Conflicting demands on angiosperm xylem: Tradeoffs among storage, transport and biomechanics

Abstract

The secondary xylem of woody plants transports water mechanically supports the plant body and stores resources. These three functions are interdependent giving rise to tradeoffs in function. Understanding the relationships among these functions and their structural basis forms the context in which to interpret xylem evolution. The tradeoff between xylem transport efficiency and safety from cavitation has been carefully examined with less focus on other functions, particularly storage. Here, we synthesize data on all three xylem functions in angiosperm branch xylem in the context of tradeoffs. Species that have low safety and efficiency, examined from a resource economics perspective, are predicted to be adapted for slow resource acquisition and turnover as characterizes some environments. Tradeoffs with water storage primarily arise because of differences in fibre traits, while tradeoffs in carbohydrate storage are driven by parenchyma content of tissue. We find support for a tradeoff between safety from cavitation and storage of both water and starch in branch xylem tissue and between water storage capacity and mechanical strength. Living fibres may facilitate carbohydrate storage without compromising mechanical strength. The division of labour between different xylem cell types allows for considerable functional and structural diversity at multiple scales.

Coordination and trade‐offs between leaf and stem hydraulic traits and stomatal regulation along a spectrum of isohydry to anisohydry

“…The high vessel fraction could contribute to high transport efficiency (Gleason et al, ). However, using a data set collected from chaparral shrubs growing at field sites in southern California, Pratt and Jacobsen () found no relationship between stem capacitance and stem‐specific conductivity. There are two likely explanations for these inconsistent results.…”

“…Second, the six species in our study differed substantially in their degree of anisohydry (12‐fold relative range in hydroscape area) as well as in stem water storage (sixfold relative range in capacitance). The species studied by Pratt and Jacobsen () formed a larger data set but yielded a relatively narrow range in stem water storage (fourfold relative range in stem capacitance).…”

Wood allocation trade‐offs between fiber wall, fiber lumen, and axial parenchyma drive drought resistance in neotropical trees

“…Furthermore, capacitance buffers the drop of leaf and branch xylem water potential during moments of high evaporative demand. In this way, capacitance helps to avoid plant desiccation that would initiate xylem embolism, leaf turgor loss and eventually drought‐induced mortality (Pratt & Jacobsen, ; Tyree, Engelbrecht, Vargas, & Kursar, ; Wolfe, ).…”

Mechanisms for minimizing height‐related stomatal conductance declines in tall vines

“…Summed for an entire plant, the water supply used in transpiration may be greater than the amount of water taken up at the same time from the soil because of the use of stored water. Plant utilization of water stored in their tissues can contribute a significant fraction of overall water loss, especially in lianas made of soft and elastic tissues (Pratt & Jacobsen, ; Scholz, Phillips, Bucci, Meinzer, & Goldstein, ). Water storage capacity (capacitance) can be defined as the amount of water withdrawn from a given volume or area of a plant tissue per ΔΨ .…”