Linking changes in chlorophyll a fluorescence with drought stress susceptibility in mung bean [Vigna radiata (L.) Wilczek].

In the present study, the mung bean cv. NM-13-1Tol was selected as drought-tolerant and NM-54Sens as drought-sensitive. The effects of progressive drought (16 days) on the photosystem II (PSII) activity was assessed using OJIP and JIP-test in the selected two mung bean cultivars differing in drought tolerance. Drought stress reduced the relative water content to 70% (at threshold) and 62% (below the threshold) in cv.NM-13-1Tol and NM-54sens , respectively. The greater reduction in quantum yield of PSII in cv.NM-54sens due to drought stress was due to PSII photodamage. Raw OJIP induction curves and Fo and Fm normalised curves showed that significant changes in fluorescence occurred at the O, J, I and P steps only in cv. NM-54sens . Double normalised differential kinetics indicated adverse effects at the antennae, oxygen-evolving complex and intersystem electron acceptors in cv.NM54sens . Moreover, JIP-test analysis showed that drought stress caused a greater decrease in performance index (PIABS ) in cv.NM-54sens as compared to that in cv. NM-13-1Tol , which is associated with an increase in Vj , rate of accumulation of closed reaction centres (Mo ), energy fluxes for absorption (ABS/RC), trapping (TRo /RC), electron transport (ETo /RC), and dissipation of absorbed energy as heat (DIo /RC). In conclusion, two-week drought stress reduced the RWC below the threshold in cv.NM54sens , which resulted in damages at the donor and acceptor sides of PSII. However, cv.NM-13-1Tol somehow maintained the RWC around the threshold and thus protected PSII. Of various JIP-test parameters, PIABS , Fv /Fm , Vj and Mo are key indicators of drought stress tolerance in mung bean cultivars.

Peroxidase activity and operation of photo-protective component of NPQ play key roles in drought tolerance of mung bean [Vigna radiata (L.) Wilcziek].

Developing drought-tolerant cultivars is mainly restricted due to poor knowledge of the mechanism behind drought tolerance. In the present work, available germplasm of Vigna radiata (mung bean) was screened for drought tolerance using multiple agronomic and physiological parameters and used to selected one drought-tolerant (NM-13-1) and one drought-sensitive (NM-54) cultivar for further studies. Plant water status and PSII activity were found to be potential physiological discriminating traits. Changes in PSII and PSI activity, accumulation of proline, oxidative damage, and antioxidants were further assessed in selected drought-sensitive and drought-tolerant cultivars. Drought stress reduced PSII efficiency and electron transport in both mung bean cultivars. Drought increased NPQ and Y(NPQ), a greater increase in NPQ and Y(NPQ) was found in the drought-tolerant cv NM-13-1, indicating that the drought-tolerant cultivar managed over-excitation of PSII by safe heat dissipation via photo-protective component of NPQ. A decrease in PSI efficiency with an increase in donor end limitation of PSI in both mung bean cultivars further confirmed that the electron transport through PSII became down-regulated. However, the drought-sensitive cv. NM-54 had poor ability to manage over-excitation of PSII through buildup of Y(NPQ) thereby causing greater oxidative stress. Mung bean cultivars counteracted oxidative stress by accumulation of proline and increasing POD activities. Drought-tolerant cv. NM-13-1 had higher proline accumulation and antioxidant potential than in the drought-sensitive cultivar. Overall, drought tolerance in the mung bean cultivars can be related to plant water status, PSII activity, Y(NPQ), and POD activity, which can be effectively used for selecting mung bean cultivars for drought tolerance.

Exogenous melatonin regulates chromium stress-induced feedback inhibition of photosynthesis and antioxidative protection in Brassica napus cultivars.

