Effects of ultraviolet-B radiation on cotton (Gossypium hirsutum L.) morphology and anatomy

Abstract

Cotton (Gossypium hirsutum L.) crop, cultivated between 40 degrees N and 40 degrees S, is currently experiencing 2-11 kJ m-2 d-1 of UV-B radiation. This is predicted to increase in the near future. An experiment was conducted to study the effect of enhanced UV-B radiation on vegetative and reproductive morphology and leaf anatomy of cotton in sunlit, controlled environment chambers. From emergence to harvest, cotton plants were exposed to 0, 8 or 16 kJ m-2 d-1 of UV-B in a square wave approach for 8 h from 0800 to 1600 h. Changes in plant height, internode and branch length, mainstem node number, leaf area, length and area of petals and bracts, and anther number per flower were recorded. Epidermal cell and stomatal density, stomatal index, leaf thickness, and epidermal, palisade and mesophyll tissue thickness were also measured. Initial chlorotic symptoms on leaves turned into necrotic patches on continued exposure to enhanced UV-B. Exposure to high UV-B reduced both vegetative and reproductive parameters and resulted in a smaller canopy indicating sensitivity of cotton to UV-B radiation. Enhanced UV-B radiation increased epicuticular wax content on adaxial leaf surfaces, and stomatal index on both adaxial and abaxial leaf surfaces. Leaf thickness was reduced following exposure to UV-B owing to a decrease in thickness of both the palisade and mesophyll tissue, while the epidermal thickness remained unchanged. The vegetative parameters studied were affected only by high levels of UV-B (16 kJ m-2 d-1), whereas the reproductive parameters were reduced at both ambient (8 kJ m-2 d-1) and high UV-B levels. The study shows that cotton plants are sensitive to UV-B at both the whole plant and anatomical level.

Fig. 1. Effects of ultraviolet‐B radiation (0, 8 or 16 kJ m^–2 d^–1) on internode length of cotton plants recorded at 66 d after emergence. Error bars are ± s.e. of mean of nine plants in each treatment.

Fig. 2. Effects of ultraviolet‐B radiation (0, 8 or 16 kJ m^–2 d^–1) on cotton branch lengths at various internode positions on the mainstem as observed at 66 d after emergence. Error bars are ± s.e. of mean of nine plants in each treatment.

Fig. 3. Epicuticular wax content of cotton leaves at squaring and flowering following exposure to ultraviolet‐B radiation. Error bars are ± s.e. of mean of three leaves in each treatment.

Fig. 4. Epicuticular wax morphology on leaf surfaces of cotton plants exposed to enhanced ultraviolet‐B radiation treatments. The figure shows wax tubes on both adaxial (A–C) and abaxial (D–F) leaf surfaces exposed to 0 (A and D), 8 (B and E) and 16 (C and F) kJ m^–2 d^–1 of UV‐B radiation. An increase in wax tube (WT) density is observed only on the adaxial surface with increase in ultraviolet‐B intensity. The image also shows epidermal cells (E), stomata (S) and glandular trichomes (G).

Fig. 5. Changes in total leaf thickness and thickness of various leaf ultrastucture component layers on exposure of cotton plants to ultraviolet‐B radiation treatments. Error bars indicate ± s.e. of mean of three leaves in each treatment.

Fig. 6. Anatomical features of cotton leaves exposed to 0 (A), 8 (B) and 16 (C) kJ m^–2 d^–1 of ultraviolet‐B radiation. The diagrams are traced from images obtained using a Leica TCSNT Confocal laser scanning microscope attached to a pictomicrography system. The adaxial (ADE), abaxial epidermis (ABE), palisade layer (P) mesophyll layer (M) stomata (S) and intercellular cavities (IC) can be clearly seen in the diagrams. Bar = 60 µm for length and width in all panels.