Vergara - 1976 - Physiological and morphological adaptability of ri

324 CLIMATE AND RICE
Photosynthesis tno-cozcm-Ks")
iso
I20 __
Maize
O -4 -B -|2 -l6
Leaf water potmtlul (bun)
1. Leaf elongation and net photosynthesis in maize having various leaf wvater
ptttentials. Note the responsiveness of leaf elongation even at high leaf Witter
potentials and compare this with the response of net photosynthesis. The 30-dayold
plants were growing in soil from which water was withheld. For the leaf
enlargement data. the plants were placed in a dark. humid chamber tor 36 hr at
26‘C. and measurements of water potential and leaf length were made before and
afier the last 24 hr under those conditions. Net photosynthesis of intact shoots was
measured by infrared gas analysis. Leaf water potentials were determined with a
thermocouple psychmmeter using the isopiestic technique (Boyer. 1969) on leaf
disks lrom the same leaves on which the physiological measurements were made
(data adapted from Boyer. l970a).
enlarging cell. In growing cells, the yielding of the cell walls under the action
0f turgor effectively places a limit on the tnaximurn turgor that cart occur. This
in turn controls the water potential of the cell relative to its environment. and
hence a gradient in water potential is formed. Thus. the water potential ofleaves
remains below the water potential of the water supply for as long as growth
occurs. ("rrowth then reflects a balance bettveen expansion on the one hand and
the gradient in water potential that supplies the ivater for expansion, on the
other (Boyer. 1968). This appears to be the reason that leaf water potentials
are —l.5 to —2.5 bars instead of virtually zero n=hen plants are growing in
\‘t’6ii—\\'€ll€f€d soil.
Dcsiccation of short duration. such as might occur during a clear summer
day. appears to have a reversible effect on leaf growth (Acevedo ct al.. 1971).
and rapid rates resume when leaf nutter potentials return to —l.5 to —2.5
bars. If leaf water potentials are continuously less than optimum for several
days. l1o\\-'e\~'cr. there is evidence that leaves itiay not grovr at the original rate
upon rewatering (Boyer. 1970a). Therefore. the duration of a drought has an
effect on the subsequent regrovath of the vegetative plant.