KEY MESSAGE: Melatonin is an early player in chromium stress response in canola plants; it promotes ROS scavenging and chlorophyll stability, modulates PSII stability and regulates feedback inhibition of photosynthesis conferring chromium tolerance. The development of heavy metals, especially chromium (Cr)-tolerant cultivars is mainly constrained due to poor knowledge of the mechanism behind Cr stress tolerance. In the present study, two Brassica napus contrasting cultivars Ac-Excel and DGL were studied for Cr stress tolerance by using chlorophyll a fluorescence technique and biochemical attributes with and without melatonin (MT) treatments. Cr stress significantly reduced the PSII and PSI efficiency, biomass accumulation, proline content and antioxidant enzymes in both the cultivars. The application of MT minimized the oxidative stress, as revealed via a lower level of reactive oxygen species (ROS) synthesis (H2O2 and OH-). Enhanced enzymatic activities of important antioxidants (SOD, APX, CAT, POD), proline and total soluble protein contents under MT application play an effective role in the regulation of multiple transcriptional pathways involved in oxidative stress responses. Higher NPQ and Y(NPQ) observed in Cr stress tolerant cv Ac-Excel, indicating that the MT-treated tolerant cultivar had better ability to protect PSII under Cr stress by increasing heat dissipation as photo-protective component of NPQ. Reduced PSI efficiency along with increased donor end limitation of PSI in both canola cultivars further confirmed the lower PSII activity and electron transport from PSII. The Cr content was higher in cv. DGL as compared to (that in Ac-Excel). The application of MT significantly decreased the Cr content in leaves of both cultivars. Overall, MT-induced Cr stress tolerance in canola cultivars can be related to improved PSII activity, Y(NPQ), and antioxidant potential and these physiological attributes can effectively be used to select cultivars for Cr stress tolerance.

Photosynthetic activity and metabolic profiling of bread wheat cultivars contrasting in drought tolerance.

The rapid increase in population growth under changing climatic conditions causes drought stress, threatening world food security. The identification of physiological and biochemical traits acting as yield-limiting factors in diverse germplasm is pre-requisite for genetic improvement under water-deficit conditions. The major aim of the present study was the identification of drought-tolerant wheat cultivars with a novel source of drought tolerance from local wheat germplasm. The study was conducted to screen 40 local wheat cultivars against drought stress at different growth stages. Barani-83, Blue Silver, Pak-81, and Pasban-90 containing shoot and root fresh weight >60% of control and shoot and root dry weight >80% and 70% of control, respectively, P (% of control >80 in shoot and >88 in root), K+ (>85% of control), and quantum yield of PSII > 90% of control under polyethylene glycol (PEG)-induced drought stress at seedling stage can be considered as tolerant, while more reduction in these parameters make FSD-08, Lasani-08, Punjab-96, and Sahar-06 as drought-sensitive cultivars. FSD-08 and Lasani-08 could not maintain growth and yield due to protoplasmic dehydration, decreased turgidity, cell enlargement, and cell division due to drought treatment at adult growth stage. Stability of leaf chlorophyll content (<20% decrease) reflects photosynthetic efficiency of tolerant cultivars, while ~30 µmol/g fwt concentration of proline, 100%-200% increase in free amino acids, and ~50% increase in accumulation of soluble sugars were associated with maintaining leaf water status by osmotic adjustment. Raw OJIP chlorophyll fluorescence curves revealed a decrease in fluorescence at O, J, I, and P steps in sensitive genotypes FSD-08 and Lasani-08, showing greater damage to photosynthetic machinery and greater decrease in JIP test parameters, performance index (PIABS), maximum quantum yield (Fv/Fm) associated with increase in Vj, absorption (ABS/RC), and dissipation per reaction center (DIo/RC) while a decrease in electron transport per reaction center (ETo/RC). During the present study, differential modifications in morpho-physiological, biochemical, and photosynthetic attributes that alleviate the damaging effects of drought stress in locally grown wheat cultivars were analyzed. Selected tolerant cultivars could be explored in various breeding programs to produce new wheat genotypes with adaptive traits to withstand water stress.

Mixing tannery effluent had fertilizing effect on growth, nutrient accumulation, and photosynthetic capacity of some cucurbitaceous vegetables: A little help from foe.

