ALBERT

Notes: The canopies of large broad-leaf trees exhibit significant heterogeneity in both micro-environmental conditions and leaf morphology. Whether the visible differences in the size and shape of leaves from the top and bottom of the crown are determined prior to bud break or result from different patterns of leaf expansion is not known. Analysis of ontogenetic changes of both the degree of lobing and vein density in Quercus rubra demonstrates that leaves throughout the crown are identical in size and shape at the time of bud break. Morphological adaptation to the local micro-environment takes place during the expansion phase and starts after the determination of the vascular architecture has been completed. Leaves from the bottom of the crown undergo greater expansion in the tissue close to the main veins than occurs either in the more peripheral tissue of the same leaf or anywhere in leaves from the top of the crown. This results in a water transport system that is well suited to the low evaporative rates near the bottom of the crown, but inadequate for the conditions found at the top of the tree. Acclimation of leaf form and function based upon differential expansion may be entirely driven by the local hydraulic demand during the expansion phase, resulting in leaf size and vein density being determined during development by the same hydraulic properties which will constrain the size of leaf that can be functionally supported at maturity.

Notes: Sieve tubes are primarily responsible for the movement of solutes over long distances, but they also conduct information about the osmotic state of the system. Using a previously developed dimensionless model of phloem transport, the mechanism behind the sieve tube's capacity to rapidly transmit pressure/concentration waves in response to local changes in either membrane solute exchange or the magnitude and axial gradient of apoplastic water potential is demonstrated. These wave fronts can move several orders of magnitude faster than the solution itself when the sieve tube's axial pressure drop is relatively small. Unlike the axial concentration drop, the axial pressure drop at steady state is independent of the apoplastic water potential gradient. As such, the regulation of whole-sieve tube turgor could play a vital role in controlling membrane solute exchange throughout the translocation pathway, making turgor a reliable source of information for communicating change in system state.

Notes: The functional importance of water storage in the arborescent palm, Sabal palmetto, was investigated by observing aboveground water content, pressure-volume curve parameters of leaf and stem tissue and leaf epidermal conductance rates. The ratio of the amount of water stored within the stem to the leaf area (kg m−2) increased linearly with plant height. Pressure-volume curves for leaf and stem parenchyma differed markedly; leaves lost turgor at 0.90 relative water content and –3.81 MPa, while the turgor loss point for stem parenchyma occurred at 0–64 relative water content and −0.96 MPa. Specific capacitance (change in relative water content per change in tissue water potential) of stem parenchyma tissue was 84 times higher than that of leaves, while the bulk modulus of elasticity was 346 times lower. Leaf epidermal conductance rates were extremely low (0.32–0.56 mmol m−2 s−1) suggesting that S. palmetto are able to strongly restrict foliar water loss rates. Structurally, stems of S. palmetto appear to be well suited to act as a water storage reservoir, and coupled with the ability to restrict water loss from leaf surfaces, may play an important role in tree survival during periods of low water availability.

Notes: For decades, botanists have considered Winteraceae as the least modified descendents of the first angiosperms primarily because this group lacks xylem vessels. Because of a presumed high resistance of a tracheid-based vascular system to water transport, Winteraceae have been viewed as disadvantaged relative to vessel-bearing angiosperms. Here we show that in a Costa Rican cloud forest, stem hydraulic properties, sapwood area- and leaf area-specific hydraulic conductivities of Drimys granadensis L. (Winteraceae) are similar to several co-occurring angiosperm tree species with vessels. In addition, D. granadensis had realized midday transpiration rates comparable to most vessel-bearing trees. Surprisingly, we found that D. granadensis transpired more water at night than during the day, with actual water loss being correlated with wind speed. The failure of stomata to shut at night may be related to the occlusion of stomatal pores by cutin and wax. Our measurements do not support the view that absence of xylem vessels imposes limitations on water transport above those for other vesselled plants in the same environment. This, in turn, suggests that a putative return to a tracheid-based xylem in Winteraceae may not have required a significant loss of hydraulic performance.

Notes: 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 (KSP) 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, KSP in deciduous and brevi-deciduous species encompassed a range of responses, from an insignificant increase in KSP following rains in some species, to a nine-fold increase in others. Amongst deciduous species, the minimum KSP during the dry season ranged from 6 to 56% of wet season KSP, 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 (KL) was strongly related to the photosynthetic capacity of the supported foliage, although leaf photosynthesis became saturated in species with high KL. 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 KL 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.

Notes: Stem water storage capacity and diurnal patterns of water use were studied in five canopy trees of a seasonal tropical forest in Panama. Sap flow was measured simultaneously at the top and at the base of each tree using constant energy input thermal probes inserted in the sapwood. The daily stem storage capacity was calculated by comparing the diurnal patterns of basal and crown sap flow. The amount of water withdrawn from storage and subsequently replaced daily ranged from 4 kg d–1 in a 0·20-m-diameter individual of Cecropia longipes to 54 kg d–1 in a 1·02-m-diameter individual of Anacardium excelsum, representing 9–15% of the total daily water loss, respectively. Ficus insipida, Luehea seemannii and Spondias mombin had intermediate diurnal water storage capacities. Trees with greater storage capacity maintained maximum rates of transpiration for a substantially longer fraction of the day than trees with smaller water storage capacity. All five trees conformed to a common linear relationship between diurnal storage capacity and basal sapwood area, suggesting that this relationship was species-independent and size-specific for trees at the study site. According to this relationship there was an increment of 10 kg of diurnal water storage capacity for every 0·1 m2 increase in basal sapwood area. The diurnal withdrawal of water from, and refill of, internal stores was a dynamic process, tightly coupled to fluctuations in environmental conditions. The variations in basal and crown sap flow were more synchronized after 1100 h when internal reserves were mostly depleted. Stem water storage may partially compensate for increases in axial hydraulic resistance with tree size and thus play an important role in regulating the water status of leaves exposed to the large diurnal variations in evaporative demand that occur in the upper canopy of seasonal lowland tropical forests.

