Water deficit and salinity are two of the most important abiotic stresses that limit both productivity and quality of crops [1
]. Plant responses to these stresses involve adaptive changes and, frequently, deleterious effects [2
]. Water deficit prevents plant development by decreasing the plant’s relative water content and water potential in their tissues, and consequently the closure of the stomatal complex. This leads to osmotic stress, limited nutrient uptake, reduced photosynthetic activity, oxidative stress, and growth inhibition [3
]. Likewise, net photosynthesis can be lowered by mesophyll conductance restriction or by impairing CO2
fixation reactions [4
]. The effect of salinity on plants is more complex and involves two phases: osmotic stress, similar to the one induced by water deficit, and toxic effect produced by the ionic accumulation on leaves that gives rise to imbalances between nutrients and induces metabolic damage [5
]. Plants have developed different adaptive strategies to face these stresses, such as osmotic adjustment by accumulation of compatible solutes, stomatal closure, reactive oxygen species (ROS) detoxification and, in the case of salt tolerance, mechanisms of ionic exclusion or intercellular ion compartmentalization [7
Cultivated tomato (Solanum lycopersicum
L.) is very sensitive to water stress, especially during flowering and fruit enlargement stages [8
]. In addition, tomato is moderately sensitive to salinity levels higher than 2.5–3.0 dS m−1
]. Genetic variability for salt and water stresses tolerance in tomato has been found limited; nonetheless, sources of tolerance have been reported in its wild relatives that can be exploited in breeding [11
]. However, most of these species are difficult to cross with cultivated tomato, giving rise to embryo abortion and making difficult the progress of breeding works. This fact and the complex genetic control of the tolerance to salinity and water stress have hampered the introgression of salt/drought tolerance from distant wild relatives into cultivated tomato [12
The wild species Solanum pimpinellifolium
has a bushy growth type and inhabits the coastal regions of Ecuador, Peru, and northern Chile. The natural range of S. pimpinellifolium
encompasses such differing environments as the northern coastal Ecuadorian tropical rainforests and the Peruvian coastal desert [15
]. In central and southern Peru, S. pimpinellifolium
is restricted to cultivated fields, roadsides and dumping grounds, and its distribution is sparse, whereas in northern Peru and Ecuador it is found in wild and dense populations located in undisturbed areas [16
]. This species with such a wide range of distribution is a potential source of alleles of interest for tolerance to abiotic stresses. Numerous quantitative trait loci (QTLs) conferring tolerance to abiotic stresses have been identified in S. pimpinellifolium
]. Several accessions have been characterized as tolerant to salt stress and are promising sources of genes and alleles for improvement of salinity tolerance in the cultivated tomato [18
]. Rao et al. [29
] tested a collection of 94 accessions of S. pimpinellifolium
to saline stress and found five accessions possessing better survival traits, seven with good yield traits and two combining both under salt stress. Later, the same group conducted an association study to identify variation linked to salt tolerance traits in four candidate genes [30
] including the dehydration responsive element binding (DREB1A) and Pyrophosphatase (VP1.1) genes. Recently, the accession LA0480 has been sequenced and tested for its tolerance to salinity [31
], suggesting that S. pimpinellifolium
offers a wealth of breeding potential for desirable traits and concretely an enrichment in genes involved in biotic and abiotic stresses responses [32
The Solanum lycopersicum
grows spontaneously worldwide in tropical and subtropical regions [33
]. It has been collected in a wide range of habitats, from deserts to very humid regions at altitudes that range from sea level to 2400 m [34
], although it prefers humid areas below 1200 m. It is widely distributed close to human-modified areas, such as irrigation canals, home gardens and orchards. This botanical variety has been far less studied than S. pimpinellifolium
. Its adaptation to water deficit and salinity has been recently tested by Diouf et al. [35
] using a Multi-parent Advanced Generation Inter-Cross (MAGIC) population derived from the cross of eight parental lines, four of them belonging to S. l.
and four to S. l.
