Melatonin Mitigates Drought Induced Oxidative Stress in Potato Plants through Modulation of Osmolytes, Sugar Metabolism, ABA Homeostasis and Antioxidant Enzymes

Abstract

The effect of melatonin (MT) on potato plants under drought stress is still unclear in the available literature. Here, we studied the effect of MT as a foliar application at 0, 0.05, 0.1, and 0.2 mM on potato plants grown under well-watered and drought stressed conditions during the most critical period of early tuberization stage. The results indicated that under drought stress conditions, exogenous MT significantly (p ≤ 0.05) improved shoot fresh weight, shoot dry weight, chlorophyll (Chl; a, b and a + b), leaf relative water content (RWC), free amino acids (FAA), non-reducing sugars, total soluble sugars, cell membrane stability index, superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase (G-POX), and ascorbate peroxidase (APX) compared to the untreated plants. Meanwhile, carotenoids, proline, methylglyoxal (MG), H₂O₂, lipid peroxidation (malondialdehyde; MDA) and abscisic acid (ABA) were significantly decreased compared to the untreated plants. These responses may reveal the protective role of MT against drought induced carbonyl/oxidative stress and enhancing the antioxidative defense systems. Furthermore, tuber yield was differentially responded to MT treatments under well-watered and drought stressed conditions. Since, applied-MT led to an obvious decrease in tuber yield under well-watered conditions. In contrast, under drought conditions, tuber yield was substantially increased by MT-treatments up to 0.1 mM. These results may imply that under water deficiency, MT can regulate the tuberization process in potato plants by hindering ABA transport from the root to shoot system, on the one hand, and by increasing the non-reducing sugars on the other hand.

Keywords: alpha-ketoaldehyde methylglyoxal; carbonyl stress; solanum tuberosum L.; tuberization and water stress.

Figure 1

Effect of exogenous melatonin (MT; 0, 0.05, 0.1 and 0.2 mM) on shoot fresh weight (A), shoot dry weight (B), Chl a (C), Chl b (D), Chl a + b (E) and carotenoids (F) of potato plants grown under drought stress during the early stage of tuberization. Bars represent standard deviation (SD) of the means (n = 3). Different letters indicate significant differences among the treatments at p ≤ 0.05, according to Duncan’s multiple range test. Chl, chlorophyll.

Figure 2

Effect of exogenous melatonin (MT; 0, 0.05, 0.1 and 0.2 mM) on leaf relative water content; RWC (A), free amino acids; FAA (B), proline (C), reducing sugars (D), non-reducing sugars (E) and total soluble sugars (F) of potato plants grown under drought stress during the early stage of tuberization. Bars represent standard deviation (SD) of the means (n = 3). Different letters indicate significant differences among the treatments at p ≤ 0.05, according to Duncan’s multiple range test.

Figure 3

Effect of exogenous melatonin (MT; 0, 0.05, 0.1 and 0.2 mM) on cell membrane stability index; CMSI (A), methylglycoxal; MG (B), H₂O₂; (C), malondialdehyde; MDA (D) of potato plants grown under drought stress during the early stage of tuberization. Bars represent standard deviation (SD) of the means (n = 3). Different letters indicate significant differences among the treatments at p ≤ 0.05, according to Duncan’s multiple range test.

Figure 4

Effect of exogenous melatonin (MT; 0, 0.05, 0.1 and 0.2 mM) on superoxide dismutase; SOD (A), catalase; CAT (B), guaiacol peroxidase; G-POX (C), ascorbate peroxidase; APX (D) of potato plants grown under drought stress during the early stage of tuberization. Bars represent standard deviation (SD) of the means (n = 3). Different letters indicate significant differences among the treatments at p ≤ 0.05, according to Duncan’s multiple range test.

Figure 5

Effect of exogenous melatonin (MT; 0, 0.05, 0.1 and 0.2 mM) on abscisic acid; ABA (A), the relationship between ABA concentration and leaf relative water content; RWC (B) of potato plants grown under drought stress during the early stage of tuberization. Bars represent standard deviation (SD) of the means (n = 3). Different letters indicate significant differences among the treatments at p ≤ 0.05, according to Duncan’s multiple range test. ** (p ≤ 0.01).

Figure 6

Effect of exogenous melatonin (MT; 0, 0.05, 0.1 and 0.2 mM) on tuber yield of potato plants grown under drought stress during the early stage of tuberization. Bars represent standard deviation (SD) of the means (n = 3). Different letters indicate significant differences among the treatments at p ≤ 0.05, according to Duncan’s multiple range test.

Figure 7

The timeline infographic for the treatments of melatonin as a foliar application and sampling of potato plants subjected to the well watered conditions and progressive drought stress during the tuberization stage.

Figure 8

Simplified model for the suggested effect of exogenous melatonin as foliar spray on potato plants grown under drought stress during the early stage of tuberization. In drying soil, ABA that is synthesized in roots can be carried by xylem stream to the shoot system leading to stomata closing and reducing the rate of transpiration. Moreover, soluble sugars tends to accumulate in shoots as reducing sugars (not suitable form to sugar transport) leading to decrease the rate of tuberization. Conversely, melatonin treatments led to decrease ABA in shoots as resulting to enhancement of leaf water status and antioxidant capacity. This effect may also be explained by restriction of ABA transport from root to the shoot system. Furthermore, MT-treated plants tended to accumulate the non-reducing sugars (the most suitable form to sugar transport and starch synthesis) leading to improve the tuberization. ETC, electron transport chain in chloroplast and mitochondria; ABA, abscisic acid; MG, methylglyoxal; ROS, reactive oxygen species.