Photoacoustic spectroscopy, a nondestructive method for sensitive analysisof disease in plants

The Photoacoustic (PA) spectra of wheat leaves infected by brown rust were recorded at modulation frequencies, 18 Hz and 228 Hz, using a 300W Xenon Ar e lamp. The PA signals at 6328.8 nm radiation of He-Ne laser were recorded at different modulation frequencies to ascertain depth of penetration of th e disease in the infected leaf tissue. For an abaxial surface of the leaf f acing the incident radiation, the PA signal from leaves severely infected w ith brown rust disease is higher than that obtained from healthy leaves. Th is is in a marked contrast to a situation observed when an adaxial surface faces the incident radiation (the PA signal from severely infected leaves i s lower than that from the intact leaf). In case of a horizontal spread of a disease, the tendency towards the stem side of the leaf was observed. Our results also reveal the progress of disease vertically through the leaf.