research news particularly controversial, however, is its unclear relation to the thermodynamic basis of plant water relations. Thermodynamics dictate that since water must always flow, energetically speaking, 'down hill', then the water in the xylem elements must have some property that reduces its 0 to at least slightly lower than the 0 of the water in the soil. Numerous experiments have indicated that xylem water contains very few dis- solved solutes (dissolved solutes are one way 0 can be reduced), so a hydrostatic ten- sion appears the only other alternative. An additional complication in this debate is that the measurements of little or no tension in xylems have also been used to question the validity of measurements made with a device that is widely used to estimate tension in the xylem of plants, known as a pressure chamber I3. This side of the debate has given rise to some novel experimental ap- proaches, one being the spinning of a length of branch about its center to create a known tension based on centrifugal forces14. The re- sults of such experiments were a reasonable correspondence between the pressure cham- ber measurements and the tension expected from centrifugal forces, to a substantial ten- sion of about -2 MPa (equivalent to the ten- sion in a 200 m tall column of water). Hence, it would appear that tensions can be created in xylem vessels and measured accurately by the pressure chamber. Some of the debate surrounding the pressure chamber device may stem from the initial belief that it gave a direct measure of the tension in the xylem per se 13, whereas a more accurate descrip- tion is that it measures the matric compo- nent of 0 in the leaf apoplast15,16,which may not be equivalent to the tension in the xylem under all circumstances. The ongoing debate So how does water get to the top of tall trees, or for that matter, even short ones in drying soil? If it is by tension-cohesion, then how is the system engineered to avoid all the technical difficulties of reliably transporting metastable water, sometimes under harsh environmental conditions? The question of whether the transport is by a strictly thermo- dynamic mechanism of water flowing down a gradient in potential energy, or by an in- teresting hybrid mechanism involving some as yet unrecognized biological processes, is at the heart of the current debate. I suspect that as long as the controversy does not extend itself to plant consciousness, raising doubts about the plant's ability to transport water, our food and fibre supply should be safe! Acknowledgements K.S. is grateful for the sabbatical support of the CRC/Plant Science, the helpful com- ments of J. Passioura and R. Munns, and the lively discussions of these issues by the Pomology Dept's Friday Regular Open Graduate Section (FROGS). Ken Shackel Dept of Pomology, University of CNifornia, Davis, CA 95616, USA References I Klein, H.A. (1988) The Science of Measurement: a Historical Survey, Dover 2 Hales, S. (1727) Vegetable Staticks, or, an Account of some Statical Experiments on the Sap in Vegetables, Oldbourne 3 Dixon, H.H. and Joly, J. (1895) On the ascent of sap, Philos. Trans. R. Soc. London Ser. B 186, 563-576 4 Nobel, P.S. (1991) Physicochemical and Environmental Plant Physiology, Academic Press 5 Hayward, A.T.J. (1971) Negative pressure in liquids: can it be harnessed to serve man? Am. Sci. 59, 434-443 6 Zimmermann, U. et al. (1994) Xylem water transport: is the available evidence consistent with the cohesion theory? Plant Cell Environ. 17, 1169-1181 7 Smith, A.M. (1991) Negative pressure generated by octopus suckers: a study of the tensile strength of water in nature, J. Exp. Biol. 157, 257-271 8 Balling, A. and Zimmermann, U. (1990) Comparative measurements of the xylem pressure of Nicotiana plants by means of the pressure bomb and pressure probe, Planta 182, 325-338 Passioura, J.B. (1991) An impasse in plant water relations? Bot. Acta 104, 405-411 Hayward, A.T.J. (1970) New law for liquids: don't snap, stretch! New Sci. 45, 196-199 Balling, A. et al. (1988) Direct measurement of negative pressure in artificial-biological systems, Naturwissenschaflen 75,409-411 Canny, M.J. A new theory for the ascent of sap - cohesion supported by tissue pressure, Ann. Bot. 75, 343-357 Scholander, P.F., Bradstreet, E.D. and Hemmingsen, E.A. (1965) Sap pressures in vascular plants, Science 148, 339-346 Holbrook, N.M., Burns, M.J. and Field, C.B. (1995) Negative xylem pressures in plants: a test of the balancing-pressure technique, Science 270, 1193-1194 Tyree, M.T. and Hammel, H.T. (1972) The measurement of the turgor pressure and the water relations of plants by the pressure- bomb technique, J. Exp. Bet. 23, 267-282 Passioura, J.B. (1980) The meaning ofmatric potential, J. Exp. Bot. 31, 1161-1169 9 10 11 12 13 14 15 16 The gene-for-gene relationship: from enigma to exploitation Genetics has transformed plant pathology on three occasions: at the turn of the cen- tnry, when disease resistance was observed in plants as yet another Mendelian trait; in the mid-1940s, when H.H. Flor proposed the gene-for-gene hypothesis to explain the correspondence of disease resistance with Mendelian traits (so-called avirulence genes) in the parasite; and in recent years, when several research groups first used molecular genetics to identify and decode the DNA sequences of plant genes required for disease resistance. It was auspicious timing, therefore, that the 1995 president of the British Society for Plant Pathology (BSPP), Ian Crute (HRI-Wellesbourne), selected a theme for the society's annual meeting entitled, 'The gene-for-gene rela- tionship: from enigma to exploitation'*. The meeting proved to be of wide inter- national appeal, with more than 200 participants from around the world - nearly double the usual attendance at BSPP meetings. Twenty-six speakers were assembled to present the results of their research on the genetics of a wide range of host and para- site interactions. Ian Crute set the stage with a presidential address in which he drew on examples from his own career (transcript to be published in Plant Pathology). He was responsible for pioneer- ing work on the genetics of host and para- site genes involved in lettuce downy mildew, with the result that it has become one of the best genetically defined exam- ples of plant parasitic symbiosis. Practical *BSPP Presidential Meeting, University of Warwick, UK, 12-15 December 1995. benefits resulting from his work were demonstrated by a description of how let- tuce breeders and commercial growers can now effectively manage the 'boom and bust cycles' caused by continual adaptation of the parasite to resistant cultivars and the fungicide metalaxyl. A further recent con- tribution he has made has been to establish downy mildew in Arabidopsis as a prime model for molecular genetic analyses of host-parasite recognition. Many of the talks were devoted to genetic analyses of host resistance. Some of the topics included: fine structural analyses of regions in plant genomes containing a complex of parasite recognition genes; mu- tatienal dissection of signal transduetion for primary host defence; and molecular characterisation of genes required for disease resistance. The genetic analysis of 1 O6 April 1996, Vol. 1, No. 4 © 1996 Elsevier Science Ltd