Comparative epidermal ultrastructure of cotton (Gossypium hirsutum L.) leaf, bract and capsule wall

Cotton leaves are more physiologically active than the bract and the capsule wall of the fruiting structures. To elucidate the disparities in their physiological behaviour, epidermal cell density, stomatal index, stomatal size, trichome density and type, and epicuticular wax ultrastructure of cotton leaf, bract and capsule wall were delineated using scanning electron microscopy (SEM). The epidermal cells of the outer periclinal walls on both surfaces of the leaf and bract were raised and convex. Conversely, the capsule wall epidermal cells were polygonal with hat outer periclinal walls. The stomatal complex in the leaf and bract was paracytic, whereas in the capsule wall the stomatal complex was anomocytic. The adaxial and abaxial stomata of the leaf were coplanar to the epidermal surface, as opposed to the raised adaxial stomata on the bract. On the contrary, the stomata on the capsule wall surface appeared to be slightly sunken. Furthermore, the capsule wall stomata were larger than the stomata on either surface of both the leaf and the bract. The stomatal index was greater on the abaxial surfaces of the leaf and the bract (18.4 and 9.4, respectively) than their corresponding adaxial surfaces (14.4 and 4.7, respectively). Leaves had the highest stomatal index followed by the bract and the capsule wall. The indumentum consisted of glandular and nonglandular trichomes, the density of which was greater on the abaxial surfaces than on the adaxial surfaces of the leaf and bract. The capsule wall indumentum lacked nonglandular trichomes. Epicuticular wax occurred in the form of striations. However, the striation pattern varied among the organs. This study clearly illustrates morphological disparities in the epidermal features of leaf, bract and capsule wall, helping to explain their physiological divergence. (C) 2000 Annals of Botany Company.