Salt-tolerant alternative crops as sources of quality food to mitigate the negative impact of salinity on agricultural production.

An increase of abiotic stress tolerance and nutritive value of foods is currently a priority because of climate change and rising world population. Among abiotic stresses, salt stress is one of the main problems in agriculture. Mounting urbanization and industrialization, and increasing global food demand, are pressing farmers to make use of marginal lands affected by salinity and low-quality saline water. In that situation, one of the most promising approaches is searching for new sources of genetic variation like salt-tolerant alternative crops or underexploited crops. They are generally less efficient than cultivated crops in optimal conditions due to lower yield but represent an alternative in stressful growth conditions. In this review, we summarize the advances achieved in research on underexploited species differing in their genetic nature. First, we highlight advances in research on salt tolerance of traditional varieties of tomato or landraces; varieties selected and developed by smallholder farmers for adaptation to their local environments showing specific attractive fruit quality traits. We remark advances attained in screening a collection of tomato traditional varieties gathered in Spanish Southeast, a very productive region which environment is extremely stressing. Second, we explore the opportunities of exploiting the natural variation of halophytes, in particular quinoa and amaranth. The adaptation of both species in stressful growth conditions is becoming an increasingly important issue, especially for their cultivation in arid and semiarid areas prone to be affected by salinity. Here we present a project developed in Spanish Southeast, where quinoa and amaranth varieties are being adapted for their culture under abiotic stress targeting high quality grain.

Unraveling the Strategies Used by the Underexploited Amaranth Species to Confront Salt Stress: Similarities and Differences With Quinoa Species.

Yield losses due to cultivation in saline soils is a common problem all over the world as most crop plants are glycophytes and, hence, susceptible to salt stress. The use of halophytic crops could be an interesting alternative to cope with this issue. The Amaranthaceae family comprises by far the highest proportion of salt-tolerant halophytic species. Amaranth and quinoa belong to this family, and their seeds used as pseudo-cereal grains have received much attention in recent years because of their exceptional nutritional value. While advances in the knowledge of salt tolerance mechanisms of quinoa have been remarkable in recent years, much less attention was received by amaranth, despite evidences pointing to amaranth as a promising species to be grown under salinity. In order to advance in the understanding of strategies used by amaranth to confront salt stress, we studied the comparative responses of amaranth and quinoa to salinity (100 mM NaCl) at the physiological, anatomical, and molecular levels. Amaranth was able to exhibit salt tolerance throughout its life cycle, since grain production was not affected by the saline conditions applied. The high salt tolerance of amaranth is associated with a low basal stomatal conductance due to a low number of stomata (stomatal density) and degree of stomata aperture (in adaxial surface) of leaves, which contributes to avoid leaf water loss under salt stress in a more efficient way than in quinoa. With respect to Na+ homeostasis, amaranth showed a pattern of Na+ distribution throughout the plant similar to glycophytes, with the highest accumulation found in the roots, followed by the stem and the lowest one detected in the leaves. Contrarily, quinoa exhibited a Na+ includer character with the highest accumulation detected in the shoots. Expression levels of main genes involved in Na+ homeostasis (SOS1, HKT1s, and NHX1) showed different patterns between amaranth and quinoa, with a marked higher basal expression in amaranth roots. These results highlight the important differences in the physiological and molecular responses of amaranth and quinoa when confronted with salinity.

Traditional Tomato Varieties Improve Fruit Quality Without Affecting Fruit Yield Under Moderate Salt Stress.

