Canopy Reflectance and Its Association with Nitrate Petiole In Cotton Plants.

Three cotton cultivars with differences in seed size were compared in relation to leaf area index (LAI), biomass, boll weight, chlorophyll, nitrate on petioles, and lint yield in a hot irrigated environment in Arizona. The cotton cultivars employed were PH4-745-WRF for large seed (9039 seed/lb), ST4-498-B2RF for intermediate seed size (9869 seed/lb), and DP1-64B-2RF for small seed size (11493 seed/lb). The three cultivars were planted in early May, 2009 in a seeding rate of 168 kg/ha in a randomized complete block design with a factorial arrangement of nitrogen treatments. Biomass, LAI, chlorophyll, and nitrate on leaf petiole were determined during the cropping season. Plants were harvested at November using a picker machine. Biomass and LAI showed little differences among the three cultivars in early growth stages, but the cultivar of big seed size showed lower biomass and LAI at peak bloom and cut-out. The boll weight was also lower in this cultivar, indicating that there was less boll number compared to the cultivars with medium and small seed size. Chlorophyll content estimated with the SPAD-502 showed also differences among the three cultivars, where the cultivar with the bigger seed size displayed the lowest content. Similarly, this cultivar also showed lower nitrate content on leaf petioles than the cultivars with medium and smaller seed size. Nitrate on petioles showed a direct relationship with a canopy spectral reflectance index when the three cultivars were compared. The spectral index NDRE (normalized difference red edge) displayed significant associations with nitrate content on petioles (r ranged from 0.41-0.75). The cultivar with bigger seeds was characterized for presenting low chlorophyll content, low nitrogen content on petioles, and low lint yield (1599 kg/ha) compared to the cultivars with medium and small seed size (2079 and 2022 kg/ha, respectively).