Reforestation efforts are being challenged as water stress is hampering the sapling growth and survival in arid to semiarid regions. A controlled experiment was conducted to evaluate the effect of foliar application of salicylic acid (SA) on water stress tolerance of Conocarpus erectus and Populus deltoides. Saplings were watered at 90%, 60%, and 30% of field capacity (FC), and half of the saplings under 60% and 30% FC were sprayed with 1.0 mM SA. Results indicated that dry weight production decreased significantly in Populus deltoides under both water deficit conditions, and leaf gas exchange parameters decreased significantly in both the species under both soil water deficit conditions. Foliar application of SA resulted in a significant increase in leaf gas exchange parameters, and compatible solutes, thereby increasing the dry weight production in both of the species under soil water deficit. Oxidative stress (hydrogen peroxide and superoxide anions) increased under soil water deficit and decreased after the foliar application of SA and was parallel to the increased antioxidant enzymes activity (superoxide dismutase, catalase, peroxidase, and ascorbate peroxidase). Therefore, it can be concluded that foliar application of 1.0 mM SA can significantly improve the water stress tolerance in both species, however, positive impacts of SA application were higher in Conocarpus erectus due to improved photosynthetic capacity and increased antioxidant enzyme activity.

Shifting toward stress-resistant planting material has become a pre-requisite for fruit tree culture under arid to the semiarid environment. No information currently exists on the physiological basis of water stress tolerance in Ziziphus jujuba at the saplings stage, an important fruit tree of semiarid regions of Central Punjab. Therefore, an experiment was conducted under controlled conditions where plants were subjected to three watering regimes: control irrigation, a medium, and a high-water deficit treatment. During the experiment, growth, biomass production and allocation in different plant organs (leaves, stem, and root) were measured. The physiological response of Z. jujuba under water deficit was determined through a gas exchange system and CO₂ assimilation rate, stomatal conductance, and water use efficiency were evidenced. Biomass production/allocation in leaves and stem decreased significantly with an increase in soil water deficit. Root biomass production remained unchanged (p = 0.907), but allocation increased significantly under high soil water deficit (p < 0.001). Net CO₂ assimilation rate (A) remained similar at 6.54 ± 0.32 μmol m⁻² s⁻¹ but, stomatal conductance (gs) decreased significantly from 94.0 to 69.2 mmol m⁻² s⁻¹ under high water deficit treatment. Water use efficiency (ratio between Net CO₂ assimilation rate and transpiration rate) also increased significantly (p = 0.012) under high water deficit treatment. Ziziphus jujuba showed a medium degree of plasticity under water deficit conditions, which was mainly driven by a sustained CO₂ assimilation rate and more biomass allocation for root development.

Conocarpus lancifolius is a fast-growing and drought tolerant tree species with phytoremediation potential in arid environments. The present study was conducted to evaluate the phytoaccumulation potential under wastewater treatment. The experiment was performed in a greenhouse where 3-month-old seedlings were irrigated with industrial wastewater and growth, biomass and physiological parameters were measured. Concentrations of zinc (Zn), lead (Pb), and cadmium (Cd) in leaves, shoots, and roots along with translocation and tolerance index were also determined. The results showed that under wastewater treatment total biomass increased from 24.2 to 31.5 g, net CO₂ assimilation rate increased from 9.93 to 13.3 μmol m⁻² s⁻¹, and water use efficiency increased from 1.7 to 2.42. Similarly, heavy metals (Zn, Pb, and Cd) accumulation in stem, leaves, and roots increased significantly under wastewater treatment where the highest concentration of Zn, Pb and Cd was found in roots followed by leaves and stem, respectively. Tolerance index was found >1, and translocation factor of all heavy metals was found >1. The study revealed that phytoaccumulation potential of C. lancifolius was mainly driven by improved net CO₂ assimilation rate and water use efficiency.

Low water availability predicted under climate change is a major abiotic factor limiting plants growth and productivity. In this study a greenhouse experiment was conducted on three important tree species of arid environment: Conocarpus erectus (CE), Acacia modesta (AM), and Salix tetrasperma (ST). Young saplings were subjected to control (C), medium (MWD) and severe soil water deficit (SWD) treatments and response was evaluated. Results showed that in all the three species leaf, stem and root dry weight production remained similar to C under MWD treatment but decreased significantly under SWD. The highest decrease in total dry weight was noticed in ST and the lowest was evidenced in AM under SWD. Root:shoot ratio increased significantly in both CE and AM under MWD and SWD. Furthermore, chlorophyll content decreased while proline content increased significantly in both MWD and SWD treatments. The production of oxidants (hydrogen peroxide and superoxide anions) and antioxidants (superoxide dismutase, catalase, peroxidase and ascorbate peroxidase) increased significantly under both MWD and SWD treatments and were the highest in AM in both MWD and SWD treatments. Therefore, we may conclude that all the three species can tolerate medium water stress due to increased root production and an effective antioxidant defense mechanism.