Identification of tomato varieties able to exhibit higher accumulation of primary and secondary metabolites in their fruits is currently a main objective in tomato breeding. One tool to improve fruit quality is to cultivate the plants under salt stress, although improvement of fruit quality is generally accompanied by productivity losses. However, it is very interesting to implement strategies aiming at enhancing fruit quality of tomato by means of growing plants in moderate salt stress that allows for a sustainable fruit yield. The traditional tomato varieties adapted to the Mediterranean environmental constraints may be very attractive plant materials to achieve this goal, given the wide range of fruit quality traits because of their genetic diversity. Here, agronomic responses and fruit quality traits, including primary and secondary metabolites, were analyzed in fruits of two Mediterranean traditional tomato varieties named "Tomate Pimiento" ("TP") and "Muchamiel Aperado" ("MA") because of the pepper and pear shape of their fruits, using as reference the commercial cultivar "Moneymaker" ("MM"). Plants were grown without salt (control) and with moderate salt stress (50 mM NaCl), which did not affect fruit yield in any variety. "TP" is of great interest because of its high soluble solids content (SSC) in control, which is even higher in salt, whereas "MA" is very attractive because of its high Brix yield index (SSC × fruit yield), used as overall fruit quality measure. Similitude between both traditional varieties were found for primary metabolism, as they significantly increased sucrose contents compared with "MM" in red ripe fruits from plants in control and, especially, salt stress conditions. The most remarkable difference was the high constitutive levels of total amino acids in "TP" fruits, including the three major free amino acids found in tomato fruit, GABA, glutamate, and glutamine, which even increased under salinity. Regarding secondary metabolites, the most interesting change induced by salinity was the increase in α-tocopherol found in red ripe fruits of both "TP" and "MA." These results reveal the interest of traditional varieties as sources of genetic variation in breeding because of their improvement of tomato fruit quality without production losses under moderate salt stress.

The Salt Sensitivity Induced by Disruption of Cell Wall-Associated Kinase 1 (SlWAK1) Tomato Gene Is Linked to Altered Osmotic and Metabolic Homeostasis.

Tomato cell wall-associated kinase 1 (SlWAK1) has only been studied in biotic stress response and hence its function in abiotic stress remains unknown. In a screening under salinity of an insertional mutant collection of tomato (Solanum lycopersicum L.), a mutant exhibiting lower degree of leaf chlorosis than wild type (WT) together with reduced leaf Na+ accumulation was selected. Genetic analysis of the mutation revealed that a single T-DNA insertion in the SlWAK1 gene was responsible of the mutant phenotype. Slwak1 null mutant reduced its shoot growth compared with WT, despite its improved Na+ homeostasis. SlWAK1 disruption affected osmotic homeostasis, as leaf water content was lower in mutant than in WT under salt stress. In addition, Slwak1 altered the source-sink balance under salinity, by increasing sucrose content in roots. Finally, a significant fruit yield reduction was found in Slwak1 vs. WT under long-term salt stress, mainly due to lower fruit weight. Our results show that disruption of SlWAK1 induces a higher sucrose transport from source leaf to sink root, negatively affecting fruit, the main sink at adult stage.

The res (restored cell structure by salinity) tomato mutant reveals the role of the DEAD-box RNA helicase SlDEAD39 in plant development and salt response.

Increasing evidences highlight the importance of DEAD-box RNA helicases in plant development and stress responses. In a previous study, we characterized the tomato res mutant (restored cell structure by salinity), showing chlorosis and development alterations that reverted under salt-stress conditions. Map-based cloning demonstrates that RES gene encodes SlDEAD39, a chloroplast-targeted DEAD-box RNA helicase. Constitutive expression of SlDEAD39 complements the res mutation, while the silencing lines had a similar phenotype than res mutant, which is also reverted under salinity. Functional analysis of res mutant proved SlDEAD39 is involved in the in vivo processing of the chloroplast, 23S rRNA, at the hidden break-B site, a feature also supported by in vitro binding experiments of the protein. In addition, our results show that other genes coding for chloroplast-targeted DEAD-box proteins are induced by salt-stress, which might explain the rescue of the res mutant phenotype. Interestingly, salinity restored the phenotype of res adult plants by increasing their sugar content and fruit yield. Together, these results propose an unprecedented role of a DEAD-box RNA helicase in regulating plant development and stress response through the proper ribosome and chloroplast functioning, which, in turn, represents a potential target to improve salt tolerance in tomato crops.

Recovering Tomato Landraces to Simultaneously Improve Fruit Yield and Nutritional Quality Against Salt Stress.

