Introduction

Being rooted in place, plants cannot evade attacks by pathogens and predators and they are permanently exposed to their physical environment, with changes in seasonal and diurnal cycles and extreme and stressful climatic variations. Among the abiotic factors, on which we will focus, a number of conditions are meaningful stresses because they can be life threatening and in combination determine plant distribution and the productivity of crop species (1). Prolonged drought, high or fluctuating salinity, and low and freezing temperatures account for most production losses, but flooding, high light, ozone, ion deficiency or imbalance, heavy metals, and soil structure are other factors that threaten plant life.

The foremost abiotic stress is water deficit. Water must supply the needs of both agriculture and a still increasing human population. This competition for water resources poses problems in areas where water is an increasingly precious commodity, most significantly in Australia, countries of the Middle East, North Africa, the west and midwest of the United States, parts of the Indian subcontinent, and central Asia (http://www.undp.org/ popln/fao/water.html). Even in areas with typically ample precipitation, transient drought can lead to economic hardship for farmers or inconvenience urban populations. At the same time, predictions of possible global climatic changes seem to indicate that the distribution of rain might become more erratic than in the past. Lack of water prolongs the agricultural growing cycle, increases vulnerability to pathogens, and ultimately results in decreased yield. Also, agricultural practices jeopardize productivity in many irrigated areas because long-term irrigation leads to the buildup of sodium chloride and other salts in the soil, with up to half of the area under irrigation affected (2,3). What is happening in many growing areas with elaborate irrigation schemes is reminiscent of events that led to the decline of ancient civilizations, the former "fertile crescent" from Egypt to Persia being the prime example. How will we provide a stable supply of food, feed, and fiber for a human population that may reach 9 billion people within the next two generations (4)? One approach is to study plant responses to abiotic stresses and to understand why and how some plants can tolerate water deficits while maintaining at least some productivity.

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