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Rice has the evolutionary advantage of being semi-aquatic, making it one of
the world's most important crops. As a result, it has fewer water-scarcity responses
and is especially vulnerable to drought stress. Climate change has put rice
production in jeopardy today. According to recent studies, rice yield decreases as
result of climate change such that when drought frequency and rainfall pattern
changes, rice output is lowered. Rice is the principal staple grain grown in Kerala,
with around 600 types used across the state's paddy fields. Uma (MO 16),
developed by Kerala Agricultural University's Rice Research Station, Moncompu,
is one of the most popular rice varieties in Kerala, which is currently planted in
more than 60% rice growing areas in the state. It also has adaptability towards wide
range of environmental conditions as well, even though environmental variations
limit the potential yield of variety Uma.
In this context, the present study on differential and dynamic response of
variety Uma under drought stress at vegetative stage and its impact on stress
responses of progeny, were attempted with the main objective of identifying
potential drought stress specific biomarkers representing a trend in metabolite
dynamics across generations. This approach was aimed to demonstrate in a
simple way that a natural drought stress exposure can influence stress
responses of rice variety, Uma in the present as well as future generations, with an
aim of identification and selection of potential traits for inclusion in crop
improvement programmes. The study was carried out at the Department of
Plant Physiology, College of Agriculture, Vellanikkara, Thrissur as two
experiments- a drought stress imposition study in the net house facility and a
laboratory study in the next generation. As part of the first experiment,
drought stress (60% field capacity; FC) was imposed to rice variety, Uma at
vegetative stage by gravimetric approach, simultaneously maintaining a nonstressed control at 100% FC by regular watering. The experiment was laid out in
completely randomized design in customized rain-out shelter like structures in the
net house. Physiological parameterswere assessed immediately after stress exposure
for assessment of stress damage and it was noted that there was a significant
reduction in chlorophyll content (36.2%), membrane thermostability (78.5%) and
relative water content (23.5%) under stress in comparison to non-stressed control
tissues, ensuring effective stress exposure. Biochemical markers such as proline and
malondialdehyde (MDA) estimated immediately after stress and after recovery
across different phenophases, revealed a differential pattern in their regulation. The
levels of both the biomarkers were significantly higher (14% and 70.7%
respectively) under stress which dropped drastically at panicle initiation stage,
followed by an increase in the grain filling stage. However, it was interesting to
note that correspondingly both the biomarker metabolites exhibited lesser
accumulation (95.1% and 25.2% respectively) in comparison to the non-stressed
control plants in grain filling stage. The differential dynamics of proline as well as
MDA over different phenophases when related to the protein levels estimated at
panicle initiation stage, revealed that there was no concomitant recovery in protein
levels even after stress recovery. The non-stressed control and stressed plants were
raised to maturity and harvested seeds were used for the second experiment for
assessing the stress memory response in the next generation. Yield attributes
assessed also showed significant difference between the stressed and non-stressed
plants.
The second experiment to assess the stress memory response aimed at
comparative stress response assessment under salinity as well as osmotic stresses at
seedling stage in the next generation. In this context, a standardization experiment
was carried out using different concentrations of NaCl and polyethylene glycol
(PEG 6000) to impose salinity and osmotic stresses respectively, to uniformly
germinated seedlings of rice variety, Uma, to ascertain sub-lethal stress levels.
From the standardization experiment, sub-lethal levels of salt (100 mM NaCl) and
osmotic (-4 bar) stresses were arrived at based on the percent reduction in growth
over control. The progenies of the first experiment along with a lot of fresh
seeds were exposed to stress using the standardized doses of salt as well as
osmotic stresses. Priorto stress exposure, germination speed and the vigour index of
the seedlings were estimated, which led to the discovery of two classes of stress
exposed plants of first experiment with their seeds differing in vigour viz. moderate
and high vigour. Four classes of seed materials- control and stressed (high and
moderate vigour) seed sets from first generation and a fresh set of seeds of rice
variety, Uma were used for the stress response experiment using the sub-lethal
stress levels ascertained. The high vigour seedlings from the stressed plants showed
better growth under salt stress compared to control set of seedlings from first
generation as evidenced by lesser percent reduction in growth as well as recovery
responses. Subsequently, biochemical marker MDA was estimated in the different
treatments revealing that moderate as well as high vigour seedlings of the stress
class accumulated lesser (64.5% and 18.6% respectively) MDA levels in
comparison to the non-stressed control, with a consistent pattern under salinity
stress. GC-MS/MS analysis was performed using shoot tissues of the seedlings
across the different treatments, with an attempt to unravel any possible stress
responsemechanism. GC spectra displayed a number of highly abundant as well as
uniquely present compounds in the plants. Diphenyl sulfone, a potent sulfone
derivative with antioxidant activity was differentially detected in all classes of plant
materials used for the stress response study. There were corroborative inferences
from the fold changes in diphenyl sulfone as well as the MDA contents, wherein
there were proportional variations in the two especially in NaCl induced salinity
stress in all treatments, indicating its probable role in maintaining lower MDA
levels in the progenies of plants that were subjected to a prior drought stress
exposure in the previous generation.
A round of stress exposure at vegetative stage in rice variety, Uma
influenced several morpho-physiological and biochemical parameters at different
stages of the crop. There was an indication of the transmission of stress induced
signal even in the next generation as evidenced by profuse growth and lower MDA
contents under different stresses at seedling stage. It will be rewarding to observe
the stress response/acclimation behaviour of Uma, under different abiotic stress
exposure regimes across different phenophases.