Pakistan lacks forests because of high rates of deforestation, pressure from an increasing population, and climate change. To counteract salinity, reforestation, and water logging, several eucalypt species were introduced into Pakistan. Due to Pakistan's predominantly arid and semi-arid climate, water is a scarce resource. High water content is necessary for eucalyptus to reach its full development potential. It is crucial to choose species that are suitable for low water circumstances and can flourish there. In this study, the growth behaviors of two of the most significant eucalyptus species i.e., Eucalyptus globulus and Eucalyptus camaldulensis—under water stress situations are compared. Water stress changed the growth behavior of eucalyptus species in terms of biomass allocation above and below ground. Regardless of the species, low water regimes had a significant impact on every feature assessed as the soil moisture content decreased. The number of leaves, plant height, stem diameter, root diameter, stem fresh weight, leaf moisture content, root fresh weight, root dry weight, total biomass, root biomass, and stem root ratio were all significantly lower in both species compared to the control. The ability of a plant to respond to drought is largely dependent on its root length. The roots of E. globulus, which had a shorter mean root length (17.7±4.11 cm) than the control (19.8±4.49), and E. camaldulensis grew longer than the control. For both the control and the species, the leaf biomass was similar. Under water stress conditions, E. camaldulensis retained more water as evidenced by the fact that its dry weight was greatly influenced and that it lost more moisture than E. globulus. It is noteworthy that under stress conditions, E. globulus had a higher root fresh weight (5.32±0.48 gm) than E. camaldulensis (4.9±0.27 gm), despite the latter having a longer root length. This suggests that the former possessed greater lateral root growth and greater root mass. Between treated and control plants, there was a substantial difference in total biomass. Comparable to E. camaldulensis (7.8±0.29 gm), E. globulus had a total biomass of 7.78±0.27 gm. The stem-root ratio was significantly impacted in both species; surprisingly, E. globulus exhibited a higher ratio than E. camaldulensis. The selection of genotypes tolerant to drought appeared to be aided by four morphological markers: root length, stem fresh biomass, stem dry biomass, and root moisture content. In comparison to the control, E. globulus underwent greater morphological changes in response to stress and generated biomass that was like that of E. camaldulensis. Consequently, it can be inferred that in situations of water deprivation and for general plantation, E. globulus is more adaptable than E. camaldulensis.

Keywords: Eucalyptus; Water deficit; Morphological plasticity; Reforestation; Afforestation; Biomass