Abstract
5-aminolevulinic acid (ALA) increases plant tolerance to low-temperature stress, but the physiological and biochemical mechanisms that underlie its effects are not fully understood. To investigate them, cucumber seedlings were treated with different ALA concentrations (0, 15, 30 and 45 mg/L ALA) and subjected to low temperatures (12/8 °C day/night temperature). The another group (RT; regular temperature) was exposed to normal temperature (28/18 °C day/night temperature). Low-temperature stress decreased plant height, root length, leaf area, dry mass accumulation and the strong seedling index (SSI), chlorophyll contents, photosynthesis, leaf and root nutrient contents, antioxidant enzymatic activities, and hormone accumulation. Exogenous ALA application significantly alleviated the inhibition of seedling growth and increased plant height, root length, hypocotyl diameter, leaf area, and dry mass accumulation under low-temperature stress. Moreover, ALA increased chlorophyll content (Chl a, Chl b, Chl a+b, and Carotenoids) and photosynthetic capacity, net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 concentration (Ci), and transpiration rate (Tr), as well as the activities of superoxide dismutase (SOD), peroxidase (POD, catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR) enzymes, while decreasing hydrogen peroxide (H2O2), superoxide (O2•−), and malondialdehyde (MDA) contents under low-temperature stress. In addition, nutrient contents (N, P, K, Mg, Ca, Cu, Fe, Mn, and Zn) and endogenous hormones (JA, IAA, BR, iPA, and ZR) were enhanced in roots and leaves, and GA4 and ABA were decreased. Our results suggest the up-regulation of antioxidant enzyme activities, nutrient contents, and hormone accumulation with the application of ALA increases tolerance to low-temperature stress, leading to improved cucumber seedling performance.
Highlights
Plants are challenged by a variety of biotic and abiotic stresses throughout their life cycle [1]
The results of the present study indicate that low-temperature stress had a negative effect on cucumber seedling growth and the strong seedling index (SSI), but these were significantly enhanced by exogenous aminolevulinic acid (ALA) application (Table 1)
Root length, hypocotyl diameter, leaf area, plant dry weight, and SSI of cucumber seedlings were significantly reduced by 41.73%, 42.11%, 26.55%, 44.78%, 48.65%, and 36.84%, respectively, in low-temperature stress (CK), and by 14.93%, 19.14%, 5.19%, 18.93%, 24.32%, and 10.53%, respectively, in T2 (30 mg/L ALA), when compared to RT
Summary
Plants are challenged by a variety of biotic and abiotic stresses throughout their life cycle [1]. Low-temperature stress limits agriculture production severely in large parts of the world, especially northern parts of China [2,3,4]. Numerous studies have shown that low temperatures negatively affect plant nutritional uptake and accumulation, chlorophyll content, photosynthetic capacity, oxidative stress, metabolic processes, defense system, and hormonal imbalance [5], in addition to having adverse effects on almost all developmental stages from seed germination to maturation [6]. Plants exposed to low temperatures can increase the overproduction of 1O2, O2−, H2O2, and OH, known as reactive oxygen species (ROS), which damage chloroplasts and mitochondria and can lead to cell death [7]. Phytohormones (abscisic acid (ABA), indole-3-acetic acid (IAA), gibberellin (GA), brassinosteroid (BR), jasmonic acid (JA), and indole-3-propionic acid (iPA)) play key roles in increasing antioxidant enzyme activities under various kinds of abiotic stress to reduce their harmful effects [5,9,10,11]
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