Summary Ten groundnut genotypes were grown under adequately irrigated conditions or subjected to drought during the pod filling phase (83–113 days after sowing) in a medium deep Alfisol at the ICRISAT Centre during the 1986–1987 post‐rainy season. Crop growth was measured in both treatments, but transpiration (7) and water‐use efficiency (W) were quantified only in the drought treatment. Leaf samples from both treatments were assayed for discrimination against 13CO2 fixed in leaves (Δ) to examine the relationships between Δ, crop growth, and W under field conditions. The shoot dry matter accumulated during the period of drought (Y) ranged from 72–150 g m‐2 and was closely related to transpiration. This indicates scope for selection of traits and practices to increase T. Water‐use efficiencies ranged from 1.38–2.50 g kg‐1 and were inversely related to Δ in eight out of the 10 genotypes. For the other two genotypes, there was evidence that T was underestimated by field measurements. Water‐use efficiency and transpiration were not correlated suggesting that these two traits might be combined through breeding. Variation between genotypes was greatest for the partitioning of total dry matter to pods (73%), followed by water‐use efficiency (31%) and transpiration (29%). Crop growth rates were negatively related to Δ under irrigated conditions but not under drought.

Water deficits can affect peanut yields differentially depending on time of occurrence and intensity. Previous reports have shown that moderate water deficit during the preflowering phase increases pod yields by 13 to 19% relative to the fully irrigated control. This paper describes, in a more quantitative way, crop physiological factors that contribute to these increased yields. In the 1980 to 1981 and 1981 to 1982 postrainy seasons, a study was conducted on a medium deep Alfisol at the International Crop Research Institute for the Semi-Arid Tropics (ICRISAT) in central India, to examine the responses of peanut (Arachis hypogaea L.), cv. Robot 33-1, under three levels of soil water deficits during preflowering phase relative to an adequately irrigated control. Three intensities of drought were created (from 10-50 d after sowing [DAS]) using-line source sprinkler irrigation, while the control was irrigated at 10-d intervals. Moderate water deficits during the preflowering phase increased subsequent crop growth and pod growth rates in 1980 to 1981 but not in 1981 to 1982. Differences in flowering and total pod numbers between treatments were relatively small when compared to the subsequent differences in pod setting and maturity. In both seasons, greater synchrony of pod set in the moderately stressed plots resulted in a greater proportion of mature pods at final harvest. The most severely stressed plot had lower yields despite adequate irrigation subsequent to the preflowering water deficits.

Experiments are described to quantify the effects of temperature and leaf wetness duration on infection of groundnut by Phaeoisariopsis personata. Temperature response curves for conidial germination and infection were similar, with optima close to 20°C and minimum and maximum temperatures of about 8°C and 34 C, respectively. The effect of temperature on infection between 15°C and 26°C was slight. Lesions developed only if the leaf wetness period exceeded about 20 h, and the total wetness period necessary for maximum infection exceeded 160 h. The number of lesions resulting from a fixed amount of inoculum was several times greater if leaves were exposed to alternate wet and dry periods (intermittent wetness), compared with continuous wetness. With intermittent wetness the length of the dry period had little effect on the number of lesions, providing it exceeded 2 h. The response curve relating total wetness periods to lesion density was an exponential asymptote.

SUMMARYFour genotypes of groundnut grown with limited irrigation in a medium depth Alfisol in Central India transpired similar total amounts of water (220–226 mm) over the season, but produced different amounts of shoot dry matter (390–490 g m−2). The extraction front of Kadiri 3 moved most rapidly down the soil profile which may have enabled it to maintain the fastest rates of transpiration when soil water depletion was greatest. Tap root extension rates of Kadiri 3 in the first 32 days after sowing were also the fastest. NC Ac 17090 was more efficient than the other genotypes in extracting water immediately after irrigation from the upper 40 cm of the soil, but this had little value in determining the pattern of water availability in this experiment. Differences in the water extraction characteristics of these genotypes explain little of the variation in dry matter:water ratio, and do not account for the major variation in harvest index associated with drought.

The effect of temperature on the latent periods of rust, late leaf spot and early leaf spot diseases of groundnut caused by Puccinia arachidis, Phaeoisariopsis personata and Cercospora arachidicola, respectively, was studied. The latent periods (LP) of rust, late leaf spot and early leaf spot ranged from 12–49 days, 13–38 days and 13–39 days, respectively, between 12°C and 33°C An equation relating the rate of pathogen development (1/LP) to temperature was fitted using daily mean temperatures to provide three cardinal temperatures: the minimum (Tmln), optimum (Topl), and maximum (Tmax), Tmln was about 12°C for rust and about 10°C for the two leaf‐spot diseases. Topt, for all three diseases was close to 25°C. Tmax was 31°C for early leaf spot, and extrapolated values for late leaf spot and rust were about 35 and 40°C, respectively. For P. personata, a temperature response curve was fitted using data only from controlled environment experiments. This curve was used to simulate latent periods from both mean daily and mean hourly temperatures in the field. There was substantially better agreement between observed and simulated latent period with hourly temperatures, provided the developmental rate of the pathogen was determined at a constant temperature.