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.

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.