Casuarina equisetifolia is drought tolerant, salt tolerant, and able to grow in barren environments. It is often used to reduce wind damage, to prevent sand erosion, and to help establish plant communities in tropical and subtropical coastal zones. To determine the basis for its drought tolerance, we conducted transcriptomic and metabolic analyses of young branchlets under a non-drought treatment (D_0h) and 2-, 12-, and 24-h-long drought treatments (D_2h, D_12h, and D_24h). A total of 5033 and 8159 differentially expressed genes (DEGs) were identified in D_2h/D_0h and D_24h/D_0h. These DEGs were involved in plant hormone signal transduction, jasmonic acid (JA) biosynthesis, flavonoid biosynthesis, and phenylpropanoid biosynthesis. A total of 148 and 168 differentially accumulated metabolites (DAMs) were identified in D_12h/D_0h and D_24h/D_0h, which were mainly amino acids, phenolic acids, and flavonoids. In conclusion, C. equisetifolia responds to drought by regulating plant hormone signal transduction and the biosynthesis of JA, flavonoid, and phenylpropanoid. These results increase the understanding of drought tolerance in C. equisetifolia at both transcriptional and metabolic levels and provide new insights into coastal vegetation reconstruction and management.

Woody plant encroachments in arid and semiarid ecosystems are widely reported but the physiological mechanisms still need to be further revealed. In the current study, we aim to determine whether differences in leaf physiological traits help explain grassland susceptibility to woody plant encroachment and whether distinctive physiological adaptations allow some shrub species to invade grasslands. We compared physiological traits (photosynthesis, leaf water status, pigment compositions and leaf antioxidant capacities) of six species representing three functional groups: woody encroachers (Prosopis velutina, Larrea tridentata), woody non-encroachers (Acacia greggii, Lycium fremontii) and C4 grasses (Bouteloua curtipendula, Bouteloua barbata) which are naturally growing in a botanical garden in University of Arizona, USA. We infer that P. velutina (encroacher) but not A. greggii or L. fremontii (non-encroachers) is encroaching in grasslands because the former species has higher water and light utilization efficiencies (instantaneous water use efficiency, instantaneous light use efficiency, and Fv/Fm). The extremely high carotenoid and total antioxidant capacity in its leaves appears to help the shrub L. tridentata (encroacher) survive high ambient oxidative damage caused by both drought and high light stresses in this grassland. The two C₄ grass species, B. curtipendula and B. barbata, grow well in the arid ecosystem but may be susceptible to disturbances.