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

Abstract

The ability to transport water through tall stems hydraulically limits stomatal conductance (gs), thereby constraining photosynthesis and growth. However, some plants are able to minimize this height‐related decrease in gs, regardless of path length. We hypothesized that kudzu (Pueraria lobata) prevents strong declines in gs with height through appreciable structural and hydraulic compensative alterations. We observed only a 12% decline in maximum gs along 15‐m‐long stems and were able to model this empirical trend. Increasing resistance with transport distance was not compensated by increasing sapwood‐to‐leaf‐area ratio. Compensating for increasing leaf area by adjusting the driving force would require water potential reaching −1.9 MPa, far below the wilting point (−1.2 MPa). The negative effect of stem length was compensated for by decreasing petiole hydraulic resistance and by increasing stem sapwood area and water storage, with capacitive discharge representing 8–12% of the water flux. In addition, large lateral (petiole, leaves) relative to axial hydraulic resistance helped improve water flow distribution to top leaves. These results indicate that gs of distal leaves can be similar to that of basal leaves, provided that resistance is highest in petioles, and sufficient amounts of water storage can be used to subsidize the transpiration stream.

Tip‐to‐base xylem conduit widening as an adaptation: causes, consequences, and empirical priorities

“…3) are the mechanical and storage roles of secondary xylem, but it is clear that the conduction‐storage‐support tradeoff is key to understanding xylem evolution (Montes‐Cartas et al ., 2017; Pratt & Jacobsen, 2017). That vessels scale similarly across self‐ and nonself‐supporting plants suggests that mechanics has no influence on the rate of tip‐to‐base conduit widening (Rosell & Olson, 2014; see also Domec et al ., 2019). Recent studies in angiosperms (Olson et al ., 2014, 2020) indicate that vessels become slightly more abundant per square millimeter at the base with increase in tree height than would be expected given the simple geometric tradeoff in which vessel density should scale linearly with the square of vessel diameter (Ewers et al ., 2007; Zanne et al ., 2010).…”

“…Gravity and its contribution to resistance, for example, does not appear to be the main driver favoring conduit widening. If gravity were the main selective agent, then prostrate, climbing, and other nonvertical habits would have differing conduit widening patterns, but they are apparently identical to erect self‐supporting plants (Rosell & Olson, 2014; see also Domec et al ., 2019). However, the relative influences of gravity and pathlength on conduit widening remain to be disentangled in detail.…”

Filling the gaps in gene banks: Collecting, characterizing, and phenotyping wild banana relatives of Papua New Guinea

“…Musa ingens is the tallest banana species, reaching up to 15 m. This high stature enables it to compete for sunlight with surrounding trees (Supplemental Figure S4). This however would imply a higher hydraulic resistance (Domec et al., 2019) as water is transported over a huge distance. As a result, it is reasonable to assume that M. ingens is restricted to high altitudes because the low temperature, frequent fogs, and low evaporative demands create the conditions to transport water 15 m upwards without cavitating its xylem vessels.…”