WATER RELATIONS OF WALL-FISSURE PLANTS

Water relations of the wall-fissure plants Cymbalaria muralis and Pari etaria judaica were studied during fall and winter 1991. On their drou ght-prone habitats these plants have to utilize available water quite efficiently. Alternatively, their drought tolerance must be rather hig h. Drought tolerance increased in Cymbalaria from August to December f rom a critical saturation deficit of 27% to 66%. This was achieved bot h by osmotic adjustment and by changed leaf elasticity, as shown with pressure-volume (pV) analysis. The osmotic potential at full turgor de creased from -0.7 MPa in August to -1.31 MPa in December. Concomitantl y, the turgor loss point was lowered from -0.72 to -1.5 MPa and the re lative water content at this point from 98 to 82%. Leaf elasticity inc reased during this time as indicated by a decrease of the bulk modulus of elasticity for 43 to 12 MPa, accompanied by an increase of the apo plastic water content from 10 to 28%. No such changes could be detecte d when leaf tissue of Parietaria judaica was subjected to pV-analysis and drought resistance treatments. The critical water saturation defic it always occurred at values around 48%. This species showed a low osm otic potential at water saturation of approximately -1.45 MPa througho ut the period of study. It lost leaf turgor at 92% relative water cont ent. Bulk elastic modulus ranged between 34 and 38 MPa, the apoplastic water content lay between 24 and 30%. It is concluded that the observ ed seasonal changes of the bulk water relations in Cymbalaria indicate a process of drought hardening which makes it easier to overcome wint er drought. It is speculated that this ability of Cymbalaria for droug ht hardening during fall and the inability of Parietaria to do so migh t be important for the different ranges of occurrence of the two speci es.