. Even using the eight parents of the same species, they were able to identify 54 QTLs linked to fruit quality traits, flowering and ripening earliness, and vegetative traits. Impact of water deficit and salt stress was found for most traits, with a positive effect on soluble solids content. Remarkably, some lines increased, simultaneously, fruit weight and soluble solid content under both stresses [35
]. Using the high-resolution mapping constructed from the MAGIC population it was possible to detect candidate genes, to specify the allelic effect of each parental line at the QTL, and the sequence information of the eight parental lines.
We are constructing a MAGIC population from eight parents, four belonging to S. pimpinellifolium
and four to S. l.
species. These genotypes were selected to maximize their genetic diversity and origin from diverse habitats and environmental conditions [36
]. In order to evaluate the adaptability and response of each accession to both water and salinity stresses and to further study their genetic control in the MAGIC population, we have measured different physiological parameters like the CO2
assimilation rate (AN
), stomatal conductance to water vapor (gs
), instantaneous carboxylation (AN
/Ci) and water use efficiency (AN
/E), as well as several morphological and agronomic vegetative, flowering and fruits traits in an experiment comparing three treatments: water deficit, salinity, and a control. The relation between the responses and the origin of their climatic conditions and accession domestication level is discussed herein.
Water stress and salinity are two of the major abiotic constraints to tomato cultivation. Despite tomato being considered moderately sensitive to salinity compared to other Solanaceae’s species, yield loss may be high and depends on both the salt concentration and the duration of the stress [43
]. In addition, water stress can have important consequences for tomato production, as it might result in yield reduction of up to 50% in the case of an equivalent reduction in irrigation [46
]. In this work, we explore the adaptation to water deficit and salinity of seven parents of a MAGIC population, four belonging to S. pimpinellifolium
and three to S. l.
Overall, there was an evident high variability in the response of the tested genotypes under the experimental conditions, demonstrating the existence of genetic variability susceptible of being exploited by breeders. The conducted ANOVA revealed the existence of significant differences between the tested accessions for all recorded traits. However, when looking at the results of the LSD test it can be concluded that there is not a clear effect of the “species” factor on the results. In our opinion, this result shouldn’t be taken as a general rule, but as the consequence of the specific group of accessions tested in this experiment. These accessions were selected to maximize their genetic diversity, morphological traits, geographic origin, and degree of domestication, as they are the founders of a MAGIC population. As an example, Ceras1 grows as a wild and resembles phenotypically to S. pimpinellifolium
. In the same way, the four accessions of S. pimpinellifolium
were collected covering a wide range of distribution of this species, showing remarkable different morphology in some plant, inflorescence and fruit traits, as well as adaptation to different environmental conditions. Hence, the maximization of the diversity between the accessions belonging to each species probably masked the differences in the response between species. Regarding the applied treatments, the effect of the water stress treatment was less harmful than the one produced by salinity. These results have also been described in other crops, such as pepper [47
], as a consequence of the specific toxicity produced by salt ion accumulation, mainly Na+
, when plants are subjected to salinity stress.
In this experiment, S. pimpinellifolium
accessions did not show a decrease in AN
at 75 DAT under water stress. At 120 DAT only Pim4 decreased AN
, remaining this parameter unaltered in Pim1 and Pim3 and increasing in Pim2. However, the stomatal conductance diminished at 120 DAT in three accessions except in Pim2. In spite of the stomatal closure detected in Pim1 and Pim3, it did not interfere with the photosynthetic rate (AN
) indicating that stomatal closure was more sensitive to water stress than AN
]. This could be explained by the fact that only critical levels of gs
, described as low as 0.1 mol H2
O/m·s are able to affect photosynthesis [49
]. At the end of the productive period, the instantaneous carboxylation efficiency increased in Pim2 and Pim3, diminishing in Pim1 and Pim4. The decrease in AN
/Ci indicated that water stress affected the photosynthesis by metabolic limitations in Pim1 and Pim4 [50
]. Water use efficiency (AN
/E) remained unaltered in the four genotypes. Considering these data as a whole, we consider the accessions Pim2 and Pim3 as tolerant to the water stress treatment applied in this experiment. Regarding the recorded agronomic and morphological traits, most of them remained unaffected, decreasing only the number of flowers per truss in Pim3 and increasing the number of fruits per truss and the fruit set in Pim2. The vegetative vigor, measured as the leaves size, decreased at 75 DAT, but this effect disappeared as the treatment progressed. The fruit weight was only reduced in Pim4. The °Brix increased in almost all Pim accessions as a consequence of the water deficit treatment. This response is in agreement with the one found by other authors [52
]. According to Albert et al. [54
], this can be due to a reduction in fruit water content and not to increased synthesis of sugars, although Ripoll et al. [55
] found higher fructose and glucose synthesis in tomato fruits submitted to water deficit stress at different stages of fruit development, indicating that both dilution effect and higher sugar synthesis are responsible of fruit quality enhancement in tomato under water deficit conditions.
