Root anatomy underlies transport processes. Root hydraulics can be only interpreted with sufficient knowledge about the anatomy of roots. Root anatomy varies a lot depending on the way plants are grown and in response to the developmental state of roots (Steudle and Peterson 1998). For example, in maize, rates of uptake of water and that of nutrient ions were significantly reduced during drought stress (Stasovsky and Peterson 1993). To minimize water losses, roots developed a suberized interface between living tissue and soil, which reduced both the ability of roots to take up water and water losses, both measured as the hydraulic conductivity of roots (root Lpr in m3 m~2 MPa-1 s-1). Taleisnik et al. (1999) grew plants under conditions of water shortage, where their ability to retain water was increased in the presence of an exodermis, which was not present in the controls. Older roots being more suberized released less water to the dry environment than younger ones. It was concluded that the changes in root hydraulics were brought about by the presence of apoplastic barriers rather than by changes in membrane properties of cells. Similarly, earlier results from Kramer and coworkers (summarized by Kramer and Boyer 1995, on pp. 130, and 184/5) indicated that suberization substantially reduced the ability of roots of woody species to take up water. However, it was stressed that older suberized roots did still contribute to the overall uptake of water and nutrients.
Zimmermann and Steudle (1998) compared the hydraulics of young roots of corn with and without an exodermis. When the seedlings were grown in hydroponics, they did not develop a continuous exodermis. But when grown in mist culture they did, which reduced the hydraulic conductivity by a factor of 3.6 at constant water permeability of root cells. Recently, the thicker roots of Iris germanica have been studied by Meyer et al. (2010). When exposed to air, these roots developed a continuous multiseriate exodermis, which was uniseriate, when roots were brought up in hydroponics. As for the young corn roots, the treatment caused substantial changes in the root hydraulics measured by combining the root pressure probe and the pressure chamber. When present, the multiseriate exodermis was limiting water flow (as for corn grown in mist culture).
The above examples suggest that the simple textbook notion that the endodermis would usually represent the dominating hydraulic resistance in roots (and the osmotic barrier) may be questioned, at least for roots that develop an exodermis. For roots grown under harsh conditions of water shortage, the effects of suberiza-tion may be dramatic. This means that, during restoration from the desiccated state, roots may have a problem to acquire water from the rewetted soil in view of a suberized exo- and endodermis. In addition, plants grown at low water potentials of the soil develop less extended root systems. Unfortunately, there are, for technical reasons, no quantitative data for these roots to show whether or not their hydraulics may encounter severe problems or may even become limiting during water uptake following periods of desiccation. This knowledge would be of great importance when judging about role of root pressure during the recovery of the function of xylem during these time periods.
The discovery of aquaporins (water channels; AQPs) by Agre and coworkers around 1990 (Denker et al. 1988; Zeidel et al. 1992; Preston et al. 1992) caused a focus on the cell-to-cell rather than on the apoplastic passage of water across roots as emphasized in the previous paragraph (Maurel 1997; Maurel et al. 2009). AQPs are proteins of a molecular weight of about 30 kDa, which span the membrane six times to form the pore by a folding back of two loops of the protein into the bilayer. The actual pore of a length of about 2 nm (20 A) has a diameter, which is adapted to the size of the water molecule (about 4 A). It has a mouth part and a constriction part in the center, which determines its selectivity according to size and polarity. Molecules which are similar in size and polarity as water may also pass through the AQPs. The break in the water structure at the constriction and electrostatic barriers provide that no charge can pass (Burykin and Warshel 2004). The latter point is of major importance with respect to a potential passage of protons or hydroxyl ions (Grotthus mechanism), which would otherwise short circuit pH gradients across membranes and substantially affect membrane potentials, proton motive forces, and the like.
The open/closed state or activity of AQPs may be affected by different external and internal factors that cause a conformational transition from open to closed state. This gating of AQPs happens under conditions of stress such as in the presence of heavy metals, high concentrations of osmolytes or salinity, low temperature, oxidative stress and hypoxia, or mechanical stress (Azaizeh et al. 1992; Tyerman et al. 1999; Ye and Steudle 2006; Henzler et al. 2004; Ye et al. 2004; Lee et al. 2005a, b; Birner and Steudle 1993; Wan et al. 2004). The water stress hormone abscisic acid has been shown to have an ameliorative effect on AQPs tending to reopen closed AQPs (Freundl et al. 1998; Hose et al. 2000). ABA is also of key importance in physiology of desiccation as described in Chaps. 9, 12, and 16. For technical reasons, it has not been possible yet to measure effects of just low water potential as it occurs during drought or desiccation. However, the results obtained in the presence of high concentrations of osmolytes ("physiological drought") for roots or individual algal cells used as model systems may mimic the situation at low water potential, at least to some extent. The results obtained so far indicate that low water potential induces a "shrinkage" of AQPs. The osmolytes present on both sides of the membrane are excluded from the pores and, therefore, extract water from the pore, which would eventually and reversibly collapse under a certain stress (Ye et al. 2004). The experimental results obtained from both corn root cells and Chara internodes were in favor of the dehydration or "cohesion/tension" model, which also allowed to estimate pore volumes (Ye et al. 2005). When one extrapolates to the effects of low water potential and high concentration to conditions as they occur, and when plant tissue is subjected to desiccation (or just recovers from it), AQP activity should then soon become zero. The remaining water permeability of the bilayer in the absence of AQP activity may be around 10% of that in its presence. However, this should also disappear, when protoplasts collapse.
It is evident that in tissues such as the root, the radial flow of water may be either apoplastic or cell-to-cell, and the contribution of each pathway would depend on the hydraulic resistance along the two parallel pathways. In other words, due to the composite structure of roots (or of other tissue) transport should be composite in nature. This is dealt with in detail in the next section.
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