CATION CONCENTRATIONS OF PLANT TISSUES OF FRUIT-VEGETABLE CROPS AS AFFECTED BY THE EC OF THE EXTERNAL NUTRIENT SOLUTION AND BY HUMIDITY

Abstract

In a series of experiments effects of EC values in the root environment and differences in climatic conditions were studied in relation to the uptake of cations. Tomato, cucumber and sweet pepper were used as test crops. Differences with EC values were realised by the addition of various concentrations of nutrients, whereby the mutual ratios between the nutrients were kept constant. Differences of climatic conditions were realised by ventilation, heating, screening regimes and evaporation of water from open water basins. For all crops a spring and a fall grown crop were incorporated in the study. The uptake of the cations was determined by tissue samples. Young fully grown leaves and ripe fruits were sampled and analysed systematically. Accidentally, old leaves and tops were sampled and analysed, while in some cases the fruits were separated in a proximal part and a distal part. The K concentration in the plant tissues was increased and the Ca concentration was decreased by an increasing EC in the external solution. The Mg concentration in the plant tissues was affected less evidently by an increased EC. The overall effect of the humid climate was a decrease of the K and Mg concentrations in the leaves and a small increase of the Ca concentration. The K, Ca as well the Mg concentrations in the fruits were increased by the high humidity in the greenhouse. However, these effects were not of the same size as occurred with the EC and were not consistent for all crops. The average Ca concentration in the distal end of the tomato fruits was 40% lower than in the proximal part; for sweet pepper this difference was only 10%. Increased nutrient concentrations in equal ratios, to enhance the EC in the external solution, had only slight effects on the uptake of cations in comparison with the effects of nutrient concentrations realised by different ratios in the external solution at an equal EC value. In this way it was shown that great differences can occur in the cation uptake by plants, while the cation concentrations in the external solution were equal. Therefore, with the interpretation of analytical data of cation concentrations in the external solution in substrate systems, beside the concentration at such, especially the relation to the other nutrient cations should be seriously taken into account. Guidelines for interpretation are discussed. INTRODUCTION In hydroponic and hydroponic related systems the composition of the nutrient solution in the root environment is regularly analysed to check the availability of plant nutrients. The small root volumes in charge and the high uptake of nutrients require specific knowledge with interpretation of the analytical data. The traditional models used for field grown crops are an insufficient basis for an interpretation. These models are developed for big root volumes and for the sub-optimal to optimal domain of nutrient supply, while in hydroponic systems small root volumes and mostly the optimal to luxurious domain of nutrient supply is in view. Because of the differences between the volumes, with equal analytical data huge differences occur in the quantities of nutrients available for field and substrate grown crops (Sonneveld, 1981) and because of the different domains of nutrient supply in view linear and curvilinear uptake models should 1 † Gerard Welles died on October 27 2004 when this paper was ready to publish. I have lost with him a valuable colleague and a good friend. Proc. IS on Soilless Cult. and Hydroponics Ed: M. Urrestarazu Gavilan Acta Hort. 697 ISHS 2005 378 be taken into account in relation to luxurious and sub-optimal nutrient supply (Sonneveld, 1991) respectively. With equal EC levels mostly a close relationship has been found between concentrations in the root environment and those in plant tissues (Adams, 2002; Maruo et al., 2004; Sonneveld and Voogt, 1985; Sonneveld and Voogt, 1986). However, this is not the case when the EC values differ, surely not when EC values differ by addition of ions with equal mutual ratios (Sonneveld and Voogt, 1990). Another factor that affects the nutrient uptake is the humidity. Differences in humidity will affect the uptake of nutrients at equal concentration in the root environment. At the Applied Plant Research, Division Glasshouse Horticulture at Naaldwijk a series of experiments was carried out in which effects of different EC values in the root environment and different climatic condition on the uptake of nutrients were studied. The results offer possibility to discuss the effects of these factors on the interpretation of analytical data in relation to the uptake of nutrients. In this paper data of cation uptake will be presented. MATERIALS AND METHODS Tomato, cucumber and sweet pepper were grown in a greenhouse in rock wool strips placed in gutters, furnished with a drip irrigation system with which the nutrient solution was circulated. The greenhouse consisted of different compartments covered by glass, equipped with a movable polythene thermal screen and a heating system. Nutrient solutions were adjusted to the crop in agreement with the recommendations to growers as published by Sonneveld and Straver, (1994). The primary water was rain water or desalinated water. The different EC values in the root environment varied roughly between 1.5 and 7 dS m and were realised by increasing all nutrients with equal ratios. The differences in climatic conditions were focussed on humidity, obtained by differences for ventilation, heating, screening and evaporation from open water basins. The temperature between the humidity treatments was kept constant in principle. The equipment to realise the climatic differences was used such that the crop development never seriously was hindered. In the present paper data of the driest and the most humid climate will be presented. The driest climate represented more or less the climatic conditions as usual in The Netherlands and therefore will be mentioned “standard”. The differences in humidity varied strongly by the daily weather conditions and the growing season of the crop. The average differences of vapour pressure deficits (vpd) realised under standard and humid conditions were in spring 0.73 and 0.53 kPa and in autumn 0.72 and 0.54 kPa for the standard and the humid climatic conditions respectively. Two crops per year were grown, one from December until June and the second from June until November, called spring crop and fall crop respectively. The circulating solution was sampled and analysed frequently to control EC, pH and nutrient concentrations. Tissue samples of young fully grown leaves and ripe fruits were gathered of all crops at proper times. With the fall crop of sweet pepper no fruit samples were available. For tomato and sweet pepper sometimes fruits were split in a proximal and a distal part, because of the great differences of nutrient concentrations, especially Ca, expected between such parts (Adams, 1990; Ehret and Ho, 1986). With cucumber also plant tops and old leaves were sampled, to compare the composition of those plant parts with those of young leaves. In the present publication the concentrations of cations in the plant tissues will be discussed in relation to the EC and the cation concentrations in the root environment and the different climatic conditions. Cation concentrations in the root environment are expressed as mmol l and the EC as dS m; the cation concentrations in the plant tissues are expressed as mmol kg dry matter. RESULTS The results of the tissue tests are listed in the Tables 1-6 for the spring and fall crops of tomato, cucumber and sweet pepper respectively.