Department of Biological Sciences
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Item Inhibition of mitochondrial electron transport is the prime cause behind proline accumulation during mineral deficiency in Oryza sativa(Springer, 1993) Bhagavatula, VaniThe concentration of proline in shoots of rice (Oryza sativa) seedlings raised in distilled water was about 3.3 times higher than in the seedlings raised in modified B5 medium. The shoots of seedlings raised in B5 medium which was depleted of calcium, iron, magnesium or potassium had a higher concentration of proline than those grown in standard B5 medium. The shoots of seedlings raised in distilled water with iron had a lower level of proline than those in distilled water. These results suggest that iron deficiency leads to high proline accumulation.Item Effect of short-term heat treatment of rice seedlings on sensitivity of thylakoid membranes to photoinhibition(Springer, 1996) Bhagavatula, VaniThe after-effects of 24 h high temperature (35 or 45 ~ treatment on the photochemical activities and photooxidative lipid peroxidation, subsequent to their irradiation were studied in 7-d-old etiolated rice (Oryza sativa) seedlings. Photosystem (PS) 1 and PS 2 mediated photoreactions of thylakoids isolated from the seedlings exposed to high temperature did not differ significantly from the thylakoids isolated from control seedlings (25 ~ Hence, all kinds of tested thylakoids were equally efficient in capturing and utilizing radiant energy. The high irradiance induced loss in PS 2 activity and lipid peroxidation measured in terms of malondialdehyde production was more rapid in thylakoids isolated from stressed seedlings as compared to that of control seedlings. Thus the thylakoids isolated from the stressed seedlings were more prone to photodamage than those from the control seedlings.Item Alteration in NAD+/NADH ratio regulates salt stress induced proline accumulation in Vigna radiata.(NAS, 1996) Bhagavatula, VaniFive-day-old vigna radiata seedlings exposed to NaCI stress induced a significant enhancement in proline levels accompanied by reduction in growthItem Structure-function correlation during the etioplast-chloroplast transition in cucumber cotyledonary leaves(IJPR, 1998) Bhagavatula, VaniWe studied the development of chloroplasts from etioplasts in the cotyledonary leaves of 4-d-old dark-grown cucumber (Cucumis sativus) seedlings after irradiation (20 μmol m-2 s-1). Upon irradiation, the triggering of chlorophyll (Chl) synthesis and accumulation showed a relatively short lag phase. The irradiation of etiolated seedlings initiated the synthesis of apoproteins of pigment-protein complexes. While Chl-protein 2 (CP2) was detected at 6 h after irradiation, CP1 only after 29 h. The appearance and accumulation of some of the apoproteins were monitored by Western-blotting. LHC2 apoprotein was detected after a 6 h-irradiation. The amounts of D1 protein of photosystem (PS) 2 and PsaA/B protein of PS1 were quantitated by ELISA. Further, the thylakoid membrane function during this time period in terms of PS1- and PS2-mediated electron transfer activity and intersystem electron pool size were analyzed. While PS1 activity was detected after 4 h, PS2-mediated O2 evolution was detected only after a 17 h-irradiation. Fv/Fm value of Chl a fluorescence measurements indicated that the photochemical efficiency of these leaves reached its maximum after 29 h of irradiation. The intersystem pool size of cotyledonary leaves was equivalent to that of the control cotyledonary leaves grown for 25 h under continuous irradiation. Thus etioplasts develop into fully functional chloroplasts after approximately 25 h when 4 d-dark grown cucumber seedlings are continuously moderately irradiated. The development of photosynthetic electron transport chain seems to be limited in time at the level of PS2, possibly at the donor side.Item Heat stress on rice leaves probed by the fluorescence transient OKJIP.(Springer, 1999) Bhagavatula, VaniItem Alteration in chloroplast structure and thylakoid membrane composition due to in vivo heat treatment of rice seedlings: correlation with the functional changes(Elsiever, 2001) Bhagavatula, VaniExposure of 25 °C-grown, seven-day-old rice seedlings to mild heat stress of 40 °C for 24 h in dark did not cause any change in protein or pigment content of the thylakoids, but produced major disorganization of chloroplast ultrastructure. This heat induced disorganization of thylakoid structure/organization caused significant (∼65 percnt;) loss in PSII activity, slight loss in PSI activity, and brought about a decrease in relative quantum efficiency of PSII. The herbicide 14C atrazine binding assay revealed a decreased number of binding sites of the herbicide and altered the herbicide dissociation constant, suggesting that the heat induced disorganization of the thylakoids affects the acceptor side of PSII. Cation induced Chla fluorescence analyses at room temperature and low temperature indicated thatin vivo heat exposure of rice seedlings altered the extent of energy transfer in favor of PSIItem Characterization of high temperature induced stress impairments in thylakoids of rice seedlings(NISCAIR, 2001-08) Bhagavatula, VaniExposure o f isolated thylakoids or intact plants to elevated temperature is known to inhibit photosynthesis at multiple sites. We have investigated the effect of elevated temperature (40°C) for 24 hr in dark on rice seedlings to characterize the extent of damage by inl vivo heat stress on photofunctions of photosystem II (PSII). Chi a fluorescence transient analysis in the intactrice leaves indicated a loss in PSII photochemistry (Fv) and an associated loss in the number of functional PSII units. Thylakoids isolated from rice seedlings exposed to mild heat stress exhibited > 50% reduction in PSII catalyzed oxygen evolution activity compared to the corresponding control thylakoids. The ability of thy lakoid membranes from heat exposed seed lings to photooxidize artificial PSII electron donor, DPC, subsequent to washing the thylakoids with alkaline Tris or NH2OH was also reduced by ~40% compared to control Tris or NH2OH washed thylakoids. This clearly indicated that besides the disruption of oxygen evolving complex (OEC) by 40°C heat exposure for 24 hr, the PSII reaction centers were impaired by inl vivo heat stress. The analysis of Mn and manganese stabilizing protein (MSP) contents showed no breakdown of 33 kDa extrinsic MSP and only a marginal loss in Mn. Thus, we suggest that the extent of heat induced loss of OEC must be due to disorganization of the OEC complex by in vivo heat stress. Studies with inhibitors like DCMU and atrazine clearly indicated that in vivo heat stress altered the acceptor side significantly. [14C] Atrazine binding studies clearly demonstrated that there is a significant alteration in the QB binding site on D1 as well as altered QA to QB equilibrium. Thus, our results show that the loss in PSII photochemistry by in vivo heat exposure not only alters the donor side but significantly alters the acceptor side of PSII .