Sprinkler irrigation efficiency declines when applied water intercepted by the crop foliage, or gross interception (Igross), as well as airborne droplets and ponded water at the soil surface evaporate before use by the crop. However, evaporation of applied water can also supply some of the atmospheric demands usually met by plant transpiration. Any suppression of crop transpiration from the irrigated area as compared to a non-irrigated area can be subtracted from Igross irrigation application losses for a reduced, or net, interception (Inet) loss. This study was conducted to determine the extent in which transpiration suppression due to microclimatic modification resulting from evaporation of plant-intercepted water and/or of applied water can reduce total sprinkler irrigation application losses of impact sprinkler and low energy precision application (LEPA) irrigation systems. Fully irrigated corn (Zea Mays L.) was grown on 0.75 m wide east-west rows in 1990 at Bushland, TX in two contiguous 5-ha fields, each containing a weighing lysimeter and micrometeorological instrumentation. Transpiration (Tr) was measured using heat balance sap flow gauges. During and following an impact sprinkler irrigation, within-canopy vapor pressure deficit and canopy temperature declined sharply due to canopyintercepted water and microclimatic modification from evaporation. For an average day time impact irrigation application of 21 ram, estimated average Igross loss was 10.7%, but the resulting suppression of measured Tr by 50% or more during the irrigation reduced Igross loss by 3.9%. On days of high solar radiation, continued transpiration suppression following the irrigation reduced Igross J. A. Tolk ([]) 9

No abstract

Significant genetic variation in leaf photosynthetic rate has been reported in grain sorghum [Sorghum biocolor (L.) Moench]. The relationships between leaf photosynthetic rates and total biomass production and grain yield remain to be established and formed the purpose of this experiment. Twenty two grain sorghum parent lines were tested in the field during the 1988 growing season under well-watered and water-limited conditions. Net carbon assimilation rates were measured at mid-day during the 30 day period from panicle initiation to head exertion on upper-most fully expanded leaves using a portable photosynthesis system (LI-6200). Total biomass and grain production were determined at physiological maturity. The lines exhibited significant genetic variation in leaf photosynthetic rate, total biomass production and grain yield. Significant positive correlations existed between leaf photosynthesis and total biomass and grain production under both well-watered and water-limited conditions. The results suggest that leaf photosynthetic rate measured prior to flowering is a good indicator of productivity in grain sorghum.

The effects of varying degrees of water stress on stomatal activity, photosynthesis, and nitrate reductase activity were examined in field grown cotton (Gossyplum hirsutum L., cv ‘Dunn 56C’). A relationship between plant water status and activity of each measured physiological parameter was established.Slight increases in leaf diffusive resistance were observed as leaf water potentials decreased although complete stomatal closure due to water stress was not generally observed. In many cases, visibly wilted leaves with zero turgor potentials exhibited minimal diffusive resistances. Morning and afternoon values of leaf diffusive resistance were distinctly different even though no correlation between leaf water potential and diffusive resistance was evident.Water stress substantially reduced photosynthesis in both vegetative and reproductive leaves of cotton. Photosynthetic rates of each leaf type responded differently to declining leaf water potentials. The data suggest that the photosynthetic reduction could not be attributed to stomatal closure.The activity of nitrate reductase was adversely affected by declining leaf water potentials. The nocturnal activity of nitrate reductase (respiratory linked) was also reduced by severe water stress. However, the reduction from maximum daily activity to minimum night time activity was similar in both stressed and non‐stressed plants. These data suggest that inhibition of nitrate reductase activity could be due to long term water stress effects rather than temporal changes in plant water status.The data presented indicate that stomata of field grown cotton are relatively insensitive to water stress, at least within the range of leaf water potentials observed in this study. Measurement of stomatal activity may not be a good criterion for assessing plant water status of cotton. The measurement of one or more physiological processes may prove a better index of plant water status as well as providing sensitive selection criteria for breeding more drought tolerant varieties.

The Southern High Plains of Texas represents the largest contiguous cotton (Gossypium hirsutum L.) production area in the USA. Water supply represents the greatest limitation to production under rainfed conditions. Where supplemental irrigation is used, growing season length represents a major limitation to attainment of high yields of desirable quality fiber and seed. The primary objective of this research project was to determine the inter‐relationships between H2O, N, and heat unit supplies as they affect lint yield of cotton. Field experiments were conducted during a 4‐yr period at a sandy soil (fine, loamy, mixed, thermic family of Aridic Paleustalf) site. Water supply was varied through irrigation with treatments ranging from dryland to fully irrigated. Superimposed on the water supplies were N rate treatments applied preplant and sidedress in a factorial design. Lint yield (LY) was defined as a function of components including plant density, bolls per plant and average boll size. Regression analysis was used to determine LY response to treatments. Lint yield was most highly correlated with boll number per unit ground area with equal contribution from plant density and bolls per plant. Water supply was most responsible for boll number; however, increasing N supply within each H2O regime resulted in a positive response in boll number per plant. Multiple regression analysis revealed that LY responded to H2O and N supplies during the fruiting period to a greater extent than to preflower supplies. Within any heat unit regime, LY was maximized as water supply increased by maintaining a constant ratio of 0.2 kg N ha−1 mm−1 H2O.