Harnessing salt for woody biomass production.

In medieval times, salt (sodium chloride, NaCl) was called ‘white gold’ because it was a costly commodity that was balanced with gold. Although salt has many positive usages and applications, it is currently considered more a threat than an asset. Soil salinization has endangered ancient as well as modern agriculture and forestry (Owens 2001). Inappropriate soil management practices have led to and continue to cause soil salinization. Reasons for salt accumulation include changes in the hydrological balance of soils caused by clearing of perennials and replacement by annual plants, as well as irrigation with salt-contaminated water and insufficient drainage in dry hot areas. Salt-affected soils can be found in almost all climatic regions, but the majority occur in arid and semiarid climatic regions (FAO 2009). Therefore, from a plant’s perspective, water limitations due to lack of water or the unavailability of water from salty solutions and the accumulation of salt in the plant body with internal changes in the water balance are often co-occurring stresses. Consequences of these constraints are severe limitations on food and biomass production. Notably, even in very hot, dry and saline deserts some woody species exist, and these have obviously acquired specific measures to adapt to these hostile conditions. In this issue, Ma et al. (2011) report on an interesting example of stress adaptation, the succulent xerophyte Zygophyllum xanthoxylum. Zygophyllum xanthoxylum is a fodder shrub that colonizes arid areas in China and Mongolia (Flora of China Editorial Committee 1988–2013). Ma et al. (2011) show that this species responds to salt with increased growth, a feature typical for halophytes (Flowers et al. 1977) and moreover becomes more tolerant to drought in the presence of moderate salinity. They conducted detailed ecophysiological analyses to dissect the contribution of salt to the adjustment of the plant water balance in response to changes in the external conditions. The analyses of the different components of the forces acting within the plant and at the cellular level can be integrated into a growth model that sets a physiological framework for the observed responses (Figure 1). Under unstressed conditions, cells adjust the osmotic potential of their cytoplasm (Ψsc) below that of the apoplast (≈ leaf water potential ΨL), thus maintaining a moderate positive turgor pressure (ΠT, Figure 1, C). In non-growing tissues, ΠT is almost completely compensated for by the mechanical cell wall pressure (PCW, Figure 1, C). Major osmo-regulators in this situation are usually potassium, other ions and organic solutes. Plants pre-adapted to saline soils such as Populus euphratica, Thellungiella halophila and sugar beet (Beta vulgaris) maintain high concentrations of sugars or other compatible solutes in leaves, even in the absence of stress (Zhao et al. 2009, Hoffmann 2010, Janz et al. 2010) and therefore have only a small requirement for the adjustment of osmotic pressure and stress signalling in response to salt (Amtmann 2009, Brinker et al. 2010, Ding et al. 2010, Qiu et al. 2011). Salt-adapted species such as Z. xanthoxylum and P. euphratica accumulate Na preferentially in the vacuole and not the cytosol (Ottow et al. 2005, Wu et al. 2011). Ma et al. (2011) showed elegantly that cellular ion uptake resulted in a strong increase in turgor pressure under mild salt stress compared with nonsalt-exposed Z. xanthoxylum plants, because under these conditions, the decrease in cytosolic osmotic potential was stronger than the decrease in leaf water potential (Figure 1, S). We can suppose that the generation of this increased pressure is relatively ‘cheap’, since the cell can concentrate costly compatible solutes like sugars or proline in the cytoplasm counterbalancing Na in the vacuole (Figure 1, S). In fact, decreased organic solute concentrations were found in Z. xanthoxylum grown with mild salt stress (Cai et al. 2011), supporting the idea that Na can at least partly compensate for organic solutes. The increased turgor pressure due to Na may result in cell Commentary