Effects of Nitrogen Source and Water Limitation on Growth, Transpiration Efficiency and Carbon-Isotope Discrimination in Peanut Cultivars

Abstract

Effects on growth of nitrogen nutrition and its interactions with water availability were explored in two experiments using cultivars of peanut (Arachis hypogaea L.). In one experiment plants of the two cultivars, Tifton-8 and Chico, were given relatively high (high-N) or low (low-N) amounts of nitrogen, supplied as ammonium nitrate. The low-N plants accumulated less dry matter and used less water than the high-N plants. Transpiration efficiency, the ratio of dry matter produced to water used (W), was not significantly affected by the nitrogen treatment and neither was carbon-isotope discrimination (Δ). Isolated measurements showed leaf gas-exchange characteristics during pod filling were not affected in Tifton-8, but assimiliation rate was reduced in the low-N treatment, compared to the high-N treatment, in Chico. The ratio of intercellular CO₂ pressure to ambient CO₂ pressure, p_i/p_a, in the short term was consistent with Δ in leaves, a longer term measurement of p_i/p_a. The allocation of dry matter to roots was affected in Tifton-8 but not in Chico. The ratio of plant dry matter to leaf area was unaffected by nitrogen level. In another experiment, with four cultivars-Tifton-8, Chico, Robut 33-1 and TMV2-the nitrogen source was varied by giving plants ammonium nitrate or having them acquire nitrogen through nodules. Two levels of water availability were incorporated with the nitrogen treatments in a factorial design. Plants were either watered well or grown on water stored in the soil, in addition to having different nitrogen sources. In this experiment, the method of nitrogen acquisition caused different reductions in dry-matter accumulation among cultivars when plants were water-limited. Dry-matter accumulation in nodulated plants was less than in plants given ammonium nitrate, and nodulated plants had smaller values of W than plants which were given ammonium nitrate. Growing plants on stored soil water also affected dry-matter accumulation but there was no significant effect on W. The differences among cultivars in W were maintained over the two experiments so that the ranking of cultivars in W was conserved. Dry matter allocation among plant parts was affected by water limitation and by the nitrogen source. Water limitation had a greater effect on dry-matter accumulation of plants that were given ammonium nitrate than it did on dry matter of nodulated plants. Plants that were nodulated allocated more dry matter to the pods than did those that were not nodulated and this response varied among cultivars. Cultivars which accumulated the most dry matter had the greatest W but these cultivars did not necessarily allocate the most dry matter to pods. Irrespective of treatment, there was a strong negative correlation between W and Δ in both experiments. The small variations in the relationship between W and Δ among cultivars indicated that there may have been differences among cultivars in the carbon cost of nitrogen acquisition when the plants were nodulated.