Effect of Mineral Nutrient Sprays on Photosynthesis and Stomatal Opening of Water-stressed and Unstressed Apple Seedlings II. Potassium Sulfate Sprays1

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Abstract
‘York Imperial’ apple seedlings (Malus domestica Borkh.) grown in nutrient solution cultures with decreased water potential to− 1.0 bar by polyethylene glycol (PEG) increased water consumption, photosynthesis rate (Pn), and stomatal conductance (Cs). High light preconditioning of the plants used in this experiment was probably the reason why− 1.0 bar water potential in the nutrient solution was not low enough to induce apple seedling responses typical of water-stressed plants. However, application of PEG stress (−1.0 bar), to K-sprayed (K2SO4, −0.5%) trees lowered seedling water consumption Pn, and Cs. Potassium sprays alone did not significantly affect water consumption, Pn or Cs. When the water potential of the nutrient solution of PEG stressed plants was further decreased to −2.5 bars, unsprayed trees started to wilt within 2 days while sprayed trees did not. It is proposed that earlier stomatal closure of K-sprayed trees when stressed, already at low level of water stress (−1.0 bar), prevented plant water depletion when stress level was increased. This in turn delayed commencement of plant wilting. Potassium sprays also increased root:shoot ratio and root K concentration in PEG-stressed plants. These responses of K-sprayed trees could also contribute to greater tolerance to higher levels of water stress.

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