4.1 Water balance and leaf permeability in Capparis
odoratissima
Previous studies of C. odoratissima showed that, unlike other
species coexisting in the same environment, C. odoratissimaincreased its biomass when the canopy, but not the soil, were supplied
with water (Díaz & Granadillo, 2005). However, the magnitude and
pathway of water uptake by individual leaves was not quantified. Our
detailed evaluations of water evaporation through time (i.e. epidermal
evaporation), and entry of water (measured as the increase of leaf
weight) from detached leaves serves to constrain the extreme values of
water inputs and outputs. During the initial epidermal evaporation with
the petiole sealed, water loss rates of C. odoratissima leaves
were 0.041mg cm-2 h-1, which results
in a permeance (P ), calculated with the saturation concentration
of water vapour at 20ºC (according to Kerstiens, 1996), of 0.66
10-5 m s-1. Compared with the 85
species for which the permeance was calculated from the mass loss of
detached leaves, C. odoratissima leaves fall within the lowest
ranges observed, similar to the values of species adapted to xeric
environments such as Opuntia camanchica , with a P value of
0.210-5 m s-1 (Pisek and Berger,
1938). Recent evaluations critically reviewed the meaning of permeances
calculated by Kerstiens, pointing to a high variability of this
parameter among species and biomes (Duursma et al., 2019). Our results
suggest that the leaves of C. odoratissima retain water
efficiently, although we could not be entirely sure that our leaves were
initially saturated with water (our measurements started sometime after
the leaves were detached). Furthermore, the dehydration rate of C.
odoratissima leaves in natural conditions is likely slower than in our
lab conditions, not only because the leaves are not detached, but also
because the relative humidity of the atmosphere oscillates between 50%
and 90% (Diaz & Granadillo, 2005).
Capparis odoratissima leaves immersed in water increased in
weight at a rate of 0.30 mg cm-2h-1, which is approximately seven times greater than
rates of epidermal evaporation. The rate of water absorption by C.
odoratissima leaves, however, is four times lower than inTillandsia mullenbecki , which grows in the Atacama Desert in
Chile, and is entirely dependent on foliar uptake (Raix et al., 2020).
Nevertheless, we estimate that 15h continuous exposure of the leaves to
surface moisture may result in a 50% increase in leaf water content.
Unlike in Tillandsia, we found that rates of water loss
immediately following leaf immersion were large (on average 0.27 mg
cm-2 h-1), and we interpret this as
an evidence that water is weakly retained in the millions of interstices
formed by the idioblasts, which were likely responsible for the high
initial rate of water loss. The fact that leaves increased in weight
over subsequent immersion and drying cycles demonstrates that water was
being transferred to the mesophyll.
The major barriers for water absorption and/or evaporation are the leaf
cuticles. Traditionally, the thickness of lipid coatings such as
cuticles, were associated with higher water retention of leaves
(Shields, 1950). Yet, the leaf cuticle is gradually revealing as a
permeable layer to absorb water and electrolytes (Fernández et al.,
2014; 2017). Thus, cuticle thickness may not have a direct relationship
with impermeability (Schuster et al., 2017), but rather its biochemistry
may be more important for water exchange between the mesophyll and the
atmosphere. In our experiments, the leaves loaded adaxially gained less
water than the ones loaded abaxially, pointing to a difference in
absorption by each surface, and indirectly pointing to their different
evaporative losses. The anatomy of leaves further played a role in these
capacities, such as the compact and convoluted cuticle in the lower side
of the leaves of C. odoratissima, which is thinner than that of
the upper surface. Stomata embedded in this cuticle, despite being
covered by a carpet of peltate hairs, which should reduce
evapotranspiration, are probably ways of water scape. Pubescence is a
typical character of xerophilous species, and functionally related with
protection against desiccation and thus temperature regulation (Shields,
1950; Fahn, 1986). Future work is needed to explore the impact of the
peltate hairs of the abaxial epidermis on reducing rates of water loss
by stomata in vivo .