EVALUATION OF SALT TOLERANCE IN ROOTSTOCKS OF CUCURBITACEAE

Research was carried out at the Experimental Farm of University of Tuscia to investigate the salt tolerance of different commercial rootstocks for watermelon in a closed soilless system (NFT). A split-plot design with three replicates was used to compare six concentrations of NaCl (main plot) × five genotypes (sub-plot) which included four commercial rootstocks widely used for watermelon crop and a watermelon cultivar. The salinity treatments were: 0, 20, 40, 60, 80 and 100 mM of NaCl. Genotypes were four commercial rootstocks of Lagenaria spp. [‘Emphasis’ (S&G) and ‘Macis’ (Nunhems)] and Cucurbita spp. [‘P360’ (SAIS) and ‘Polifemo’ (Esasem)] and a cultivar of watermelon [(Citrullus lanatus (thumb.)Mansf.cv ‘Star’ (Petoseed)]. Cucurbita rootstocks produced higher dry weight biomass of shoots and roots than those observed on Lagenaria spp. and watermelon cultivar. Moreover, increasing NaCl concentration decreased shoot dry weight of all genotypes and especially on watermelon cultivar. Finally, the higher salt tolerance observed in Cucurbita spp. and Lagenaria spp. compared to watermelon cultivar was associated to a lower Na uptake. INTRODUCTION Salinity is one of the major limiting factor of growth and yield of vegetable crops in Mediterranean regions. There are three major constraints for plant growth under saline conditions: water deficit, ion toxicity and nutrient imbalance. In some vegetable crops, the physiological damage caused by excessive accumulation of toxic ions (Na and Cl) in vegetative organs can be reduced by grafting plants on salt-tolerant rootstocks (Romero et al., 1997; Fernandez-Garcia et al., 2003). In addition Santa-Cruz et al. (2002) found that at moderate salt levels, grafting improved salt tolerance of tomato plants as a result of the higher capability of this plant to use higher concentration of ions proceeding from the substrate for osmotic adjustment. Starting from the above considerations, research activities were conducted at the Experimental Farm of University of Tuscia to investigate the salt tolerance of different commercial rootstocks for watermelon in a closed soilless system. MATERIAL AND METHODS The experiment was carried out in a greenhouse at Viterbo University Experimental Farm, Central Italy (12° 08' E, 42° 25' N, 310 m alt.). A split-plot design with three replicates was used to compare six concentrations of NaCl (main plot) × five genotypes (sub-plot) which included four commercial rootstocks widely used for watermelon crop and a watermelon cultivar. The salinity treatments were: 0, 20, 40, 60, 80 and 100 mM of NaCl. Genotypes were four commercial rootstocks of Lagenaria spp. [Emphasis (S&G) and Macis (Nunhems)] and Cucurbita spp. [P360 (SAIS) and Polifemo (Esasem)] and a commercial hybrid of watermelon [(Citrullus lanatus (thumb.)Mansf.cv ‘Star’ (Petoseed)]. Seeds of Cucurbitacee genotypes were sown at the beginning of October into rockwool cubes filled with vermiculite and the plantlets at the cotyledonal stage were placed on plastic channels (5 m long × 0.16 m wide, with a slope of 1.5% and oriented N-S) at a density of 25 rockwool cubes per m. In each plastic channel, the nutrient solution was continuously pumped from independent tank (50 L) at the elevated end of the channel and allowed to run slowly down the trough past the rockwool blocks, and the excess was drained back to the tank for later recirculation (NFT system). Plants were fertilized with the following complete nutrient solution (mg/L): NO3-N (196), NH4-N (14), S (64), P (31), K (234), Ca (160), Mg (48), Fe Proc. IS on Soilless Cult. and Hydroponics Ed: M. Urrestarazu Gavilán Acta Hort. 697 ISHS 2005 470 (0.6), Mn (0.5), Cu (0.02), Zn (0.05), B (0.5), Mo (0.01). The EC of the solution was 2.5 dS/m and a pH was 6.0. The saline nutrient solutions had the same basic composition plus an additional amount of NaCl according to the experimental treatments. The amount of water consumed by the plants was replenished automatically during the day. Moreover the EC and pH were adjusted daily in all the nutrient solution