Non-invasive monitoring of the impact of potassium and calcium malnutrition in apple trees (Malus × domestica Borkh.)

The seasonal changes of leaf potassium and calcium status were studied in apple trees potted in sand for the seasons 2019 and 2020. The potassium concentrations tested were 0.0, 3.4, 6.8, 8.5, 10.2, 13.6, and 17.0 g L^‑1 in the stock solution. Potassium and calcium contents were assayed in the apple tree leaves after recording their hyperspectral imaging during the vegetation season. The maximum concentration of potassium in the stock solution was determined (8.5 g L^‑1); its further increase did not further increase the leaf potassium content, higher concentrations (13.6 g L^‑1) caused death of the plants. The potassium concentrations above 6.8 g L^‑1 drove the leaf calcium content significantly below the optimum level and caused an aberrant pattern of seasonal changes on the leaf nutrient content. We also used hyperspectral reflectance imaging to monitor the chlorosis accompanying the nutrient imbalance induced damages at leaf level. The decline in leaf chlorophyll could serve as a proxy for detection of the potassium malnutrition-induced disorders in the orchards and assess their severity.