Rainer Lösch

Tagesverlauf von Wasserpotential und Wasserbilanz bei Pflanzen verschiedener Standorte des fränkischen Wellenkalkes

Summary The daily rhythm of water potential, osmotic potential, and transpiration of plants from different associations on a xeric limestone site was investigated. The courses of potential evapotranspiration, light intensity, relative humidity, and temperature were measured simultaneously. Water potential, as measured with a pressure bomb, was largely dependent upon transpiration. This relationship between the daily course of water potential and transpiration was especially apparent during the midday depression on clear, cloudless days. During this period the water potential rises despite the great evaporative demand. This behaviour can be explained by a reaction of stomatal closure under increasing evaporation conditions independent of the actual plant water status. Similar results have been obtained in laboratory investigations and on desert plants. During the course of a day the water potential is often considerably below the osmotic potential. The dependence of plants upon the water availability on their respective sites is also indicated by the widely differing water potential values found for the same species but growing in associations with different microclimates.

Die Hitzeresistenz der Pflanzen des kanarischen Lorbeerwaldes

Summary Leaves of trees and shrubs of the macaronesian laurel forest were tested for their heat resistance. Laurus azorica proved to be the most resistant species. It became damaged at temperatures only higher than 56°C. The least resistance showed Myrsine canariensis with serious lesions even at about 48°C Herbs from the forest floor endured not as high temperatures; Canarina canariensis was killed already by a heat treatment of 40- 42°C. The climax vegetation of the laurel forest is often replaced at the Canary Islands by a tree·heath vegetation called Fayal-Brezal. The species of it on average do not reach a temperature tolerance of 50°C. In general a correlation exists between higher heat resistance and sclerophyllous leaves. No important differences in heat resistance can be found if canarian species are compared with allied plants of the mediterranean and west European area. The heat resistance of the macaronesian laurel forest species and the temperature conditions of their habitat are discussed finally with regard to the vegetation history of these plants.

Die Kaliumverteilung in den Schließzellen leptosporangiater Farne unterschiedlicher Stomatatypen

Summary The potassium content of open and closed stomata has been investigated in 46 fern species using the Macallum-test. About the half of the species showed substomatal ion-absorbent structures as they were recently described by S tevens and M artin (1977). Others showed a staining behaviour as it is known from many angiosperms. In the latter case there were higher amounts of potassiwn in open stomata than in closed ones. The occurrence of substomatal ion-absorbent structures could be correlated with diacytic stomata. A diffuse distribution of staining deposits occurred only in polocytic and pericytic stomata, where one subsidiary cell surrounds more than three fourths or the total circumference of a stoma. It may bc supposed that the substomatal ion-absorbent structures are involved in exchange processes between the guard cells and the two polar neighbouring cells.