Why do plants slow their growth during drought?
Slower growth during drought is not caused solely by an inability to photosynthesise. Water uptake and cell expansion, cell-wall properties, hormonal signals and resource allocation change together. Growth can become restricted even before conspicuous wilting appears.
The process and its underlying mechanism
1. Cell expansion requires water For a new leaf to enlarge, cells must extend their walls and take in water to increase their volume. Internal pressure, called turgor, contributes to the physical driving force for expansion. When water potential around roots falls, supplying water to growing regions may become harder. Pressure alone does not determine growth, however: the capacity of the wall to loosen also matters. Cells may partially maintain their water balance yet expand more slowly because the properties of their walls have changed. 2. Slower growth can precede visible wilting Leaf expansion and cell division can be sensitive to water shortage. The production of new shoots may slow while a plant still looks upright. This shows that growing tissues can respond early to environmental signals. Waiting for obvious wilting is consequently not always sufficient for assessing water status. Slower growth alone does not diagnose drought either. Insufficient light, low temperature, root problems and natural periods of reduced activity can create similar outcomes that require separate investigation. 3. Stomata and carbon acquisition Narrowing stomata to limit water loss can also reduce carbon dioxide entry into a leaf. Photosynthesis and the carbon supply for new tissues may then be restricted. Prolonged or severe stress can additionally affect photosynthetic metabolism directly. The earliest decline in growth is not always explained by complete sugar depletion, however. Carbon may accumulate in some conditions because restricted expansion reduces its use. The sequence in which growth and photosynthesis change depends on both the severity of stress and the species.
Environmental conditions and biological limits
4. Resources are not distributed equally among organs Under moderate water shortage, some species reduce shoot growth while maintaining elongation of particular roots for a time. This may help them access additional water. It does not follow that all roots grow faster in drought: severe stress can stop root development as well. Root depth, soil strength and the distribution of moisture influence the outcome. An increased root-to-shoot ratio may sometimes result from shoots being suppressed more strongly, rather than from an actual increase in root production. 5. Hormonal regulation and the cost of adjustment Abscisic acid and other signalling systems regulate gas exchange and development during water shortage. Cells may accumulate compatible dissolved compounds to help maintain water balance. These adjustments require energy and resources. They may support survival without allowing production to continue at its previous rate. Drought tolerance and high yield are therefore not the same characteristic. A plant remaining alive through an extended dry period does not show that growth or reproduction has escaped damage under those conditions. 6. Recovery when water becomes available again Some growth processes can recover quickly when suitable water conditions return. Lost leaf area, damaged roots or problems in conducting tissues may nevertheless delay recovery. Older leaves remaining small can preserve evidence of earlier limitations even after stress ends. Recovery rates vary with species and stress duration. Watering cannot instantly reverse every injury. Keeping the medium continually saturated may also deprive roots of oxygen, so care after drought should be based on the actual condition of the root environment.