4.3 The biochemistry of foliar water uptake in Capparis
odoratissima
The pathway of atmospheric water entry in the leaves of C.
odoratissima involves hygroscopic materials deposited in the leaf
coatings (Gouvra & Gamatipoulus, 2003). In the current work, we first
describe the thick walled structures composing the multicellular peltate
hairs, which project pectins to the external part, suggesting their
involvement in the initial water capture when condensation happens in
the abaxial side. A few reports have revealed the presence of pectins in
trichomes, such as species from semi-arid forests like Crombretum
leprosum (Pina et al., 2016), the tropical species Drymis
brasiliensis (Eller et al., 2013), or in the trichomes of Fagus(Schreel et al., 2020). In addition, we revealed that both epidermal and
spongy mesophyll cells show a high concentration of un-esterified
pectins in their cell walls. This is in line with the ubiquity of
pectins within the leaf mesophyll. In the leaves of C.
odoratissima , the tight association of mesophyll cells and their
exposed pectins with the numerous idioblasts may imply a pulling force
for water deposited on the leaf surfaces, using idioblasts as carriers.
The idioblasts of C. odoratissima are highly hygroscopic, with
cellulosic walls that contain polar molecules for water attachment, but
also partially lignified, suggesting a secondary role in defense and
structural support. A finely tuned water uptake in C.
odoratissima is revealed by our experiments with the apoplastic dye
tracer Lucifer Yellow. Water entering from the surrounding atmosphere to
the idioblasts flows through an intricate network of crenations and
branches within the idioblasts that connect all leaf tissues and the
exterior. Most strikingly is the fact that epitopes belonging to
arabinogalacatan proteins are specifically located within these
channels. AGPs are highly branched proteins with a short amino acid
backbone attached to the plasma membrane of cells, and a large
saccharidic part that has been related with nutritive and/or signaling
functions between cells (Ellis et al., 2010). AGPs play different roles
in plant development, including mate recognition and support during
reproduction, proper early seedling development, and many others
(Majewska-Sawka & Nothnagel, 2000; Vaughn et al., 2007; Pereira et al.,
2016). However, the role of AGPs in plant hydration has been scarcely
studied. Remarkably, works evaluating the composition of the cell walls
in the resurrection plant Craterostigma wilmsii , which can
completely dry out and subsequently regain water, point to the
hygroscopic properties of AGPs as critical players in this rapid and
effective rehydration (Vicré et al, 2004). AGP-related proteins have
been previously related with the tensile strength of stems due to their
participation in secondary cell wall composition, such as in the vessels
or fibers (Ito et al., 2005; Liu et al., 2013). But the presence of AGPs
have never been reported before in idioblasts with such specific pattern
as in the current work. What this reveals are the biochemical complexity
of sclerenchymatous tissues, which are possible effectors enabling
hydration of tissues during periods where water deficits in the soil
combines with water saturation in the atmosphere (Figure 8). Thus, AGPs
may be secreted to the lumen of the idioblasts during development,
serving as bridges of water uptake between the atmosphere and the leaf
mesophyll. Future works should explore the presence of AGPs in
sclerenchymatous tissues of other species, as well as their putative
role in leaf capacitance.