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

Abstract

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 (PI ABS ) 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 PI ABS , electron transport per reaction center (ET/RC), reduced variable fluorescence at J step (V J ) 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.

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

“…Previous studies suggest the destructive effect of Cr on reaction center and ETC which consequence reduced net PSII quantum yield (Oukarroum et al 2015). While, according to (Ayyaz et al 2020) foliar aapplication of MT under Cr stress prevents photosynthetic pigment degradation along with enhanced overall photosynthetic process in B. napus. The formation of ROS can also play a potential role in protein synthesis and gene expression regulation accompanied with plant stress defense.…”

“…Previous studies suggested that exogenously applied MT under Cr stress considerably enhanced plant growth, photosynthetic e ciency, antioxidants activity along with reduced the chromium toxicity damage to crop plants (Altaf et al 2020). In another study foliar application of MT in Brassica against chromium stress showed increased performance index (PI), indicating higher energy conservation of excited electron to reduce PSI (Ayyaz et al 2020). Brassica napus is the main cash crop that is widely cultivated in both the north and south hemispheres during annuals or biennials seasons and has an annual value of USD 41 billion.…”

Regulation of Osmotic Balance and Increased Antioxidant Activities under Heat Stress in Abelmoschus esculentus L. Triggered by Exogenous Proline Application

“…A high root and shoot weight (fresh and dry) is positively related to the number of leaves that contain the photosynthetic apparatus and that convert the solar energy into biomass [37]. Similarly, deep and long roots are helpful in the uptake of water and nutrients from the soil and of light from the sun, respectively [41]. In heat-tolerant genotypes, all of these characteristics helped in achieving high values in 131 morphological parameters that outperformed the heat-sensitive okra genotypes.…”

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

“…In the present work, remarkable variation for Cr stress tolerance was observed in canola cultivars. However, based on our previous study (Ayyaz et al 2020) two canola cultivars Ac-Excel and DGL were further evaluated. It is presumed that such variability in Cr stress tolerance in canola cultivars might have been because of variation in physio-biochemical attributes (Farooq et al 2013).…”

“…Plants were sprayed twice a day with foliar application of Melatonin MT (10 µM) on lower and upper surface of leaves until runoff before one week of Cr treatment. Based on our previous study (Ayyaz et al 2020), 28 days old canola plants were applied with chromium (K2Cr2O7) salt solution (50 µM) with Hoagland's nutrients for 14 days. Two cultivars, Three Cr levels and four melatonin levels having ve replicates for each treatment were used in the CRBD (completely randomized block design) experiment.…”