M. T. Estañ

Agronomical and physiological characterization of salinity tolerance in a commercial tomato hybrid

The salt tolerance of the commercial F1 tomato hybrid (Lycopersicon esculentum Mill) Radja (GC-793) has been agronomically and physiologically evaluated under greenhouse conditions, using a control (nutrient solution), a moderate (70 mM NaCl added to the nutrient solution) and a high salt level (140 mM NaCl), applied for 130 days. The results show that Radja is a Na+-excluder genotype, tolerant to moderate salinity. Fruit yield was reduced by 16% and 60% and the shoot biomass by 30% and more than 75% under moderate and high salinities, respectively. At 90 days of salt treatment (DST), the mature leaves feeding the 4th truss at fruiting accumulated little Na+ (178 mmol kg-1 DW). At this time, the sucrose concentration in these leaves even increased with moderate salinity and the amino acid proline was not accumulated under salt conditions as compared to control. At 130 DST, Na+ was accumulated mainly by the roots in proportion to the salt level applied, while in leaves appreciable amounts were found only at high salinity (452 mmol kg-1 DW). In the leaves, Cl- was always accumulated in proportion to the salt level and in a very much greater amounts than Na+ (until 1640 mmol kg-1 DW). The sucrose content was reduced in all plants by salinity, and was distributed preferentially toward the distal stem and peduncle of a truss at fruiting under moderate salinity, and toward the basal stem and root at high salinity. Moreover, proline was accumulated in different organs of the plant only at high salinity, coinciding with Na+ accumulation in leaves. Attempts are made to find a clear relationship between physiological behaviour triggered by stress and the agronomical behaviour, in order to assess the validity of physiological traits used for salt-tolerance selection and breeding in tomato.

Response of tomato cultivars to salinity

The responses of five tomato cultivars (L. esculentum Mill) of different degrees of salt tolerance were examined over a range of 0 to 140 mM NaCl applied for 3 and 10 weeks. Judged by both Na and Cl accumulations and maintenance of K, Ca and Mg contents with increasing salinity, the most tolerant cultivars (Pera and GC-72) showed different responses. The greater salt tolerance of cv Pera was associated with a higher Cl and Na accumulation and a lower K content in the shoot than those found in the other cultivars, typical of a halophytic response to salinity. However, the greater salt tolerance of cv GC-72 was associated with a retention of Na and Cl in the root, restriction of their translocation to the shoot and maintenance of potassium selectivity under saline conditions. The salt tolerance mechanisms that operated in the remaining cultivars were similar to that of cv GC-72, as at first they excluded Na and Cl from the shoots, accumulating them in the roots; with longer treatment, the ability to regulate Na and Cl concentrations in the plant was lost only in the most salt sensitive cultivar (Volgogradskij), resulting in a massive influx of both ions into the shoot.The salt sensitivity of some tomato cultivars to salinity could be due to both the toxic effect of Na and Cl ions and nutritional imbalance induced by salinity, as plant growth was inversely correlated with Na and Cl contents and directly correlated with K and Ca contents. This study displays that there is not a single salt tolerance mechanism, since different physiological responses among tomato cultivars have been found.

Salinity tolerance of normal-fruited and cherry tomato cultivars

The salinity tolerances (NaCl) of 8 normal-fruited tomato cultivars (Lycopersicon esculentum Mill.) and 4 cherry tomato cultivars (L. esculentum var.cerasiforme) were determined by yield-substrate EC response curves, according to the Mass-Hoffman model, modified by van Genuchten and Hoffman (1984). The same model was used to determine the response curves of leaf dry-weight, stem dry-weight, and plant height against substrate EC and also between yield and leaf concentrations of Cl- and Na ions.According to the salinity-threshold (maximum EC-value without yield reduction) and slope (yield decrease per unit EC increase) parameters, determined from the yield-EC response curves, the cherry tomato cultivars were more salt-tolerant than the normal-fruited ones. However, on the basis of vegetative growth characters-EC response curves, cherry tomato cultivars and normal-fruited ones were similarly affected by NaCl.The ranking of the cultivars by their salinity tolerance, determined from the plots of yield vs. leaf concentrations of Cl- and Na ions, was the same as that evaluated from the yield vs. substrate EC plots.

