A THEORETICAL-ANALYSIS OF NITROGEN AND RADIATION EFFECTS ON RADIATIONUSE EFFICIENCY IN PEANUT

Radiation use efficiency (RUE) of well-watered crops, measured as gram s of biomass accumulated for each megajoule of intercepted total solar radiation, is affected by the level of leaf nitrogen in the canopy an d 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 can opy SLN levels. It is possible that these discrepancies between measur ed 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 use d it to explain the causes of discrepancies between theory and practic e. The framework is structured to determine photosynthesis of a layere d crop canopy by distributing incident radiation among sunlit and shad ed leaves in each layer. It allows for variation in incident direct an d diffuse radiation associated with location (latitude), time of year, time of day, and atmospheric condition, which is expressed as the deg ree of transmission of extra-terrestrial radiation. It also allows for variation in photosynthetic capacity associated with average SLN of t he canopy and its distribution in the canopy. Daily canopy photosynthe sis, intercepted radiation, and RUE are obtained by numerical integrat ion of instantaneous values calculated at specific times of the day. T he 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 1.1 g MJ-1. The major cause of previous underestimation of RUE was found t o be variation in RUE associated with the level of incident radiation flux density as affected by the degree of atmospheric transmission. RU E 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. Pa rtitioning incident radiation into direct and diffuse components and c onsideration of canopy gradients in SLN both had significant effects o n 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-1 , 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 p rocesses and integrating appropriately to the canopy scale, this theor etical framework has explained the causes of discrepancies between mea sured RUE and previous theoretical estimates.