ACROPETAL LEAF DIFFERENTIATION IN QUERCUS RUBRA (FAGACEAE)

Northern red oak (Quercus rubra L.) leaves were shown to mature progressively from base to tip of the lamina based on studies of growth rates, anatomical differentiation, and ¹⁴C-transport. Lamina expansion in both length and width ceased in the basal quarter of the leaf before the apical quarter. Cell expansion and tissue differentiation were more advanced at the base than at the tip of leaves at 10%–20% of full expansion. Physiological data supported the morphological and anatomical data. Sink activity was examined by following the distribution of ¹⁴C imported into sink leaves with direct vascular connections to the source leaf to assure uniform assimilate supply to the sink leaves. Leaves approximately 50% of full expansion imported five to seven times more ^l4C-assimilates into the tip than into the base of the leaf, consistent with continued sink activity in the leaf tip after import by the leaf base has ceased. Transport of ¹⁴C from portions of the leaf, indicating source activity, occurred first in the basal portion of the lamina. The base functioned as a source at approximately 40% of full expansion; the tip, at approximately 60%. Thus, northern red oak displays an acropetal pattern of leaf expansion and differentiation, unlike the more typical pattern of basipetal leaf development defined in many other dicotyledonous genera with simple leaves.     

ORGANIZATION AND ONTOGENY OF THE VASCULAR SYSTEM IN THE PETIOLE OF EASTERN COTTONWOOD

The topologic arrangement of petiolar bundles varies within the length of the cottonwood petiole. Each petiolar bundle is formed by the subdivision and aggregation of acropetally differentiating subsidiary bundles in a predictable pattern. The subsidiary bundles provide vascular continuity between the stem and specific portions of the leaf lamina. Spot-labeling of individual veins with ¹⁴CO₂, freeze substitution, and microautoradiography were used to establish the relation between the secondary veins of the lamina and the vasculature of the petiole. Within the petiole vasculature each subsidiary bundle was continuous with a specific portion of the lamina and seemed to have a separate function. Subsidiary bundles continuous with the central leaf trace were closely related functionally to the tip region of the lamina, while the subsidiary bundles continuous with the lateral leaf traces were functionally related to the middle and basal portions of the lamina.     

Interacting elevated CO2 and tropospheric O3 predisposes aspen (Populus tremuloides Michx.) to infection by rust (Melampsora medusae f. sp. tremuloidae)

We investigated the interaction of elevated CO₂ and/or (Ozone) O₃ on the occurrence and severity of aspen leaf rust (Melampsora medusae Thuem. f. sp. tremuloidae) on trembling aspen (Populus tremuloides Michx.). Furthermore, we examined the role of changes in leaf surface properties induced by elevated CO₂ and/or O₃ in this host–pathogen interaction. Three‐ to five‐fold increases in levels of rust infection index were found in 2 consecutive years following growing‐season‐long exposures with either O₃ alone or CO₂ + O₃ depending on aspen clone. Examination of leaf surface properties (wax appearance, wax amount, wax chemical composition, leaf surface and wettability) suggested significant effects by O₃ and CO₂ + O₃. We conclude that elevated O₃ is altering aspen leaf surfaces in such a way that it is likely predisposing the plants to increased infection by aspen leaf rust.     

Photosynthesis, light and nitrogen relationships in a young deciduous forest canopy under open-air CO2 enrichment

Leaf photosynthesis (Ps), nitrogen (N) and light environment were measured on Populus tremuloides trees in a developing canopy under free‐air CO₂ enrichment in Wisconsin, USA. After 2 years of growth, the trees averaged 1·5 and 1·6 m tall under ambient and elevated CO₂, respectively, at the beginning of the study period in 1999. They grew to 2·6 and 2·9 m, respectively, by the end of the 1999 growing season. Daily integrated photon flux from cloud‐free days (PPFD_day,sat) around the lowermost branches was 16·8 ± 0·8 and 8·7 ± 0·2% of values at the top for the ambient and elevated CO₂ canopies, respectively. Elevated CO₂ significantly decreased leaf N on a mass, but not on an area, basis. N per unit leaf area was related linearly to PPFD_day,sat throughout the canopies, and elevated CO₂ did not affect that relationship. Leaf Ps light‐response curves responded differently to elevated CO₂, depending upon canopy position. Elevated CO₂ increased Ps_sat only in the upper (unshaded) canopy, whereas characteristics that would favour photosynthesis in shade were unaffected by elevated CO₂. Consequently, estimated daily integrated Ps on cloud‐free days (Ps_day,sat) was stimulated by elevated CO₂ only in the upper canopy. Ps_day,sat of the lowermost branches was actually lower with elevated CO₂ because of the darker light environment. The lack of CO₂ stimulation at the mid‐ and lower canopy was probably related to significant down‐regulation of photosynthetic capacity; there was no down‐regulation of Ps in the upper canopy. The relationship between Ps_day,sat and leaf N indicated that N was not optimally allocated within the canopy in a manner that would maximize whole‐canopy Ps or photosynthetic N use efficiency. Elevated CO₂ had no effect on the optimization of canopy N allocation.     

Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO2 and/or O3

Leaf gas exchange parameters and the content of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) in the leaves of two 2‐year‐old aspen (Populus tremuloides Michx.) clones (no. 216, ozone tolerant and no. 259, ozone sensitive) were determined to estimate the relative stomatal and mesophyll limitations to photosynthesis and to determine how these limitations were altered by exposure to elevated CO₂ and/or O₃. The plants were exposed either to ambient air (control), elevated CO₂ (560 p.p.m.) elevated O₃ (55 p.p.b.) or a mixture of elevated CO₂ and O₃ in a free air CO₂ enrichment (FACE) facility located near Rhinelander, Wisconsin, USA. Light‐saturated photosynthesis and stomatal conductance were measured in all leaves of the current terminal and of two lateral branches (one from the upper and one from the lower canopy) to detect possible age‐related variation in relative stomatal limitation (leaf age is described as a function of leaf plastochron index). Photosynthesis was increased by elevated CO₂ and decreased by O₃ at both control and elevated CO₂. The relative stomatal limitation to photosynthesis (l_s) was in both clones about 10% under control and elevated O₃. Exposure to elevated CO₂ + O₃ in both clones and to elevated CO₂ in clone 259, decreased l_s even further – to about 5%. The corresponding changes in Rubisco content and the stability of C_i/C_a ratio suggest that the changes in photosynthesis in response to elevated CO₂ and O₃ were primarily triggered by altered mesophyll processes in the two aspen clones of contrasting O₃ tolerance. The changes in stomatal conductance seem to be a secondary response, maintaining stable C_i under the given treatment, that indicates close coupling between stomatal and mesophyll processes.     

ANATOMICAL CHANGES DURING LEAF ONTOGENY IN POPULUS DELTOIDES

The leaf plastochron index (LPI) was used to interpret the anatomical changes during leaf ontogeny in the developing leaf zone of young cottonwood trees and to relate leaf anatomical structure to physiological function. The lamina tip matured precociously with respect to both structure and function. Below the lamina tip the intercellular spaces, stomates, and minor veins matured basipetally, while the major veins developed acropetally. Ontogenetically, maturation progressed from LPI –1.0, which was anatomically immature except for its lamina tip, to the first fully expanded leaf at LPI 6.0, which was anatomically mature. Physiological maturity also occurred at LPI 6.0, thus signifying a transition with respect to both structure and function. By evaluating the anatomical observations in conjunction with physiological data collected at comparable LPI's in other experiments, it could be demonstrated that anatomical development was a limiting factor in photosynthesis and translocation of assimilates.     

Contrasting fine-root production,survival and soil CO2 efflux in pine and poplar plantations

Tree root activity, including fine-root production, turnover and metabolic activity are significant components of forest productivity and nutrient cycling. Differences in root activity among forest types are not well known. A 3-year study was undertaken in red pine (Pinus resinosa Ait.) and hybrid poplar (Populus tristis X P. balsamifera cv `Tristis no. 1') plantations to compare belowground root dynamics. We measured fine-root production, mortality and standing crop, as well as soil CO₂ efflux. Pine fine-root production was only 2.9% of that of poplar during three years; 85 pine roots were observed in minirhizotron tubes compared with 4088 poplar roots. Live-root density oscillated seasonally for both species with late winter minimum and autumn maximum. Poplar reached constant maximum live-root length within the first growing season, but pine continued to increase observed fine-root length for three growing seasons. Within the first 100 days following initial appearance, 22% of the pine roots disappeared and 38% of the poplar roots disappeared. Median fine-root longevity of pine was 291 days compared with 149 days for poplar roots. Fine-root longevity increased with depth in the soil, and was greater for roots with initial diameter >0.5 mm. The probability of poplar root death from late February to May was more than three times that in any other season, regardless of root age. Despite the greater poplar root production and live-root length, fine-root biomass and soil CO₂ efflux was greater in pine. Greater metabolic activity in the pine stand may be due to greater fine-root biomass or greater heterotrophic respiration. This revised version was published online in June 2006 with corrections to the Cover Date.     

Sink to Source Transition of Populus Leaves

Development of the Populus leaf is presented as a model system to illustrate the sequence of events that occur during the sink to source transition. A Populus leaf is served by three leaf traces, each of which consists of an original procambial trace bundle that differentiates acropetally and continuously from more mature procambium in the stem and a complement of subsidiary bundles that differentiates bidirectionally from a leaf basal meristem. During development these subsidiary bundles maintain continuity through the meristematic region of the node. The basipetally developing subsidiary bunles form phloem bridges that serve to integrate adjacent leaf traces of the stem vasculature. Distal to the node the acropetally developing bundles from all three leaf traces are reoriented in a precise and orderly sequence to form tiers of petiolar bundles. These tiers of bundles extend into the midrib where bundles diverge at intervals as the major lateral veins. The dorsal-most tier of bundles extends to the lamina tip and each successive tier of bundles contributes to lateral veins situated more proximally in the lamina. Although the midrib and the major vein system differentiate acropetally in the lamina, they mature basipetally. Maturation of the mesophyll and other lamina tissues also mature basipetally. As a consequence of the basi-petal maturation process, the lamina tip matures very early and begins exporting photosynthates while the lamina base is still importing from other leaves. The transition of a leaf from sink to source status must therefore be considered as a progression of structural and functional events that occur in synchrony.