Effect of greenhouse microclimate and water quality on nutrient uptake in pepper hydroponic crop

The nutrient to water uptake ratios, henceforth termed “uptake concentrations” (UC), remain relatively constant over time under similar climatic conditions for a particular plant species and developmental stage. Under greenhouses with low temperature (LT) conditions, the uptake of nutrients may be altered in a different manner than that of the water and thus their UC may be different than in greenhouses with standard temperature (ST) conditions. In the Mediterranean regions, sweet pepper is frequently cultivated in unheated greenhouses in which the temperature during the winter may drop to suboptimal or even lower levels. In these areas, the available irrigation water frequently contains sodium chloride but also calcium bicarbonate, which at excessively high concentrations in closed hydroponic crops can impose Ca accumulation in the recycled NS and concomitantly negatively affect fruit yield and quality of the produce.Taking the above into consideration there were established three studies:In the first study, pepper plants of the cultivars ‘Sammy’ and ‘Orangery’, self-grafted or grafted onto two commercial rootstocks ('Robusto' and 'Terrano'), were cultivated in a greenhouse under either ST or LT conditions. The aim of the study was to test the impact of grafting and greenhouse temperature on total yield, water use efficiency and nutrient uptake. The LT regime reduced yield by about 50% in ‘Sammy’ and 33% in ‘Orangery’, irrespective of the grafting combination. Grafting of ‘Sammy’ onto both 'Robusto' and 'Terrano' increased the total fruit yield by 39% and 34% compared with the self-grafted control, while grafting of ‘Orangery’ increased yield only when the rootstock was ‘Terrano’. The yield increase resulted exclusively from enhancement of the fruit number per plant. Both the water consumption and the water use efficiency were suppressed by the LT regime but the temperature effect interacted with the rootstock/scion combination. The LT increased the UC of K, Ca, Mg, N, and Mn, while it decreased strongly that of P and slightly the UC of Fe, and Zn. The UC of K and Mg were influenced by the rootstock/scion combination but this effect interacted with the temperature regime. In contrast, the Ca, N, and P concentrations were not influenced by the grafting combination. The results of the present study show that the impact of grafting on yield and nutrient uptake in pepper depend not merely on the rootstock genotype but on the rootstock/scion combination.In the second study, mean UC of macro- and micronutrients were determined during five developmental stages in different pepper cultivars grown in a closed hydroponic system by measuring the water uptake and the nutrient removal from the RNS. The experiment was conducted in a Mediterranean environment and the tested cultivars were ‘Orangery’, ‘Bellisa’, ‘Sondela’, ‘Sammy’ self-grafted and ‘Sammy’ grafted onto the commercial rootstock `RS10'. ‘Sondela’ exhibited significantly higher ΝΟ3, Μg, Ca and B UC in comparison with all other cultivars, while Bellisa exhibited higher K UC. The UC of all nutrients were similar in the grafted and the non-grafted ‘Sammy’ plants. The UC of macronutrients estimated in the second study (mmol L-1) ranged from 2.4 to 3.7 for Ca, 1.0 to 1.5 for Mg, 6.2 to 9.0 for K, 11.7 to 13.7 for N, and 0.7 to 1.1 for P. The UC of N, K, Ca and Mg were appreciably higher than the corresponding values found under Dutch climatic conditions, while that of P was similar in both environments during the vegetative stage and higher thereafter. The UC of Fe, Zn and B tended to decrease with time, while that of Mn increased initially and subsequently decreased slightly during the reproductive developmental stage.In the third study, irrigation water containing 1.5, 3.0, 4.5 and 6.0 mM was used to prepare NS in a closed hydroponic crop of sweet pepper cultivated in RNS. The aim of the study was to determine maximum Ca levels that do not harm the crop and to simulate the pattern of Ca accumulation when the Ca concentration in the irrigation water is excessive. At 1.5 mM Ca, no Ca accumulation was observed in the RNS, while at 3.0, 4.5 and 6.0 mM the Ca concentration in the RNS, and concomitantly in the root environment, increased to 17, 28 and 37 mM, corresponding to 6.4, 9.0 and 10.8 dS m-1. The accumulation of Ca in the RNS affected both tissue nutrient concentrations and UC of Ca, S and Mg, but this was not the case for N and K. Growth, yield and plant water uptake were restricted at moderate and high external Ca levels. Our results showed that in soilless sweet pepper crops with zero discharge of fertigation effluents, the Ca concentration in the irrigation water should be lower than 3.0 mM to avoid yield restrictions due to salinity.