Gas Exchange and Growth
163
8.8.1 Water Deficits
It is well known that growth at the cellular and organ levels is more sensitive
to water deficits than gas exchange (e.g., Bradford and Hsiao 1982). In the
study of the effects of water deficits there has been some confusion between
the effects of dehydration and water shortage per se. However, gas exchange and growth may be affected by low water availability without tissue
dehydration (Davies and Zhang 1991; see also Chap. 10, this Vol.).
In the long term, water deficits reduce the rate of leaf expansion, final
leaf size, and cr, and the rate of initiation of leaf primordia and the release
ofaxilary buds (Pereira 1989; Pereira and Chaves 1992, see also Chap. 10,
this Vol.). The relative importance of the effects of water deficits in each
of these processes may change with plant genotype and stage of plant development. In many cases, differences in leaf area may change growth
rates without important changes in net photosynthetic rates or in photosynthetic capacity, as shown by a reduction in the average biomass production rate between ca. 30 and 60% in three E. globulus clones grown in a soil
with one quarter of the field capacity of well-watered controls (Osorio and
Pereira unpublished). These plants not only had similar photosynthesis
but also had the same leaf nitrogen concentration as the well-watered controls, thus ruling out the likelihood that water shortage creates nitrogen
deficiency. However, lower soil water content may interact with nutrient
uptake. Gollan et al. (1992) showed that the concentration of nitrate (and
phosphate) in the xylem sap decreased with decreasing soil water content in
plants kept fully turgid. Because growth is reduced, this could occur without
changes in the nitrogen concentrations in leaves but result in reduced growth
rates by interference with growth regulators such cytokinins (see below).
Nevertheless, studies in the field with almond trees failed to show any
decrease in cytokinin concentration in xylem sap with increasing water stress
under field conditions (Heilmeier, personal communication). Alternatively,
the response of the plant to ABA might be modulated by nitrate uptake, as
suggested by Schurr et al. (1992) for stomatal closure.
Root growth is often less affected by water deficits than shoots, and this
results in an increase in root/shoot ratio when water availability is low.
As in the case of shoots, hormonal control has been invoked together with
osmotic adjustment to explain the response of roots to water deficits. In the
case of roots, however, it has been suggested that increases in endogenous
abscisic acid ABA act differentially to maintain root elongation and inhibit
leaf elongation with water deficits (Sharp 1990).
After a period of water stress even if gas exchange rates are re-established,
whole-plant growth is reduced mainly due to the decrease in F [see Eqs. (3)
and (4)]. The reduction in water loss by stomatal closure is often not enough
to prevent tissue dehydration and this may cause a decrease in the photosynthetic capacity of leaf tissues. It has been shown that in many cases
photosynthetic capacity may remain unchanged at least until relative water
163
8.8.1 Water Deficits
It is well known that growth at the cellular and organ levels is more sensitive
to water deficits than gas exchange (e.g., Bradford and Hsiao 1982). In the
study of the effects of water deficits there has been some confusion between
the effects of dehydration and water shortage per se. However, gas exchange and growth may be affected by low water availability without tissue
dehydration (Davies and Zhang 1991; see also Chap. 10, this Vol.).
In the long term, water deficits reduce the rate of leaf expansion, final
leaf size, and cr, and the rate of initiation of leaf primordia and the release
ofaxilary buds (Pereira 1989; Pereira and Chaves 1992, see also Chap. 10,
this Vol.). The relative importance of the effects of water deficits in each
of these processes may change with plant genotype and stage of plant development. In many cases, differences in leaf area may change growth
rates without important changes in net photosynthetic rates or in photosynthetic capacity, as shown by a reduction in the average biomass production rate between ca. 30 and 60% in three E. globulus clones grown in a soil
with one quarter of the field capacity of well-watered controls (Osorio and
Pereira unpublished). These plants not only had similar photosynthesis
but also had the same leaf nitrogen concentration as the well-watered controls, thus ruling out the likelihood that water shortage creates nitrogen
deficiency. However, lower soil water content may interact with nutrient
uptake. Gollan et al. (1992) showed that the concentration of nitrate (and
phosphate) in the xylem sap decreased with decreasing soil water content in
plants kept fully turgid. Because growth is reduced, this could occur without
changes in the nitrogen concentrations in leaves but result in reduced growth
rates by interference with growth regulators such cytokinins (see below).
Nevertheless, studies in the field with almond trees failed to show any
decrease in cytokinin concentration in xylem sap with increasing water stress
under field conditions (Heilmeier, personal communication). Alternatively,
the response of the plant to ABA might be modulated by nitrate uptake, as
suggested by Schurr et al. (1992) for stomatal closure.
Root growth is often less affected by water deficits than shoots, and this
results in an increase in root/shoot ratio when water availability is low.
As in the case of shoots, hormonal control has been invoked together with
osmotic adjustment to explain the response of roots to water deficits. In the
case of roots, however, it has been suggested that increases in endogenous
abscisic acid ABA act differentially to maintain root elongation and inhibit
leaf elongation with water deficits (Sharp 1990).
After a period of water stress even if gas exchange rates are re-established,
whole-plant growth is reduced mainly due to the decrease in F [see Eqs. (3)
and (4)]. The reduction in water loss by stomatal closure is often not enough
to prevent tissue dehydration and this may cause a decrease in the photosynthetic capacity of leaf tissues. It has been shown that in many cases
photosynthetic capacity may remain unchanged at least until relative water