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Peterson, 1998). When osmotic gradients are present, the water flow is negligible in the apoplast (Quintero et al., Excised 20-d-old sunflower roots (Helianthus annuus 1998; Steudle and Peterson, 1998) and transport occurs L. cv. Sun-Gro 380) with different Ca2+ status were mainly from cell to cell (Steudle and Frensch, 1989; used to study the effects of root Ca2+ status and Steudle, 1994). In recent years, it has been shown that abscisic acid (ABA) on the exudation rate (Jv), the membrane-integral proteins, or aquaporins, act as waterhydraulic conductivity of the root (Lp r ), the flux of specific channels (Chrispeels and Maurel, 1994; Maggio exuded Ca2+ (J Ca ), and the gradient of osmotic pres- and Joly, 1995). The presence of aquaporins in roots sure between the xylem and the external medium. Jv ( Yamamoto et al., 1991; Niemietz and Tyerman, 1997) and Lp r increased in direct proportion to the Ca2+ suggests that radial flow of water could be mainly transstatus of the root. Addition of ABA (4 mM) at the onset cellular (Chrispeels and Maurel, 1994; Maggio and Joly, of exudation in the external medium made Jv and Lp r 1995) and may be controlled by the opening or closing rise, and this effect also increased with the Ca2+ of these water channels (Chrispeels and Maurel, 1994; status. The effects of HgCl 2 and its interaction with Steudle and Henzler, 1995). ABA on water transport in the root were also studied. The existence of a regulatory mechanism which controls Addition of HgCl 2 (1 mM) 2 h after the onset of exuda- radial transport of water in the root has yet to be tion in the external medium quickly inhibited Jv, inde- determined (Quintero et al., 1998). It is known that roots pendently of the presence of ABA in the root medium. oVer the greatest resistance to water flow ( Weatherley, The results recorded here point to the involvement of 1982), and that hydraulic conductivity of the root (Lp r ) ABA and Ca2+ in the regulation of root water flow, may be aVected by diverse forms of abiotic stress, such as well as the existence of aquaporins in the cell as salinity (Munns and Passioura, 1984; Joly, 1989), membranes of sunflower roots. anaerobiosis (Zhang and Tyerman, 1991), drought (North and Nobel, 1991) or nutritional stress (Radin and

Water transport in sunflower root systems: effects of ABA, Ca2+ status and HgCl2