**4. Temperature stress**

### **4.1. High temperature**

In general, a transient elevation in temperature (usually 10–15o C above environment) causes heat shock or heat stress [89]. High-temperature effects can be seen at the biochemical and molecular level in plant organs (especially leaves). Heat stress induces decrease in duration of developmental phases, leading to fewer organs, smaller organs, reduce light perception over the shortened life cycle, and finally play an important role in losing the product [90, 91, 92].

High-temperature stress often induces the overproduction of ROS [93] which can cause membrane lipid peroxidation, protein denaturation, and nucleic acid damage [94, 95]. Many studies have demonstrated that ROS scavenging mechanisms play an important role in protecting plants from high-temperature stress [96, 97]. BRs applications decrease ROS levels and increase antioxidant enzyme activities to provide thermotolerance to elevated tempera‐ tures [98].

#### **4.2. Low temperature**

Chilling and frost stresses affect growth, development, survival, and crop productivity in plants [99, 100, 101]. However, BRs treatments enhance seedling tolerance to chilling stress [101] and increase the height, root length, root biomass, and total biomass of rice under lowtemperature conditions [102, 103]. In another study, Krishna [104] reported the same results in maize. They postulated that treatments with BRs promoted growth recovery of maize seedlings following chilling treatment (0–3°C).

Chilling stress increases the proline, betaine, soluble protein, soluble sugar contents of plants [79, 105]. Studies showed that BRs treatment enhanced proline content and therefore increased plant chilling resistance and cell membrane stability [99, 100, 106, 107].

Chilling stress could trigger the production of antioxidant enzymes in plants to prevent the chilling injury [108]. In the previous investigations, it was reported that treatment with BRs further increased the activities of antioxidant enzymes under chilling stress as well [99, 100, 107, 109]. The enhanced activities of the antioxidative enzymes as a result of BRs applications may occur with increasing de novo synthesis or activation of the enzymes, which is mediated through transcription and/or translation of specific genes to gain tolerance [57].
