How does salinity affect the growth and photosynthesis in plants? Elucidate the avoidance mechanisms of salt stress by plants.
Salinity severely impacts plant growth and photosynthesis. High salt concentrations in soil create osmotic stress, making it difficult for roots to absorb water, leading to physiological drought. Excess ions like Na+ and Cl- become toxic, disrupting enzyme activity, damaging cell membranes, and causing nutrient imbalances. For photosynthesis, osmotic stress forces stomata to close, reducing CO2 uptake. Ion toxicity also directly damages photosynthetic machinery (chloroplasts), leading to reduced efficiency and oxidative stress.
Plants employ various avoidance mechanisms. Ion exclusion involves preventing excessive salt uptake by roots (e.g., in some mangroves). Some plants use specialized salt glands to excrete excess salt from their leaves (e.g., Atriplex species). Others utilize succulence, accumulating water in fleshy tissues to dilute internal salt concentrations. Leaf abscission is another strategy, shedding old, salt-laden leaves. These mechanisms collectively aim to prevent salt from accumulating to toxic levels in sensitive parts, thus maintaining growth.
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