Effects of silicon and salinity on fruit yield and quality of tomato grown hydroponically

The effects of silicon added to the nutrient solution either at a standard electrical conductivity (EC) of 2.2 dS m or at an increased EC of 4.8 dS m on yield, nutritional status and fruit quality were investigated in a tomato crop grown in a closed hydroponic system. Si was added in form of a water-soluble potassium silicate compound at a reference concentration of 2.25 mM. The EC was raised either by NaCl or by extra addition of K, Ca, and Mg salts of nitrates and sulfates at rates resulting in the same K:Ca:Mg and NO3:SO4 ratios and NH4, P, and micronutrient concentrations in all treatments. The average EC values in the drainage water were 3.7 and 3.4 dS m in the low EC treatments without and with Si supply, respectively, and 5.8, 5.7 and 5.8 dS m in the salinity treatments involving NaCl addition, NaCl and Si addition, and extra nutrients combined with Si addition, respectively. The increase of the EC up to 4.8 dS m by adding NaCl had no significant influence on the fruit yield of tomato, when Si was added to the saline nutrient solution. In contrast, the fruit yield per plant was significantly restricted at this level of salinity, when no Si was added to the NaCl-enriched nutrient solution, or when Si was included but salinity was induced by extra addition of major nutrients. In both cases, the yield depressions were exclusively due to a lower mean fruit weight. The β-carotene and lycopene contents of fruit were significantly increased by Si and nutrient-induced salinity. Both Si and EC enhanced the fruit firmness and the contents of total solid solutes and vitamin C in the tomato fruit. Moreover, the addition of Si significantly restricted the occurrence of blossom-end rot in tomato fruit when the plants were not exposed to salinity. INTRODUCTION Although silicon is not considered an essential element for plant nutrition, many authors report on beneficial effects when its supply to various cultivated plants is enhanced. In most cases, the favorable effects of Si on crop plants seem to originate from reinforcement of the cell walls due to deposition of Si in form of amorphous silica (SiO2nH2O) and opal phytoliths (Inanaga and Okasaka, 1995; Epstein, 1999). For instance, Si increases the thickness and erectness of rice plants (Yoshida et al., 1969) and the roughness of wheat leaves and awns (Rafi and Epstein, 1997) thus improving light reception, which results in enhanced yield (Mitsui and Takatoh, 1963). Moreover, the mechanical strength provided by Si to the plant tissues increases their resistance to several bacterial, fungi and insect diseases (Adatia and Besford, 1986; Menzies et al., 1991; Epstein, 1999) and decreases the occurrence of the physiological disorder bent neck in gerbera (Savvas et al., 2002). In other reports, Si was implicated to ameliorate the adverse effects of aluminium toxicity (Barcelo et al., 1993; Hammond et al., 1995), manganese toxicity (Horst and Marschner, 1978; Iwasaki et al., 2002), and salinity (Bradbury and Ahmad, 1990; Liang et al., 1996). In commercial hydroponics, plants are grown on inert substrates and, therefore, their supply with Si depends mainly on the silicon concentration in the raw water used to prepare nutrient solution. Thus, if the Si concentration in the raw water is low, some plant species may benefit from extra addition of Si to the nutrient solution. On the other hand, A. Stamatakis, N. Papadantonakis, N. Lydakis-Simantiris and P. Kefalas Mediterranean Agronomic Institute of Chania, Chania, Greece D. Savvas T.E. I. of Epirus Department of Floriculture and Landscape Architecture, Arta, Greece Proc. IS on Greenhouse Salinity Eds: A. Pardossi et al. Acta Hort 609, ISHS 2003 142 too high salt concentrations in the available irrigation water, and hence exposure of greenhouse crops to salinity, is a frequent problem worldwide (Sonneveld, 2000). Based on the above mentioned reports regarding the interactions between Si and salt, it seems likely that the supply of Si via the nutrient solution might alleviate the adverse effects of salinity on some soilless grown crop species. The objective of this study was to test whether tomato grown hydroponically either at a standard or at an increased salinity level responds positively to the supply of Si via the nutrient solution. MATERIALS AND METHODS Tomato plants were grown in bags filled with perlite in an unheated glasshouse at the Mediterranean Agronomic Institute of Chania. The plants were supplied with nutrient solution having a standard (2.2 dS m) or a high (4.8 dS m) total salt concentration and a low (0.2 mM) or a high (2.25 mM) reference Si concentration. When no Si was applied, the increase of the electrical conductivity (EC) to 4.8 dS m was achieved by adding appropriate amounts of NaCl to the standard nutrient solution. However, when Si was also included, two salinity treatments were established by injecting either NaCl or extra salts of K, Ca and Mg with sulfates and nitrates at the same K:Ca:Mg and NO3:SO4 ratios as in the standard EC treatments. The concentration of 0.2 mM Si originated exclusively from the raw water, while the increase to 2.25 mM Si was attained by adding a liquid potassium silicate compound (Sonneveld, 2000) from a separate stock solution tank. The five nutrient solution treatments were applied by means of a computer-controlled system as described in a previous paper (Savvas and Adamidis, 1999). At each irrigation, the entire volume of drainage water collected after the previous nutrient solution application was automatically recycled according to Savvas and Manos (1999). The target mean ionic concentrations in each nutrient solution treatment, which were attained by replenishing the reused drainage with nutrients according to the concept of a reference composition (Savvas, 2002), are given in Table 1. The pH of the supplied nutrient solution was set at 5.6 in all treatments. The plants were planted on November 1, 2001 and arranged into twenty experimental units at a plant density of 2.4 plants per m. Thus, there were four replications of each treatment with 16 experimental plants per replication. Harvesting of commercially ripe fruits was commenced on 21 March and terminated on 13 July 2002. During the whole growing period, the harvested fruits from all treatments were counted, weighed and graded. To determine the influence of Si and salinity on the uptake of nutrients, 4 young, fully expanded leaves were sampled from each experimental unit on May, 22. The leaves were collected immediately above the trust bearing the most recently set fruits. Moreover, on June 12, samples of fruits at commercial ripening were collected from each experimental unit. The above samples were dried at 65 C to constant weight, ground, and used to determine the fruit firmness and the K, Ca, Mg, P, Na, and Cl concentrations. Moreover, a sub-sample was obtained from each fruit sample, which was used for the determination of β-carotene, lycopene, lutein, vitamin C and total solute solids contents. The concentrations of K, Ca, Mg, P, and Na from the plant tissue samples were measured by inductively-coupled plasma spectroscopy (Isaac and Johnson, 1998) using a PS 1000 AT, Leeman Labs Inc spectrometer, after dry ashing at 550C for 5 h and extraction with 2 N HCl (Miller, 1998). Leaf Cl concentrations were measured by an ion specific electrode after a water extraction of the dried and ground material (Liu, 1998). The extraction of β-carotene, lycopene and lutein was performed by weighing 5 g of homogenized tomato fruit tissue, adding 2 g of silica gel 60 and 10 ml methanol, concentrating the mixture to dryness on a rotary evaporator, then adding 10 ml acetone plus 10 ml tetrahydrofuran Z.A. , treating it with ultra sound, concentrating it again to dryness, adding ethyl acetate and distilled water to separate the pigments from the sugars, using sodium sulfate to dry over the organic phase, concentrating the mixture to dryness on a rotary evaporator, and taking up the residue using 10 ml ethyl acetate. Subsequently, the concentrations of β-carotene, lycopene and lutein were determined by means of HPLC