Notes: Concurrent, independent measurements of stomatal conductance (gs), transpiration (E) and microenvironmental variables were used to characterize control of crown transpiration in four tree species growing in a moist, lowland tropical forest. Access to the upper forest canopy was provided by a construction crane equipped with a gondola. Estimates of boundary layer conductance (gb) obtained with two independent methods permitted control of E to be partitioned quantitatively between gs and gb using a dimensionless decoupling coefficient (Ω) ranging from zero to 1. A combination of high gs (c. 300–600 mmol m−2 s−1) and low wind speed, and therefore relatively low gb (c. 100–800 mmol m−2 s−1), strongly decoupled E from control by stomata in all four species (Ω= 0.7–0.9). Photosynthetic water-use efficiency was predicted to increase rather than decrease with increasing gs because gb was relatively low and internal conductance to CO2 transfer was relatively high. Responses of gs to humidity were apparent only when the leaf surface, and not the bulk air, was used as the reference point for determination of external vapour pressure. However, independent measurements of crown conductance (gc), a total vapour phase conductance that included stomatal and boundary layer components, revealed a clear decline in gc with increasing leaf-to-bulk air vapour pressure difference (Va because the external reference points for determination of gc and Va were compatible. The relationships between gc and Vc and between gs and Vs appeared to be distinct for each species. However, when gs and gc were normalized by the branch-specific ratio of leaf area to sapwood area (LA/SA), a morphological index of potential transpirational demand relative to water transport capacity, a common relationship between conductance and evaporative demand for all four species emerged. Taken together, these results implied that, at a given combination of LA/SA and evaporative demand scaled to the appropriate reference point, the vapour phase conductance and therefore transpiration rates on a leaf area basis were identical in all four contrasting species studied.

Notes: The shoots of cultivated tomato (Lycopersicon esculentum cv. T5) wilt if their roots are exposed to chilling temperatures of around 5 °C. Under the same treatment, a chilling-tolerant congener (Lycopersicon hirsutum LA 1778) maintains shoot turgor. To determine the physiological basis of this differential response, the effect of chilling on both excised roots and roots of intact plants in pressure chambers were investigated. In excised roots and intact plants, root hydraulic conductance declined with temperature to nearly twice the extent expected from the temperature dependence of the viscosity of water, but the response was similar in both species. The species differed markedly, however, in stomatal behaviour: in L. hirsutum, stomatal conductance declined as root temperatures were lowered, whereas the stomata of L. esculentum remained open until the roots reached 5 °C, and the plants became flaccid and suffered damage. Grafted plants with the shoots of one genotype and roots of another indicated that the differential stomatal behaviour during root chilling has distinct shoot and root components.

Notes: In this work, the common assumption that phloem sap is in water potential equilibrium with the surrounding apoplast was examined. With a dimensionless model of phloem translocation that scales with just two dimensionless parameters (R̂and F̂), a ‘map’ of phloem behaviour as a function of these parameters was produced, which shows that the water potential equilibrium assumption (R̂F̂ 〉〉 1) is valid for essentially all realistic values of the relevant scales. When in water potential equilibrium, a further parameter reduction is possible that limits model dependence to a single parameter (F̂), which describes the ratio of the solution's osmotic strength to its axial pressure drop. Due to the locally autonomous nature of individual sieve element/companion cell complexes, it is argued that long-distance integrative control is most efficient when F̂ is large (that is, when the pressure drop is relatively small), permitting the sieve tube to regulate solute loading in response to global changes in turgor. This mode of transport has been called ‘osmoregulatory flow.’ Limitations on the pressure drop within the transport phloem could require that sieve tubes be shorter than the long axis of the plant, and thus arranged in series and hydraulically isolated from one another.

Notes: The hydraulic conductance of the leaf lamina (Klamina) substantially constrains whole-plant water transport, but little is known of its association with leaf structure and function. Klamina was measured for sun and shade leaves of six woody temperate species growing in moist soil, and tested for correlation with the prevailing leaf irradiance, and with 22 other leaf traits. Klamina varied from 7.40 × 10−5 kg m−2 s−1 MPa−1 for Acer saccharum shade leaves to 2.89 × 10−4 kg m−2 s−1 MPa−1 for Vitis labrusca sun leaves. Tree sun leaves had 15–67% higher Klamina than shade leaves. Klamina was co-ordinated with traits associated with high water flux, including leaf irradiance, petiole hydraulic conductance, guard cell length, and stomatal pore area per lamina area. Klamina was also co-ordinated with lamina thickness, water storage capacitance, 1/mesophyll water transfer resistance, and, in five of the six species, with lamina perimeter/area. However, for the six species, Klamina was independent of inter-related leaf traits including leaf dry mass per area, density, modulus of elasticity, osmotic potential, and cuticular conductance. Klamina was thus co-ordinated with structural and functional traits relating to liquid-phase water transport and to maximum rates of gas exchange, but independent of other traits relating to drought tolerance and to aspects of carbon economy.