Plant water stress and K+ starvation reduce absorption of foliar applied K+ by olive leaves

Three-month-old mist-rooted 'Picual' olive cuttings were transplanted into 2-l plastics pots containing perlite as substrate and fertigated with a complete nutrient solution containing 0.05, 0.1 or 2.5 mM KCl depending on the experiment. In the first experiment, plants were sprayed with RbCl (Rb+ is a K+ analog) at a rate of 4% at 63 days after transplanting (DAT). Foliar Rb+ uptake through leaves increased with K+ concentration in the nutrient solution, indicating that foliar Rb+ uptake was lower when plants were K+ deficient than when they were adequate. On the contrary, it was observed that translocation of Rb+ from leaves to other organs of the plant was higher under K+ deficiency conditions. In the second and third experiments, when differences appeared on shoot length due to K+ nutritional status (0.05 or 2.5 mM KCl) at 63 DAT, a group of plants were subjected to water stress during 7 weeks. On the other hand, another group of plants (control plants) did not receive any water stress treatment during the experiments. After this period of 7 weeks, all plants were sprayed with RbCl at 4%. Leaf K+ concentration diminished in water-stressed plants independently of plants nutritional status. Foliar Rb+ uptake through leaves was restricted by water stress either in plants with low K+ (0.05 mM KCl) or plants with high K+ (2.5 mM KCl). Translocation of Rb+ from leaves was greater under water stress conditions in both K+ nutritional statuses. In conclusion, the results obtained could explain the irregular response in olive trees to foliar K+ sprays, particularly, when they grow in rainfed orchards.