Phloem Transport and the Regulation of Growth of Sorghum bicolor (Moench) at Low Temperature.

Leaf expansion in Sorghum bicolor (Moench) was severely retarded by low night temperatures (5 C). However, this was not reflected in the early measurements of relative growth rate, indicating that the response was not associated with a deterioration of the photosynthetic system. For plants grown at 30/25 C (day/night) and subsequently held at an ambient temperature of 30 C, phloem transport, as measured either by the movement of (14)C-photosynthate through a zone of controlled temperature or by accumulation of dry matter distal to this zone, was inhibited by temperatures below 10 C. The speed of movement of (32)P through the temperature controlled zone was more sensitive to temperature with reductions apparent below 20 C. Although there was some recovery in the movement of (32)P following 3 days equilibration at low temperature (1 to 10 C), the new values (approximately 100 centimeters per hour) were still only about one-third of those obtained in the high temperature controls. For plants held at an ambient temperature of 21 C, which is well below the optimum for growth, translocation was only inhibited by temperatures below 5 C. Although low temperature may reduce the carrying capacity of the phloem of S. bicolor, this is unlikely to be an important factor in regulating the growth of the plants at low temperatures.