Drought and Water Deficit Effects on Plants

Although all droughts lead to plant water deficit, not all plant water deficits result from drought. Disease or pests that damage roots can also cause plant water deficit that cannot be overcome by adding water to the crop. Visual symptoms of root pests and diseases may be indistinguishable from symptoms of drought. Similarly, plants with shallow root systems may suffer water deficit if upper soil layers dry, even though water is available in soil layers below the root zone.

As the basic medium of life, water plays many roles in plant growth and development. Similarly, water deficit affects numerous plant processes. For crop plants, it is convenient to consider four broad groups of responses to water deficit: germination processes, turgor-mediated phenomena, substrate-mediated phenomena, and desiccation effects.

Germination

Crops first require water for germination. Seeds must imbibe enough water to initiate biochemical processes needed to break down the storage compounds to provide the energy and substrate for growth. Seeds are very resis tant to drought as long as they have not begun the process of germination. Once seeds begin breaking down the storage compounds, the radicle begins to elongate seedlings, which become highly susceptible to water deficit.

For the most part, there is a close relationship between crop transpiration and drought sensitivity (figure 2.1). It is only during the short period after seedling emergence that crops are highly susceptible to drought, even though their transpiration rates are low. A light rain may allow seeds to imbibe enough water to begin germination, but if additional rains or irrigation do not follow within a few days, either seedlings may not have enough water to emerge from the soil, or if they have emerged, roots may be too shallow to take up the water needed for the shoots. One of the most common economic losses to rain-fed farming systems results from farmers having to replant crops that fail to establish with the first rains.

Turgor-Mediated Phenomena

Once seedlings have established, water deficits tend to follow a cyclic pattern, with stress levels increasing as the time increases between successive rainfall or irrigation events (figure 2.2). The first water deficit effect, which occurs under relatively mild stress levels, is the slowing or inhibition of cell expansion. A reduction of expansive growth is reflected in smaller leaves and shorter plants. As stress becomes more severe, leaves wilt or roll. One turgor-mediated phenomenon is reduction in stomatal conductance (or stomatal closure). Although partial or complete closure of stomata will conserve moisture within the plant, it also reduces the uptake of carbon dioxide needed for photosynthesis. Similarly, leaf rolling and wilting may reduce the load of radiant energy on the plant and decrease transpiration

Emergence Expansion Flowering Seed Fill Maturation

Figure 2.1 General pattern of changes in relative drought susceptibility and relative crop transpiration during crop development.

Emergence Expansion Flowering Seed Fill Maturation

Figure 2.1 General pattern of changes in relative drought susceptibility and relative crop transpiration during crop development.

Figure 2.2 Idealized graphs showing the severity of drought and time between irrigation or rainfall events for crops receiving frequent, well-distributed (top), or infrequent, poorly distributed rainfall (bottom). Arrows indicate rainfall or irrigation events.

from leaf surface area, which also would conserve water at the expense of reduced capacity for assimilation.

In some plants, water deficits lead to accumulation of solutes within cells in a process called osmotic adjustment. Simple sugars are the principal solutes that contribute to osmotic adjustment, though any dissolved molecules will reduce the solute potential of a cell. An increased concentration of solutes may theoretically allow cells to maintain turgor at lower water potential levels, but there has been no documented case where osmotic adjustment has increased crop productivity under water-limited conditions.

In cereal crops, stems elongate rapidly to force emergence of the reproductive organ from the whorl of leaves. Water deficit during this period can delay or prevent the emergence of the reproductive organ, which in turn causes severe yield loss. Many studies have reported that water deficit increases the root/shoot ratio while reducing both roots and shoots because the deficit reduces shoot growth more than it reduces root growth.

Substrate-Mediated Phenomena

Although the photosynthetic apparatus of leaves is relatively drought resistant, carbon dioxide assimilation declines because of stomatal closure, leaf wilting, and leaf rolling, which occur as a result of moderate to severe water deficit conditions. Reductions in carbon dioxide assimilation reduce substrate available for cell division and growth. Lack of growth substrate may delay ontogenic development, which is seen in reduced rates of leaf appearance, tillering or branching, or in the delay of the reproductive growth phase.

Perhaps the most devastating substrate-mediated response to water deficit is lack of seed set. Near the time of pollination, depending on the availability of assimilates from photosynthesis or stored carbohydrates, plants establish the number of seeds that will develop further. If drought restricts carbon dioxide assimilation or the rate or transfer of stored assimilates, then fewer seeds will develop. Once a plant aborts a seed, the seed can grow no further and potential yield is irrevocably lost.

Desiccation Effects Even at relatively moderate degrees of drought severity, plant extremities (such as leaf tips beginning to fire, turning brown, and desiccate) take place. As drought intensifies, leaves die and abscise. Eventually, shoots, and then roots, die. Only viable seeds can survive desiccation. The following sections explain the distribution of the selected major crops and their response to water deficits.

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