Daytime Sprinkler Irrigation Effects on Net Photosynthesis of Maize and Alfalfa

During sprinkler irrigation some water is lost due to drift and evaporation. After irrigation, plant-intercepted water is lost due to evaporation. The water loss causes microclimatic changes, which may involve positive or negative plant physiological responses. We studied the changes in net photosynthesis of maize (Zea mays L.) and alfalfa (Medicago sativa L.) associated with irrigation with a solid-set sprinkler system. For each species, measurements were made simultaneously in two plots, one being irrigated and the other not being irrigated. Two automatic canopy chambers connected to two CO2 infrared gas analyzers were used. Sprinkler irrigation decreased air temperature (1.5 degrees C on maize, 1.7 degrees C on alfalfa), air vapor pressure deficit (VPD) (0.44 kPa for both crops) and canopy temperature (5.1 degrees C on maize, 5.9 degrees C on alfalfa). Sprinkler irrigation decreased maize net photosynthesis on 80% of the days and the mean reduction was 19%. Sprinkler irrigation increased alfalfa net photosynthesis on 36% of days, decreased it on 14% of days, and had no effect on half of the days. The decrease of maize net photosynthesis during sprinkler irrigation was linked to the high leaf wettability (water contact angles from 60-80 degrees) and the decrease in temperature below the optimum range for photosynthesis. The higher hydrophobicity of alfalfa leaves (water contact angles >120 degrees) and the wide range of optimum temperature for alfalfa photosynthesis may be the reasons why photosynthesis remained unaffected by sprinkler irrigation. The results suggest that daytime sprinkler irrigation with solid-set should be avoided for maize but can be used for alfalfa.