Spectral Response to Phosphorus Deficiencies in Wheat.

The phosphorus (P) deficiency symptom appears much later while the crop growth is already affected. To know the nutrient stress timely and accurately, remote sensing plays a vital role. To investigate the wavelengths for P stress, the glass house and field experiments were conducted in wheat with different levels of applied P. The leaf hyperspectral reflectance measurements were taken in the range of 200-1100 nm wavelengths from different growth stages. The separable wavelengths of glass house experiment were presented after validation with field experiment data. The reflectance curves of glass house experiment show that the low levels of P (zero, 5 and 10 percent) were separable at 334 - 1100 nm in Ultraviolet (UV), visible and Near Infrared (NIR) to the 100 percent P plant at two leaf stage (Z12). At tillering initiation (Z20) low levels of P plant were separable in UV, visible and NIR. The UV to blue (377-433 nm) and NIR (940-990 nm, 1035-1038 nm and 1060-1100 nm) were found to separate deficient P plants from full nutrient plant at stem elongation (Z30). Like wise, at booting (Z40) blue to green (496-528 nm), green to red (538 -718 nm) and NIR (751-945 nm) were separable wavelengths.  In grain filling (Z70) the very low P plants (zero and 5 percent) were significantly separable with rest of the P treated plants in red edge (706-751 nm) and NIR (760–1050 nm) while 10, 15, 25, 50 and 100 percent P plants were not significantly separable among themselves. Thus, in early stage of plant growth; UV, visible and NIR are useful for detection of the P deficiency, while with the advancement of plant growth the separable wavelengths depend on the growth stages and degree of P deficiencies. Hence, the deficient P plants could be identified from the hyperspectral reflectance before the deficiency symptom appears.