Disguised Blessings: A Mechanistic Understanding of the Beneficial Outcomes Triggered by Partial K Replacement With Na in Two <i>Eucalyptus</i> Species Under Drought Stress

While not essential for most plants, sodium (Na⁺) can partially substitute for potassium (K⁺) in some metabolic functions. Thus, understanding the mechanisms underlying K⁺ and Na⁺ uptake, transport, utilization, and ion replacement is crucial to sustain forest production. A pot experiment was designed with 6 K/Na ratios (100/0, 85/15, 70/30, 55/45, 40/60, and 0/0%) and two water conditions (well-watered, W+; and water-stressed, W-) on two Eucalyptus species with contrasting drought tolerance. In a multi-level analysis, we measured morphological, nutritional, physiological, biochemical, molecular, and anatomical traits. Low to moderate K replacement with Na (85/15%-55/45%) provided partial and faster stomatal closure (lower δ¹³C), thereby enhancing plants' water status (WUE, RLWC, Ψ_PD, Ψ_MD), photosynthetic capacity (g_s, E, A, C_i/C_a), photoprotection (NPQ, qP, ETR, F_v/F_M, ΦPSII), and growth (height, collar diameter, LA, TDM) relative to exclusive K supply. The 70/30% K/Na replacement was defined as the ideal ratio, upregulating K⁺ and water uptake (overexpression of AKT1, PIP2;5, PIP2;7 and TIP1;3), maximizing enzymatic antioxidant performance and biomass production, and reducing oxidative stress. High K replacement with Na (40/60%) and K deficiency (0/0%) led to incomplete stomatal closure reduced water status, photosynthetic capacity, photoprotection, and growth, especially in the species with low drought tolerance.