ABSTRACT

In the present study the linkage between hydraulic, photosynthetic and phenological properties of tropical dry forest trees were investigated. Seasonal patterns of stem-specific conductivity ( K SP ) described from 12 species, including deciduous, brevi-deciduous and evergreen species, indicated that only evergreen species were consistent in their response to a dry-to-wet season transition. In contrast, K SP in deciduous and brevi-deciduous species encompassed a range of responses, from an insignificant increase in K SP following rains in some species, to a nine-fold increase in others. Amongst deciduous species, the minimum K SP during the dry season ranged from 6 to 56% of wet season K SP, indicating in the latter case that a significant portion of the xylem remained functional during the dry season. In all species and all seasons, leaf-specific stem conductivity ( K L ) was strongly related to the photosynthetic capacity of the supported foliage, although leaf photosynthesis became saturated in species with high K L . The strength of this correlation was surprising given that much of the whole-plant resistance appears to be in the leaves. Hydraulic capacity, defined as the product of K L and the soil-leaf water potential difference, was strongly correlated with the photosynthetic rate of foliage in the dry season, but only weakly correlated in the wet season.

Environmental and physiological regulation of transpiration were examined in several gap-colonizing shrub and tree species during two consecutive dry seasons in a moist, lowland tropical forest on Barro Colorado Island, Panama. Whole plant transpiration, stomatal and total vapor phase (stomatal + boundary layer) conductance, plant water potential and environmental variables were measured concurrently. This allowed control of transpiration (E) to be partitioned quantitatively between stomatal (g ) and boundary layer (g) conductance and permitted the impact of invividual environmental and physiological variables on stomatal behavior and E to be assessed. Wind speed in treefall gap sites was often below the 0.25 m s stalling speed of the anemometer used and was rarely above 0.5 m s, resulting in uniformly low g (c. 200-300 mmol m s) among all species studied regardless of leaf size. Stomatal conductance was typically equal to or somewhat greater than g . This strongly decoupled E from control by stomata, so that in Miconia argentea a 10% change in g when g was near its mean value was predicted to yield only a 2.5% change in E. Porometric estimates of E, obtained as the product of g and the leaf-bulk air vapor pressure difference (VPD) without taking g into account, were up to 300% higher than actual E determined from sap flow measurements. Porometry was thus inadequate as a means of assessing the physiological consequences of stomatal behavior in different gap colonizing species. Stomatal responses to humidity strongly limited the increase in E with increasing evaporative demand. Stomata of all species studied appeared to respond to increasing evaporative demand in the same manner when the leaf surface was selected as the reference point for determination of external vapor pressure and when simultaneous variation of light and leaf-air VPD was taken into account. This result suggests that contrasting stomatal responses to similar leaf-bulk air VPD may be governed as much by the external boundary layer as by intrinsic physiological differences among species. Both E and g initially increased sharply with increasing leaf area-specific total hydraulic conductance of the soil/root/leaf pathway (G ), becoming asymptotic at higher values of G. For both E and g a unique relationship appeared to describe the response of all species to variations in G. The relatively weak correlation observed between g and midday leaf water potential suggested that stomatal adjustment to variations in water availability coordinated E with water transport efficiency rather than bulk leaf water status.

We show the fault-tolerant encoding, measurement, and operation of a logical qubit via quantum error detection.

Many trees in tropical dry forests flower during the dry season when evaporative demand is high and soil water levels are low. In this study the factors influencing the water balance of flowers from three species of dry forest trees were examined. Flowers had greater mucilage contents than leaves, high intrinsic and absolute capacitances, long time constants for water exchange and high transfer resistances. Flower water potentials were higher than in leaves and did not fluctuate over the lifespan of the flower. Flower water content also remained constant even though evaporation rates were high, suggesting that water was being supplied from the stem. In two of the species, the water potential gradient between flowers and leaves was opposite to that necessary for water transport from stem to flowers through the xylem, and it was therefore hypothesized that water may enter the flower through the phloem. Calculations showed that nectar production in these flowers could drive a sink of sufficient magnitude to allow water input via the phloem equal to water lost from the flower to the atmosphere.