Studies on anatomical behaviour of PaLCuV infected papaya (Carica papaya L.)

Abstract: Papaya leaf curl virus (PaLCuV) of geminiviridae family is a major threat to papaya plants in the world. The major visual characteristics of PaLCuV infected plants are downward and inward rolling and curling of leaves in the form of an inverted cup and thickening of veins. Microscopic observation showed that in the healthy papaya leaf, stomata guard cell size was 19-20 µm. However, it increased significantly in infected plant leaves up to 29-30 µm. This observation suggested that stomatal density and guard cell size were changed due to puckered anatomy of leaf. SEM analysis revealed that subsidiary or accessory cells of guard cells were less turgid and the arrangement of starch grains was disturbed as compared to healthy plant leaves. Light microscopic, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses of symptomatic leaves showed the puckering leaf lamina due to presence of loosened cells in its tissues and hyper-accumulation of Ca²⁺ ions. High accumulation of Ca²⁺ in PaLCuV infected leaves as compared to healthy leaves which might be the cause of hypertrophy and thickening of veins of infected papaya leaves. Uniform trichomes/hairs/nodular structures were present in midrib of healthy leaf but were missing in infected leaf. The infected midrib showed scantly distributed floret-like structures instead of a smooth trichomes/hairs/nodular structures in midrib of the healthy leaf.