Linking Mineral Nutrition and Fruit Quality to Growth Intensity and Crop Load in Apple

Diagnostic of the nutritional status in fruit trees can be estimated using different models. However, these models usually do not fully match the character of the perennial crops and their accuracy needs to be improved. This study aimed at deepening the understanding of the aspects controlling mineral content of macro- and micronutrients and fruit quality of the apple trees with respect to the crop load and vegetative shoot growth. The focus was given on description of the nutrient content variability in time and space, its relation to important physiological events, and conditions altering their gradient in leaf and fruit. The evaluation was carried out with six years old ’Golden Delicious’ apple trees on M9, planted in an experimental orchard. Flowering rate, total yield, shoot growth intensity, mean fruit weight and quality along with mineral content of leaves, and fruit in different development stages were analysed according to intensity of natural or regulated flowering and fruit set, respectively. We found that aside of macroelements, iron, and manganese depended on the crop load. For nitrogen, phosphorus, and potassium content, their gradient increased up to 10–25% within the sufficiency range as the trees tried to maintain certain level of these nutrients in leaves and fruit simultaneously during deficit. In contrary, the higher content of Mg, Fe, and Mn related to the avoidance of deficit through up to 20–30% higher uptake in trees with higher crop load. Phosphorus, potassium, and calcium content in short fruiting shoots was more stage dependent, showing relation with the timing of cessation of particular meristematic growth activity. If no intense deficit occurred, the trees kept unrestricted initial investment of nutrients in the fruit during cell division stage whatever the crop load was. Their content at the later development stages appeared to be more dependent upon the consumption by particular sink. These findings allowed to improve the interpretation of the fruit trees nutrition status due to adaptation of the reference nutrient content in low cropping trees by a reduction of 5–10% N and P, 10–15% Ca, as well as by an increase of 15–20% K content in leaves at the stage with fruit size of 20 mm, and without change in P content at the stage of about 70% of final fruit size.