Plant size has long been implicated in plant response to drought stress. This
study is the second in a series of two intended to examine the effect of plant
size on plant performance under the effect of various agents of drought
stress. Variable plant size (in terms of plant height and shoot biomass)
independent of genetic background effects was experimentally achieved using
rht (tallest),
Rht1 and
Rht2 (medium) and
Rht3 (shortest) homozygous height
isogenic lines of spring wheat (Triticum aestivum)
cultivars Bersee and April-Bearded.
Plants were grown in hydroponic culture in the growth chamber. In the first
experiment, juvenile plants were challenged by osmotic stress using
polyethylene glycol (PEG) in the nutrient solution giving a water potential of
–0.55 MPa. The control nutrient solution was at –0.05 MPa. Plant
growth, shoot biomass, leaf area, relative water content (RWC) and osmotic
adjustment (OA) were measured. In the second experiment, effects on growth
rate of chronic heat stress and abscisic acid (ABA) in the root medium of
juvenile plants were evaluated.
Potential plant size as determined by shoot biomass in the controls at 25 days
after emergence was greatest in rht, medium in
Rht1 and
Rht2, and smallest in
Rht3 genotypes. Potential growth
rate and leaf area were greater in plants of larger potential biomass
(rht) than in plants of smaller potential biomass
(Rht3). Growth reduction by
osmotic stress was inversely related to plant size, while the extent of
osmotic adjustment during osmotic stress was directly related to plant size.
RWC did not vary with plant size. Relative growth reduction by heat stress and
by ABA also decreased in smaller plants. ABA did not alleviate the depressing
effect of heat on growth.
Despite the greater stress tolerance of smaller
(Rht3) plants, the absolute growth
and biomass of large (rht) plants under stress
conditions was always better than that of smaller plants. The results of these
series of experiments suggest that greater stress tolerance of small plants is
derived from their relatively smaller size and slower growth rate.
Consequently, we conclude that growth under stress is sustained by potential
growth rate and plant size of the genotype when stress is mild and by plant
tolerance (even at the expense of potential growth rate and size) when stress
is more severe.
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