Interaction between zinc deficiency and boron toxicity on growth and mineral nutrition of sour orange seedlings

Sour orange (Citrus aurantium L.) seedlings were grown for 3 months in diethylenetriamine pentaacetate (DTPA)‐buffered nutrient solutions to study the effect of Zn stress on the plants’ sensitivity to high boron concentration in the root environment. There were three zinc treatments: 21 μM Zn (LOW Zn‐DTPA), 69 μM Zn (NORMAL Zn‐DTPA) in the nutrient solution, or 12 weekly foliar sprays with ZnSO₄ (FOLIAR‐Zn). In the FOLIAR‐Zn treatment, the nutrient solution contained 21 μM Zn. Zn activities calculated with a chemical equilibrium model, Geochem PC, and expressed as pZn=‐log(Zn⁺²), were 10.2 and 9.7 in the LOW Zn‐DTPA and NORMAL Zn‐DTPA nutrient solutions, respectively. One half of the plants in each Zn treatment were grown in 51 μM B (NORMAL‐B) and the other half in 200 μM B (HIGH‐B) nutrient solution. Seedlings grown in LOW Zn‐DTPA/NORMAL‐B nutrient solution developed Zn deficiency symptoms such as: reduced shoot growth, small and chlorotic leaves, and white roots with visibly shorter and thicker laterals than in Zn sufficient plants. The HIGH‐B treatment decreased shoot growth, leaf and stem dry weight, leaf area, and induced severe leaf B toxicity on seedlings grown in the LOW Zn‐DTPA nutrient solution but the effect was either absent or less pronounced in the NORMAL Zn‐DTPA or FOLIAR‐Zn treatments. Seedlings in the LOW Zn‐DTPA FOLIAR‐Zn treatments but they had lower B concentration on a whole plant basis indicating less B uptake per unit of dry weight. The FOLIAR‐Zn and NORMAL Zn‐DTPA treatments were equally effective in alleviating leaf B toxicity symptoms. The FOLIAR‐Zn treatment, however, was less effective than the NORMAL Zn‐DTPA treatment in alleviating the deleterious effect of high B on leaf dry weight even though the B concentrations in leaves, stems, and roots of the foliar‐sprayed seedlings were similar to the NORMAL Zn‐DTPA seedlings. Leaf concentrations of phosphorus, potassium, magnesium, iron, mangenese, and copper were within the optimal range for citrus with the exception of Ca which was low. Although B and particularly Zn treatments modified the concentration of some of these elements in leaves and roots, these changes were too small to explain the observed growth responses. The observation that B toxicity symptoms in Zn‐deficient citrus could be mitigated with Zn applications is of potential practical importance as B toxicity and Zn deficiency are simultaneously encountered in some soils of semiarid zones.