PhD

The study entitled “Physiological, biochemical and molecular studies in medicinal rice (Oryza sativa L.), Njavara, as influenced by abiotic stresses” was conducted during 2015 to 2019 at the Department of Plant Physiology, College of Agriculture, Vellayani, Thiruvananthapuram. The objective of the study was to elicit information on the physiological, biochemical and molecular attributes associated to secondary metabolites accumulation due to abiotic stresses viz., shade, drought and UV-B stress in medicinal rice Njavara. The study was conducted as three different experiments with black glumed and yellow glumed njavara.
Experiment one was a pot culture study laid out in Completely Randomized Design (CRD) with two varieties and five treatments. The study included a combination of two levels of shade stress and two levels of water deficit stress and a control with four replications and with three pots in each replication. The observations were taken at different critical stages of the crop viz., vegetative stage, panicle initiation stage, flowering stage and harvesting stage. The results revealed that the morphological characters such as plant height, leaf area index and specific leaf area were higher under 40% shade whereas under 50% field capacity, they were found to be highly reduced. Number of tillers per plant was higher under control (T5) compared to shaded and water deficit conditions. Leaf gas exchange parameters viz., photosynthetic rate, stomatal conductance and transpiration rate were lesser under both experimental conditions compared to the control. However maximum reduction of leaf gas exchange parameters were recorded at 50% field capacity (T3) followed by 40% shaded condition (T2) compared to the control (T5). Among the biochemical characters, maximum chlorophyll content was found under 40% shaded condition (T2). The biochemical parameters such as flavonoid, phenol and proline content of leaves were found higher under 50% field capacity (T3) at all the growth stages studied. The total flavonoid
content in the grains was found higher under 75% field capacity (T4) followed by 20% shade treatment (T1) in both the varieties.
The second experiment was carried out in pot culture in CRD with four treatments, three replications and four pots per replication. The crop was subjected to UV-B (280-320 nm) radiation with the help of UV-B fluorescent tubes during the different critical stages of plants i.e. from vegetative stage, from panicle initiation stage and from flowering stage till harvesting in ventilated polyhouse. The UV-B tubes were switched on for 4 hours daily from 10 am to 2 pm and the average intensity of UV-B radiation at the canopy level of plants was maintained at 4 Wm-2. The control was maintained in another compartment of polyhouse without UV-B tubes. The results indicated that the morphological characters viz., plant height, leaf area index, specific leaf area and tiller number reduced significantly under UV-B radiation treatment and the maximum reduction was observed in treatment T1 (UV-B treatment from vegetative stage). Leaf gas exchange parameters as well as chlorophyll content decreased significantly under UV-B radiation treatments compared to the treatment without UV-B radiation. However flavonoid, phenol and proline contents of leaves were found to increase under UV-B treatments (T1, T2 and T3). The accumulation of total flavonoid in grains was found significantly higher in treatment T2 (UV-B radiation treatment given from panicle initiation stage) in both the varieties.
The treatments which resulted in the highest accumulation of flavonoid content in grains were selected from experiment 1 (20% shade and 75% field capacity) and experiment 2 (UV-B radiation treatment from panicle initiation stage) for molecular analysis in experiment 3. Protein profiling was done in leaves using SDS-PAGE in which, there was variation in the intensity of large subunit (55 kDa) as well as small subunit (16 kDa) of RuBisCO, between the varieties and the treatments. The intensity of those bands were found higher in 20% shade (T1), 75% field capacity (T2) and control (T4) whereas under UV-B radiation treatment from panicle initiation stage (T3)
relatively lesser intensity was exhibited. Gene expression study in grains using qRT-PCR revealed relatively higher expression of chalcone synthase (CHS) and CYP75B4 genes in black glumed njavara (V1) than the yellow glumed njavara (V2) variety. Also the gene expression study revealed that both the genes were over expressed under T3 (UV-B radiation treatment from panicle initiation stage) and T2 (75% field capacity). But both the genes (CHS and CYP75B4) were found down regulated under T1 (20% shade) condition.
The present study revealed that the flavonoid content in grains is higher in black njavara (V1) compared to yellow njavara (V2). The study also indicated that the various parameters studied viz., growth, physiological and biochemical were found to have positive influence on the flavonoid accumulation of grains. But the chlorophyll content was found to have negative influence on the flavonoid content of grains. Proline content was less in black njavara (V1) under UV-B radiation compared to yellow njavara (V2). Based on the present study it is concluded that the black glumed njavara performed better than yellow glumed njavara under all the stress conditions studied and can be exploited better for its therapeutic value. Application of mild stress levels viz., water deficit (75% field capacity) or UV-B radiation treatment from panicle initiation stage may be utilized to enhance the medicinal quality of this crop.