treatments by adding concentrated nutrient solution and acid mixture with the same anionic ratio of nutrient solution, respectively. To maintain the target NaCl concentration in the NaCl-enriched solution, the Na concentration was measured by inductively coupled plasma emission spectrophotometer (Karla, 1998) and adjusted twice weekly. Moreover, the nutrient solutions were replaced and renewed from all tanks fortnightly. To prevent large fluctuations in EC, pH and ionic concentrations in the intervals between the replacements of the nutrient solutions, a relatively high volume of nutrient solution per plantlets (1.5 L) was recirculating in all treatments. At the 4-7 leaf stage, plantlets were harvested and separated into aerial part (stems and leaves), and roots; plant organs were weighted and oven dried at 80C for dry matter determination. Sodium and Cl were determined on 2 g samples of aerial plant part and roots. Samples were digested in 70% HClO4 and concentrated HNO3 (1:2 v/v), for Na determination by atomic absorption. Chloride was determined by the mercuric thiocynate method, reading the absorbance at 455 nm with a spectrophotometer. Liner regression analysis was performed using the GraphPad Prism package (1999). RESULTS AND DISCUSSION In Figure 1 is the production of dry weight reported, from the aerial parts of the plantlets as a function of NaCl concentration in the nutrient solution. Increasing NaCl concentration decreased the shoot dry weight of all genotypes which ranged from 286 to 60 mg/plantlets for ‘Star’, 880 to 304 mg/plantlets for ‘Emphasis’, 942 to 251 for ‘Macis’, 1,270 to 546 mg/plantlets for ‘P360’; 910 to 461 mg/plantlets for ‘Polifemo’. Cucurbita genotypes exhibited the highest shoot dry weight due to a more vigorous plant development. Moreover, Lagenaria genotypes gave intermediate values, while the lowest shoot dry weight was observed on watermelon genotype. The relationship between NaCl concentration in the nutrient solution and relative shoot dry weight showed that the decremented rate of shoot dry weight (slope) was more pronounced on watermelon genotype while ‘Emphasis’, ‘P360’ and ‘Polifemo’ were more tolerant to NaCl concentration. Root:shoot dry weight increased with NaCl concentration (data not shown) especially in watermelon genotype. Concentrations of Na in shoots and roots increased in response to increased salinity especially in watermelon genotype (Fig. 2). In all genotypes, Na concentration was higher in roots than in shoots confirming that watermelon, Lagenaria spp. and Cucurbita spp. can be considered as “excluder plants” in line with the results reported by Graifenberg et al. (1996) on zucchini squash. In all genotypes, Cl concentrations of shoot and root strongly increased with NaCl applications (Fig. 3). The low shoot Na concentration found in the commercial rootstocks may represent a positive attribute since increasing of photosynthesis has been linked to the low flux of Na into the xylem (Fernandez-Garcia et al., 2003). The concentration of Cl in shoots and roots were similar suggesting that the genotypes tested were unable to exclude Cl ions from the shoots presenting a different behavior in comparison with Na. The results confirm the potential of Cucurbita and Lagenaria genotypes as rootstocks to improve the salt tolerance of watermelon since they decreases Na uptake as reported for tomatoes by Fernandez-Garcia et al. (2003). However, before proceeding to the selection of rootstocks it is necessary to take into account the salt tolerance mechanism of the shoot genotype as reported by Santa-Cruz et al. (2002). CONCLUSIONS Results showed that Cucurbita spp. produced higher shoot and root biomass than Lagenaria rootstocks and watermelon genotype. Moreover, increasing NaCl concentration decreased shoot biomass of all genotypes and especially on watermelon genotype. Results on Na uptake showed that rootstocks of Cucurbita spp. and Lagenaria spp. have the potential to reduce the Na uptake improving the salt tolerance of scion. Literature Cited Fernandez-Garcia, N. , Cerda, A. and Carvajal, M. 2003. Grafting a useful technique for