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R. Losch and E.-D. Schulze
of plant water potential. Generally, long-term water stress is caused by
decreases of soil water potential, while short-term changes in plants occur
mainly on a daily basis in leaves.
Bates and Hall (1981), studying Vigna unguiculata, found that leaf conductance declined although total leaf water potential remained constant over
a wide range of soil water deficits. Turner et al. (1985) and Gollan et al.
(1985) extended these findings with experiments using Helianthus annuus
and Nerium oleander. They found that leaf conductances and photosynthesis
decreased when about two-thirds of the soil extractable water had been
utilized. Kiippers et al. (1988) found Vigna leaf water contents nearly unchanged when the soil extractable water content decreased from 60 to 0%.
Maximal leaf conductance and assimilation values decreased dramatically
once the amount of soil extractable water was reduced to 20%. Berard and
Thurtell (1991) reported also reduced photosynthetic rates of maize as a
consequence of decreased leaf conductances when soil moisture was low but
leaf water potentials high in a humid atmospheric environment.
Gollan et al. (1986) experimentally decoupled leaf and soil water potentials. They compensated for decreased leaf water potential by applying
pneumatic pressure within a modified pressure chamber and kept wheat and
sunflower leaves turgid while their rooting substrate dried. Leaf conductances decreased under such treatments, when soil water content was reduced to 30% of field capacity. This suggests that information about root
water relations is mediated to leaves without change in leaf water status.
Gowing et al. (1990) discuss three possibilities for signals from roots to
shoots. (1) A negative message coming from turgid roots that promotes
growth and stomatal opening. (2) A positive root signal produced from roots
under water shortage which increases in intensity with increasing drought
conditions in the root zone. (3) An altered transport rate for substances
moving in the transpiration stream from roots to the shoot if the former
are affected by dry soil conditions. Gowing et al. (1990) analyzed these
possibilities using vegetatively propagated apple plants which were cultivated with a split root system. One half of the root system was droughted
while the overall plant water potential remained unchanged because of
water supply from the other half of the root system. Nevertheless, leaf
growth was inhibited when one half of the root system was dried. This
growth inhibition was alleviated after the dry roots were excised. This
suggests that there is a positive inhibitor, produced by drying roots and
transported to the shoot.
The experiment by Gowing et al. (1990) repeats an earlier split-root study
using maize (Blackman and Davies 1985). Plants with half their root systems
subjected to drought displayed the same water, solute, and turgor potentials, but significantly lower leaf conductances, than the completely
watered controls. Also the bulk leaf ABA contents of treated and control
plants did not differ. The stomatal closure of the insufficiently watered
plants became reversed in discs which were removed from the leaves and
R. Losch and E.-D. Schulze
of plant water potential. Generally, long-term water stress is caused by
decreases of soil water potential, while short-term changes in plants occur
mainly on a daily basis in leaves.
Bates and Hall (1981), studying Vigna unguiculata, found that leaf conductance declined although total leaf water potential remained constant over
a wide range of soil water deficits. Turner et al. (1985) and Gollan et al.
(1985) extended these findings with experiments using Helianthus annuus
and Nerium oleander. They found that leaf conductances and photosynthesis
decreased when about two-thirds of the soil extractable water had been
utilized. Kiippers et al. (1988) found Vigna leaf water contents nearly unchanged when the soil extractable water content decreased from 60 to 0%.
Maximal leaf conductance and assimilation values decreased dramatically
once the amount of soil extractable water was reduced to 20%. Berard and
Thurtell (1991) reported also reduced photosynthetic rates of maize as a
consequence of decreased leaf conductances when soil moisture was low but
leaf water potentials high in a humid atmospheric environment.
Gollan et al. (1986) experimentally decoupled leaf and soil water potentials. They compensated for decreased leaf water potential by applying
pneumatic pressure within a modified pressure chamber and kept wheat and
sunflower leaves turgid while their rooting substrate dried. Leaf conductances decreased under such treatments, when soil water content was reduced to 30% of field capacity. This suggests that information about root
water relations is mediated to leaves without change in leaf water status.
Gowing et al. (1990) discuss three possibilities for signals from roots to
shoots. (1) A negative message coming from turgid roots that promotes
growth and stomatal opening. (2) A positive root signal produced from roots
under water shortage which increases in intensity with increasing drought
conditions in the root zone. (3) An altered transport rate for substances
moving in the transpiration stream from roots to the shoot if the former
are affected by dry soil conditions. Gowing et al. (1990) analyzed these
possibilities using vegetatively propagated apple plants which were cultivated with a split root system. One half of the root system was droughted
while the overall plant water potential remained unchanged because of
water supply from the other half of the root system. Nevertheless, leaf
growth was inhibited when one half of the root system was dried. This
growth inhibition was alleviated after the dry roots were excised. This
suggests that there is a positive inhibitor, produced by drying roots and
transported to the shoot.
The experiment by Gowing et al. (1990) repeats an earlier split-root study
using maize (Blackman and Davies 1985). Plants with half their root systems
subjected to drought displayed the same water, solute, and turgor potentials, but significantly lower leaf conductances, than the completely
watered controls. Also the bulk leaf ABA contents of treated and control
plants did not differ. The stomatal closure of the insufficiently watered
plants became reversed in discs which were removed from the leaves and