Vergara - 1976 - Physiological and morphological adaptability of ri

PH‘t'SIOLOGI(,‘.+-\L AND tvltiRPHOLOGttfAL ADAPTABILITY 77
the tropical countries (Moomaw and Vergara. 1965). No bleaching can be seen
under field conditions resulting from strong light intensity’. Many authors have
shown that leaves developed under high light intensity have more cells. more
highly‘ organized cells. higher chlorophyll content, and more stomata per unit
leaf area than leaves developed under low light intensity’ (Pearce and Lee, 1969).
This is an adaptation for full utilization of the high solar energy available. The
chlorophyll content of Japanese varieties has been reported to be higher than
that of Indian varieties (Nagai, I962).
PRECIPITATION
During the main cropping season of rice in the tropics. rainy and cloudy days
are abundant. These two factors affect others such as light intensity. light duration.
temperature. and humidity.
Rainfall has been known to aggravate leaf and plant lodging. The long. heatty.
wet leaves tend to bend and stick together. A more rigid, short-tipright leafblade
WOLlltl naturally be more resistant to leaf lodging while a short-statured. stiffstrawed
plant would be more resistant to plant lodging. This is one of the plant
characters that have increased the adaptability of the modern rice varieties to
fertilizers. typhoons. solar energy utilization. etc.
Decrease in light intensity compounded by the higher temperature during the
rainy season also has a marked effect on the plant type that develops. Traditional.
photopericid-nonsensitive rice varieties. like Peta. planted during the wet
season. have a different plant t_vpe or groivth pattern when planted during the
dry season". the plants are usually taller and have more leaves on the main culm
‘when planted during the xvct season (Tanaka el al.. I964). These effects are more
pronounced in traditional varieties than in improved varieties. Modern varieties
have greater adaptability to changes in environment.
Perhaps the main indirect effect of rainfall on rice growth is the availability
of water. either too much or too little of it. The rice plant is adapted to anaerobic
soil conditions. having air canals from leaves to roots. Varietal differences have
been noted on the rate of oxyjgen absorption and amount of air spaces. Latematuring
varieties are capable of transferring oxygen from aboveground parts
to the roots faster than the early maturing varieties (Fujii and Tanaka. 1955).
Also, lowland rices have a greater volume of air and a greater percentage of air
space in their leaves than the upland rice varieties (Katayarna, 1961). Development
of intercellular spaces in rice is influenced by the air content of the soil.
being three times as great in non-aerated soil as in aerated soil. The adaptability
of rice to anaerobic conditions is apparent when one compares the larger intercellular
spaces of rice with those in corn or broad beans (Katayama. 1961).
In the case of excess water or deep-water stress, some specialized rice varieties
have the ability to elongate and keep up xvith the water rise, an adaptation that
is common even in wild rices. This specific adaptability can, how/ever, be incorporated
for general adaptability in rice varieties without a decrease in grain