Xylem vessel length, hydraulic efficiency, and safety from drought-induced cavitation
The function of xylem within woody plants includes water transport, mechanical support for the plant body, and the storage of resources such as carbohydrates. An important feature of xylem transport is a safety-efficiency tradeoff, whereby species with xylem safety from cavitation tend to be less efficient with respect to water transport. The structure of xylem vessels is important in explaining the safety-efficiency tradeoff, including the two structural traits of diameter and length of the xylem vessels. However, the mean length of vessels has been little studied compared to other vessel traits, which motivated our work.
We hypothesized that vessel length would be linked to xylem transport safety and efficiency. We predicted that species with greater vessel length would be more vulnerable to water stress-induced cavitation. Furthermore, based on a safety-efficiency tradeoff, we predicted that longer vessels would be more efficient water transport. We measured resistance to cavitation due to water stress, xylem specific conductivity, and mean vessel lengths and diameters for 18 chaparral shrub species. To determine relationships among traits we can correlation analyses.
We found strong evidence for a safety and efficiency tradeoff in stem xylem. Species with greater xylem hydraulic efficiency (Ks) and wider diameter vessels were more vulnerable to drought-induced cavitation (P50; Ks R=-0.574, P=0.013; vessel diameter R=0.697, P=0.001). Mean vessel length was positively correlated with xylem vessel diameter (R=0.721, P<0,001), but was not correlated with Ks (R=0.251, P=0.315). Species with longer vessels were more vulnerable to drought-induced cavitation (R=-0.510, P=0.031).
Vessel length is an important trait that affects the safety-efficiency tradeoff in xylem. Specifically, longer vessels are less safe from cavitation. Longer vessels may be more vulnerable to drought-induced cavitation because of an increased pit area that leads to a greater probability of a rare vulnerable pit. The lack of a relationship between vessel length and Ks suggests these traits vary independently. Interestingly, vessel length and vessel diameter were correlated suggesting a complex relationship between vessel length, diameter, and Ks. Other traits such as pit conductivity, pit area, and connectivity of vessels comprising the xylem network are likely interacting to create different ways in which traits vary to enhance Ks.