COS 50-3
The number of vessel connections is linked to xylem transport safety and efficiency in stems and roots of chaparral shrubs
The functions of xylem tissue in woody plants are to transport water, mechanically support the plant body, and to store resources such as carbohydrates. The structure of xylem is intimately associated with its function. One structural characteristic of the xylem that broadly differs among species is how many unique vessels are connected to a given vessel via pits, referred to here as connectivity. There has been little research quantifying the influence of vessel connectivity on hydraulic function of the xylem.
We hypothesized that vessel connectivity would be linked to xylem transport safety and efficiency. We predicted that more connections among vessels, allowing for more pathways for gas to spread, would lead to xylem that is more vulnerable to cavitation, or less safe. We also predicted that greater connectivity would facilitate greater transport efficiency.
Resistance to drought-induced cavitation (safety), xylem specific conductivity (transport efficiency), and connectivity were measured for stems of 36 chaparral shrub species. The same measurements were taken for roots of 25 chaparral shrub species from a common garden. We quantified connectivity as the mean number of unique vessels sharing cell walls with surrounding vessels in cross section. To assess if traits were associated with one another we ran correlation analyses of the measured values as well as values adjusted for phylogenetic relationships among the sampled taxa (phylogenetic independent contrasts).
Results/Conclusions
Lower connectivity was associated with greater resistance to cavitation in stems for all species. Also in stems, greater xylem specific hydraulic conductivity was associated with lower resistance to cavitation. The same was true for roots. However, in roots there was a positive association between transport efficiency and connectivity, whereas stems had no significant association between these two variables. The phylogenetic independent contrast results did not change these conclusions.
We conclude that connectivity is an important structural feature of xylem that is linked to cavitation resistance and water transport efficiency. These two factors may be associated because connectivity determines the pathways for gas and water to move between vessels and throughout the vessel network, leading to greater vulnerability to cavitation and efficiency in water transport. Connectivity within the xylem network may be an important determinant of the tradeoff between xylem safety and efficiency in woody angiosperms.