringsoybean plants was determined by growing plants under a 7 or14 h light regime. As has been reported previously, the rateof starch accumulation by leaves was inversely related to daylength. High sucrose content was associated with a high rateof starch accumulation. Stem tissue showed diurnal fluctuationsin starch content and the rate of accumulation was also inverselyrelated to day length. This starch resulted from photosynthesiswithin the stem itself. A negligible amount of starch was foundin root tissue of both sets of plants. The rate of starch breakdown in leaves of 7 h plants was significantlyless than that in 14 h plants. Nevertheless, leaf starch inshort day length plants was depleted at least 4 h prior to theend of the dark period. In both sets of plants, degradationof stem starch started simultaneously with that in the leavesand continued throughout the dark period, although at a muchlower rate than that of leaves. Thus, stem starch acted as abuffer once leaf starch was depleted, providing carbohydratesto the plant, although in small quantities. To determine if soybean leaves adjust their rate of starch accumulationduring the light period to different dark period temperatures,plants were grown under temperature regimes of 30/20 ^°Cand 30/30 ^°C. Plants did not differ in rate of starch accumulationor CO₂ exchange rate, but did show large differences in growthcharacteristics. High temperature plants had significantly greaterleaf area and tended to have greater leaf area ratio. Thus,despite similar rates of starch accumulation on a leaf areabasis, high temperature plants accumulated greater amounts ofstarch on a per plant basis. Glycine max(L.)Merr., soybean reserve carbohydrates, remobilization, source-sink realtionships