Salt tolerance strategies of the ancient Andean crop quinoa

Soil salinization is a serious threat to agriculture, jeopardizing global food security and causing huge economic losses every year. Plants face formidable challenges when growing under saline conditions. The high concentration of ions in the root environment hampers water uptake, yet the accumulation of ions in plant tissues induces cytotoxicity. Unfortunately, soil salinization is likely to increase in the context of climate change, which lead to sea level rising and shortage of fresh-water sources. Hence, crop production might rely on irrigation with brackish/ saline water. In addition, the increasing global food demand calls for cultivation in less favourable soils including saline deltas around the world. Improvement of salt tolerance of existing crops might not be sufficient under increasing salinization, so the exploration of new, more salt tolerant crops appears to be essential to ensure food supply. Chenopodium quinoa is -at least outside S. America- a relatively new crop that has gained ground due to its resilience to withstand abiotic stress, especially soil salinity. Quinoa can potentially contribute to ensure food security and it has a healthy balance of proteins with all the essential amino acids, carbohydrates (starch), fibre and seed oil. In this thesis novel insights on the mechanisms and processes that underlie the remarkable salt tolerance of quinoa are provided and traits that may be used to improve crop salt tolerance are described.In Chapter 2 we evaluate responses of quinoa varieties to long-term salinity (0- 400 mM NaCl) with respect to growth and ion homeostasis. We found that salinity reduced the seed yield of the varieties (grown as spaced plants in the greenhouse) by on average 29 % at 100 mM NaCl and up to 88 % at 400 mM NaCl. We observed that quinoa varieties utilise salt exclusion strategies to produce relatively high yields under mild salinity or short-term stress, while tissue tolerance mechanisms enable the plants to survive and even reproduce under severe and prolonged salinity stress. In addition, [K+] in young leaves increased under salinity, especially at high salt concentrations. Likely, K+ plays a role in leaf osmotic adjustment, and also protects the cells from metabolic failure due to a low K+/Na+ when Na+ reaches high concentrations in the leaves. We also examined the potential contribution of epidermal bladder cells (EBCs) to the ion homeostasis of quinoa. EBCs accumulated 5.4 % and 6.5 % of the total Na+ and Cl− in the leaves, which indicates that the storage of salt in EBCs is not likely to contribute significantly to reduce levels of ions in the leaves.In Chapter 3 we focused on changes in leaf and stem cell wall composition under a salt stress of 400 mM NaCl. We found that salinity alters the cell wall composition by decreasing the lignin and cellulose content and increasing pectin content. The highest variation in pectic monosaccharides was found for arabinose that increased by 160 % in stems and 60 % in leaves. The mineral composition of the cell wall was also affected by salinity: Ca²⁺ decreased by 30 % and 65 % while Na⁺ increased by 140 % and 70 % in stem and leaf cell walls, respectively. We suggest that these changes increase the flexibility and hydration of the cell wall, which might help the cells to cope with changes in osmotic potential and turgor imposed by salinity.In Chapter 4 we investigated the salinity-induced changes in water relations of two varieties with different responses to salinity: Pasto and selRiobamba. To this end, we implemented the Plantarray 3.0 phenotyping platform® to monitor changes in growth and transpiration with a high temporal resolution. Salinity reduced the cumulative transpiration of both varieties by 60 % at 200 mM NaCl and by 75 % and 82 % at 300 mM NaCl for selRiobamba and Pasto, respectively. Stomatal conductance was reduced by salinity, but at 200 mM NaCl Pasto showed a lower reduction (15 %) than selRiobamba (35 %), along with decreased specific leaf area. Both varieties had increased water use efficiency under salt stress. We propose that contrasting water management strategies contribute to the differences in salt tolerance between Pasto and selRiobamba. Pasto adopted a “conservative-growth” strategy, saving water at the expense of growth, while selRiobamba used an “acquisitive-growth” strategy, maximising growth in spite of the stress.In Chapters 2, 3 and 4 responses of quinoa to salinity were explored using single plants as experimental units. In Chapter 5 we switched to field-like conditions, using crop-like plant densities (~50 plants.m¯²) and a salt treatment of 250 mM NaCl. We evaluated the agronomic performance (seed yield, thousand seed weight, harvest index) and physiological responses to salinity of six commercial varieties. In addition, we used bi-parental crosses from varieties with contrasting salt tolerance responses to discover genetic factors of traits contributing to salt tolerance of quinoa. The salt tolerance of the varieties based on seed yield ranged from 68 to 92 %. The average salt tolerance was 67 % for the mapping population Atlas x Red Carina and 75 % for the Pasto x Red Carina population. We found QTLs for all the analysed traits and identified putative alleles donated by Pasto associated to lower Na⁺ and Cl¯ contents, higher K⁺ retention and lower SLA that likely contribute to salt tolerance in the Pasto x Red Carina population.Natural variation in agronomical and salt stress tolerance traits within species is a pre- requisite for crop improvement through breeding. In Chapter 6 we evaluated the genetic diversity of 22 genotypes selected as good representatives of the genetic diversity of quinoa in Ecuador. These accessions were grown under a 12-h light photoperiod and field-crop density under control conditions and a salt treatment of 250 mM NaCl. All the genotypes proved to be highly salt tolerant with an average Salt Tolerance Index based on seed yield of 78 %. This collection was highly diverse for agronomical traits: the seed yield under control conditions ranged from 890-1145 g/m² and the thousand seed weight ranged from 2.4-4.4 g/ 1000 seeds. This collection was diverse for salt tolerance traits as well. Na+ exclusion was the preferred mechanism under this, for quinoa, “mild” salinity stress, but some genotypes accumulated high concentrations of Na⁺ in the leaves. The results reported in this Chapter demonstrate that the Inter-Andean valley ecotype from the Ecuadorian highlands is highly diverse and constitutes an interesting genetic resource for salt tolerance and other breeding purposes.In Chapter 7 I integrate the insights presented in this thesis on how quinoa responds to salt stress and analyse the potential use of this species to identify traits that might increase the salt tolerance of current crops. I propose an ideotype for salt tolerance characterized by a high rate of Na⁺ and Cl⁻ exclusion from leaves, high K⁺ retention (when Na⁺ in the shoot rises), responsive control of stomata opening, among other plastic traits associated with an “acquisitive” growth under moderate stress and “conservative” growth under severe salt stress. Finally, I reflect on how salt tolerance in particular, and resilience to abiotic stresses in general, are quinoa’s strongest asset to strengthen as an emerging staple crop.