by breeders (IRRI 1976).
During the 1970s, physiologists were screening root
growth of large numbers of genotypes, as required by
the breeding program. Using the root box technique,
200 varieties were screened for root depth in 1975
(IRRI 1976); 768 were screened for root depth in 1977,
of which 256 were classified as deep, mostly upland
varieties (IRRI 1978); and 1081 were screened for
deep root:shoot ratio in 1979 (IRRI 1980). A number
of traits were explored that were hypothesized to be
representative of deep root growth, including root
pulling force in flooded paddies and time to flowering,
for which shorter time to flowering was reported to be
correlated with deep root:shoot ratio.
Root study methods at IRRI in the 1970s
Of all the root techniques used at IRRI during the
1970s, perhaps the most distinct was “root pulling
force” (Fig. 3), in which individual hills were
vertically pulled out of the soil, and the force required
to do so was reported to be correlated with root growth.
This measurement was conducted in flooded paddies,
using about 10 hills per plot. Other root measurements
in the field included core sampling coupled with the
use of photographs to estimate total root length (IRRI
1978). Both vertical and lateral root distribution were
assessed, and the need for uniformity of field soil was
emphasized for root studies.
Greenhouse root studies during the 1970s included
the use of tanks (hydroponic systems) and soil-filled
boxes, and a unique greenhouse setup for maintaining
constant soil water potential (Fig. 4). Containers were
also used to conduct large-scale screenings on drought
tolerance under limited rooting depth (IRRI 1979).
Description of a plant ideotype for improved yield
under drought
Water stress at different growth stages was recognized
to affect yield differently (IRRI 1971). A collection of
terms was used to describe the different types of
drought stress responses that allow a plant to produce
grains under stress (O’Toole and Chang 1978): 1)
escape (e.g., early flowering, or matching crop
duration and development to the rainy season length),
2) avoidance (e.g., deep root growth to allow continued
water uptake), 3) drought tolerance (e.g., the
ability to withstand very negative soil water potentials),
and 4) drought resistance, an overall term for the
ability to produce grains through any of the above
mechanisms (Fig. 5). Root morphology and rate of
development were thought to be the most important
characteristics for drought resistance (Krupp et al.
1972).
w