Respostas fisiológicas e bioquímicas do Noni (Morinda citrifolia l.) ao estresse salino

Abstract

The study evaluated the effects of salt stress on physiological and biochemical aspects of noni plant, with a view to a better understanding of the mechanisms of salt tolerance in this species. Therefore, two experiments design was completely randomized with different salinity levels in the nutrient solution. The noni plants were grown in greenhouse under floating hydroponic system. The first experiment was carried out with five levels of NaCl (0, 25, 50, 75 or 100 mM NaCl) and four replications. After 40 days of stress, plant height, number of leaves, stem diameter, leaf, stem and root dry masses, leaf area, leaf succulence, sclerophylly index and the Na+, K+ and Cl- contents in the different plant organs were evaluated. The second experiment was conducted in the absence and presence of 100 mM NaCl and five periods of collections, with four replications. The growth conditions were identical to those of the first experiment. Samples were collected at 1, 10, 20, 30 and 40 days of salt stress. The same growth variables of the first experiment, as well as the variables of gas exchange and the photochemical efficiency (F0, Fm, Fv/Fm, YII, YNO and YNPQ) were determined. Besides those, we also determined the water relative content, the damage membrane and chlorophyll content, carotenoid, soluble carbohydrates, free amino acids, soluble proteins, proline, Na+, Cl-, K+, N and P. Salinity reduced growth of all plant organs. In contrast, juiciness and sclerophylly rate increased by 32% in the saline level compared to control plants. The contents of Na+ and Cl- in the leaves, stems and roots increased and K+ decreased with increasing salinity. The highest K+ levels were observed in the roots, and the Na+ and Cl- in the leaves. Salinity also reduced N and P in the leaves. The levels of soluble carbohydrates and free amino acids decreased in the treatment of stress. The salinity increased proline levels more in roots than in leaves. Salinity did not affect the efficiency of water use, but reduced net CO2 assimilation, stomatal conductance, transpiration, carboxylation efficiency and the contents of chlorophyll a, b, total and carotenoid. Salinity caused no major changes in the emission of chlorophyll a fluorescence, however the stressed plants showed a decrease in photoprotection capacity by the cycle of xanthophylls. Salinity did not affect the water status of the leaves, but damage to the integrity of the membranes were observed with the time of exposure to salt. Data from this study indicate that salinity changes the ionic homeostasis of noni plant, which can at least partly explain the reduction in growth induced by salt. Quantitatively, proline does not contribute substantially to osmotic potential of noni, however its increase suggests that either it plays a role in the acclimation of salt stress or it is an indicator of metabolic disorders induced by salinity. The noni presents as a mechanism of salt tolerance stomatal closure, reducing water loss by transpiration and maintaining the water status of the plant.