The salinization of soils is a very serious agricultural problem. Because saline soils can adversely affect the establishment, growth, and development of crop plants, leading to huge losses in yield, the significance of soil salinity in reducing agricultural productivity is enormous.
Unfortunately, the salinity of croplands is increasing steadily in many parts of the world, especially in arid and semiarid regions. And the effects of “global weirding” will likely exacerbate the situation.
Despite efforts to address this problem using better soil management and cropping practices, it remains a growing threat to agriculture worldwide.
It’s clear that these improved management practices must also be accompanied by a better array of salt-tolerant crop plants available to farmers.
But where will such plants come from?
Breeding, Grafting, and Genetically-Engineering Crop Plants for Increased Salt Tolerance
Some salt-tolerant crop plants already exist, such as some varieties of barley. Traits that help confer salt tolerance in these barley varieties may be bred into other cereals, such as wheat, for example. Plant breeders may also use wild relatives of existing crop species as sources of “salt-tolerance” genes.
A recent report regarding the evolution of salt tolerance in grasses may explain why it’s been so difficult to breed salt tolerance into cereals despite the fact that there are many naturally salt-tolerant grasses in the world. Briefly, this study indicates that wild grasses may be able to adapt to saline conditions relatively quickly (from an evolutionary perspective), but then discard these adaptations if not needed. This suggests that plant breeders probably have a good chance of being able to generate salt tolerant cereal crop plants, but it also says that these adaptations likely come with significant costs to the plant.
Another approach that may be viable, using crops such as tomato, is grafting salt-sensitive, but desirable, varieties onto salt-tolerant rootstocks. (e.g., see ref. 2 below)
As plant biologists learn more precisely about how salt tolerance works at the cellular and molecular levels in plant cells, specific genes may also be introduced into crop plants by genetic engineering.
Such examples include proteins involved in the membrane transport of salts in plant cells. (e.g., see Ref. 3 below)
Instead of trying to make current crop plants more salt tolerant, why not domesticate plants that are already salt tolerant to use as crop plants?
As described in Ref. 4 below: “Our approach has been to domesticate wild, salt-tolerant plants, called halophytes, for use as food, forage and oilseed crops. We reasoned that changing the basic physiology of a traditional crop plant from salt-sensitive to salt-tolerant would be difficult and that it might be more feasible to domesticate a wild, salt-tolerant plant. After all, our modern crops started out as wild plants.”
Unfortunately, this approach has not proved to be successful so far, except in limited cases of use as forage plants.
Can Mycorrhizae Help Save the Day?
Mycorrhizae are symbiotic relationships that form in the soil between certain fungi and most native plants. One of the benefits plants may get by forming mycorrhizal associations is improved salt tolerance, which has been well documented. (for example, see Ref. 5 below)
It may be possible to improve salt tolerance in crop plants by encouraging mycorrhizal associations under cultivated conditions.
Also, by learning how mycorrhizal fungi help improve plant salt tolerance may help in developing salt-tolerant crops.
Seawater Greenhouses in the Desert?
Finally, it’s worth mentioning that there are people who are using seawater in a much different approach in trying to grow crop plants in deserts.
Briefly, this approach is to use solar power to generate electricity to convert seawater into freshwater. This freshwater is then used to irrigate plants in greenhouses. (For example, please see references 6, 7 & 8 below.)
Bottom line: At least two things seem clear. Firstly, we need to know a lot more about the basic biology of salt-tolerance mechanisms in plants. Secondly, solutions to the problem of ever-increasing saline soils will likely come from multiple approaches.
1. Barrett-Lennard, E. G. and T. L. Setter (2010) “Developing saline agriculture: moving from traits and genes to systems.” Functional Plant Biology, Vol. 37 , pp. iii-iv. (PDF)
2. Sanchez-Bel, P., Egea, I., Flores, F. B. and Bolarin, M. C. (2012) “Tomato: Grafting to Improve Salt Tolerance.”, in Improving Crop Resistance to Abiotic Stress, Volume 1 & Volume 2 (eds N. Tuteja, S. S. Gill, A. F. Tiburcio and R. Tuteja), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany. doi: 10.1002/9783527632930.ch42 (Abstract)
3. Mian, A. A., et al. (2010) “Improving crop salt tolerance: anion and cation transporters as genetic engineering targets.” Plant Stress 5 (PDF)
4. Glenn, E. P., J. J. Brown and J. W. O’Leary (1998) “Irrigating crops with seawater.” Scientific American, August 1998, pp. 76-81. (PDF)
5. Evelin, H., R. Kapoor, and B. Giri (2009) “Arbuscular mycorrhizal fungi in alleviation of salt stress: a review.” Annals of Botany, Vol. 104 pp. 1263-1280. (PDF)
6. Jonathan Margolis (2012) “Growing food in the desert: is this the solution to the world’s food crisis?.” The Observer, Saturday 24 November 2012. (Full Text)
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