Seedling plasticity and phytomass allocation of Stipa tenacissima L. populations in arid region of Tunisia

Seedling plasticity in Poaceae, particularly in Stipa tenacissima L., represents a key adaptive mechanism for survival in arid steppes. This plasticity is essential for the species’ persistence and success under harsh environmental conditions. In the present study, we examined the leaf and root morphological traits and plasticity in relation to the seed-source rainfall of Stipa tenacissima (Alfa grass), a key species in the arid and semi-arid environments of the Mediterranean Basin. Populations representing a rainfall range of 100 to 400 mm per year at the seed collection sites were grown in a common garden. Populations differed significantly in TB, leaf traits, and several root traits, with Hassi El Frid showing higher above-ground biomass and leaf architecture, and Sfax populations exhibiting larger root systems. Specific root length (SRL) and leaf traits were positively correlated, while mean root diameter (MRD) decreased with increasing aridity. Plasticity indices revealed high variability among populations, with Sbeitla showing the greatest trait plasticity. Principal component analysis grouped populations according to functional trait expression, explaining 72.7% of total variance. Our results indicate that seedling trait variation is influenced by both local climate and genetic differentiation. Resource allocation patterns support the optimal partitioning theory, with xeric populations favoring root investment and mesic populations favoring leaf development. These findings highlight the adaptive strategies of S. tenacissima seedlings to cope with aridity, providing insights for predicting species responses to climate change and informing conservation of Mediterranean drylands.