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 .