Analysis on the tapering of Xylem conduits as a compensation mechanism for hydraulic limitations to tree growth

In trees, water flows throughout a network of xylem conduits by mean of a negative pressure gradient between sites of consumption (leaves) and absorption (roots), the distance of which can be longer than 120 m in the world's tallest specimens. The architecture of the transport system must guarantee an adequate water supply and the mechanical stability of the tree. In fact, increased hydraulic constraints cause strong limitations to height growth. Trees adopt a number of structural modifications to compensate for the effect of these constraints and the tapering of xylem conduits is probably the most effective. Recent studies seemed to support the WBE model's hypothesis about the universality of the degree of conduit tapering in vascular plants, which should converge to a threshold value that minimizes the effect of path length on the total hydraulic resistance, which hence would remain substantially constant with increasing tree height. Therefore, it has been hypothesized that hydraulic constraints, which lead to the decline in the rate of height growth, are consequent to a reduced degree of conduit tapering, whereas the maximum height is substantially set by the absolute dimensions of xylem conduits, in particular those at the apex, where the most hydraulic resistance is concentrated. The aim of this work was to evaluate the combined effect of the dimension of xylem conduits and their tapering in compensating the inhibition-action of hydraulic constraints on height growth. Among the discussed topics, the first is a thorough analysis of the mathematical and logical structure of the WBE model, while the others describe empirical experiments. The first consists of an analysis of the structural modifications of the xylem transport system during ontogenesis in A. pseudoplatanus. Specimens from two age/size classes (young/small and old/tall) were selected and the use of grafted plants (i.e., clones of the parent plants selected in the field) as controls allowed to decouple the auto-correlation between age and dimensions, and thus to determine which of these two factors is responsible for the structural modifications. In the fourth and fifth chapter, the importance is discussed of conduit tapering in coniferous species (Pinus cembra, Larix decidua and Picea abies) at the treeline, where low temperatures inhibit the formation of wide conduits and height growth is strongly limited. In P. cembra, the analysis of conduit tapering was extended to roots, whereas on the selected specimens of P. abies a heating system was applied to the developing apical bud for two successive growing seasons. In chapter 6, a new hypothesis is elaborated regarding the heartwood formation in A. pseudoplatanus, where the intra-ring sectoriality of water flows, tested by a dye injection into the innermost ring of 5-6 year-old, 1 m long branches, and conduit tapering might play a key role. In all the experiments, wooden samples were extracted at different distances along the longitudinal axis of each stem/branch. Samples were embedded in paraffin, cut with a rotary microtome at 10-12 ?m and sections were stained with safranine and fixed permanently on slides. Slides were observed under the microscope, the lumen areas of conduits measured and the averaged hydraulically weighted diameter assessed for each distance. In A. pseudoplatanus, an overall reduction in the degree of conduit tapering and an increase in width of the apical conduits of old/tall trees compared to young/small ones was not detected in the relative grafts. The degree of tapering in the small plants, irrespective of their meristematic age, resulted in agreement with the WBE model's principle of minimization and stabilization of the total hydraulic resistance. These structural modifications were therefore connected to the size variations rather than to proper ageing. Moreover, the safranine injected into the innermost ring of branches of the same species flowed until the apex, remaining confined within the innermost ring at each internode, i.e., without any radial spread between successive rings, thus demonstrating a substantial intra-ring sectoriality of water flows. At the treeline, trees showed a degree of conduit tapering on average in agreement with the WBE model's prediction. Moreover, warming applied to the developing apical buds promoted the formation of wider apical conduits and apical shoots resulted as longer, thus supporting the hypothesis of a hydraulic limitation to height growth also in trees at the treeline. Lastly, it was observed that the tapering of xylem conduits is not confined to the aboveground organs, but also continues along the roots, where the degree of tapering resulted as higher than that of the stem. The analyses presented in this work demonstrates the importance of conduit tapering as a fundamental mechanism of compensation for the limiting effect of hydraulic constraints on growth in height, whose maximum seems to substantially depend on the possibility of forming wide apical conduits.