Water deficit during root development: effects on the growth of roots and osmotic water permeability of isolated root protoplasts

The effect of low water potentials on root growth of flax (Linum usitatissimum L. cv. Ariane), rape (Brassica napus L. de Candolle, cv. Bristol), hard wheat (Triticum turgidum L. cv. Cham1) and soft wheat (Triticum aestivum L. cv. Ritmo) was studied by measuring the osmotic water permeability (P,,,) of root protoplasts and the protein abundance of PIPI and PIP2 aquaporins. These different species require more or less water, the most sensitive to water deficit being flax and rape. Ritmo, is a cultivar of wheat adapted to temperate zones, while the other cultivar Cham1 is adapted to low-rainfall areas. The seedlings were germinated and grown in water, salt or sugar solutions at different water potentials. The values of P-OS for flax, rape and Cham1 wheat were normally distributed and could be characterized by mean +/- SD. Root protoplasts from water-grown seedlings had P-OS values of 485+/-159 mum s(-1) (flax), 582+/-100 mum s(-1) (rape), and 6.3+/-3.5 mum s(-1) (Cham1). At the same age, the protoplasts from Ritmo exhibited a much wider range of values than the protoplasts of Cham1. When seedlings were grown under conditions of osmotic or salt stress, the mass of the roots was reduced for all species. With 0.25 mol kg(-1) sorbitol or 0.125 M NaCl, the P-OS, for flax, rape and Cham1 remained constant or slightly increased, while for Ritmo the reduction in the mass of the roots was paralleled by a reduction in P,,,. Only Cham1 and Ritmo were able to germinate at a lower potential (0.5 mol kg-1 sorbitol). For Ritmo the reduction in the mass of the roots was paralleled by a reduction in P,,, when grown in this stress condition and both wheats exhibited low P,,, values. The expression of the PIP I and PIP2 aquaporins families was also studied by immunoblotting. We did not observe any difference in protein expression for any of the species, whatever the growing conditions. We suggest that the high P-OS values for flax and rape could play a role in the sensitivity of these plants at low water potential. The low native P-OS for Cham1 in spite of the expression of both families of aquaporins may reflect its adaptation to low-rainfall conditions by a functional regulation of the water channels. For a similar reason, the low-water-potential-induced P,,, of Ritmo may also correspond to a down-regulation of the aquaporins, reflecting adaptation of this wheat to water-deficit conditions.