Drought-Related Characteristics of Important Cereal Crops - Monitoring and Predicting Agricultural Drought: A Global Study - page 14

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[14],

Line
——
-0.9
——
Long
PgEn

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
dis tributed 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 princi-
pal solutes that contribute to osmotic adjustment, though any dissolved
molecules will reduce the solute potential of a cell. An increased con-
centration 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 repro-
ductive 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.

[14],

Su bstrate-Mediated Phenomena
Although the photosynthetic apparatus of leaves is relatively drought re-
sistant, carbon dioxide assimilation declines because of stomatal closure,
leaf wilting, and leaf rolling, which occur as a result of moderate to se-
vere 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.