Salt stress generally induces important negative effects on tomato (Solanum lycopersicum) productivity but it may also cause a positive effect improving fruit quality, one of the greatest challenges in nowadays agriculture. Because of the genetic erosion of this horticultural species, the recovery of locally adapted landraces could play a very important role in avoiding, at least partially, production losses and simultaneously improving fruit quality. Two tomato landraces endemic of the Spanish Southeast area, characterized by the harsh climatic conditions of the Mediterranean basin, have been selected: Negro Yeste (NY) characterized by its dark-red colored fruits and Verdal (V), which fruits did not achieve the characteristic red color at ripening. Here the agronomic, physiological, and metabolic responses of these landraces were compared with the reference tomato commercial cv. Moneymaker (MM), in plants grown without salt (control) and with salt stress (100 mM NaCl) for 70 days. The higher salt tolerance of both landraces was mainly reflected in the fruit number, as NY only reduced the fruit number in salt stress by 20% whereas in MM it was reduced till 43%, and in V the fruit number even showed an increase of 33% with salt stress. An important fruit quality parameter is soluble solids content, which increases induced by salinity were significantly higher in both landraces (60 and 78% in NY and V, respectively) compared with MM (34%). Although both landraces showed a similar response in relation to the high chlorophyll accumulation detected in their fruits, the fruit metabolic profiles were very different. Increased carotenoids levels were found in NY fruits, especially lycopene in ripe fruit, and this characteristic was observed in both control and salt stress. Contrarily, the carotenoid biosynthesis pathway was disrupted in V ripe fruits, but other metabolites, such as Ca2+, mannose, formate, and glutamate were accumulated. These results highlight the potential of tomato landraces to improve nutritional fruit quality and maintain fruit yield stability under salt stress.

Identification of key genes involved in the phenotypic alterations of res (restored cell structure by salinity) tomato mutant and its recovery induced by salt stress through transcriptomic analysis.

BACKGROUND: The res (restored cell structure by salinity) mutant, recently identified as the first tomato mutant accumulating jasmonate in roots under non-stressful conditions, exhibits a remarkable growth inhibition and morphological alterations in roots and leaves, which are suppressed when the mutant plants are exposed to salinity. In order to understand the molecular basis of the phenotype recovery induced by salt stress in the res mutant, we carried out a comparative transcriptomic analysis in roots and leaves of wild-type and res plants in absence of stress (control) and when the phenotypic recovery of res mutant began to be observed upon salt stress (5 days of 200 mM NaCl). RESULTS: The number of differentially expressed genes was three times greater in roots than in leaves of res vs WT plants grown in control, and included the down-regulation of growth-promoting genes and the up-regulation of genes involved in Ca2+ signalling, transcription factors and others related to stress responses. However, these expression differences were attenuated under salt stress, coinciding with the phenotypic normalisation of the mutant. Contrarily to the attenuated response observed in roots, an enhanced response was found in leaves under salt stress. This included drastic expression changes in several circadian clock genes, such as GIGANTEA1, which was down-regulated in res vs WT plants. Moreover, the higher photosynthetic efficiency of res leaves under salt stress was accompanied by specific salt-upregulation of the genes RUBISCO ACTIVASE1 and ALTERNATIVE OXIDASE1A. Very few genes were found to be differentially expressed in both tissues (root and leaf) and conditions (control and salt), but this group included SlWRKY39 and SlMYB14 transcription factors, as well as genes related to protein homeostasis, especially protease inhibitors such as METALLOCARBOXYPEPTIDASE INHIBITOR, which also seem to play a role in the phenotype recovery and salt tolerance of res mutant. CONCLUSIONS: In summary, in this study we have identified genes which seem to have a prominent role in salt tolerance. Moreover, we think this work could contribute to future breeding of tomato crops with increased stress tolerance.

The drought-tolerant Solanum pennellii regulates leaf water loss and induces genes involved in amino acid and ethylene/jasmonate metabolism under dehydration.

Breeding for drought-tolerant crops is a pressing issue due to the increasing frequency and duration of droughts caused by climate change. Although important sources of variation for drought tolerance exist in wild relatives, the mechanisms and the key genes controlling tolerance in tomato are little known. The aim of this study is to determine the drought response of the tomato wild relative Solanum pennellii (Sp) compared with the cultivated tomato Solanum lycopersicum (Sl). The paper investigates the physiological and molecular responses in leaves of Sp and Sl plants without stress and moderate drought stress. Significant physiological differences between species were found, with Sp leaves showing greater ability to avoid water loss and oxidative damage. Leaf transcriptomic analysis carried out when leaves did not as yet show visual dehydration symptoms revealed important constitutive expression differences between Sp and Sl species. Genes linked to different physiological and metabolic processes were induced by drought in Sp, especially those involved in N assimilation, GOGAT/GS cycle and GABA-shunt. Up-regulation in Sp of genes linked to JA/ET biosynthesis and signaling pathways was also observed. In sum, genes involved in the amino acid metabolism together with genes linked to ET/JA seem to be key actors in the drought tolerance of the wild tomato species.

