PhD

Rice production is subject to many biotic and abiotic stresses of which rise in
temperature is a crucial one in the present context of global warming. Hence, a study on
management of high temperature stress in aerobic rice was taken up at the Department of
Agronomy, College of Horticulture, Vellanikkara with the main objective of assessing the
varietal response to high temperature as well as to find out sensitivity of various growth
phases of rice to elevated temperature. In two pot culture experiments conducted in the
first year of study (Dec 2017- Mar 2018), selected aerobic and wetland rice varieties were
exposed to temperature 2-3 ℃ higher than ambient air temperature at different growth phases.
A field experiment to study the effect of nitrogen and potassium nutrition in mitigating high
temperature stress was carried out in second year of study during Jan 2019 – April 2019.
Four aerobic rice varieties [Swarnaprabha, Sharada (MAS 946-1), Vaishakh and
MDU-6] were included in Experiment I and four wetland rice varieties (Kanchana, Jyothi,
Uma and Aiswarya) were tested in Experiment II. Temperature stress (2-3 ℃ above
ambient) was artificially imposed at three different growth stages of rice i.e., from tillering
to panicle initiation, panicle initiation to flowering and flowering to maturity. For imposing
temperature stress, portable chambers with the dimensions of 2 m x 2 m x 1.5 m with top
covered with UV stabilized polythene sheet and sides covered with polythene sheet, were
fabricated (Singh, 2010) leaving a gap of 15cm from the ground level at the base for free
air passage. Each variety was maintained under ambient condition throughout the crop
growth period as control (i.e., without stress).
High temperature during vegetative phase enhanced the growth parameters in all
varieties. Among rice varieties, Vaishakhh and Aiswarya exposed to stress from tillering
to panicle initiation (PI) showed increased plant height, tiller number, leaf area index and
dry matter production compared to stress at other stages and control. Earliness in flowering
was observed when stress was imposed from tillering to PI stage in all varieties and they
produced higher number of panicles under ambient temperature compared to elevated
temperature condition. It was found that in varieties Sharada and Aiswarya, comparable
number of productive tillers was seen when stress was imposed at various growth phases
compared to plants grown under ambient condition. The highest number of filled grains per
panicle in both aerobic and wetland rice varieties were recorded under ambient condition.
The highest reduction in filled grains per panicle and 1000 grain weight was observed when
high temperature stress was imposed from flowering to maturity and this resulted in grain
yield reduction in all the varieties. At this stage the varieties Vaishakh and Aiswarya
ii
produced higher number of filled grains per panicle, indicating varietal suitability to high
temperature. These varieties produced higher grain yield compared to other varieties when
stress was imposed at this sensitive stage (Vaishakh-13.81g/hill and Aiswarya-13.35g/hill).
The performance of these two varieties were superior under ambient condition also.
Physiological parameters such as photosynthetic rate, stomatal conductance,
transpiration rate, chlorophyll content, relative water content (RWC), soluble protein,
nitrate reductase activity, membrane stability index (MSI), proline and superoxide
dismutase (SOD) decreased under high temperature whereas increase in malondialdehyde
(MDA) content was observed. Grain yield was positively correlated with chlorophyll
content, membrane stability index and soluble protein content, but negatively correlated
with MDA and proline content. The varieties Vaishakh and Aiswarya showed high values
of transpiration rate, soluble protein, nitrate reductase acitivity, SOD and canopy air
temperature difference under high temperature compared to other varieties, which points
to their tolerance to high temperature. Higher proline content was observed for
Swarnaprabha, Vaishakh and Kanchana when exposed to high temperature. Variation in
heat units like GDD, HTU and PTU were not significant under heat stress compared to
control.
The third experiment was laid out in randomized block design with six treatments
which included two rice varieties Vaishakh and Aiswarya grown with three N and K2O
levels (60:30 kg, 90 :45 kg , 120:60 kg N:K2O/ha). Phosphorus was applied uniformly @
30 kg/ha to all treatments basally. High temperature stress was imposed during flowering
to maturity period by using portable polythene chamber. The study revealed that increase
in N and K levels from 60:30 kg N: K2O/ha to 120:60 N:K2O kg /ha resulted in increased
growth and yield attributes and grain yield along with a remarkable increase in RWC,
chlorophyll content and MSI. Also, application of 90 kg N and 45 kg K2O produced
comparable grain yield to that of 120 kg N and 60 kg K2O/ha in both Vaishakh (2485
kg/ha) and Aiswarya (2401 kg/ha) . The increase in grain yield over recommended dose of
60:30 kg N: K2O/ha was about 20 per cent. Hence, it can be inferred that yield reduction
due to high temperature stress in aerobic system of rice cultivation can be managed to some
extend by nitrogen and potassium nutrition. The net return was also high with application
of N: K2O @ 90:45 kg /ha when compared to 60:30 N: K2O kg/ha.