RESPONSE OF DIGITARIA-DECUMBENS LEAF CARBOHYDRATE-LEVELS AND GLUCAN DEGRADING ENZYMES TO CHILLING NIGHT TEMPERATURE

Experiments were performed to analyze how chilling temperatures presen t during the night period affect leaf carbohydrate metabolism in pango lagrass (Digitaria decumbens Stent.), a warm-season forage grass. Plan ts were exposed to three consecutive 14-h night periods of either 25 o r 10 degrees C. Control plants (25 degrees C nights) displayed an aver age 75% decline in leaf glucose levels during the night. This decline was more than 50% smaller in chilled plants (10 degrees C nights) duri ng all three nights. In control plants, leaf sucrose levels declined 7 8% during the night period. In chilled plants, this decline was inhibi ted to less than 2% of the available sucrose during the first night, a nd 47 and 44%, respectively, during two subsequent chilling nights. Gr eater than 90% of leaf starch was mobilized in control plants during t he night, whereas 52, 46, and 16% of leaf starch was mobilized during three consecutive chilling nights, respectively. Chilling did not caus e observable alterations in the ex planta activity of specific amylase enzymes; however, two diurnally regulated extrachloroplastic amylolyt ic enzymes were identified. Total leaf alpha-1,4-glucan phosphorylase (GP) activity was diurnally regulated with highest activity at the end of the night period, and activity increased in chilled plants after t wo consecutive chilling nights. Therefore, during three consecutive ch illing nights the diurnal fluctuations in neither leaf carbohydrate le vels nor GP activity remained constant. This dynamic response indicate s that previous chilling night exposure influenced subsequent night-pe riod carbohydrate metabolism despite each night period being separated with a warm (25-33 degrees C) 10-h d period.