In biology, stress is the result of the action of an external factor on an organism. Moisture stress indicates the action of lack of or excess of water on plants. The term moisture stress is generally used for deficit moisture conditions though it is applicable to excess moisture also.
Development of Moisture Stress
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Water deficits occur in the plant whenever transpiration exceeds absorption.
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It may be due to excessive water loss, reduced absorption of water from the soil, or both.
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After irrigation or rain, water deficits develop gradually. Though there is a sufficient amount of soil moisture, water deficits develop due to higher transpiration than absorption, especially on hot middays. This temporary wilting is known as incipient wilting or middy depression.
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As the days progress, the soil moisture is depleted and plants cannot absorb a sufficient amount of water to meet the high demand. Water deficits, therefore, occur for a prolonged period, stomata are open for a short period in the morning and evening and partially closed during the rest of the day. When the soil moisture reaches about –15 bars, plants show wilting symptoms most of the day but do not die and it is known as a permanent wilting point. Plants recover when irrigation is given or rain is received.
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If the soil moisture is further allowed to deplete to a level of -60 bars, plants die permanently and this is known as the ultimate wilting point.
Effects of Moisture Stress
Water relations
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It alters the water status by its influence on absorption, translocation, and transpiration.
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Moisture-stressed plants manifest a typical reduction in leaf water potential and relative water content.
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Water deficits also cause an increase in leaf and canopy temperature.
Photosynthesis
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Photosynthesis is reduced by moisture stress due to a reduction in photosynthetic rate, chlorophyll content, leaf area, and an increase in assimilate saturation in leaves.
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The photosynthetic process i.e. entry of carbon dioxide into the leaf, photochemical reactions, and reduction of carbon dioxide are affected by moisture stress.
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Reduction in photosynthesis due to moisture stress is mainly by the reduction in leaf area than by photosynthetic rate.
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The translocation of assimilates is affected by water stress. Between the two processes viz. Translocation of photosynthates and photosynthesis, translocation is more reduced due to moisture stress than net photosynthesis.
Respiration
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Respiration increases with mild stress. However, as the water deficits become severe, it decreases
Anatomical changes
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Periodical water stress leads to anatomical changes like a decrease in the size of the cells and intercellular spaces, thicker cell walls, and greater development of mechanical tissue.
Metabolic reaction
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Accumulation of sugars and amino acids takes place under moisture stress. Proline, an amino acid, accumulates whenever there is moisture stress.
Hormonal relationships
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As a consequence of water deficits, hormonal balance is altered. The activity of growth-promoting hormones like cytokinin, gibberellic acid, and auxins decreases, and growth-regulating hormones like abscisic acid, and ethylene, increase.
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Abscisic acid content is inversely related to leaf water potential. Abscisic acid acts as a water deficit sensor
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Betain is another hormone produced by moisture-stressed plants and it is used as an indicator of moisture stress.
Nutrition
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Moisture stress affects the fixation, uptake, and assimilation of nitrogen.
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Nitrogen fixation by leguminous plants is reduced due to a reduction in leghaemoglobin in nodules, specific nodules activity, and several nodules.
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Nitrogenase activity in the nodules is less in stressed plants.
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Moisture stress also delays nodule formation in leguminous crops.
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Nutrient uptake is the product of nutrient content and dry matter produced.
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Moisture stress may or may not reduce nutrient content but reduces dry matter production considerably. As a result, nutrient uptake is reduced.
Growth
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Generally, the organ growing most rapidly at the time of stress is the one most affected.
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The expansion of cells and cell division are reduced due to moisture stress resulting in a decrease in the growth of leaves, stems, and fruits
Development
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In general, moisture stress delays maturity.
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The general rule is that if stress occurs before flowering, the duration of the crop increases, and when it occurs after flowering, the duration decreases.
Reproduction and Grain Growth
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Moisture regime during flowering and grain development determines the number of fruits and individuals’ grain weight respectively.
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For many crop plants, especially cereals, moisture stress at panicle initiation is critical.
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Anthesis is another important moisture-sensitive stage
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However, vegetative and grain-filling stages are less sensitive to moisture stress.
Yield
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The effect of water stress on yield depends largely on what proportion of the total dry matter produced is considered useful material to be harvested when the yield consists of mostly aerial parts like forage crops, tobacco, etc. the effect of moisture stress is the same as those on total growth.
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When the yield consists of underground storage organs as in potatoes, sugarbeets, etc., it will be as sensitive as total growth.
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When the yield is fiber or chemicals where the economic product is a small fraction of total dry matter, moderate stress on growth does not have an adverse effect on yields.