Drought-Related Characteristics of Important Cereal Crops - Monitoring and Predicting Agricultural Drought: A Global Study - page 13

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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 transpira-
tion 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 ir-
rigation 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.

[13],

Tu rgor-Mediated Phenomena
Once seedlings have established, water deficits tend to follow a cyclic pat-
te rn, 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

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PgEn

[13],

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