Low night temperatures during late January and early February coinciding with early
pollen microspore (EPM) development of rice (Oryza sativa) is a major factor
limiting productivity in the Riverina region of New South Wales (NSW). This project
primarily examined genotypic differences in cold damage that are associated with low
temperature during reproductive development. The objectives were to: (1) investigate
the effects of low temperature on physio-morphological traits of rice plants, with
particular emphasis on reproductive traits; (2) examine the consistency of expression
of cold tolerance in different screening environments; and (3) quantify the effects of
temperature and daylength on the phonological development among cultivars.
Results from three screening environments including temperature-controlled rooms, a
cold water facility and field experiments are reported. Over 50 cultivars from diverse
origins, including cold tolerant cultivars from Eastern Europe, Japan and California
were screened. Cultivars were exposed to day/night air temperatures of 27°/13°C in
temperature-controlled rooms and a constant temperature of 19°C in the cold water
facility. Exposure time for plants was from panicle initiation (PI) to 50% heading.
To increase the likelihood of inducing cold damage in field experiments, several
techniques such as multiple sowing dates, shallow water depths (5cm) and high
nitrogen rates (300kgN ha-1) were used.
The three screening methods induced sufficient levels of spikelet sterility to identify
genotypic differences and consistently categorise cold tolerant cultivars. Among the
common cultivars there was a highly significant relationship for spikelet sterility
between temperature-controlled rooms and field experiments (r2=0.52, p<0.01, n=31),
temperature-controlled rooms and the cold water facility (r2=0.63, p<0.01, n=21) and
the cold water facility and field experiments (r2=0.53, p<0.01, n=21). Screening for
cold tolerance in temperature-controlled rooms or the cold water facility was
preferred to field screening because of the reliability of exposure to low temperature
in both environments. However, it is still important to combine a controlled
environment screen with field observations since some cultivars varied in their
response under different screening methods.
Several flowering traits such as the number of engorged pollen grains per anther,
anther length and anther area produced significant genotypic variation and were
negatively related to spikelet sterility at maturity. When low temperature coincided
with reproductive development Australian and Californian cultivars were inefficient
at producing filled grains, despite them having a similar number of engorged pollen
grains and similar sized anthers to cultivars from other origins. This inefficiency may
be partly related to a small stigma area.
Several cold tolerant cultivars (M103, HSC55, Plovdiv 22, M104 and Jyoudeki) and
cold susceptible cultivars (Sasanishiki, Doongara, Nippon Bare, Sprint and Reiziq)
were identified. However, many of the cold tolerant cultivars had a shorter growth
duration leading to lower yield potential compared to commercial cultivars.
Therefore, two shorter duration cold tolerant cultivars, HSC55 and Plovdiv 22, were
hybridised with two NSW commercial cultivars, Illabong and Millin, to determine if
cold tolerance could be improved. The progeny were evaluated for cold tolerance in
temperature-controlled rooms and there was found to be no relationship between
growth duration and spikelet sterility. Although, it should still be possible to produce
cold tolerant cultivars with appropriate growth duration for Australian conditions.
Phenological development was examined in sequentially sown field experiments by
exposing plants to low temperatures and providing several different temperature and
daylength conditions. Amaroo and Millin were identified as mildly photoperiod
sensitive, whilst M103 and HSC55 were found to be photoperiod insensitive. A crop
phenology model was developed for Amaroo and used to predict an optimum sowing
date based on historical weather data from 1955 to 2002. The model minimizes the
possibility of exposure to low temperatures during the young microspore and
flowering stages. The analysis indicated that the 15th October was the optimal sowing
date for Amaroo. Nevertheless, sowing up to November 1st when seasonal
temperatures are average also minimizes the risk of encountering low temperatures.
Increasing the photoperiod sensitivity of cultivars above that of Amaroo may further
reduce the risk of encountering low temperatures and at the same time increase
Results from this project have improved our understanding of the mechanisms of
genotypic response to low temperature during reproductive development and provided
methods to develop cold tolerant cultivars.