Developmental and physiological constraints on transport efficiency

In addition to the mechanical constraints on hydraulics discussed previously, there are other considerations that limit efficiency. To achieve Murray-type architecture in combination with higher conduit furcation numbers (i.e., moving up on the Murray optimum in Figure 4.2B), the xylem conduits in the major branches and main stem must be able to increase in diameter.

Developmental factors may limit conduit diameter growth. Intracellular transport and physiology can limit maximum cell size. This is probably part of the reason for the single-celled tracheid having a smaller maximum diameter [43] (about 80 ^m) than vessels [27] (about 500 ^m), which are composed of multiple single-celled vessel elements. Cambial development may also limit mature conduit width in secondary xylem. Diameter growth of a conduit beyond the width of its antecedent cambial initial may be limited by how much the development of flanking cambial derivatives can be modified.

Safety vs. efficiency trade-offs in transport function also limit effective conduit diameters. Large diameter vessels are known to be much more vulnerable to freezing-induced cavitation [44] and would seemingly be maladaptive where the growing season is frost prone. Larger vessels also tend to be more vulnerable to cavitation by water stress [45], in part because they tend to have greater area of interconnecting pits and hence greater chance of air-seeding cavitation [46]. These kinds of limitations probably combine to limit maximum conduit sizes and thus conduit furcation numbers and hydraulic efficiency.

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