The four tested accessions in this experiment come from areas with very different climatic conditions, ranging from the desertic climate of the coastal northern Peru (Pim2) and the coastal south Peru (Pim4), to the highly humid and hot areas or Manabí (Pim1) in Ecuador and Amazonas province in the north of Peru (Pim3). Nakazato et al. [56
] conducted an interesting study using S. pimpinellifolium
accessions from different western slopes of the Andes and the adjacent coastal regions of Peru, to demonstrate that environmental factors drive the phenotypic differentiation and adaptation to environmental stresses, and showing evidence of trait-environment associations. They found a negative correlation between tolerance to water stress, measured as the number of days to wilting, and the annual precipitation, indicating that individual populations that occur in arid environments are more likely drought tolerant [56
]. Interestingly, Pim2, one of the most tolerant accessions to water stress in our experiment due to photosynthesis preservation, comes from desert areas with scarce precipitation. This behavior fits with the results found by Nakazato et al. [56
]. Nevertheless, in a study conducted by Albert et al. [54
], in which some S. pimpinellifolium
accessions from different origins were tested, no correlation was found between the climate and the adaptation to water deficit, showing different adaptation to water stress of accessions coming from the same province. Herein, Pim3, coming from Amazonas province (Peru) with climatology clearly different to Piura (Pim2) was also found tolerant to the water stress conditions applied in this study. Zuriaga et al. [15
] conducted a comprehensive experiment to study the molecular variability of S. pimpinellifolium,
including 247 accessions covering all its range of distribution. They found a clear genetic differentiation among the studied accessions, related to their geographical origin [15
]. The accessions used herein belong to groups genetically and environmentally differentiated by Zuriaga et al. [15
], demonstrating that the differences found have a genetic basis and can be exploited in breeding.
In contrast to the response to water stress observed in S. pimpinellifolium
, salinity treatment severely affected the photosynthetic rate, as well as the stomatal conductance, particularly at the end of experiment. The ion imbalance, ion toxicity and osmotic stress produced by the salinity treatment interfered with both parameters, as described by other authors [7
]. Herein, salt treatment lead to a decrease in chlorophyll levels, as demonstrated by the significant differences shown by SPAD measurements in Pim1 and Pim2, not observed under water stress conditions. As a consequence of the decrease in photosynthesis efficiency, the vegetative development also decreased, as shown by the smaller leaf and leaf terminal lobe sizes, compared to control. Despite that, fruit set (except for Pim4) and flowering earliness were not affected. Finally, a negative effect was observed on fruit fresh weight, which has been described as one of the most important effects of sensitivity to salinity, being more notable in bigger fruits [30
]. The decrease in fruits as small as the ones of S. pimpinellifolium
, shows the negative effects of salinity in the tested accessions.
Regarding S. l.
