NITROGEN ASSIMILATION, ROOT GROWTH AND WHOLE PLANT RESPONSES OF SOYBEAN TO ROOT TEMPERATURE, AND TO CARBON DIOXIDE AND LIGHT IN THE AERIAL ENVIRONMENT *

Abstract

SUMMARYVegetative soybean plants were exposed to root temperatures of 18, 24, and 30 °C in each of three aerial environments varying in light and CO2. Under high light (700 mUE m‐2 s‐1) at ambient CO2 (400 mUl l‐1) nitrate absorption and root growth were less at 18 and 30°C than 24°C. Both nitrate absorption and root growth were more sensitive to increasing root temperature under low light (325 μE m‐2 s‐1) and less sensitive under enriched CO2 (1000 μl l‐1). As indicated by dry matter accumulation, net flux of photosynthate to the roots was correlated with the changes in aerial environment. It was concluded from these relationships that the variation in root growth and nitrate uptake resulted from altered utilization of carbohydrates in the roots and that altered utilization of carbohydrates with increasing root temperature resulted from disproportionate partitioning of carbohydrate among root functions.Although differences occurred in the amount of nitrogen translocated out of the root, nitrogen accumulation in the shoot was primarily a function of nitrate absorbed. Restrictions in dry matter accumulation in the shoot were similar to restrictions observed in the root. A decrease in emergence of new leaves was often the first response observed, with decreases in leaf area and dry wt occurring later. The integrated plant response to moderate root temperature stress is interpreted as evidence for an interdependent plant system, predominantly regulated by carbohydrate flux to the root and nitrogen flux to the shoot.