Aspectos citoquímicos e bioquímicos da germinação e crescimento de plântulas de gergelim sob estresse salino

Abstract

Salinity is one of the abiotic stresses that most limit the growth and crop productivity. Sesame (Sesamum indicum L.) is one of the oldest oilseeds used by the man, with high adaptability to different conditions, and has been cultivated in many countries. The objective of this work is evaluate the effects of salinity on germination, reserves mobilization and seedling growth of sesame, cv. BRS SEDA. Seeds were sown in gerbox over germitest paper moistened with a solution of distilled water (NaCl 0 mM) or saline whater (NaCL 50 or 100 mM), been maintained in B.O.D at 25ºC under the 12h light photoperiod. The parameters of germination and growth were determined and proceeded the cytochemical analysis and determination of levels of lipids, total protein, soluble carbohydrates, free amino acids, Na+, Cl- and K+, as well as the activity of isocitrate lyase (ICL) and malate synthase (MLS) enzymes, in quiescent seeds [developmental stage (DS 0)], and obtained from seedlings with radicle protusion (DS 1), seedlings with radicle between 3-5 mm long (DS 2) and seedlings with radicle greater than 1.0 cm and evident hypocotyl (DS 3). Salinity increased the average time of germination and reduced germination percentage and seedling growth. In relation to reserve compounds, lipids were the most abundant totaling 52.7% of the dry weight of the quiescent seed. During the growth, the contents of lipids only varied in saline conditions (NaCl 50 and 100 mM) without significantly differ from those on DS 0. In the quiescent seed, there was predominance of oleic (59.6%) and linoleic (30.3%) acids among the fatty acids detected. At higher NaCl concentration, the contents of palmitic, oleic and stearic acids remained significantly lower than shown in the treatment with 0 mM NaCl, while the oleic acid was superior at all developmental stages analyzed. The protein levels, the second most abundant reserve on the quiescent seed (25.4%), did not significantly change in the course of development neither in function of salinity. However, levels of free aminoacids increased during the observed period accompanied by a strong reduction on the amount of protein bodies in cotyledons and endosperm cells in the DS 3, compared to DS 0. Soluble carbohydrates constituted 2.8% of the dry mass of the quiescent seed, and gradually accumulated according the NaCl concentration increased in all DS, composed mostly by non-reducing carbohydrates. The content of Na+ and Cl- increased with salinity in all DS, whereas the K+ was significantly reduced only in DS 3. Consequently, the Na+/K+ ratio was increased with salinity, especially in DS 3. The activity of both enzymes of the glyoxylate cycle increased progressively in the course of development. In DS 2 and DS 3, the ICL activity in the salt conditions was significantly higher than the control treatment (0 mM NaCl), however, in all ED, there was low or no variation in the activity of MLS in function of salinity. Based on these results, can conclude that salinity had a greater effect on seedling growth than on germination. Furthermore, significant changes in cell morphology and composition of reserves observed from the cytochemical analysis according to development or salinity did not correlate well with the changes detected by the analytical methods employed, making necessary further studies to evaluate how the catabolic pathways and transport of compounds to the embryonic axis are affected by salinity in sesame.