The SlCBL10 Calcineurin B-Like Protein Ensures Plant Growth under Salt Stress by Regulating Na+ and Ca2+ Homeostasis.

Characterization of a new tomato (Solanum lycopersicum) T-DNA mutant allowed for the isolation of the CALCINEURIN B-LIKE PROTEIN 10 (SlCBL10) gene whose lack of function was responsible for the severe alterations observed in the shoot apex and reproductive organs under salinity conditions. Physiological studies proved that SlCBL10 gene is required to maintain a proper low Na+/Ca2+ ratio in growing tissues allowing tomato growth under salt stress. Expression analysis of the main responsible genes for Na+ compartmentalization (i.e. Na+/H+ EXCHANGERs, SALT OVERLY SENSITIVE, HIGH-AFFINITY K+ TRANSPORTER 1;2, H+-pyrophosphatase AVP1 [SlAVP1] and V-ATPase [SlVHA-A1]) supported a reduced capacity to accumulate Na+ in Slcbl10 mutant leaves, which resulted in a lower uploading of Na+ from xylem, allowing the toxic ion to reach apex and flowers. Likewise, the tomato CATION EXCHANGER 1 and TWO-PORE CHANNEL 1 (SlTPC1), key genes for Ca2+ fluxes to the vacuole, showed abnormal expression in Slcbl10 plants indicating an impaired Ca2+ release from vacuole. Additionally, complementation assay revealed that SlCBL10 is a true ortholog of the Arabidopsis (Arabidopsis thaliana) CBL10 gene, supporting that the essential function of CBL10 is conserved in Arabidopsis and tomato. Together, the findings obtained in this study provide new insights into the function of SlCBL10 in salt stress tolerance. Thus, it is proposed that SlCBL10 mediates salt tolerance by regulating Na+ and Ca2+ fluxes in the vacuole, cooperating with the vacuolar cation channel SlTPC1 and the two vacuolar H+-pumps, SlAVP1 and SlVHA-A1, which in turn are revealed as potential targets of SlCBL10.

The phenotype alterations showed by the res tomato mutant disappear when the plants are grown under semi-arid conditions: Is the res mutant tolerant to multiple stresses?

The res (restored cell structure by salinity) mutant, recently identified as the first tomato mutant accumulating jasmonate (JA) without stress, exhibited important morphological alterations when plants were grown under control conditions but these disappeared under salt stress. Since the defense responses against stresses are activated in the res mutant as a consequence of the increased expression of genes from the JA biosynthetic and signaling pathways, the mutant may display a tolerance response not only to salt stress but also to multiple stresses. Here, we show that when res mutant plants are grown under the summer natural conditions of the Mediterranean area, with high temperatures and low relative humidity, the characteristic leaf chlorosis exhibited by the mutant disappears and leaves become dark green over time, with a similar aspect to WT leaves. Moreover, the mutant plants are able to achieve chlorophyll and fluorescence levels similar to those of WT. These results hint that research on res tomato mutant may allow very significant advances in the knowledge of defense responses activated by JA against multiple stresses.

The tomato mutant ars1 (altered response to salt stress 1) identifies an R1-type MYB transcription factor involved in stomatal closure under salt acclimation.

A screening under salt stress conditions of a T-DNA mutant collection of tomato (Solanum lycopersicum L.) led to the identification of the altered response to salt stress 1 (ars1) mutant, which showed a salt-sensitive phenotype. Genetic analysis of the ars1 mutation revealed that a single T-DNA insertion in the ARS1 gene was responsible of the mutant phenotype. ARS1 coded for an R1-MYB type transcription factor and its expression was induced by salinity in leaves. The mutant reduced fruit yield under salt acclimation while in the absence of stress the disruption of ARS1 did not affect this agronomic trait. The stomatal behaviour of ars1 mutant leaves induced higher Na(+) accumulation via the transpiration stream, as the decreases of stomatal conductance and transpiration rate induced by salt stress were markedly lower in the mutant plants. Moreover, the mutation affected stomatal closure in a response mediated by abscisic acid (ABA). The characterization of tomato transgenic lines silencing and overexpressing ARS1 corroborates the role of the gene in regulating the water loss via transpiration under salinity. Together, our results show that ARS1 tomato gene contributes to reduce transpirational water loss under salt stress. Finally, this gene could be interesting for tomato molecular breeding, because its manipulation could lead to improved stress tolerance without yield penalty under optimal culture conditions.

