**5.1. Proline**

Proline is the most extensively studied osmolyte because of its great importance in stress tolerance [26]. The exogenous application of proline can increase its endogenous levels in plant tissues subjected to waterstress conditions which help maintain osmotic adjustment in plant tissues. It may be a good source of minimizing the adverse effects of water stress on plants, and triggering their growth also depends upon the type of plant species and its concentration [27].

The production of proline is widely present in higher plants and normally accumulates in large quantities in response to environmental stresses [28]. For osmotic adjustment, proline contributes to stabilizing subcellular structures (e.g., membranes and proteins), scavenging free radicals, and buffering cellular redox potential under stress conditions. A rapid breakdown of proline upon relief of stress may provide sufficient reducing agents that support mitochondrial oxidative phosphorylation and generation of ATP for recovery from stress and repairing of stress-induced damages [29]. Iqbal et al. [30] have reported that the accumulation of proline in drought-tolerant and drought-sensitive cultivars has revealed the significance of this osmolyte. The role of proline in induced PEG experiment gave evidence that the higher levels of proline are due to the emergent need of stressed plant. This osmolyte is able to control the osmotic regulation of the cellular environment because of its high water solubility and its accumulation in the leaves of many halophytic higher plants grown in saline environment. Proline protects membranes against adverse effects of high concentration of inorganic ions and temperature extremes. It is also functional as a proteincompatible hydrotope and as a hydroxyl radical scavenger [31].
