ESTRUTURAS SECRETORAS ENVOLVIDAS COM A PROTEÇÃO DOS MERISTEMAS VEGETATIVOS DE BEGONIA CURTII L.B.Sm. & B.G.Schub (Begoniaceae): CARACTERIZAÇÃO ANATÔMICA E HISTOQUÍMICA

Abstract

The stem apexes are considered essential for the construction of the plant's body and damage to this region brings a high cost to the plants. Because it is a fragile and extremely important structure, it is common to find mechanical and chemical protective structures that provide protection or resistance against herbivores, desiccation, and solar radiation. Field observations verified the presence of a sticky secretion accumulated between the stipules of Begonia curtii, protecting the apical meristem, which motivated the present study. Thus, the objective of this work was to identify and characterize the secretory structures involved with the production of this secretion as well as to identify the chemical nature of this secretion. For this, samples of young leaves and stipules, covering the apical meristem of B. curtii were collected and submitted to the usual techniques for light microscopy and scanning electron microscopy studies. Two types of secretory structures responsible for the secretion observed in the field were identified: colleters and mucilaginous idioblasts. In the stipules, on the adaxial side, colleters were observed, which occur in clusters of three or four and rarely solitary, a pattern not yet described in the literature. In the leaf lamina there were solitary glandular trichomes, functionally called colleters, near the median vein, on the abaxial side. Mucilaginous idioblasts occur in pairs, in a mirrored fashion, both in the leaf lamina, petiole and stipules. These idioblasts are found close to the vascular bundles, and in the case of the leaf lamina, occur internal to a layer of phenolic idioblasts that contours the entire mesophyll. Accumulation of secretion was observed in the substomatal chambers, connecting the epidermis with the phenolic sheath. This pattern indicates that the secretion produced by the idioblasts is released through the stomata. The presence of mucilage both in the stipules and leaves of B. curtii may act in the storage of water and carbohydrates, in the water balance and in the resistance against drought, since this species inhabits soils with low water availability.