The tomato res mutant which accumulates JA in roots in non-stressed conditions restores cell structure alterations under salinity.

Jasmonic acid (JA) regulates a wide spectrum of plant biological processes, from plant development to stress defense responses. The role of JA in plant response to salt stress is scarcely known, and even less known is the specific response in root, the main plant organ responsible for ionic uptake and transport to the shoot. Here we report the characterization of the first tomato (Solanum lycopersicum) mutant, named res (restored cell structure by salinity), that accumulates JA in roots prior to exposure to stress. The res tomato mutant presented remarkable growth inhibition and displayed important morphological alterations and cellular disorganization in roots and leaves under control conditions, while these alterations disappeared when the res mutant plants were grown under salt stress. Reciprocal grafting between res and wild type (WT) (tomato cv. Moneymaker) indicated that the main organ responsible for the development of alterations was the root. The JA-signaling pathway is activated in res roots prior to stress, with transcripts levels being even higher in control condition than in salinity. Future studies on this mutant will provide significant advances in the knowledge of JA role in root in salt-stress tolerance response, as well as in the energy trade-off between plant growth and response to stress.

Heterologous expression of the yeast HAL5 gene in tomato enhances salt tolerance by reducing shoot Na+ accumulation in the long term.

For salt tolerance to be achieved in the long-term plants must regulate Na(+)/K(+) homeostasis over time. In this study, we show that the salt tolerance induced by overexpression of the yeast HAL5 gene in tomato (Solanum lycopersicum) was related to a lower leaf Na(+) accumulation in the long term, by reducing Na(+) transport from root to shoot over time regardless of the severity of salt stress. Furthermore, maintaining Na(+)/K(+) homeostasis over time was associated with changes in the transcript levels of the Na(+) and K(+) transporters such as SlHKT1;2 and SlHAK5. The expression of SlHKT1;2 was upregulated in response to salinity in roots of transgenic plants but downregulated in the roots of wild-type (WT) plants, which seems to be related to the lower Na(+) transport rate from root to shoot in transgenic plants. The expression of the SlHAK5 increased in roots and leaves of both WT and transgenic plants under salinity. However, this increase was much higher in the leaves of transgenic plants than in those of WT plants, which may be associated with the ability of transgenic leaves to maintain Na(+)/K(+) homeostasis over time. Taken together, the results show that the salt tolerance mechanism induced by HAL5 overexpression in tomato is related to the appropriate regulation of ion transport from root to shoot and maintenance of the leaf Na(+)/K(+) homeostasis through modulation of SlHKT1 and SlHAK5 over time.

Understanding the mechanisms of chilling injury in bell pepper fruits using the proteomic approach.

In order to advance in the understanding of CI in pepper fruits, the cell ultrastructure alterations induced by CI and the physiological and metabolic changes have been studied along with the proteomic study. When stored at low temperatures bell pepper (Capsicum annuum) fruits exhibited visual CI symptoms and important alterations within the cell ultrastructure, since peroxisomes and starch grains were not detected and the structure of the chloroplast was seriously damaged in chilled tissues. Physiological and metabolic disorders were also observed in chilled fruits, such as higher ethylene production, increased MDA content, changes in sugar and organic acids and enzymatic activities. The comparative proteomic analysis between control and chilled fruits reveals that the main alterations induced by CI in bell pepper fruits are linked to redox homeostasis and carbohydrate metabolism. Thus, protein abundance in the ascorbate-glutathione cycle is altered and catalase is down-regulated. Key proteins from glycolysis, Calvin cycle and Krebs cycle are also inhibited in chilled fruits. Enolase and GAPDH are revealed as proteins that may play a key role in the development of chilling injury. This study also provides the first evidence at the protein level that cytosolic MDH is involved in abiotic stress.

Overexpression of dehydrin tas14 gene improves the osmotic stress imposed by drought and salinity in tomato.