Tannery effluent contains a number of organic and inorganic elements as pollutants which reduce plant growth. To overcome shortage of water, use of diluted industrial wastewater such as tannery effluent can be a viable strategy for improving crop growth and yield. A pot experiment was conducted to determine the effects of tannery effluent and its various dilutions on physiological and biochemical characteristics of five cucurbitaceous vegetables. Tannery effluent was applied 0, 25, 50, 75 and 100% to 3-week-old plants of five cucurbitaceous vegetables (Cucurbita maxima, Luffa cylindrica, Citrullus vulgaris, Cucumis melo, and Praecitrullus fistulosus) for 4 weeks. Tannery effluent reduced the growth of all five cucrbitaceous vegetables. Diluted tannery effluent (25%) improved the growth of Cucurbita maxima, Citrullus vulgaris, and Cucumis melo. Moderately diluted (50%) did not affect the growth of Citrullus vulgaris and Cucumis melo. Toxic effects of tannery effluent were associated with high accumulation of heavy metals Cr, Cd, Mn, and Fe in leaves and roots. High accumulation of heavy metals in leaves reduced the accumulation of nutrients in leaves (N, P, K) and reduced photosynthetic pigments and photosynthetic rate. Changes in photosynthetic rates of all vegetable species due to tannery effluent were not associated with stomatal limitations (stomatal conductance, transpiration rate, internal CO2). Toxic effects of tannery effluent on plants also include changes in N-metabolism (amino acid and protein). However, extent of these adverse effects of tannery effluent on vegetables was species specific. It is suggested that Cucurbita maxima can be grown by supplying 25% tannery effluent, whereas Citrullus vulgaris and Cucumis melo can be grown with moderately diluted (50%) tannery effluent.

Role of mineral nutrients, antioxidants, osmotic adjustment and PSII stability in salt tolerance of contrasting wheat genotypes.

Global food production is threatened due to increasing salinity and can be stabilized by improving salt tolerance of crops. In the current study, salt tolerance potential of 40 local wheat cultivars against 150 mM NaCl stress was explored. Salt treatment at seedling stage caused less reduction in biomass, K+ and P while more decline of Na+ in tolerant cultivars due to reduced translocation and enhanced exclusion of Na+ from leaves. Principal component analysis based selected S-24, LU-26S, Pasban-90 (salt tolerant) and MH-97, Kohistan-97, Inqilab-91 and Iqbal-2000 (salt sensitive) cultivars were evaluated at adult stage applying 150 mM salinity. Osmotic adjustment by accumulation of soluble sugars and proline and accelerated antioxidant enzymes activities caused efficient scavenging of reactive oxygen species making S-24 and LU-26S tolerant while in MH-97 and Kohistan-97, high MDA represent greater membrane damage due to oxidative stress making them salt sensitive. Chlorophyll a fluorescence transients confirmed better efficiency of photosystem II in S-24 and LU-26S based on energy fluxes (ABS/RC, TRo/RC, ETo/RC and DIo/RC), performance index (PIABS) and maximum quantum yield (Fv/Fm). These findings can be correlated using molecular techniques to identify genes for salt exclusion, osmotic adjustment and photosynthetic activity for use in molecular breeding programs.

Coordinated impact of ion exclusion, antioxidants and photosynthetic potential on salt tolerance of ridge gourd [Luffa acutangula (L.) Roxb.].

The contribution of one major or a combination of several physiological processes in salt tolerance was assessed in three local varieties (Blacklong, Advanta-1103, and Dilpasand) of ridge gourd [Luffa acutangula (L.) Roxb.] at varying salt levels (0, 75, and 150 mM NaCl). Based on growth attributes, var. Dilpasand as salt-tolerant and var. Blacklong as moderately salt-tolerant, while var. Advanta-1103 as salt-sensitive. Inter-varietal differences for photosynthetic pigments and relative water content (RWC) was not observed. The salt-sensitive variety Advanta 1103 had greater Na+ accumulation (73.72%) in the leaves than those in the moderately tolerant and tolerant varieties. Total soluble proteins were relatively lower (58.25%) in the salt-sensitive variety but maximal increase (69.34%) in total free amino acids was observed. However, accumulation of proline was maximal in the salt-tolerant variety (Dilpasand). Salt-tolerant variety exhibited minimal oxidative stress (relative low levels of H2O2) and membrane damage (low content of MDA and electrolytic leakage) and higher activities of antioxidant enzymes (catalase and peroxidase). Although all ridge gourd varieties down-regulated the electron transport through PSII by increasing the safe dissipation of heat Y(NPQ) to lower the ROS generation, this was maximal in the salt-tolerant variety Dilpasand. Relatively greater reduction in Y(ND) and enhancement in Y(NA) indicated PSI-photoinhibition in salt-sensitive variety. The greater salt tolerance in var. Dilpasand was due to the coordinated impact of ion exclusion, higher accumulation of proline, better capacity to manage electron transport from PSII to PSI with higher Y(NPQ) and antioxidant capacity.

