Modelling Ca2+ accumulation in soilless zucchini crops: Physiological and agronomical responses

Soilless zucchini (Cucurbita pepo L.) crops were grown in two distinct cropping periods (spring-summer; SS and autumn-winter; AW) using irrigation water with different Ca2+ concentrations (1.5, 3, 4.5 and 6 mM). The objectives of this study were to: (i) mathematically correlate the accumulation of Ca2+ in the root environment with the respective Ca2+/water uptake ratio (namely uptake concentration; UC), and (ii) determine the UC of macronutrients (i.e., N, P, K, Ca and Mg) under these conditions. Equations of the literature, initially developed to predict NaCl accumulation in a closed hydroponic system, were further extended to fit experimental results. The evolution of Ca2+ accumulation in the drainage exhibited a sigmoid pattern with time and the relationship between the concentration of Ca2+ in the root zone and the corresponding uptake ratio Ca2+/water was better described by curvilinear functions. Validation of the model showed a very good agreement between simulated and measured values. Increasing Ca2+ levels affected both tissue concentrations and UC of Ca and N, but this was not the case for P, K, and Mg. Photosynthesis, growth, yield and plant water uptake were restricted (avg. 15% decrease) at high external Ca2+ levels, due to high total salt concentration (EC) in the recycled solution (4.2-5.5 dS m−1). Fruit quality attributes, however, remained unaffected by treatments with the exception of fruit nitrate content. The empirical model parameterized and tested in this work may serve as a tool to predict Ca2+ ion concentrations in the root environment of zucchini crops as relationships of the water absorbed by the crop. Finally, the results showed that in soilless zucchini crops with zero discharge of fertigation effluents, there is no compelling reason not to use irrigation water resources with Ca2+ concentration up to 3 mM.