Erhöhte Saugspannungswerte und morphologische Veränderungen durch transversale Einschnitte in einem Taxus-Stamm

Abstract

Summary Total suction tension (or, according to the ψ -nomenclature, total water potential) at a certain point in the plant body is the sum of three components: S = S H + S B + S R , with S H =hydrostatic tension, S B = static soil tension and S R = frictional tension due to water movement in soil and plant. Experimental increase of the total tension is usually achieved by reducing the soil water content. This approach has a readily measurable effect on S B , but it must also influence the soil component of S R to an extent not easily assessed. Since both S B and the soil component of S R act from outside the plant body, variations in the irrigation pattern do not provide a means to influence selectively the tensions in a certain region of the plant body while leaving other parts unchanged. In a large tree, however, total tensions at various heights above ground are widely different. This is due far more to an increase with height I of conduit resistances (i. e., S R ) than to the simple height effect of S H . The influence of S R on tota potentials in the crown of a large tree may be simulated experimentally by means of a double sawcut in the stem of a small tree, which increases transport resistances of the conduit. In the early spring of 1972, such an experiment was set up with a large bush of yew ( Taxus baccata , L.), and suction tensions were followed with a pressure bomb. Diurnal courses of suction tensions are presented for typical days. Some general trends become evident: 1. Tensions are always higher in the experimental part than in the control, but the extent of the difference depends on the climatic conditions. 2. Values in the control reach a minimum in the late evening, while those in the experimental twigs continue to decline during the night. An eventual coincidence of the two curves is prevented by the onset of transpiration in the morning. 3. Tensions in the control follow changes in the vapour pressure deficit rather closely, while the experimental part shows a tendency to hold a plateau value at about 21 atm under different atmospheric conditions. Thus, maximum tension differences occur whenever there is a sudden fall in Δ p-values. Such a situation leads to a rapid tension drop in the control twigs, while the experimental parts remain at their previous high level, sometimes for a prolonged period. It is supposed that differences in the tension pattern similar to those described may play an important role in the establishment of physiological and morphological differences between lower and upper crown of a high tree. Shoot and needle lengths and needle distances were compared for experimental and control parts in 1972 and 1973. Results agree with previous work on artificial drought and its morphological consequences in conifers. Since in most earlier experiments drought was induced in young plants by means of infrequent irrigation, the similarity of results obtained in an old tree by the sawcut technique seems to be of some significance.