Morphological and physiological adaptation of a desert shrub, Encelia farinosa, under drought stress

Water scarcity in arid and semi-arid regions affects ecosystem development. Therefore, elucidation of the mechanisms by which plant species in a given region respond to drought conditions may allow us to improve global vegetation. Encelia species are drought-tolerant and an important component of shrub communities in the arid and semi-arid regions of southern California, U.S.A.; these species are widely used for vegetation restoration in the region. This study aimed to investigate the morphological, and physiological responses in E. farinosa in relation to changes in soil properties under drought conditions. Plants were subjected to two levels of soil water regimes, drought and well-watered conditions (5% and 12% of gravimetric water content, respectively). After 3 months, above-ground biomass, basal diameter, leaf area, leaf and root relative water content, and root length were measured. The shoot:root ratio in dry mass and specific leaf area (SLA) were calculated. We also analyzed leaf and soil nutrients, including total C, N, and available P. In the drought treatment, E. farinosa showed significantly lower above-ground biomass, stem diameter, and shoot and root relative water content. Also, the drought treatment resulted in significantly lower leaf areas, while the SLA and root length were significantly higher. Leaf total C and N were higher in the drought treatment while leaf P was not affected by soil water content. The reduction in plant leaf area, root elongation and the increase in SLA under drought are known as drought avoidance strategies of plants, suggesting that E. farinosa hampers water loss by morphological regulations. Additionally, higher leaf content of C and N seems to be important for metabolic drought adaptation. Further research on osmotic adjustment and antioxidant defense systems will clarify the detailed drought tolerance mechanisms of E. farinosa in relation to C and N accumulation under drought conditions.