36.5: Responses to Drought and Flooding
Water plays a significant role in the life cycle of plants. However, insufficient or excess of water can be detrimental and pose a serious threat to plants.
Under normal conditions, water taken up by the plant evaporates from leaves and other parts in a process called transpiration. In times of drought stress, water that evaporates by transpiration far exceeds the water absorbed from the soil, causing plants to wilt. The general plant response to drought stress is the synthesis of hormone abscisic acid that keeps stomata closed and reduces transpiration. Additionally, plants may respond to extreme water insufficiency by shedding leaves. This method, however, reduces photosynthesis and consequently hampers plant growth.
Mitigation of drought stress in plants by microbes
Drought stress limits the growth and productivity of plants in arid and semi-arid regions. However, certain microbes present in the vicinity of plants may release physical and chemical signals that induce changes related to plant defense under drought conditions. For example, the soil bacterium Paenibacillus polymyx is reported to induce drought tolerance in Arabidopsis. The most significant effect of this bacteria was observed in the growth of legumes under water stress. Leguminous plants depend on soil rhizobium for nitrogen fixation - but rhizobia are extremely sensitive to drought stress, resulting in very low nitrogen fixation. However, soil mixed with P. polymyx resulted in increased nitrogen fixation by rhizobium and increased growth of the bean plant.
Excess water is equally as disastrous to plants as a lack of water. Too much water can suffocate plants by reducing air spaces in the soil, thereby restricting oxygen needed for cellular respiration. Certain woody plant species respond to flood conditions by developing hypertrophic growth that appears as swelling of tissues at the stem base. This hypertrophic growth may aid in the downward diffusion of oxygen as well as potential venting of toxic compounds (carbon dioxide, methane, and ethanol) formed from anaerobic metabolism. Other adaptive responses to flood stress include the formation of adventitious roots, increases in root porosity via specialized cells called aerenchyma cells, and a suberized exodermis to prevent loss of oxygen.