Effect of salt stress on growth and contents of organic and inorganic compounds in noni (<i>Morinda citrifolia</i> L.)

Abstract

Salinity is one of the abiotic stresses that most affect agricultural production, especially in arid and semi-arid regions; however, among species there are large differences in salt tolerance. In this study, the effects of salinity on the growth and accumulation of organic and inorganic solutes were evaluated in ‘noni’ seedlings at 1, 10, 20, 30 and 40 days of salt stress in a 5 x 2 completely randomized experimental design. Seedlings of ‘noni’ were grown in the nutrient solution at two salinity levels (0 and 100 mM sodium chloride). Plant height, number of leaves, stem diameter and dry matter of leaves, stems and roots, the allocation of biomass and the contents of organic and inorganic solutes were determined in the different plant organs. Salinity reduced all growth variables, being less expressive in stem diameter. The biomass allocation in leaves was higher than in roots, regardless of treatment or time considered. The contents of organic and inorganic solutes varied according to the plant part and the time of exposure to salinity. In general, salinity increased the contents of sodium ion (Na⁺), chloride ion (Cl^-) and reduced potassium ion (K⁺), nitrogen (N) and phosphorus (P). Soluble carbohydrates and free amino acids were the main organic solutes contributing to osmotic potential of noni to salt stress. The salinity increased proline content in roots more than in leaves, but the proline content in the leaves was, on average 17 and 6 times higher than that of roots of plants under control and stressed conditions, respectively. Quantitatively, proline does not contribute substantially to the osmotic potential of noni, however its increase suggests that it plays a role in the salt stress acclimation or is an indicator of salt-induced metabolic disorders.

Keywords: Amino acids, carbohydrates, toxic ions, proline, proteins