The flux of water in the soil-plant-atmosphere continuum is driven by the difference in water potential between soil, plant, and atmosphere. By definition, the water potential (T is zero in pure water (1 atm, 25 °C) and decreases with decreasing water "availability", i.e. in solutions, in vapour, etc. This can be illustrated by a few examples: in well-watered soil the water potential is -0.3 MPa; inside plants it is about -0.6 MPa; within the stomatal cavity it can reach about -6.9 MPa (=95 % relative air humidity), and outside in the atmosphere it is -95.1 MPa (=50 % relative air humidity; Nobel 1994). The movement of water molecules follows the water potential gradient from the soil to the atmosphere. If soil water availability decreases, the plant needs to adjust, i.e. lower, its internal water potential to be able to continue extracting water from the soil. The
1 Institut für Forstbotanik, Georg-August Universität Göttingen, Göttingen, Germany
2 The Robert H. Smith Institute of Plant Sciences and Genetics in Agriculture, The Hebrew University of Jerusalem, Rehovot, Israel
3 College of Life Sciences and Biotechnology, Beijing Forestry University, Beijing, PR China
M.Fladung and D.Ewald (Eds.)
Tree Transgenesis: Recent Developments
© Springer-Verlag Berlin Heidelberg 2006
water potential of a plant, usually measured in the xylem, is therefore a suitable measure for the extent of drought stress experienced by a plant (Scholander et al. 1964; Cochard et al. 2001). Within a tissue, water flux is determined by the concentration of solute molecules inside and outside the plant cell. The radial cell-to-cell transport across membranes is facilitated by aquaporins (Chrispeels and Maurel 1994; Kirch et al. 2000; Tyerman et al. 2002). Some aquaporins can also transport small solutes such as glycerol, other small organic molecules, and ions (Quigley et al. 2002; Tyerman et al. 2002), which may be relevant for osmotic adjustment. The extent to which aquaporins contribute to plant water status under favourable growth conditions and abiotic stress is not clear. Among various aquaporin genes, some were up regulated under stress (Yamaguchi et al. 1992; Yamada et al. 1997), whereas others such as PIP1a were not affected (Grote et al. 1998). PIP1b overexpression significantly increased plant growth rate, transpiration rate, stomatal density, and photosynthetic efficiency under well-watered conditions; but during drought stress, it had a negative effect, causing faster wilting (Aharon et al. 2003). Furthermore, the relative contributions of the apoplastic and symplastic routes to water transport in plants under favourable growth conditions, as well as under drought stress remain a matter of debate (Carvajal et al. 1998; Schaffner 1998; Amodeo et al. 1999; Tyerman et al. 1999). Changes in water availability may cause perturbations in solute flux and of cellular structures, alter the composition of the cytoplasm, and affect cellular functions.
Water deficits pose problems both at the cellular level, due to dehydration and turgor loss, and also for long-distance transport of nutrients. All aspects of water availability and transport are particularly relevant for growth and survival of tree species since trees have to ensure water transport into their crowns to heights of up 100 m. Additionally, trees have to acclimatise to changing water supply not only on a daily basis but also seasonally over many decades during their long life cycle.
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