Spectral estimates of the absorbed photosynthetically active radiation and light-use efficiency of a winter wheat crop subjected to nitrogen and water deficiencies†

Abstract

A linear regression equation is found relating the photosynthetically active radiation intercepted by the canopy (PAR_i), measured with hemispherical photographs, and both the normalized difference ND and the ratio NIR/R vegetation indices. On the basis of this equation, NIR/R is used to estimate PAR_i during the crop cycle. The efficiency with which the PAR absorbed by the crop is transformed into biomass (ϵ_c) is calculated for three phenological phases of the crop. Nitrogen fertilization is the main factor affecting light interception. At the booting stage, PAR_i is about 15 per cent greater for treatments with higher nitrogen levels. ϵ_c is influenced by both nitrogen and irrigation levels, and varies with the phenological phases of the crop. For the irrigated plots, ϵ_c is higher in the period going from anthesis to soft dough and not in the period from stem elongation to anthesis as most published results indicate. Water stress is the main factor affecting ϵ_c. The greatest reductions of ;ϵ_c are observed on plots with higher biomass levels when water shortage starts. The results suggest the need for a water stress index for biomass estimations of rain-fed crops in regions susceptible to drought. This would require knowledge of ϵ_cc for the crop grown under non-limiting conditions.

Notes

†Paper presented at the University of Nottingham 48th Easter School in Agricultural Science, ‘Applications of Remote Sensing in Agriculture’ 3-7 April 1989.