Phenotypic plasticity in rubber bush (Calotropis procera) along altitudinal gradient

Abstract

Calotropis procera (Aiton) W.T. Aiton is a widely distributed C3 species which is particularly prevalent in the Punjab province of the country (arid/semiarid conditions). This study is the first attempt to explore the adaptive components of this species across the region with a focus on the altitudinal gradient. Plant samples were randomly collected from six different altitudinal ranges, viz., 400, 800, 1200, 1600, 2000 and 2400 m a.s.l. Across the altitudinal gradient, this species exhibits significant variation in its morpho-anatomical and physiological characteristics, enabling it to thrive in various environments. At 400–1200 m altitude, the stomata were large and circular, followed by narrowly elliptic stomata at moderate 1600–2000 m elevation and rhomboidal stomata at high 2400 m elevation. Growth and biomass production were maximum at 400–1200 m along with most of the structural and functional features like proportion of vascular and storage tissues in stems/leaves, nutrients uptake (Na+ and Ca2+), osmolyte production (TAA, TSP, and TSS), and chlorophyll pigments. Xeromorphy was high in plants colonizing at 1600 m elevation, as indicated by the accumulation of beneficial ion (K+), elevated levels of antioxidants (SOD, CAT and POD), increased epidermal and lamina thicknesses, and an enhanced phloem region in the leaf. Leaf and stem succulence was the prominent feature of the species along altitudinal gradients. Growth attributes such as plant height and root fresh weight positively correlated (P < 0.001) with maximum temperature, shoot dry weight, root fresh and dry weight with soil ECe and Na+, while carotenoids content negatively correlated with minimum temperature at P < 0.05. In conclusion, the differential response of C. procera was linked to shifts in environmental factors along this elevation gradient, thus enabling the successful distribution of this species along this altitudinal gradient.