Discussion
Observations of Δ18OLW values in maize
and rice are consistent with the Péclet effect in the veins. Unlike the
two-pool model where the f sw is a fixed volume
and not dependent on physiological factors
(Roden & Ehleringer 1999;
Roden et al. 2015;
Song et al. 2015;
Hirl et al. 2019), a positive
relationship between f sw and transpiration rate
(E ) was observed in all genotypes of maize and rice. According to
the Farquhar and Gan model, Δ18OLW is
the accumulation of the Péclet effect in each tissue relative to the
fraction of leaf water in the tissue, but several studies have found
that the xylem water pool is the principal, if not the only, location of
the Péclet effect (Helliker & Ehleringer
2000; Gan et al. 2002;
Farquhar & Gan 2003;
Gan et al. 2003;
Holloway-Phillips et al. 2016). In
support of this observation, we found that L mestimates in the mesophyll and xylem Péclet model were essentially 0,
indicating that the mesophyll Péclet effect was likely nonexistent.
However, observed variation in Δ18OLWacross genotypes, species, and treatments was not related to the
differences in the partial volume of the vasculature, meaning that
variation in f sw was influenced by anatomical and
physiological factors regulating water movement and the relative
importance of leaf water pools.
Anatomical differences between C3 and C4grasses such as suberization of the bundle sheath regulate the movement
of water out of the vasculature and through the mesophyll, contributing
to variation in leaf water isotopic composition. For example, it is not
clear which tissues should be included in the xylem water pool
(ϕ x) of the xylem Péclet model, andϕ x could include only the vein elements or
include associated ground tissue such as the bundle sheath and the large
quantity of parenchyma in the midrib (Ganet al. 2003). In C3 rice, the estimatedϕ x (and f sw) were
relatively similar to the partial volume of the entire vasculature,
which indicates that likely conducting and ground tissues associated
with the vasculature together are isotopically one pool of water.
Previous studies have made similar conclusions
(Gan et al. 2003;
Song et al. 2015;
Holloway-Phillips et al. 2016). In
contrast, the influence of xylem water in C4 maize may
be more constrained than in rice in that ϕ x may
be confined to the vascular bundles and may not necessarily include all
ground tissue associated with the veins. This difference between
C3 and C4 grass species may lie in that
C3 plants have less suberized bundle sheaths, leading to
greater outside xylem conductivity to support higher E and a
greater isotopic influence of xylem water on the mesophyll water pool
(Sonawane et al. in review; Kocacinar &
Sage 2003). Therefore, the lower
Δ18OLW in C3 grass
species than C4 species may depend on more than
anatomical features such as interveinal distance (vein density) and
include the facility of water movement out of the xylem and outside
xylem conductivity.