Abstract
Ion-mediated enhancement of the hydraulic conductivity of xylem tissue (i.e. the ionic effect) has been reported for various angiosperm species. One explanation of the ionic effect is that it is caused by the swelling and shrinking of intervessel pit membranes due to the presence of pectins and/or other cell-wall matrix polymers such as heteroxylans or arabinogalactan-proteins (AGPs) that may contain acidic sugars. Here, we examined the ionic effect for six Acer species and their pit membrane chemistry using immunocytochemistry, including antibodies against glycoproteins. Moreover, anatomical features related to the bordered pit morphology and vessel dimensions were investigated using light and electron microscopy. The ionic effect varied from 18 % (± 9) to 32 % (± 13). Epitopes of homogalacturonan (LM18) and xylan (LM11) were not detected in intervessel pit membranes. Negative results were also obtained for glycoproteins (extensin: LM1, JIM20; AGP glycan: LM2), although AGP (JIM13)-related epitopes were detected in parenchyma cells. The mean vessel length was significantly correlated with the magnitude of the ionic effect, unlike other pit or vessel-related characteristics. Our results suggest that intervessel pit membranes of Acer are unlikely to contain pectic or other acidic polysaccharides. Therefore, alternative explanations should be tested to clarify the ionic effect.
Highlights
According to the cohesion–tension theory, long-distance water transport in plants occurs through the xylem tissue in a passive way (Askenasy 1895; Dixon and Joly 1895; Jansen & Schenk 2015)
One of the key findings of this paper is that none of the epitopes for the six antibodies tested could be detected in intervessel pit membranes of the six Acer species studied
The lack of HG and RG-I-related epitopes in intervessel pit membranes as based on LM18 and seven additional antibodies tested in previous studies (Table 1; Plavcova and Hacke 2011; Kim and Daniel 2012, 2013; Herbette et al 2015; but see Sun et al 2011) suggest that pectic polysaccharides appear to be absent in intervessel pit membranes of fully developed vessels, and that the hydrogel hypothesis does not fully explain the ionic effect
Summary
According to the cohesion–tension theory, long-distance water transport in plants occurs through the xylem tissue in a passive way (Askenasy 1895; Dixon and Joly 1895; Jansen & Schenk 2015). Individual vessel elements dissolve their primary and secondary cell wall partially to form perforated, multicellular vessels that are specialised for water transport These stacks of vessel elements are of finite length, which means that no individual vessel provides a direct connection from the roots to the canopy of a tree.
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