Vergara - 1976 - Physiological and morphological adaptability of ri

240 crrrmrs AND rues
than 80% of the total water absorbed by the roots of lowland rices grown under
upland conditions ivas obtained from soil depths of less than 2O cm (llasebe et
a1.. 1963). ly-‘ama and Murata (1961) indicated that the photosynthetic rate of rice
began to decline as soil moisture dropped below 57% of maximum water capacity’.
ln upland crops such as groundnut, wheat. barley", and soybean. photosynthesis
began to decline at a moisture level below 32 to 45% of maximum
water capacity This suggests that riee requires high soil moisture to maintain
high dry-matter production rates.
The photosynthesis of riee plants is usually reduced before wilting is visible.
The photosynthetic rate of riee leaves tyas constant up to a leaf water content
slightly less than maximum leaf turgidity’, and then decreased rapidly with
decreasing water content. though no sign of wilting was observed (Iyama and
Murata. 1961). This may be attributed to stomatal closure.
Even in rice plants grown in submerged soil. transpiration on hot, sunny‘.
windy days far exceeds water absorption. and consequently severe midday water
deficits often develop. Such deficits usually are eliminated by water absorption
during the night (T. Tanaka. 1972). Plant water deficits may be caused either by
excessive transpiration. by slow absorption of water. or by the combination of
the two. Water deficits are more severe in upper leaves receiving high S0181‘
radiation and wind than in lower leaves. The transpiration rate ofa rice population
is at a maximum during the period from booting to heading stage when the
LAI is the highest. During this period. rice leaves are liable to suffer ivater
deficit. The photosynthesis of rice leaves at booting stage was closely correlated
with leafwater content (Murata. 1961).
Little is known about the influence of humidity on photosynthesis of rice
plants. Tsuno and Sato (197'1) reported that the photosynthesis of rice leaves
attained a maximum at a relative humidity of 50-60% and above this decreased
slowly with increasing humidity When. however. half of the roots were cut off.
photosynthesis increased with increasing humidity. Thus. optimum humidity for
photosynthesis may vary depending upon interrelationships between ivater
absorption and transpiration.
Submcrgenec stabilizes the water supply of rice plants, but has some deleterious
effects. It restricts gaseous exchange between the atmosphere and the soil.
This consequently leads to the accumulation of the products of anaerobic decomposition
such as methane, organic acids. and hyjdrogen sulfide. Some of these
products interfere with the normal physiological functions of rice roots. When
the roots of rice in hydroponic culture were damaged by the addition of 11,3,
photosynthesis decreased considerably (Baba and Tajima. 1962'. Murata ct a1,
1966). Also, when root rot occurred, the riee leaves showed considerable
decrease in cytochrome oxidasc activity and increase in respiration and peroxidase
activity. '1'. Tanaka (1972) indicated that the riee population with deteriorated
roots had lower light saturation points and lower CO, uptake at high
light intensity than the population with high root activity. This difference W33
attributed to the difference in stomatal movement through water absorption.