Photoacoustic Study of Nutritional Deficiencies in Cotton Plants

Abstract

The photoacoustic (PA) spectra of powdered dry leaves containing different concentrations of nitrogen (N) and potassium (K) were recorded at a modulation frequency of 36 Hz using a 1000 W xenon arc lamp. Four units (plots) were used to induce nitrogen and potassium deficiencies in cotton plants under two different concentration levels of atmospheric carbon dioxide (360 and 720 µL L−1 CO2). The topmost fully expanded leaves from these plants, characterized by varying deficiencies, were collected from the same height of each unit and dried in the oven, at 80°C, to produce a fine powder. The PA spectra of all samples showed strong peaks at 675 nm and less intense absorption peaks at 620 and 465 nm. The relatively strong PA signals were observed from leaves containing higher concentrations of N or K. The PA results were confirmed by independent chemical analysis of the powdered leaves for N and K. The calibration curves were obtained showing the strength of PA signals at 620 and 675 nm as a function of known N and K concentration. These spectral algorithms were used to estimate the N and K concentrations in other powdered leaf samples, and the values obtained were compared with results acquired by chemical analysis. The PA spectra of fresh leaves of cotton plants having different stages of N deficiencies were also recorded by placing a portion of each leaf in a PA cell. The PA spectra of fresh leaves having higher nitrogen concentrations grown under conditions of higher N also showed PA signals larger than these from similar leaves grown with a lower nutrient application (lower leaf N concentration). The results of the study reveal the suitability of the PA technique for on‐line monitoring N or K deficiencies in the plants.