Abstract
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.
Highlights
Rapid urbanization and anthropogenic activities have been caused environmental deterioration and become major constraint to crop production due to higher accumulation of heavy metal in soil (Yang et al 2019)
MT induced tolerance in canola cultivars can be related to PSII activity, Y(NPQ), and antioxidants potential which can be effectively used for canola cultivars selection under Cr stress
Photosynthetic pigments dramatically (P< 0.001) declined in both B. napus cultivars in chromium stress (Table 1)
Summary
Rapid urbanization and anthropogenic activities have been caused environmental deterioration and become major constraint to crop production due to higher accumulation of heavy metal in soil (Yang et al 2019). Hyper-accumulation of Cr in plant tissues causes severe phytotoxicity by reducing growth and biomass accumulation, chlorophyll degradation as well as the increased production of reactive oxygen species (ROS) greatly inhibit the enzymatic activities especially in the tissues of leaves by the increment of malondialdehyde (MDA), hydrogen peroxide (H2O2) disorganized antioxidant defense system and photosynthetic efficiency (Ahmad et al 2017). This increment in the production of ROS may damage the biological macromolecules and disrupt the antioxidant system in plant. The study's main goal is to understand (1) Chromium stress intervention in B. napus cultivar growth; (2) Melatonin-induced differential antioxidative protection and against oxidative stress; and (3) Melatonin-associated PS(II) and PS(I) efficiency regulation and evaluation using chlorophyll fluorescence measurements, as well as physiological characteristics that may indicate plant chromium tolerance
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