Abstract:
This study investigated the mechanisms involved in the
regulation of stomatal closure in Douglas-fir and evaluated
the potential impact of compensatory adjustments in
response to increasing tree height upon these mechanisms.
In the laboratory, we measured leaf hydraulic conductance
(Kleaf) as leaf water potential (Yl) declined for comparison
with in situ diurnal patterns of stomatal conductance (gs)
and Yl in Douglas-fir across a height gradient, allowing us to
infer linkages between diurnal changes in Kleaf and gs. A
recently developed timed rehydration technique was used
in conjunction with data from pressure–volume curves to
develop hydraulic vulnerability curves for needles attached
to small twigs. Laboratory-measured Kleaf declined with
increasing leaf water stress and was substantially reduced at
Yl values of -1.34, -1.45, -1.56 and -1.92 MPa for foliage
sampled at mean heights of approximately 20, 35, 44 and
55 m, respectively. In situ gs measurements showed that
stomatal closure was initiated at Yl values of -1.21, -1.36,
-1.74 and -1.86 MPa along the height gradient, which was
highly correlated with Yl values at loss of Kleaf. Cryogenic
scanning electron microscopy (SEM) images showed that
relative abundances of embolized tracheids in the central
vein increased with increasing leaf water stress. Leaf embolism
appeared to be coupled to changes in gs and might
perform a vital function in stomatal regulation of plant
water status and water transport in conifers. The observed
trends in gs and Kleaf in response to changes in Yl along a
height gradient suggest that the foliage at the tops of tall
trees is capable of maintaining stomatal conductance at
more negative Yl. This adaptation may allow taller trees to
continue to photosynthesize during periods of greater water
stress.