, the photosynthetic rate did not vary in water stressed compared to control plants at 75 DAT, although Ceras1 and Ceras3 reduced their gs
, compared with control. At the end of the experiment (120 DAT), Ceras1 and Ceras2 showed no alteration of CO2
fixation, whereas Ceras3 showed a decrease of gs
values. This allowed for dynamic modulation of photosynthesis under water stress, with conservative values of AN
/Ci indicating no photosynthesis metabolic limitations in both Ceras1 and Ceras2 [47
], however in Ceras3 decrease in AN
/Ci and in gs
indicating that AN
reduced by stomatal closure and by inhibition of mesophyll conductance and/or photochemical efficiency at 120DAT, similar results were obtained in [47
]. The water use efficiency remained unaltered in Ceras1 and Ceras2 and increased in Ceras3, caused by a considerable stomata closure associated to a decrease in the transpiration rate [51
]. Regarding the phenotypic traits, only leaf size, at 75 DAT, and fruit fresh weight (not in Ceras1) decreased as consequence of the treatment. Decrease in plant vigor, as a result of the water stress treatment, was also reported for a group of 55 accessions of S. l.
and S. pimpinellifolium
tested by Albert et al. [54
] whereas the accessions’ yield remained unaffected. The authors suggested that tomato plants buffer the negative effects of water deficit by limiting their vegetative growth and by reallocating the photo-assimilates to the fruits. According to our results, the most tolerant S. l.
accessions were Ceras1 and Ceras2 for the absence of alteration of CO2
fixation, unaltered water-use efficiency, and maintenance of most of the vegetative and reproductive traits unaffected. Interestingly, Ceras1 was also tested for tolerance to water deficit by Albert et al. [54
] with similar conclusions to ours, where yield was not decreased and plant vigor was slightly reduced. Finally, Ceras3, the accession with bigger fruits, showed the most marked reduction in fruit fresh weight. The greater weight loss in larger fruited accessions has already been reported by several authors [35
Under salinity stress, at 75 DAT, no limitation of CO2
occurred in S. l.
accessions. At 120 DAT stomatal conductance and photosynthesis were negatively affected by salinity in all accessions. Herein, AN
/Ci decreased in all S. l.
accessions, suggesting stomatal constraints. Previous studies have demonstrated a positive relationship between photosynthetic capacity and growth in the plants grown under salinity [47
]. As a consequence of the considerable reduction of carbon assimilation rate, vegetative growth, leaf and leaf terminal lobe sizes decreased in all tested genotypes.
, unlike S. pimpinellifolium
grows normally in the proximity of cultivated fields or in backyards in close contact with humans [37
]. In these conditions, plants can compete with fast growing species or genotypes such as those in cultivation or in backyards, but do not need to develop special adaptive traits to abiotic stresses [56
]. Notwithstanding, S. l.
can also be found in wild conditions. This is the case of Mexico where this species is widely distributed and can be found as a wild, weedy and even as partially cultivated varieties in tropical and subtropical areas with semiarid and humid regimes [62
]. When growing in semiarid and hot climates this species is found in association with larger plants that provide them with shade. Accession Ceras1 comes from the Sinaloa desert, in Mexico, in the Pacific slopes (ca. 300–1100 m) of Sierra Madre Occidental [63
], while Ceras2 and Ceras3 come from humid and hot areas. Hence, the three S. l.
accessions come from very different conditions. In our experiment, Ceras1 has been the most tolerant accession under both treatments while Ceras2 showed an interesting behavior under water stress deficit. Given the fact that Ceras1 grows as a wild variety in semiarid conditions, it may indicate that this accession could have a natural adaptation to similar stresses conditions to those applied here. Morphologically, there are marked differences between the three accessions. Ceras1 is similar to S. pimpinellifolium
with thin stem, small leaves, and fruits of similar weight, shape and size (between 1 and 1.5 cm of diameter). In contrast, the other S. l.
accessions (Ceras2 and Ceras3) are closer to the cultivated tomato, with stronger stem and bigger leaves and fruits of more than 3 cm in diameter. The genetic and morphological variability of S. l.
was already shown by Blanca et al. [37
]. In the three accessions used in this work, strategically selected by their genetics, environmental conditions of origin and morphology, we have found different adaptation to abiotic stresses, showing that this species constitutes a valuable source of variability for tomato’s breeding.