Aridity-induced structural and functional adaptations in Solanum surattense across dryland ecosystems

Aridity is a key environmental filter that governs plant community structure by constraining species diversity, distribution, and adaptive capacity, particularly within arid and semi-arid ecosystems. Solanum surattense, a drought-resilient perennial herb, has evolved distinct morphological, anatomical, and physiological adaptations that facilitate its survival under prolonged water stress. Nevertheless, increasing anthropogenic pressure due to its high medicinal and economic value has resulted in overexploitation, leading to significant population declines and habitat fragmentation, thereby threatening its ecological sustainability and long-term persistence. This study investigates the effects of aridity on the morpho-anatomical and physiological traits of S. surratense populations, while also assessing its distribution across ecologically diverse habitats in Punjab, Pakistan. Ten populations of S. surratense were collected from ecologically distinct habitats of the Cholistan Desert and its margins to evaluate key anatomical and physiological traits contributing to its ecological success under varying aridity levels. The De Martonne aridity index¹ was used to assess aridity levels, with populations sampled from the Cholistan Desert (IDM = 8.96-7.21) and its margins (IDM = 17.49-14.99). Advanced microscopy techniques (rotary microtomy and stereo microscopy) were employed to analyze its morphological and physiological traits. Populations from the Cholistan Desert exhibited distinct xeromorphic adaptations to survive in hyper-arid environments. These included a significantly thickened epidermis, encrypted stomata (128.13 µm²), well-developed vascular tissues, and prominent storage parenchyma in both stems and leaves. Structural reinforcements such as enhanced cortical thickness (120.33 μm), pith development (118.58 μm), and vascular bundle area (74.41 µm²) contributed to improved water retention and mechanical support under drought stress. Physiologically, desert populations showed elevated levels of osmolytes-including proline (39.65 µmol g^-1 fw.), soluble sugars (13.39 µmol g^-1 fw.), and free amino acids (22.35 µmol g^-1 fw.)-which facilitate osmotic adjustment and cellular stabilization during dehydration. In contrast, populations from the Cholistan Desert margins displayed superior growth performance, characterized by greater shoot length (55.66 cm), larger leaf area (25.24 cm²), higher flower (24.35 per shoot) and fruit production (23.74 per shoot), and increased shoot biomass (41.66 g plant^-1). These marginal populations also demonstrated improved photosynthetic capacity, indicated by higher chlorophyll a and b contents (1.62 mg g^-1 fw. and 1.24 mg g^-1 fw.), increased total chlorophyll (2.94 mg g^-1 fw.), and optimized carotenoid levels (0.30 mg g^-1 fw.), supporting greater energy capture and stress mitigation. An increased stomatal density (35.95 /mm²), coupled with reduced midrib and lamina thickness and enlarged vascular tissues, contributed to improved resource transport and gas exchange. Collectively, the results reveal a pronounced ecological divergence between Solanum surattense populations inhabiting the hyper-arid core of the desert and those from its peripheral zones, indicative of an adaptive trade-off between drought resistance and growth performance. These findings underscore the species' functional plasticity and resilience, offering valuable insights into plant survival strategies under extreme climatic stress. Elucidating such intraspecific trait variability holds significant implications for restoration ecology, targeted conservation efforts, and the development of sustainable land management practices in arid and semi-arid landscapes.