Solar radiation absorption of wheat cultivars grown under different nitrogen levels and water deficit

Estimates of agronomic parameters from remotely sensed measurements are highly based on the relationship between radiation reflection and crop growth characteristics. Field reflectance measurements in the visible and near infrared regions of the electromagnetic spectrum were used to estimate accumulated absorbed photosynthetically active radiation (AAPAR) in order to verify its relationship with biomass and grain production for wheat crop (Triticum aestivum L.) during the winter seasons of 1997, 1998, 1999 and 2000 at the experimental farm of the University of Taubate, Sao Paulo State, Brazil. Wheat cultivars (IAC-24, IAC-287, IAC-289 e IAC 355) were grown under three soil water levels (100%, 50% and 25% of the field capacity) and four nitrogen levels (0, 60, 90 e 120 kg ha^-1 of N). A split-split-plot design was used to mimic a large number of wheat crop growing conditions in terms of water and nitrogen availability. It was observed that the AAPAR, estimate via remotely sensed means, had a significant correlation with biomass production and grain yield. However, the biophysical relevance of such relationships is being questioned based upon the comments of Demetriades-Shah et al. (1992) and an interesting discussion that followed their paper. Under tropical growing condition, water and nutrients are major crop growing limitation factors while radiation is not limiting growth. Results showed that the number of days of crop growing cycle (emergence to physiological maturity) was highly correlated with AAPAR (r>0.90) and was even better correlated to grain yield than AAPAR.