Identification of novel source of salt tolerance in local bread wheat germplasm using morpho-physiological and biochemical attributes.

Salt tolerant wheat cultivars may be used as genetic resource for wheat breeding to ensure yield stability in future. The study was aimed to select salt tolerant cultivar(s) to identify novel source of salt tolerance in local wheat germplasm. Initially, 40 local wheat cultivars were screened at 150 mM NaCl stress at seedling stage. Selected salt-tolerant (three; S-24, LU-26S and Pasban-90) and salt-sensitive (four; MH-97, Kohistan-97, Inqilab-91 and Iqbal-2000) wheat cultivars were further evaluated using growth, yield, biochemical and physiological attributes. Growth and yield of selected cultivars were reduced under salt stress due to decline in plant water status, limited uptake of macronutrients (N, P and K), reduced K+/Na+ ratio, photosynthetic pigments and quantum yield of PSII. Wheat plants tried to acclimate salt stress by osmotic adjustment (accumulation of total soluble sugars, proline and free amino acids). Degree of salinity tolerance in cvs. S-24 and LU-26S found to be associated with maintenance of K+/Na+ ratio, osmo-protectant and photosynthetic activity and can be used as donor for salt tolerance in wheat breeding program at least in Pakistan. These cultivars can be further characterized using molecular techniques to identify QTLs/genes for salt exclusion, osmo-protectant and photosynthetic activity for molecular breeding.

Melatonin induced changes in photosynthetic efficiency as probed by OJIP associated with improved chromium stress tolerance in canola (Brassica napus L.).

Chromium toxicity is considered as a major problem for agricultural soil that reduced crop productivity by affecting photosynthetic tissues. Exogenous application of melatonin can alleviate the adverse effects of chromium toxicity on plant growth. However, little is known about its effect on thylakoidal protein complexes responsible for conversion of solar energy to biochemical energy. Chlorophyll fluorescence a transients considered one of the best non-invasive and rapid method for the evaluation of photosynthetic (Photosystem II) efficiency of plants and plant health under environmental stress conditions. In the present study, three-week old plants of two canola cultivars AC-Excel and DGL were applied to melatonin (0, 1, 5, 10 µM) when grown under chromium stress (0, 50 and 100 µM) for further two weeks. Chromium stress reduced the growth (fresh and dry weights of shoots and roots) of both canola cultivars and exogenous application of 5 and 10 µM melatonin improved the growth of canola at 50 or 100 µM chromium stress. This improvement was greater in cv DGL than in AC-Excel. Increasing chromium decreased the photosynthetic pigments (chlorophyll a and chlorophyll b). However, 5 and 10 µM melatonin application improved chlorophyll a at 50 µM chromium stress. Structural stability and efficiency of photosystem II (PSII) measured as performance index (PIABS) and ratios of fluorescence (Fv/Fm, Fv/Fo) Fv decreased due to chromium stress. JIP-test parameters showed that chromium stress increased the absorption and trapping fluxes with decrease in electron transport fluxes which caused the damage to reaction centers (RC), detachment of oxygen evolving complex (OEC) from RC or inefficiency of electron transfer from OEC to RC. Such adverse effects were greater in cv AC-Excel. However exogenous application of melatonin improved PIABS, electron transport per reaction center (ET/RC), reduced variable fluorescence at J step (VJ) reflecting melatonin protected PSII from chromium stress induced damage by protecting OEC. Thus, OJIP fluorescence transients are quite helpful for understanding the intersystem electron transport beyond photosystem II in canola cultivars due to melatonin application under chromium stress. FINDINGS: Exogenous application of melatonin alleviated toxic effects of chromium on plant growth of canola by modulating photosynthesis, enhanced photosystem II efficiency and regulation of electron transport flux to protect photo-inhibition of PSII from oxidative damage.