One strategy to increase the level of drought and salinity tolerance is the transfer of genes codifying different types of proteins functionally related to macromolecules protection, such as group 2 of late embryogenesis abundant (LEA) proteins or dehydrins. The TAS14 dehydrin was isolated and characterized in tomato and its expression was induced by osmotic stress (NaCl and mannitol) and abscisic acid (ABA) [Godoy et al., Plant Mol Biol 1994;26:1921-1934], yet its function in drought and salinity tolerance of tomato remains elusive. In this study, transgenic tomato plants overexpressing tas14 gene under the control of the 35SCaMV promoter were generated to assess the function of tas14 gene in drought and salinity tolerance. The plants overexpressing tas14 gene achieved improved long-term drought and salinity tolerance without affecting plant growth under non-stress conditions. A mechanism of osmotic stress tolerance via osmotic potential reduction and solutes accumulation, such as sugars and K(+) is operating in tas14 overexpressing plants in drought conditions. A similar mechanism of osmotic stress tolerance was observed under salinity. Moreover, the overexpression of tas14 gene increased Na(+) accumulation only in adult leaves, whereas in young leaves, the accumulated solutes were K(+) and sugars, suggesting that plants overexpressing tas14 gene are able to distribute the Na(+) accumulation between young and adult leaves over a prolonged period in stressful conditions. Measurement of ABA showed that the action mechanism of tas14 gene is associated with an earlier and greater accumulation of ABA in leaves during short-term periods. A good feature for the application of this gene in improving drought and salt stress tolerance is the fact that its constitutive expression does not affect plant growth under non-stress conditions, and tolerance induced by overexpression of tas14 gene was observed at the different stress degrees applied to the long term.

The HAL1 function on Na+ homeostasis is maintained over time in salt-treated transgenic tomato plants, but the high reduction of Na+ in leaf is not associated with salt tolerance.

To achieve a deeper knowledge on the function of HAL1 gene in tomato (Solanum lycopersicum) plants submitted to salt stress, in this study, we studied the growth and physiological responses to high salt stress of T3 transgenic plants (an azygous line without transgene and both homozygous and hemizygous lines for HAL1) proceeding from a primary transformant with a very high expression level of HAL1 gene. The homozygous plants for HAL1 gene did not increase their salt tolerance in spite of an earlier and higher reduction of the Na(+) accumulation in leaves, being moreover the Na(+) homeostasis maintained throughout the growth cycle. The greater ability of the homozygous line to regulate the Na(+) transport to the shoot to long term was even shown in low accumulation of Na(+) in fruits. By comparing the homozygous and hemizygous lines, a higher salt tolerance in the hemizygous line, with respect to the homozygous line, was observed on the basis of fruit yield. The Na(+) homeostasis and osmotic homeostasis were also different in homozygous and hemizygous lines. Indeed, the Na(+) accumulation rate in leaves was greater in hemizygous than in homozygous line after 35 days of 100 mM NaCl treatment and only at the end of growth cycle did the hemizygous line show leaf Na(+) levels similar to those found in the homozygous line. With respect to the osmotic homeostasis, the main difference between lines was the different contribution of inorganic and organic solutes to the leaf osmotic balance. Taken together, these results suggest that the greater Na(+) exclusion ability of the homozygous line overexpressing HAL1 induces a greater use of organic solutes for osmotic balance, which seems to have an energy cost and hence a growth penalty that reverts negatively on fruit yield.

The high fruit soluble sugar content in wild Lycopersicon species and their hybrids with cultivars depends on sucrose import during ripening rather than on sucrose metabolism.

Soluble sugar content has been studied in relation to sucrose metabolism in the hexose-accumulating cultivated tomato Lycopersicon esculentum Mill, the wild relative species Lycopersicon cheesmanii Riley, in the sucrose-accumulating wild relative species Lycopersicon chmielewskii Rick, Kesicky, Fobes & Holle. and in two hexose-accumulating interspecific F1 hybrids (L. esculentum × L. cheesmanii; L. esculentum × L. chmielewskii), cultivated under two irrigation regimes (control: EC = 2.1 and saline: EC = 8.4 dS m-1). Under control conditions the total soluble sugar content (as hexose equivalents) in the ripe fruits of L. cheesmanii was 3-fold higher than in L. esculentum, while L. chmielewskii and both F1 hybrids contained twice as much as the cultivar. With the exception of L. esculentum × L. cheesmanii, salinity increased the sugar content by 1.3 (wild species) and 1.7 times (cultivar and L. esculentum × L. chmielewskii) with respect to control fruits. Wild germplasm or salinity provided two different mechanisms for the increases in fruit sugar content. The hexoses accumulated in ripe fruits were strongly influenced by those accumulated at the start of ripening, but the hydrolysed starch before start of ripening only partially explained the final hexose levels and especially the increase under salinity. The early cell wall acid invertase and the late neutral invertase activities appeared to be related to the amount of hexoses accumulated in ripe fruits. However, no metabolic parameter was positively related to the amount of sugar accumulated (including sucrose). The major differences between genotypes appeared in ripe fruits, in which up to 50% of the total amount of sugars accumulated in the wild species (mainly in L. cheesmanii) and hybrids cannot be explained by the sugars accumulated and the starch hydrolysed before the start of ripening stage. As a consequence, the higher fruit quality of the wild species compared with L. esculentum may depend more on the continuation of sucrose import during ripening than on osmotic or metabolic particularities such as the hexose / sucrose-accumulator character or specific enzyme activities.

