The double-edged sword of potassium and sodium fertilization in xylem embolism resistance of two Eucalyptus species under drought stress.

Sodium (Na+) is a beneficial element for most plants that may replace potassium (K+) in osmoregulatory process to a certain extent, increasing plant water-use efficiency. Thus, understanding coordinated mechanisms underlying the combined use of K+ and Na+ in tree drought tolerance is a key challenge for the agricultural industry in dealing with forest productivity and water limitations. A pot experiment with three ratios of K/Na (K-supplied, partial K replacement by Na and K-deficient plants) and two water regimes, well-watered (W+) and water-stressed (W-), was conducted on saplings of two Eucalyptus species with contrasting drought sensitivities. We evaluated the point of stomatal closure (Pgs90), xylem embolism thresholds (P12, P50, P88), hydraulic safety margin (HSM), leaf gas exchange (A, E, gs and dark respiration), leaf water potential (ΨPD and ΨMD), long-term water use efficiency (WUEL) and total dry mass (TDM). Partial K replacement by Na increased the leaf gas exchange, WUEL and TDM, while Pgs90, P12, P50, P88 and ΨMD decreased (more negative), compared to plants exclusively supplied with K and K-deficient plants of both species. Fertilized plants had narrower HSMs than K-deficient plants, indicating that these Eucalyptus species adopt the functional adaptive strategy of operating close to their hydraulic limits to maximize carbon uptake while increasing the risk of hydraulic failure under drought-stress.

Leaf 13C and 15N composition shedding light on easing drought stress through partial K substitution by Na in eucalyptus species.

This work aimed to investigate the partial K-replacement by Na supply to alleviate drought-induced stress in Eucalyptus species. Plant growth, leaf gas exchange parameters, water relations, oxidative stress (H2O2 and MDA content), chlorophyll concentration, carbon (C) and nitrogen (N) isotopic leaf composition (δ13C and δ15N) were analyzed. Drought tolerant E. urophylla and E. camaldulensis showed positive responses to the partial K substitution by Na, with similar dry mass yields, stomatal density and total stomatal pore area relative to the well K-supplied plants under both water conditions, suggesting that 50% of the K requirements is pressing for physiological functions that is poorly substituted by Na. Furthermore, E. urophylla and E. camaldulensis up-regulated leaf gas exchanges, leading to enhanced long-term water use efficiency (WUEL). Moreover, the partial K substitution by Na had no effects on plants H2O2, MDA, δ13C and δ15N, confirming that Na, to a certain extent, can effectively replace K in plants metabolism. Otherwise, the drought-sensitive E. saligna species was negatively affected by partial K replacement by Na, decreasing plants dry mass, even with up-regulated leaf gas exchange parameters. The exclusive Na-supplied plants showed K-deficient symptoms and lower growth, WUEL, and δ13C, besides higher Na accumulation, δ15N, H2O2 and MDA content.

Enhancing potassium content in leaves and stems improves drought tolerance of eucalyptus clones.

Eucalyptus are widely planted in regions with low rainfall, occasioning frequent drought stresses. To alleviate the stress-induced effects on plants growing in these environments, soil fertilization with potassium (K) may affect drought-adaptive plant mechanisms, notably on tropical soils with low K availability. This work aimed to evaluate the K dynamic nutrition in eucalyptus in response to soil-K and -water availabilities, correlating the K-nutritional status with the physiological responses of contrasting eucalyptus clones to drought tolerance. A complete randomized design was used to investigate the effects of three water regimes (well-watered, moderate water deficit, and severe water deficit) and two K soil supplies (sufficient and low K) on growth and physiological responses of two elite eucalyptus clones: "VM01" (Eucalyptus urophylla × camaldulensis) and "AEC 0144" (E. urophylla). Results depicted that the K-well-nourished E. urophylla × camaldulensis clone under severe water deficit maintained shoot biomass accumulation by upregulating the K-content in leaves and stems, gas exchange, water-use efficiency (WUEI ), leaf water potential (Ψw), and chlorophyll a fluorescence parameters, compared to E. urophylla clone. Meanwhile, E. urophylla with a severe water deficit showed a decreased of K content in leaves and stem, as well as a reduction in the accumulation of dry mass. Therefore, the K-use efficiency and the apparent electron transport rate through photosystem II were positively correlated in plants grown in low K, indicating the importance of K in maintaining leaf photochemical processes. In conclusion, management strategy should seek to enhance K-nutrition to optimize water-use efficiencies and photosynthesis.