Soil alkalinity and salt tolerance: adapting to multiple stresses.

Bui's commentary [1] arises from an observation made in our paper on the evolution of salt tolerance in grasses. We described the 'paradox' of salt tolerance ; that it evolves often in nature but is hard to breed into crops [2], although there has been recent success in increasing the tolerance of wheat on salt-affected soils in Australia [3]. Bui suggests that one reason for lack of success in breeding salt-tolerant crops is that researchers have focused primarily on sodium chloride in soil and have paid less attention to the effect of alkalinity. This important issue should be explored more fully, particularly with reference to Australian soils with their transient and complex salt contents [4]. However, while soils in Australia are predominantly sodic, many soils throughout the world are saline and dominated by chloride, reflecting the ion balance in seawater [5]. Screening for tolerance to sodic soils is difficult in a laboratory [6]. It requires a soil-based system [7], although the use of tanks can make this a practical proposition [8]. We agree that, given the overlap of salinity and alkalinity on the landscape [1,9], it would be interesting to ask whether there has been co-adaptation to sal-inity and alkalinity. Although clear differences in tolerance to salinity and alkalinity exist for some species (e.g. rice [7]), shared underlying mechanisms in salinity and alkalinity tolerance have been suggested for some taxa, such as Eucalyptus [10] and Chloris [1]. This leads to a broader question of whether some lineages have characteristics that make them better able to adapt to a range of environmental stresses. There may be a suite of stress-related traits that allow plants to survive in different stressful environments by switching from high-resource strategies to stress-tolerant strategies [11]. The stress resistance syndrome hypothesis [11] could explain, at least in part, the striking pattern which emerged from our study that salt tolerance has evolved often in a wide range of grasses [2]. C 4 photosynthesis has also evolved independently many times in grasses [12]. Because C 4 leads to improved water use efficiency, it is often associated with open, arid environments [12], which are also more likely to be affected by salinity and alkalinity [13]. Alkaline soils often have poor soil structure, which affects their hydraulic conductivity and water uptake by plants [14]. Furthermore, accumulation of sodium ions may interfere with normal stomatal closure, exacerbating water loss in plants growing in …