Effects of irrigation and nitrogen on growth, light interception and efficiency of light conversion in wheat

Effects of three irrigation treatments (rainfed, and irrigation at 7-day and 14-day frequencies beginning in spring) and two rates of nitrogen (0 and 150 kg N ha⁻¹) on growth, light absorption, and conversion efficiency in wheat were studied. Growth was considered in four phases extending from 95 days after sowing ( 95) to the beginning of rapid stem growth ( 120), the stem growth-phase lasting to the onset of rapid grain-filling ( 148), the grain-filling phase between 148 and 170, and the final period to harvest. The first irrigation treatments were applied at 120.Radiation interception was the major determinant of growth. Rainfed treatments captured ca. 1100 MJ m⁻² between 95 and 148, by which time they had achieved maximum above-ground biomass. Irrigated treatments continued to grow until 170. They captured ca. 1300 MJ m⁻² to 170 where no nitrogen was applied, and ca. 1500 MJ m⁻² where N was applied.In addition to effects on leaf-area duration and radiation absorption, treatments also affected conversion efficiency, ε. In the first phase, ε increased from 0.85 g MJ⁻¹ to 1.15 g MJ⁻¹ where N was applied. After 120, irrigation increased ε from a mean of 0.8 g MJ⁻¹ in rainfed treatments to 1.2 g MJ⁻¹. In the periods of rapid stem-growth and grain-filling, ε was a maximum of 1.45 g MJ⁻¹ in the frequently irrigated treatment which received N, resulting in a maximum above-ground biomass of 2100 g m⁻². Mean maximum biomass was 1670 g m⁻² in the other irrigated treatments, as compared with a mean of 1100 g m⁻² in rainfed treatments.Growth rates were compared with predicted potential rates. After accounting for differences in light absorption between treatments, rates of growth ranged between 0.4 and 0.65 of potential rates in treatments other than I_wN₁₅₀, in which the growth rate between 120 and 170 was almost 0.8 of the potential rate. These proportions were strongly correlated with estimates of ε, although the relationship varied between phases as a result of differences in global radiation. Collectively, the data suggest that physiological constraints, associated with both N and water, contributed to differences in rates of growth in addition to those imposed by leaf-area duration and radiation absorption.The yield potential of the frequently irrigated treatment which received N was, however, not realised in the field. Lodging after 162 was estimated to decrease yield from a potential of ca. 900 g m⁻² to 650 g m⁻².