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

Groundnuts (Arachis hypogaea L.) are frequently exposed to high temperatures in the semi-arid tropics. The objectives of the present research were: (i) to determine the response of groundnuts to different nitrogen sources; (ii) 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 at optimum air and ambient soil temperatures from sowing until the first flowering. Thereafter, plants were exposed to a factorial combination of two air temperatures [optimum: 28 degrees/22 degrees C (day/night) and high: 38 degrees/22 degrees C], two soil temperatures (ambient: 26 degrees/24 degrees C and high: 37 degrees/30 degrees C) and three N-sources [inoculated with Bradyrhizobium strain NC 92 (symbiotic N2); inoculated and supplied with 20 ppm inorganic N (symbiotic N2 plus 20 N); or not inoculated and supplied 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 N2 plus 20 N and least in symbiotic N2. High air or high soil temperatures significantly (P < 0.001) reduced pod yield to a similar extent and their effects were additive and without interaction. High soil, but not high air temperature, significantly (P < 0.001) 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 N2 or symbiotic N2 plus 20 N, but significantly (P < 0.05) reduced pod yield in plants dependent on inorganic N. This suggest that effectively nodulated plants with small quantities of inorganic N are potentially more adaptable to hot environments than those relying on large quantities of inorganic N.