Physiological and biochemical responses of the forage legume Trifolium alexandrinum to different saline conditions and nitrogen levels.

Salinity stress reduces plant productivity, but low levels of salinity often increase plant growth rates in some species. We herein describe the effects of salinity on plant growth while focusing on nitrogen use. We treated Trifolium alexandrinum with two nitrogen concentrations and salinity levels and determined growth rates, mineral concentrations, nitrogen use efficiency, photosynthesis rates, and nitrate reductase (NR, E.C. 1.6.6.1) and glutamine synthetase (GS, EC 6.3.1.2) activities. The T. alexandrinum growth rate increased following treatment with 100 mM NaCl in low nitrogen (LN) and high nitrogen (HN) conditions. Salt treatment also increased root volume, intrinsic water use efficiency, and nitrogen use efficiency in LN and HN conditions. These changes likely contributed to higher biomass production. Salinity also increased accumulations of sodium, chloride, and phosphate, but decreased potassium and calcium levels and total nitrogen concentrations in all plant organs independently of the available nitrogen level. However, the effect of salt treatment on magnesium and nitrate concentrations in photosynthetic organs depended on nitrogen levels. Salt treatment reduced photosynthesis rates in LN and HN conditions because of inhibited stomatal conductance. The effects of salinity on leaf NR and GS activities depended on nitrogen levels, with activities increasing in LN conditions. In saline conditions, LN availability resulted in optimal growth because of low chloride accumulation and increases in total nitrogen concentrations, nitrogen use efficiency, and NR and GS activities in photosynthetic organs. Therefore, T. alexandrinum is a legume forage crop that can be cultivated in low-saline soils where nitrogen availability is limited.

Scanning and transmission electron microscopy and X-ray analysisof leaf salt glands of Limoniastrum guyonianum Boiss. under NaCl salinity.

Leaf salt glands of Limoniastrum guyonianum were examined by scanning and transmission electron microscopes and energy dispersive X-ray analysis (EDAX) system, after growing for three months on sandy soil with or without 300 mM NaCl. Results showed that salt glands were irregularly scattered on both leaf sides and sunk under the epidermal level. Salt excretion occurred in both conditions and is mainly composed of calcium and magnesium in control plants, and essentially sodium and chloride in plants subjected to salt treatment. A salt gland is comprised of collecting, accumulating, and central compartments, and is made up of total thirty-two cells. The collecting cells were characterized by large central vacuoles. Accumulating cells contain numerous, large, and unshaped vacuoles and rudimentary chloroplasts. The central compartment was comprised of four basal cells and each one is surmounted by an apical cell. The basal cells are granulated, containing large nucleus, numerous mitochondria, endoplasmic reticulum, ribosomes, polyribosomes, and small vacuoles or vesicles. Equally, the apical cells are rich in organelles. Application of 300 mM NaCl to the culture medium increased vacuoles number and size, and organelles density especially the mitochondria which suggests energy requirement for ions transport. The reduction in size and number of vacuoles toward the interior of salt glands of treated plants and the fusion of the smallest ones with the plasma membrane substantiate the implication of such vacuoles in salt excretion process. The current study which is the first report on L. guyonianum salt gland has provided an in-depth understanding on structure-function relationship in the multicellular salt glands.