Mitigating High-Temperature Stress in Peppers: The Role of Exogenous NO in Antioxidant Enzyme Activities and Nitrogen Metabolism

1. Introduction

Peppers (Capsicum annuum L.) are the second most widely cultivated vegetable crop in China and are rich in vitamins A, C, and B6, folic acid, dietary fiber, and antioxidants. With global climate change, temperature has become a crucial environmental factor affecting plant physiology, metabolism, growth, development, and productivity [1]. Increasing evidence indicates that high-temperature (HT) stress is a significant abiotic factor that limits the growth and yield of pepper seedlings and often leads to substantial agricultural losses [2]. The physiological and biochemical responses of plants to HT include alterations in carbohydrate metabolism, disruption of photosynthesis, changes in signal transduction, and increased oxidative damage [3]. Additionally, HT stress causes fluctuations in compatible solutes, such as sugars, amino acids, and proline (Pro), which play critical roles in maintaining cellular homeostasis [4]. Studies have consistently shown that HT stress can also lead to fluctuations in sugar levels and the accumulation of certain amino acids, such as proline, which act as osmoprotectants to mitigate stress damage [5]. Plants have developed complex antioxidant defense systems comprising both enzymatic and non-enzymatic components in response to oxidative stress. Antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), play crucial roles in scavenging reactive oxygen species (ROS) and mitigating their harmful effects [6]. Additionally, non-enzymatic antioxidants, including ascorbic acid and glutathione, contribute to the redox balance and protect plants from oxidative damage [7].

Under abiotic stress, significant changes occur in various amino acid metabolites within plants, reflecting their response and adaptation mechanisms [8]. Drought, HT, and salt stress lead to increased levels of kynurenine, N-acetyl-L-tyrosine, L-methionine, urea, and creatinine. These changes are typically associated with antioxidant responses and energy metabolism, aiding in the adaptation to adverse environments [9]. Specifically, under HT stress, the level of kynurenine increases significantly, which is linked to the activation of the tryptophan metabolism pathway that may help regulate ROS production and antioxidant responses in Arabidopsis [10]. Additionally, the level of N-acetyl-L-tyrosine reflects protein degradation and changes in amino acid metabolism, which help maintain cellular homeostasis in rice under drought stress [11]. Low-temperature stress leads to an increase in L-methionine levels, as L-methionine, a precursor of antioxidants, participates in glutathione synthesis, enhances antioxidant capacity, and reduces the oxidative damage caused by HT...