SPECTRAL ESTIMATES OF THE ABSORBED PHOTOSYNTHETICALLY ACTIVE RADIATION AND LIGHT-USE EFFICIENCY OF A WINTER-WHEAT CROP SUBJECTED TO NITROGEN AND WATER DEFICIENCIES

A linear regression equation is found relating the photosynthetically active radiation intercepted by the canopy (PARi), 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 PARi during the crop cycle. The efficiency with which the PAR absorbed by the crop is transformed into biomass (ɛc) is calculated for three phenological phases of the crop. Nitrogen fertilization is the main factor affecting light interception. At the booting stage, PARi is about 15 per cent greater for treatments with higher nitrogen levels. £c is influenced by both nitrogen and irrigation levels, and varies with the phenological phases of the crop. For the irrigaied plots, ɛc 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 indicale. Water stress is the main factor affecting ɛc. The greatest reductions of ɛc 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 ɛc for the crop grown under non-limiting conditions. © 1990 Taylor & Francis Group, LLC.