Grafting raises the salt tolerance of tomato through limiting the transport of sodium and chloride to the shoot.

With the aim of determining whether grafting could improve salinity tolerance of tomato (Lycopersicon esculentum Mill.), and what characteristics of the rootstock were required to increase the salt tolerance of the shoot, a commercial tomato hybrid (cv. Jaguar) was grafted onto the roots of several tomato genotypes with different potentials to exclude saline ions. The rootstock effect was assessed by growing plants at different NaCl concentrations (0, 25, 50, and 75 mM NaCl) under greenhouse conditions, and by determining the fruit yield and the leaf physiological changes induced by the rootstock after 60 d and 90 d of salt treatment. The grafting process itself did not affect the fruit yield, as non-grafted plants of cv. Jaguar and those grafted onto their own root showed the same yield over time under non-saline conditions. However, grafting raised fruit yield in Jaguar on most rootstocks, although the positive effect induced by the rootstock was lower at 25 mM NaCl than at 50 and 75 mM NaCl. At these higher levels, the plants grafted onto Radja, Pera and the hybrid VolgogradskijxPera increased their yields by approximately 80%, with respect to the Jaguar plants. The tolerance induced by the rootstock in the shoot was related to ionic rather than osmotic stress caused by salinity, as the differential fruit yield responses among graft combinations were mainly related to the different abilities of rootstocks to regulate the transport of saline ions. This was corroborated by the high negative correlation found between fruit yield and the leaf Na(+) or Cl(-) concentrations in salt-treated plants after 90 d of salt treatment. In conclusion, grafting provides an alternative way to enhance salt tolerance, determined as fruit yield, in the tomato, and evidence is reported that the rootstock is able to reduce ionic stress.

Sucrolytic activities during fruit development of Lycopersicon genotypes differing in tolerance to salinity.

The different growth responses under control and moderate salinity (70 mM NaCl) in relation to the carbon partitioning and sucrose metabolism in developing tomato fruits [20 days after anthesis (DAA), start of ripening and ripe stages] were studied in the cultivated tomato Lycopersicon esculentum Mill (cv. H-324-1), in the wild relative species L. cheesmanii (ac. LA-530) (hexose-accumulators), L. chmielewskii (ac. LA-1028) (sucrose-accumulator) and in two interspecific F1 hybrids (hexose-accumulators) (F1-530: H-324-1 x A-530, F1-1028: H-324-1 x A-1028). The higher salt-tolerance of the wild species and hybrids with respect to the domestic tomatoes was also observed at the fruit level because these genotypes were less affected in the assimilation of dry weight (DW) under salinity. With the exception of the wild tomatoes, the sink strength, evaluated as the dry matter accumulation rate (mg DW day-1) and the sink activity, evaluated as a relative growth rate (mg DW mg-1 day-1), were reduced during the early fruit growing period (20 DAA-start ripening). However, a total recovery of growth was registered in the salinized hybrid fruits during the late growing period (start of ripening-ripe fruits). The early reduction in sink activity in the hybrid and domestic fruits was related to a sucrose accumulation and a decrease in the total sucrolytic activity at 20 DAA, especially the cytoplasmic sucrolytic activities sucrose synthase (EC 2.4.1.13) and neutral invertase (EC 3.2.1.26). The further recovery in sink strength of the hybrid fruits was related to the maintenance of the insoluble acid invertase (EC 3.2.1.25) and the induction of the cytoplasmic sucrolytic activities, namely at the start of ripening stage, demonstrating the existence of an inverse relationship between these activities, which suggests a regulatory mechanism in order to maintain the sink capacity. The roles of different enzymes in the control of assimilate import under salinity in relation to the sucrose transport and possible regulatory mechanisms are discussed.