A theoretical analysis of nitrogen and radiation effects on radiation use efficiency in peanut

Abstract

Radiation use efficiency (RUE) of well-watered crops, measured as grams of biomass accumulated for each megajoule of intercepted total solar radiation, is affected by the level of leaf nitrogen in the canopy and has been related to the canopy specific leaf nitrogen (SLN; g N m-2 leaf area). A number of field experiments on peanut have measured RUE values greater than current theories predict on the basis of their canopy SLN levels. It is possible that these discrepancies between measured and theoretical values may be caused by non-uniform distribution of SLN in the canopy, incident radiation level, and/or the influence of diffuse radiation. In this study, we developed a theoretical framework to predict the consequences of these factors on RUE in peanut and used it to explain the causes of discrepancies between theory and practice. The framework is structured to determine photosynthesis of a layered crop canopy by distributing incident radiation among sunlit and shaded leaves in each layer. It allows for variation in incident direct and diffuse radiation associated with location (latitude), time of year, time of day, and atmospheric condition, which is expressed as the degree of transmission of extra-terrestrial radiation. It also allows for variation in photosynthetic capacity associated with average SLN of the canopy and its distribution in the canopy. Daily canopy photosynthesis, intercepted radiation, and RUE are obtained by numerical integration of instantaneous values calculated at specific times of the day. The framework predicted experimentally determined RUE values accurately and quantified the contribution of each major factor to variation in RUE. On clear days, with high canopy SLN, RUE was predicted to be 1.l g MJ-1. The major cause of previous underestimation of RUE was found to be variation in RUE associated with the level of incident radiation flux density as affected by the degree of atmospheric transmission. RUE increased by up to 0.4 g MJ-1 as atmospheric transmission decreased from 0.75 (clear sky) to 0.35 (heavy cloud). However, varying incident radiation by changing time of year or latitude did not affect RUE. Partitioning incident radiation into direct and diffuse components and consideration of canopy gradients in SLN both had significant effects on RUE, but of a lesser magnitude than effects of degree of atmospheric transmission. The former caused increases in RUE of up to 0.15 g MJ-l, while the latter caused increases of up to 0.13 g MJ-1 at low canopy SLN. Hence, by quantifying the understanding of plant physiological processes and integrating appropriately to the canopy scale, this theoretical framework has explained the causes of discrepancies between measured RUE and previous theoretical estimates.