Compatible Solute Variation and Stress Adaptation in Native Qatari Plants: Focus on Proline

Osmolytes, including proline, soluble sugars, and glycine betaine (GB), are essential for plant adaptation to environmental stress. They contribute to osmotic adjustment, membrane stabilization, and the protection of cellular functions in arid and saline habitats. This study investigated major osmolytes (mainly proline and soluble sugars) in native Qatari plant species in natural field conditions and their physiological adaptation strategies. Significant interspecific variation indicated diverse mechanisms of stress acclimation. Although proline accumulation was common, it did not consistently correlate with salinity tolerance, which suggests that its accumulation may reflect stress-induced metabolic imbalance and adaptation rather than being a reliable indicator of resistance. Plant species, including crops and native plants such as Avicennia marina, Halopeplis perfoliata, Limonium axillare, Tetraena qatarensis, and Ochradenus baccatus, are presented as examples supporting this pattern. In contrast, the relative balance between soluble sugars and proline indicates coordinated carbon–nitrogen regulation that supports osmotic homeostasis and growth in fluctuating environmental conditions. Halophytic species exhibited distinct osmolyte profiles that highlight the potential role of additional compatible solutes (particularly GB) in stress adaptation. However, its occurrence and functional significance in these species have been insufficiently characterized. Given the predominance of C3 photosynthesis in Qatari flora, GB may also help mitigate photorespiratory stress in extreme conditions. The findings expand the current understanding of osmotic regulation in desert plants and highlight the potential of biotechnological approaches to enhance crop tolerance of harsh environments by manipulating compatible solutes.