Response of groundnuts dependent on symbiotic and inorganic nitrogen to high air and soil temperatures

Groundnuts (Arachis hypogaea L.) are frequently exposed to high temperature s in the semi-arid tropics. The objectives of the present research were: (i ) to determine the response of groundnuts to different nitrogen sources; (i i) to quantify the effects of high air and soil temperatures on nodulation, dry matter production, partitioning and pod yields; and (iii) to discover whether plants dependent on symbiotic dinitrogen are more sensitive to heat stress than those dependent on inorganic nitrogen (N). Plants were grown a t optimum air and ambient soil temperatures from sowing until the first flo wering. Thereafter, plants were exposed to a factorial combination of two a ir temperatures [optimum: 28 degrees /22 degreesC (day/night) and high: 38 degrees /22 degreesC], two soil temperatures (ambient: 26 degrees /24 degre esC and high: 37 degrees /30 degreesC) and three N-sources [inoculated with Bradyrhizobium strain NC 92 (symbiotic N-2); inoculated and supplied with 20 ppm inorganic N (symbiotic N-2 plus 20 N); or not inoculated and supplie d with 100 ppm inorganic N (inorganic N)]. At optimum air and ambient soil temperature dry matter and pod yields were greatest in plants dependent on inorganic N, intermediate in symbiotic NZ plus 20 N and least in symbiotic N-2. High air or high soil temperatures significantly (P < 0.001) reduced p od yield to a similar extent and their effects were additive and without in teraction. High soil, but not high air temperature, significantly (P < 0.00 1) reduced nodule numbers, nodule dry weight and 100 seed weight. High air and/or high soil temperature had no effect on pod yield in plants dependent on symbiotic N-2 or symbiotic Na plus 20 N-2 but significantly (P < 0.05) reduced pod yield in plants dependent on inorganic N. This suggest that eff ectively nodulated plants with small quantities of inorganic N are potentia lly more adaptable to hot environments than those relying on large quantiti es of inorganic N.