Daily Patterns of Apparent Photosynthesis and Evapotranspiration in a Developing Winter Wheat Crop

High temperatures strongly influence apparent photosynthesis (AP), evapotranspiration (ET), and leaf area index (LAI) of winter wheat (Triticum aestivum L.) in semi-arid regions. Our objectives were to relate temperature and short-wave solar radiation (SR) to crop AP, ET, and LAI in a field environment. Two aluminum frame chambers covered with mylar plastic and occupying 0.74 m² of soil surface area were used with infrared gas analyzers to measure AP and ET on ‘Wanser’ winter wheat grown in Ritzville silt loam (Mesic Calciorthidic Haploxerolls). Calculations of AP and ET were made from the respective differences between chamber intake and exhaust CO₂ and H₂O concentrations. On clear, high SR days, maximum AP declined from 4.2 to 2.9 g CO₂m⁻²hour⁻¹ from stem extension through the watery ripe developmental stage. During that same period, maximum daily ET ranged between 440 and 600 g H₂O m⁻² hour⁻¹, but did not decline as LAI declined from 2.0 to 1.1. A midday reduction in AP was observed when leaf temperatures (TL) approached 25 C corresponding to high daily values of ET and SR at anthesis, water ripe, and milky ripe stages. Since measurements of ET were over dry soil and assumed to represent plant transpiration, substantial stomatal closure was apparently not responsible for the decline in AP on those days because a concomitant reduction in ET was not always observed. Temperatures that were above optimal for AP most likely caused the midday decline in AP. This interpretation was consistent with water use efficiency (WUE) values, expressed as AP/ET, which were lowest at midday and generally declined as the crop matured. Total daily AP from heading through anthesis (15 days) on clear and partly cloudy days did not decline appreciably (ranging from 30.5 to 32.4 g CO₂ m⁻²1 day⁻¹) even though LAI declined 17% during that period. On clear days, high ET was associated with high SR and high temperatures. Temperatures tended to depress AP so that WUE was greater on partly cloudy and overcast days than on clear days at a given developmental stage. Under the climatic conditions of this study, WUE could be increased with early maturing cultivars that avoid the high late season heat load commonly associated with wheat production in this region.