164
l.S. Pereira
content decreases to around 70% (Chaves 1991). Stomata close at much
less severe water deficits. However, the extent and nature of nonstomatal
limitation of photosynthesis depend upon severity of dehydration, rate of
imposition of stress, and the coincidence of water deficits and other stresses.
There are, however, species differences in the response of photosynthetic
capacity to water deficits (Quick et al. 1992).
The maintenance of the photosynthetic capacity at the chloroplast level
until dehydration is too severe may be important for growth in many circumstances, especially in recovery from short dry spells. Indeed, leaf carbon
assimilation may resume at high rates as soon as stomata open after rehydration (Chaves 1991). Recovery may become more complex if dehydration
lasts for a relatively long period because then stored starch may be used up
in the growth of strong sinks, e.g., reproductive growth in annuals (Chapin
et al. 1990). In the short-term, partition of photo assimilates the accumulation
of starch is sacrificed to maintain the levels of soluble carbohydrates near
the well-watered values (Quick et al. 1992). Under the same circumstances
there may also be a decrease in Rubisco leaf concentration (Jones 1973;
Chaves 1991). In addition to the eventual decline in photosynthetic capacity,
this would result in the decrease in nitrogen stored in the leaves for future
growth. However, contradictory evidence was found for sunflower even
after 4 weeks after withholding water (Fredeen et al. 1991). It is not clear
how much of the reported decrease in photosynthetic capacity is the direct
result of tissue dehydration or is due to leaf aging.
In addition to the more or less reversible effects of dehydration on leaves,
water stress may result in accelerated acropetal leaf senescence, which may
result in a decrease in whole-canopy photosynthetic capacity (see also Chap.
10, this Vol.). The importance of drought-induced leaf senescence and
abscision is quite variable among species, and varies with the intensity and
timing of water deficits (Jordan 1983; Pereira and Pallardy 1989; Ranney et
al. 1990; Pereira and Chaves 1992).
Much less work has been done on respiratory rates under water stress,
but again the rates of metabolism per se seem to be quite resistant to
dehydration. However, both growth and maintenance respiration decrease
because growth also decreases. For example, maintenance respiration in
sorghum decreased from 50 to 30mg g-ld- 1 as the Rw decreased from 0.16
to 0.04d- 1 as a result of water stress (Wilson et al. 1980).
As mentioned above, biomass production is usually correlated with the
amount of water transpired
B = wE,
(10)
where w is the proportionality regression coefficient or water use efficiency
and E the amount of water transpired. This is not surprising, given the
leading role of stomatal aperture and leaf area in controlling both photosynthesis and transpiration. Net assimilation rate (E A ) of single plants has
also been found correlated with the rate of transpiration on a leaf area
l.S. Pereira
content decreases to around 70% (Chaves 1991). Stomata close at much
less severe water deficits. However, the extent and nature of nonstomatal
limitation of photosynthesis depend upon severity of dehydration, rate of
imposition of stress, and the coincidence of water deficits and other stresses.
There are, however, species differences in the response of photosynthetic
capacity to water deficits (Quick et al. 1992).
The maintenance of the photosynthetic capacity at the chloroplast level
until dehydration is too severe may be important for growth in many circumstances, especially in recovery from short dry spells. Indeed, leaf carbon
assimilation may resume at high rates as soon as stomata open after rehydration (Chaves 1991). Recovery may become more complex if dehydration
lasts for a relatively long period because then stored starch may be used up
in the growth of strong sinks, e.g., reproductive growth in annuals (Chapin
et al. 1990). In the short-term, partition of photo assimilates the accumulation
of starch is sacrificed to maintain the levels of soluble carbohydrates near
the well-watered values (Quick et al. 1992). Under the same circumstances
there may also be a decrease in Rubisco leaf concentration (Jones 1973;
Chaves 1991). In addition to the eventual decline in photosynthetic capacity,
this would result in the decrease in nitrogen stored in the leaves for future
growth. However, contradictory evidence was found for sunflower even
after 4 weeks after withholding water (Fredeen et al. 1991). It is not clear
how much of the reported decrease in photosynthetic capacity is the direct
result of tissue dehydration or is due to leaf aging.
In addition to the more or less reversible effects of dehydration on leaves,
water stress may result in accelerated acropetal leaf senescence, which may
result in a decrease in whole-canopy photosynthetic capacity (see also Chap.
10, this Vol.). The importance of drought-induced leaf senescence and
abscision is quite variable among species, and varies with the intensity and
timing of water deficits (Jordan 1983; Pereira and Pallardy 1989; Ranney et
al. 1990; Pereira and Chaves 1992).
Much less work has been done on respiratory rates under water stress,
but again the rates of metabolism per se seem to be quite resistant to
dehydration. However, both growth and maintenance respiration decrease
because growth also decreases. For example, maintenance respiration in
sorghum decreased from 50 to 30mg g-ld- 1 as the Rw decreased from 0.16
to 0.04d- 1 as a result of water stress (Wilson et al. 1980).
As mentioned above, biomass production is usually correlated with the
amount of water transpired
B = wE,
(10)
where w is the proportionality regression coefficient or water use efficiency
and E the amount of water transpired. This is not surprising, given the
leading role of stomatal aperture and leaf area in controlling both photosynthesis and transpiration. Net assimilation rate (E A ) of single plants has
also been found correlated with the rate of transpiration on a leaf area