Photosynthesis of Maize and Alfalfa During Daytime Sprinkler Irrigation.

Nighttime sprinkler irrigation usually results in lower wind drift and evaporation losses and better irrigation uniformity compared with daytime irrigation. However, daytime sprinkler irrigation modifies the microclimatic conditions within the crop canopy which could result in improved crop growth. We studied how the photosynthesis of maize (Zea mays L.) and alfalfa (Medicago sativa L.) crops is affected by daytime sprinkler irrigation. For each crop net photosynthesis was determined in five irrigation events of 3 to 5 h of duration. At each irrigation event, the net photosynthetic rate was determined simultaneously in one irrigated and another non-irrigated plot. To determine the rate of crop photosynthesis at each plot the CO₂ concentration was measured with an IRGA connected to an automated canopy chamber that had the top part normally open and every 15 min was closed during 50 s. Each canopy chamber covered an area of 0.75 m² and its height was 2.5 m for maize and 0.58 m for alfalfa. The rate of photosynthesis of maize was significantly reduced by 23% during the sprinkler irrigation event and by 20% during the following first hour. However, photosynthesis of alfalfa was slightly increased (not significantly) during the sprinkler irrigation event and during the following first hour. Measurement of contact angle of water with maize leaves led to mean values of 68º for the adaxial and 52º for the abaxial surfaces. For alfalfa leaves the contact angle of water showed mean values of 122º and 113º for adaxial and abaxial surfaces, respectively. The reduction of maize photosynthetic rate during sprinkler irrigation was related with the high wettability of the leaves that reduced the CO₂ uptake. However, the low wettability of alfalfa leaves precluded any reduction of CO₂ uptake.