Responses to Drought and Flooding

Insufficient or excess water can pose a serious threat to plants. 

Under normal conditions, most of the water taken up by a plant evaporates from its leaves and other plant parts, a process called transpiration. Drought stress causes plants to wilt, because the water lost through transpiration exceeds the water absorbed from the soil. 

Plants respond to drought stress by synthesizing abscisic acid and releasing it in the leaves; this hormone keeps stomata closed, preventing water loss through transpiration. 

During water scarcities, the wilted leaves of grasses fold into tube-like structures; this reduces water loss by decreasing the leaf surface area exposed to dry air. Other plants respond to drought by shedding their leaves to conserve water. However, the water-conserving strategies of leaves also reduce photosynthesis. 

Some plants respond to chemical signals released by neighboring, drought-affected plants by priming themselves for a more robust response to the imminent drought. 

Too much water, by contrast, can suffocate a plant by reducing air space in the soil and thereby restricting the oxygen needed for cellular respiration.

Some plants have developed structural adaptations that enable them to live in very wet habitats. For example, many mangrove species have specialized aerial roots called pneumatophores that allow the plants to obtain oxygen. 

In waterlogged soil, plants that lack such adaptations become oxygen-deprived, leading to the death of some cells in the root cortex; this process generates air tubes, which supply oxygen to the water-submerged roots. 

Deepwater rice is a unique rice variety that increases its height during flooding, keeping its leaves above the rising floodwaters.

When submerged, the rice plant accumulates the plant hormone ethylene, leading to the increased production of hormones called gibberellins. Gibberellins stimulate the vertical growth of the plant. 

Agriculturalists must develop such stress-resilient crops to keep up with demand amid climate change and unpredictable weather conditions.