Branch junction constriction and hydraulic limitation in two species in the Cape Proteaceae : a mechanism explaining the trade-off between longevity and degree of ramification in the Cape Proteaceae
Bachelor Thesis
2004
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University of Cape Town
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Abstract
The purpose of this study was to establish if hydraulic limitation and branch junction constriction explain the trade-off between longevity and increased ramification in species of the Cape Proteaceae. This was done by establishing if branch nodes are regions of greater resistance to xylem sap flow in the study species; attempting to quantify nodal resistance for a plant as a whole; and by determining if there is any evidence for hydraulic limitation or compensation in Leucadendron laureolum (Lam.) Fourc. and Leucospermum oleifolium (Berg.) R. Br. individuals. The extent of branch junction constriction was established by comparing the ratio of the xylem sap flow rates of adjacent branch segments of equal length. The ratio of a proximal intemodal to a distal intemodal segment was compared against the ratio of a proximal intemodal segment to a distal node. Branch nodes were found to create significant resistance to xylem sap flow rates in both species (p < .005 for L. laureolum, and p < .05 for L. oleifolium). Total nodal resistance in L. laureolum was modelled as equivalent extra branch length, and as loss of sap flow rate. Equivalent extra branch length increased exponentially with increased branching order, and increased much faster in more ramified plants. Loss of flow rate increased linearly with increased branching order and was unaffected by differences in ramification. This was because the nodes in more ramified plants had smaller basal sapwood areas, and the resistance imposed by nodes decreased with decreased basal sapwood area. As more ramified plants tend to branch earlier, and/or with greater temporal frequency, they will still accumulate greater nodal hydraulic resistance faster than less ramified plants, and thus may be limited to smaller size and younger maximum age. Branch specific conductivities (kₛ), leaf specific conductivities (LSC) and transpiration rates were investigated in higher (11 nodes) and lower (10 nodes) order branches of L. laureolum. All were found to be significantly lower in higher order branches (11 nodes), which can be related to greater nodal resistance to sap flow limiting the amount of water available to the area of developing xylem, causing the vessels to have narrower diameters. There was no significant difference in leaf to sapwood area ratios (Aₗ/As) between the two branching orders. Previous studies have suggested that changes in Aₗ/Aₛratios occur to compensate for hydraulic limitation. Reductions in Aₗ/Aₛin plants with greater hydraulic restriction allow LSC values, and thus transpiration rates, to stay as high as plants with less hydraulic restriction. As there was no such compensation, LSC values, and thus transpiration rates, were much lower in branches of higher order. Transpiration rate is a good indication of rates of photosynthesis. Lower transpiration rate in higher order branches is thus a good indication of hydraulic limitation on rates of photosynthesis with increased branching order. Hydraulic resistance in nodes thus imposes an important limitation on size and/or age of L. laureolum individuals, and explains the apparent trade-off between longevity and degree of ramification in the Cape Proteaceae.
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Slingsby, J. 2004. Branch junction constriction and hydraulic limitation in two species in the Cape Proteaceae : a mechanism explaining the trade-off between longevity and degree of ramification in the Cape Proteaceae. University of Cape Town.