Apparent Photosynthesis, Evapotranspiration, and Light Penetration in Two Contrasting Hard Red Winter Wheat Canopies

Crop canopy structure is highly variable in time and space influencing canopy light interception, apparent photosynthesis (AP), and evapotranspiration (ET). Our objectives were to relate AP and ET to light interception, leaf area index (LAI), temperature, and yield in two row-spacings of whiter wheat (Triticum aestivum L.) in a field environment. ‘Wanser’ whiter wheat was planted at 20 — (140 kg seeds ha⁻¹) and 40 — cm (70 kg seeds ha⁻¹) row spacings in Ritzville silt loam (Mesic Calciorthidic Haploxerolls). Two aluminum frame chambers covered with mylar and occupying 0.74 m² of soil surface area were used with infrared gas analyzers to measure AP and ET in each row spacing from the respective differences between chamber intake and exhaust CO₂ and H₂O concentrations. On clear days from stem extension to the milky ripe developmental stage and when incoming short-wave solar radiation (SR) was 700 War⁻² (± 140 Wm⁻²), best fit quadratic polynomial regression curves of AP and ET were obtained for each row spacing. Interception of SR across rows in each spacing was measured after anthesis with tube solarimeters 2, 27, and 53 cm above the soil surface. Values of LAI, AP, and ET were generally greater in the 20- than hi the 40-cm spacing. After anthesis, LAI declined sharply in both spacings resulting in decreasing AP values; but ET was not greatly reduced. In contrast to days before anthesis, average air temperatures after anthesis were generally greater than average leaf temperatures. Sensible heat transferred from air to foliage was apparently an energy source promoting ET late in crop development. After anthesis, SR differed significantly between spacings at 2 cm, but not at 27 or 53 cm above the soil. Plant heights in both spacings were about equal averaging 70.3 cm. At 18 days after anthesis (watery ripe stage), the 20-cm spacing had about 22% less SR 2 cm above the soil than the 40-cm spacing. At that tune AP and LAI values were about equal between spacings so that the greater SR intercepted by 20-cm spacing was apparently by tissue of low photosynthetic capability, such as dead or senescing leaves and tillers. Both spacings had statistically equal grain yields (2,102 kg ha⁻¹ average); but the 20-cm spacing had 67% more heads m⁻², resulting hi more kernels nr*, than the 40-cm spacing. The 40-cm spacing had greater kernel weights and about 2.5 more kernels/head than the 20-cm spacing. Increasing whiter wheat yields by increasing post-anthesis light interception appears unlikely unless sufficient water were available to maintain dense stands with high post-anthesis LAI values.