First insights into the functional role of vasicentric tracheids and parenchyma in eucalyptus species with solitary vessels: do they contribute to xylem efficiency or safety?

Abstract

The relationship between hydraulic specific conductivity (k) and vulnerability to cavitation (VC) with size and number of vessels has been studied in many angiosperms. However, few of the studies link other cell types (vasicentric tracheids (VT), fibre-tracheids, parenchyma) with these hydraulic functions. Eucalyptus is one of the most important genera in forestry worldwide. It exhibits a complex wood anatomy, with solitary vessels surrounded by VT and parenchyma, which could serve as a good model to investigate the functional role of the different cell types in xylem functioning. Wood anatomy (several traits of vessels, VT, fibres and parenchyma) in conjunction with maximum k and VC was studied in adult trees of commercial species with medium-to-high wood density (Eucalyptus globulus Labill., Eucalyptus viminalis Labill. and Eucalyptus camaldulensis Dehnh.). Traits of cells accompanying vessels presented correlations with functional variables suggesting that they contribute to both increasing connectivity between adjacent vessels-and, therefore, to xylem conduction efficiency-and decreasing the probability of embolism propagation into the tissue, i.e., xylem safety. All three species presented moderate-to-high resistance to cavitation (mean P values = -2.4 to -4.2 MPa) with no general trade-off between efficiency and safety at the interspecific level. The results in these species do not support some well-established hypotheses of the functional meaning of wood anatomy.

Wood density and anatomy of three Eucalyptus species: implications for hydraulic conductivity

“…As shown in Zanne et al (2010), we also observed a lack of orthogonality between S and F, and a greater influence of F on Ks than that of S. However, in contrast to their results, the variation in Ks was more influenced by F than by S. In addition, contrary to what we expected, the relative weight of S on Ks (δ coefficient) was slightly lower in each of the studied species (or similar in the pooled analysis of the three species) than in the general survey by Zanne et al (2010). This means that vessel composition, estimated by S, has a low influence on Ks, in contrast to previous findings showing a high correlation between measures of vessel diameter distribution and measured maximum Ks (Barotto et al, 2016). The relative influence of S on Ks, which was low compared to F, is also a counterintuitive result considering the high relative weight of large vessels on Ks, since this is directly proportional to the 4th power of vessel diameter (Hagen Poiseulle Law).…”

“…This is probably due to the fact that, being a species with high resistance to adverse environmental conditions, E. viminalis is usually grown in areas with some kind of growth-limiting factor (drought, low temperatures). This is in agreement with the results obtained by Barotto et al (2016) in a study of these two species planted in a common place in Argentina (analysis without environmental constraints) that compare hydraulic conductivity and vulnerability to cavitation (at branch level). Their results showed that, compared with E. globulus, E. viminalis had higher specific hydraulic conductivity (both measured and theoretically estimated from anatomy) and higher cavitation resistance.…”

“…In the three species, it seems that, at least at branch level, there is no trade-off between xylem safety and efficiency, but the opposite. Species with large vessels and high ks (such as E. viminalis) are the safest (Barotto et al, 2016). There is no information about vulnerability to cavitation at stem level, and we cannot extrapolate results from branches to stems.…”

“…However, studies using different approaches have suggested that Eucalyptus wood structure (anatomy and density of branches) is associated with functional traits involved in stress tolerance or avoidance (e.g. Monteoliva et al, 2015;Barotto et al, 2016), and that it determines the distribution patterns of different species in an aridity stress gradient (Pfautsch et al, 2016). Analysis relating branch anatomy and function in three Eucalyptus species showed that vessel diameter distribution is a key trait correlated not only with measured maximum Ks, but also with the species vulnerability to cavitation (Barotto et al, 2016).…”

“…Monteoliva et al, 2015;Barotto et al, 2016), and that it determines the distribution patterns of different species in an aridity stress gradient (Pfautsch et al, 2016). Analysis relating branch anatomy and function in three Eucalyptus species showed that vessel diameter distribution is a key trait correlated not only with measured maximum Ks, but also with the species vulnerability to cavitation (Barotto et al, 2016). That analysis did not include the traits F and S used by Zanne et al (2010), so it is not clear whether S accounts for the variation in Ks among Eucalyptus species.…”