Norman Borlaug died recently (12 September 2009) at the age of 95. Borlaug began life as an Iowa farm boy, was trained as a plant pathologist at the University of Minnesota, and went on to direct some of the most important plant breeding efforts of the 20th century. He was awarded the Nobel Peace Prize in 1970 for those efforts.
Borlaug is father of the “Green Revolution”, which began after World War II. Green Revolution agriculture is often criticized because of its reliance on pesticides, hybrid technology, and intensive water use, but it is also widely credited as having saved more human lives, particularly in the Third World, than anything else in the history of the human species. Later in life, Borlaug conceded that there might be some merit in these criticisms but that the Green Revolution was a good start in the right direction.
A good start it was, but we still face many of the same problems that Borlaug encountered. One of these is excess soil salt, or salinity. Salinity is a soil condition characterized by a high concentration of soluble salts that inhibit plant growth. Excess soil salt is a problem as old as agriculture. The civilizations of the Fertile Crescent, the area centered around modern-day Iraq, are thought to have dissipated as a result of climate change and excess soil salt that destroyed their agriculture.
Soil salinity is one of the primary abiotic stresses affecting plant growth and quality. As much as 6% of the earth’s total land area is affected by excess soil salt. Much of this arises from natural causes. Rock weathering releases soluble salts, and rainwater itself contains 6–50 ppm sodium chloride. Clearing land for cultivation and irrigation are two other causes of increased soil salinity; both raise the water table and salts are then concentrated in the root zones of plants.
Salinity is a common element of arid and semiarid lands, but it is also found in regions with moderate rainfall such as the U.S. Midwest and Northeast, particularly where irrigation is used. Poor quality irrigation water and poor drainage can make it worse. And irrigation is important in agriculture. Only about 15% of all cultivated land is irrigated, but irrigated land is about twice as productive as rained land and accounts for about 30–40% of the world’s food production. Breeding salt tolerance in plants is an important goal for plant scientists.
Now, what’s a salt? Salts are ionic compounds, and ionic compounds are characterized as having an electrostatic bond between metal and nonmetal ions. Ions are charged atoms. In water, salt dissolves as the ions composing the salt disassociate. If the water evaporates and the concentration of salt in water (in solution) becomes too great (saturated), the salt precipitates out of solution and becomes solid once more. Sodium chloride (table salt) is the primary salt involved in soil salinity; the primary ions responsible for salinization are sodium, potassium, calcium, magnesium, and chlorine.
Sensitivity to salt differs in plants. Some are tolerant while others are quite sensitive. Plants that grow in salt marshes and estuaries where the salt concentration may vary diurnally are (not surprisingly) able to thrive at much higher salinities than can woodland plants; this is easily demonstrated. But salt is so common in soils that all plants have evolved the ability to cope with and adapt to some degree of salinity.
How does salt affect plants? There are two basic ways. First, high salt concentrations in soil make it harder for plant roots to extract water from the soil. This is purely the result of osmosis, the movement of water across a semipermeable membrane, as in a plant cell, from an area of high water potential (low salt concentration) to an area of low water potential (high salt concentration). When the concentration of soil-water salt rises above a threshold, water will tend to flow out of the plant. If plants had no way of regulating this process, they would quickly dehydrate and die. Second, in a saline environment, salt enters the plant and accumulates. With time, it can reach toxic concentrations.
Both can be exacerbated by environmental factors such as sunlight, air temperature, and humidity, but of the two, osmotic stress has the most impact, and after soil-water salt exceeds a certain threshold its effect on plant growth is more or less immediate. Salt accumulation, on the other hand, has a more gradual effect. Stress from salt accumulation occurs later in the plant’s life cycle, and only at very high levels of salinity does its effect dominate.
How do plants adapt to increased salinity? Traditionally, plants have been described as either “excluders” or “includers” of salt, those that select against its uptake or those that regulate its accumulation. In most plants, a little of both strategies is seen. Other plants adapt to salinity by completing their life cycles rapidly and avoiding the toxic effects of accumulated salt altogether. These are worthwhile summaries but trivial answers to complex processes.
All plant functions ultimately result from the genes that plants possess that control and coordinate growth in concert with the constraints of the environment, and that plants mount a coordinated response to their environment is easily demonstrated. The physiological manifestations of salt tolerance and the salt-stress response have been pretty well described. Traditional plant breeding of the type that Borlaug directed has produced quite stress-tolerant crops, mainly by introducing traits from stress-adapted wild relatives. So great progress has been made, but our understanding on a molecular and cellular level is only piecemeal.
Teasing answers from several issues will provide insights into the processes that cause salt tolerance and toxicity in plants. For example, what molecular processes control salt (actually ion) compartmentalization in plants, and what accounts for tissue tolerance and osmotic adjustment? How is salt transported once inside the plant? A gene family responsible for initial entry of ions into plants has been identified and gives us some insights. One fascinating question is how do the leaves know the roots are in salty soil? Clearly, they do because leaf growth rate is reduced proportionally to the concentration of salt in the soil solution and not to the salt concentration within the leaves. What accounts for this long-distance communication within plants?
In the next few decades, we will answer these questions. And in the process, we will have taken more steps in the right direction.