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

Abstract

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, ΨPD, ΨMD), photosynthetic capacity (gs, E, A, Ci/Ca), photoprotection (NPQ, qP, ETR, Fv/FM, 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.