Effects of salinity on nitrate, total nitrogen, soluble protein and free amino add levels in tomato plants

The effects of salinity on nitrogen compounds were studied in three tomato (L. esc- ulentum Mill.) genotypes of different salt tolerance. The plants were grown under controlled conditions, and the salt treatments (0, 70 and 140 raM NaCl) were applied for three (Harvest 1) and ten (Harvest 2) weeks. The effects of salinity on total N and particularly N03 concentrations depended partly on the NaCl level and duration of the stress, but mainly on the different degrees of salt tolerance of the genotypes. In Harvest 1, the most tolerant genotype (GC-72) showed the highest N03 increase in the roots and no decreases in stem and leaf with increasing salinity; the intermediate-tolerant genotype (P-73) showed a similar response to that of GC-72 only at 70 mM NaCl. However, the most sensitive genotype (Volgogradskij) showed the greatest reductions in stem and leaf N03 concentrations with salinity. With longer durations of stress the different responses between the more tolerant genotypes were less evident; only Volgo...

Relationship between tomato fruit growth and fruit osmotic potential under salinity.

To investigate the relationship between fruit growth and fruit osmotic potential (Psi(s)) in salty conditions, a sensitive tomato cultivar (Lycopersicon esculentum Mill.) and a tolerant accession of the wild species Lycopersicon pimpinellifolium Mill. were grown in a greenhouse with 0 and 70 mM NaCl, and the growth of the fruit studied from 15 to 70 days after anthesis (DAA). L. pimpinellifolium did not reduce significantly fruit weight in salty conditions throughout the growth period, whereas L. esculentum fruit weights decreased significantly with salinity from 45 DAA. L. esculentum fruit fresh weight reductions resulted from both less dry matter and water accumulation, although the fruit water content was affected by salinity before the fruit weight. In both species, fruit osmotic potential (Psi(s)) decreased significantly with salinity during the rapid fruit growth phase, although the changes were different. Thus, fruits from L. pimpinellifolium salt treated plants showed a Psi(s) reduction at the beginning (15 DAA) twice as high as that found in L. esculentum. As the advanced growth stage (from 15 to 55 DAA), the Psi(s) reduction percentages induced by salinity were quite similar in L. pimpinellifolium fruits, while increased in L. esculentum. Under saline conditions, the solutes contributing to reduce the fruit Psi(s) during the first 55 DAA were the inorganic solutes in both species, while in the ripe fruits they were hexoses. L. esculentum fruits accumulated K(+) as the main osmoticum in salty conditions, while L. pimpinellifolium fruits were able to use not only K(+) but also the Na(+) provided by the salt.

NaCl pre-treatment at the seedling stage enhances fruit yield of tomato plants irrigated with salt water

Although salt-adaptation seems to be a widespread property of plants, the adaptive response has been rarely differentiated to the tolerance response. We report on the adaptive response of tomato plants to growing under saline conditions following a 15 day pre-treatment with a lower NaCl concentration (half) than that used during the plant growth. After 20 days of salt treatment (100 mM NaCl), the biomass of the adapted plants increased significantly with respect to that of the unadapted plants when the pre-treatment was applied to five leaf seedlings, but not at the two leaf stage. The long-term adaptive response was determined in two tomato genotypes with different tolerance to moderate salt levels. At 70 mM NaCl, the adapted-plants of the more salt-sensitive genotype produced up to 29% more fruit yield than did the unadapted plants. However, no positive effect was observed to long-term in the adapted-plants of the more salt-tolerant genotype, which suggests that the stress level necessary to trigger the adaptive response is related to the tolerance degree of genotype. The physiological response of the plants showing a positive response to the adaptation was also modified to long-term. Thus, K+ concentrations increased in the young leaves of the adapted plants, with respect to unadapted plants, and moreover these differences increased with the salinization period. These results indicate that the changes in growth and physiological responses induced by NaCl pre-treatment at the seedling stage are maintained throughout plant life cycle and this is, therefore, an interesting strategy for increasing the salt tolerance in tomato plants.