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Abstract
Salinity inhibits growth of most plants. The primary observed response to salt stress is often a reduction of leaf growth. The mechanisms involved in leaf and cell growth inhibition are not clearly understood. Nutritional status of the salinized growing cells is conventionally discussed in relation to growth inhibition. Ions may be confined to various compartments within the plant cell. Microlocalization in sub-cellular compartments may explain differences in plant response to salinity and play a role in tolerance and damage mechanisms. Knowledge of volume fractions of different cell compartments, and information on ion contents in such sub-cellular compartments is vital for evaluation of the role of cell nutritional status in cell growth inhibition under salt stress. In this project, we therefore investigated the morphometry of salinized and control developing leaf cells. The volume fraction of cytoplasm, vacuole and cell walls of mesophyll cells were quantified in order to asses which features of cell structure varied under saline conditions. Monocot leaves were chosen as a model system for this project, because of their organized and spatially distinguished elongation zone. Maize plants grown under salt (80 mM NaCl) and non-stressed (1 mM NaCl) conditions, were studied for their growth and sampled for morphometric analysis. Digital images of cross-sections throughout the growing zone of salt stressed and control maize leaves were acquired with a TEM (transmission electron microscope), and relative volume fractions of cellular compartments determined by image analysis. Salinization induced a 35% reduction in the rate of maize leaf elongation. Characterization of the spatial distribution of growth along the leaf elongation zone by marking experiments, revealed that salinity reduced the rate of leaf elongation by shortening the length of the elongation zone and decreasing the magnitude of elongation in the central and distal locations. Morphometric analysis of cells throughout the development gradient reveled that, as expected, the relative volume fraction of vacuoles increased along the developmental profile. Salinization led to decreased relative volume of vacuoles in both locations, the reduction being larger in the younger cells, while the cytoplasmic and cell wall volume fraction both increased. The reduced relative vacuolar fraction in the salt-stressed leaf might affect tissue capacity for compartmentation .
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