Diel variation in specific hydraulic conductivity ( k s ) was recorded in petioles of two savanna tree species, Schefflera macrocarpa and Caryocar brasiliense , from central Brazil. These two species have compound leaves with long petioles (10-30 cm). In both species, petiole k s decreased sharply with increasing transpiration rates and declining leaf water potentials ( y L ) during the morning. Petiole k s increased during the afternoon while the plants were still transpiring and the water in the non-embolized vessels was still under tension. Dye experiments confirmed that in both species diel variation in k s was associated with embolism formation and repair. When transpiration was prevented in individual leaves, their petiole k s and water potential remained close to their maximum values during the day. When minimum daily y L on selected branches was experimentally lowered by 0.2-0.6 MPa, the rate of k s recovery during the afternoon was slower in comparison with control branches. Several field manipulations were performed to identify potential mechanisms involved in the refilling of embolized petiole vessels. Removal of the cortex or longitudinal incisions in the cortex prevented afternoon recovery of k s and refilling of embolized vessels. When distilled water was added to petiole surfaces that had been abraded to partially remove the cuticle, k s increased sharply during the morning and early afternoon. Evidence of starch to sugar conversion in the starch sheath cells surrounding the vascular bundles of the petioles was observed during periods of rapid transpiration when the abundance of starch granules in the starch sheath cells surrounding the vascular bundles decreased. Consistent with this, petiole sugar content was highest in the early afternoon. The most parsimonious explanation of the field observations and the experimental results was that an increase in osmotically active solutes in cells outside the vascular bundles at around midday leads to water uptake by these cells. However, the concurrent increase in tissue volume is partially constrained by the cortex, resulting in a transient pressure imbalance that may drive radial water movement in the direction of the embolized vessels, thereby refilling them and restoring water flow. This study thus presents evidence that embolism formation and repair are two distinct phenomena controlled by different variables. The degree of embolism is a function of tension, and the rate of refilling a function of internal pressure imbalances.

ABSTRACT

Stomatal conductance per unit leaf area in well-irr-igated field-and gr-eenhouse-gr-own sugarcane incr-eased with leaf ar-ea up to 0.2 m' plant ', then declined so that maximum transpiration per plant tended to saturate rather than increase linearly with further incr-ease in leaf area. Conductance to liquid water tr-ansport exhibited par-allel changes with plant size. This coordination of vapour phase and liquid phase conductances r-esulted in a balance between water loss and water transport capacity, maintaining leaf water status r-emarkably constant over a wide range of plant size and growing conditions. The changes in stomatal conductance were not related to plant or leaf age. Partial defoliation caused r-apid increases in stomatal conductance, to re-establish the original relationship with remaining leaf area. Similarly, pruning of roots caused r-apid r-eductions in stomatal conductance, which maintained or improved leaf water status. These results suggest that sugarcane stomata adjusted to the ratio of total hydr-aulic conductance to total transpiring leaf area. This could be mediated by root metabolites in the transpiration stream, whose delivery per unit leaf area would be a function of the r-elative magnitudes of root system size, tr-anspir-ation rate and leaf area.

Stomatal control of crown transpiration was studied in Anacardium excelsum, a large‐leaved, emergent canopy species common in the moist forests of Central and northern South America. A construction crane equipped with a gondola was used to gain access to the uppermost level in the crown of a 35‐m‐tall individual. Stomatal conductance at the single leaf scale, and transpiration and total vapour phase conductance (stomatal and boundary layer) at the branch scale were measured simultaneously using the independent techniques of porometry and stem heat balance, respectively. This permitted the sensitivity of transpiration to a marginal change in stomatal conductance to be evaluated using a dimensionless coupling coefficient (1‐ω) ranging from zero to 1, with 1 representing maximal stomatal control of transpiration. Average stomatal conductance varied from 0.09 mol m−2 s−1 during the dry season to 0.3 mol m−2 s−1 during the wet season. Since boundary layer conductance was relatively low (0.4 mol m−2 s−1), 1‐ω ranged from 0.46 during the dry season to only 0.25 during the wet season. A pronounced stomatal response to humidity was observed, which strongly limited transpiration as evaporative demand increased. The stomatal response to humidity was apparent only when the leaf surface was used as the reference point for measurement of external vapour pressure. Average transpiration was predicted to be nearly the same during the dry and wet seasons despite a 1 kPa difference in the prevailing leaf‐to‐air vapour pressure difference. The patterns of stomatal behaviour and transpiration observed were consistent with recent proposals that stomatal responses to humidity are based on sensing the transpiration rate itself.

Dwarf mistletoe ( Arceuthobium spp.) is a hemiparasite that is said to be the single-most destructive pathogen of commercially valuable coniferous trees in many regions of the world. Although its destructive nature is well documented in many respects, its effects on the physiology of its host are poorly understood. In the present study, water and carbon relations were characterized over a range of scale from leaf to whole tree in large ( C values were 2.8‰ more negative in infected than in uninfected individuals, consistent with the absence of stomatal adjustment to diminished photosynthetic capacity. Adjustments in hydraulic architecture of infected trees thus contributed to homeostasis of water transport efficiency and transpiration on a leaf area basis, whereas both carbon accumulation and photosynthetic water use efficiency were sharply reduced at both the leaf and whole-tree scale.