Similar but unique: physiological response to drought and growth of pure species and interspecific hybrid clones of Eucalyptus

Key message

Eucalyptus grandis and hybrids with red gums and E. urophylla exhibit different strategies to deal with long-term drought, involving differences in stomatal regulation, plant hydraulics, and growth.

Abstract

Eucalyptus species are important in commercial forestry for their rapid growth and adaptability. In the context of climate change, evaluating the drought responses of different genotypes is critical for enhancing resilience and productivity. Genetic improvement often involves crossing fast-growing, high-quality species with drought-tolerant ones. Understanding trade-offs in pure species and hybrids is essential for optimizing forest management. This study examined physiologic and growth responses to water restriction (WR) of E. grandis (GG), E. grandis × E. camaldulensis (GC), E. grandis × E. tereticornis (GT), and E. grandis × E. urophylla (GU1 and GU2) clones at the sapling stage across two drought cycles. Measurements included leaf-water potential (Ψ), relative water content (RWC), stomatal conductance (g_S), pressure–volume traits, hydraulic conductivities (k_S, k_L), percentage loss of hydraulic conductivity (PLC), specific leaf area (SLA), and chlorophyll content. Results revealed different drought response strategies among clones. GC and GT hybrids were more “water spenders”, exhibiting high PLC (> 80%) due to limited stomatal closure, along with higher chlorophyll levels that maximized carbon gain and growth under drought. GC exhibited both elastic and osmotic adjustment, while GT showed only elastic adjustment. GG was the most drought-sensitive clone, relying on strong stomatal control, osmotic adjustment, and low cavitation, which limited carbon assimilation and resulted in the greatest growth reduction. GU hybrids shared physiologic similarities with GG but showed varying growth responses to WR. These findings suggest some hybrid clones may outperform pure E. grandis under WR, with significant genotype variation even among hybrids sharing similar parental species.