Low-intensity light stress on broccoli seedlings: associated mechanisms of acclamatory stress tolerance through changes in pigments, membrane lipids, and amelioration of antioxidative defense systems

Low-intensity light (LL; 100 µmol m-2 s-1PPFD) effects on growth and biochemical parameters indicative of acclimation were investigated on 21-day-old broccoli seedlings. LL stress reduces the biomass and thickness of the leaf. However, the leaf area was considerably increased. Nevertheless, LL stress noticeably increased the quantity of chloroplast. However, the length and width of stomata were reduced markedly. LL stress led to an increase in the chlorophyll level and reduced the content of anthocyanin and flavonoids. The composition of carotenoids and chloroplast membrane lipids of leaves was significantly altered by LL stress. LL stress-induced responses were closely associated with the generation of superoxide anion and hydrogen peroxide. Concomitantly, lipoxygenase activity was significantly elevated in the leaf tissue, leading to the accumulation of malondialdehyde, indicating oxidation of membrane lipids. The activities of catalase and superoxide dismutase were prominently reduced by ca. 2.11- and 1.6-fold, respectively, by LL stress. However, there was a remarkable induction in the activities of peroxidase and glutathione reductase by ca. 31.5 and 40.8%, respectively, which was influenced by LL stress. Although, no manifest effect in the activities of monodehydroascorbate reductase and dehydroascorbate reductase was noticed in LL stress and control grown leaves. Furthermore, LL stress distinctly decreases the level of ascorbate and dehydroascorbate, whereas the level of the glutathione pool (glutathione and oxidized glutathione) is markedly increased. Our results concluded that the elevated level of reactive oxygen species (ROS)-induced oxidative stress was responsible for the inhibition of growth under LL stress. The stimulated antioxidants did not effectively reverse the LL stress-induced ROS generationin leaves. The changes that occurred in the membrane lipids are possibly the consequence of modifications in the physiology of plants to endure function of the thylakoid membrane to keep physiological functions dynamic to tolerate LL stress environments. The results hypothesize that broccoli growing under shady conditions might not provide a better adaptation for plants.

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