ALBERT

Notes: The phytochrome family of signal-transducing photoreceptors provides plants with the capacity to perceive variations in the relative fluxes of red (R) and far-red (FR) radiation. This capacity has been proposed to be of ecological value in the perception of the proximity of neighbouring plants and the consequent induction of shade avoidance responses. The work reported here has evaluated this potential by determining quantitatively the effect of neighbour proximity on the growth of canopies of Populus trichocarpa×deltoides‘Beaupré’ trees, and relating the measured variables to the long-term vectoral radiation quality inside each canopy. The spectral distribution of radiation inside four canopies of Populus trichocarpa×deltoides‘Beaupre’ of different densities was monitored throughout the growing season. Spectral distributions inside the canopies were measured in 10° wedges at different heights and angles. The results are presented as PFD over 400–700 nm (PFD400–700) and PFD over 400–800 nm (PFD400–700). Results are also presented for the calculated phytochrome photoequilibrium (Pfr/P) and red:far-red ratio (R:FR). Data are presented as in-canopy angular and height profiles, and as diurnal and seasonal variations. PFD400–700 and Pfr/P were found to be reduced inside each canopy, the reduction being greatest in the most dense canopy, and least in the most open canopy. At any height within each canopy, calculated Pfr/P decreased linearly with time throughout the growing season, until leaf senescence began. The reduction was greater in the denser canopies and was found to be similar for three consecutive field seasons. Linear relationships were found between plant stem growth rate, plant spacing and Pfr/P calculated from radiation propagated approximately horizontally within the canopies. The findings support the role of phytochrome in proximity perception in the natural environment and provide a quantitative basis for investigating the competitive interactions between plants growing in dense stands. The hypothesis is proposed that the dynamics of developing or regenerating canopies can be accounted for on the basis of phytochrome-mediated perception of the proximity of neighbouring plants.

Notes: A personal view is presented of current approaches to scaling processes and variables in the context of better understanding ecosystem function and predicting the consequences of global environmental change. Issues considered include spatial and temporal scales of interest, the scaling process, scaling strategy, scaling problems, heterogeneity, patchiness and non-linearity, aggregation methodology and feedbacks.Knowledge of processes in plants and vegetation is largely at small scales. The transfer of this knowledge up to larger spatial and longer temporal scales is an open-ended process with potential errors arising from heterogeneity and patchiness in the distribution of processes and non-linearities in the functional relationships between processes and environmental variables. Scaling now covers several orders of magnitude with respect to spatial and temporal scales with attendant risks of propagating errors.At larger scales the wide diversity of vegetation classes poses a problem, and it is suggested that this can be countered by classifying classes of vegetation (not species) into a small number of ‘functional types’ of vegetation. Scaling through summation of component processes and through derivation of appropriately averaged parameters is considered. However, the increasing role of feedbacks at larger spatial and longer temporal scales is an essential feature of the scaling process. Thus, understanding the feedbacks and including them in upscaling schemes is a major priority.A scaling strategy is outlined to minimize the propagation of errors. Because the scaling process is open-ended it is essential that good models are used and tested at each increase in scale.

Notes: A simple analytical scheme, involving the distribution of nitrogen, to scale up photosynthesis from leaf to canopy is proposed. The scheme is based on the assumption that there are two pools of nitrogen in leaves: nitrogen in photosynthetic, degradable structures (Np) and nitrogen in non-photosynthetic and non-degradable structures (Ns). The rate of photon-saturated photosynthesis, Fm, is assumed to be proportional to Np and is distributed inside the canopy similarly to photon flux density (PFD). Prior assumptions of an optimum distribution of nitrogen are not a prerequisite. Calculations made with the scheme lead to development of the hypothesis that the canopy can be treated as a ‘big leaf’ on the time scales involved in acclimation of photosynthesis to PFD. Simulations using parameters for tree species with different requirements for PFD show that shade-tolerant species may have denser canopies than sun-demanding species because of smaller amounts of non-photosynthetic structural nitrogen and/or supporting tissue in their leaves.

Notes: This paper describes the construction and performance of branch bags and a CO2 control system used to fumigate branches of mature Sitka spruce trees with air enriched in CO2 (700 μmolmol-1). It contains some examples of results obtained using the system over the course of the first two growing seasons. The branch bags have run continuously for 2 years with very few problems. CO2 concentrations were within 20 μmol mol-1 of the target concentration for more than 90% of the time. Temperatures within the bags were slightly higher than ambient (1–2 °C) and this had some effect on phenology. Attenuation of quantum flux density (photosynthetically active radiation) was 10–15%. The branch bag system has enabled investigation into the effects of elevated CO2 on mature tissue without the problems and expense of fumigating whole trees. Growth in elevated CO2 resulted in an increase in starch and a decrease in soluble protein content of needles. Stomatal conductance was higher in elevated CO2 grown needles, and there was some evidence of an increase in photosynthetic capacity.

Notes: The experiments and simulations reported in this paper show that, for stomata sensitive to both CO2 and water vapour concentrations, responses of stomatal conductance (gws) to boundary layer thickness have two components, one resulting from changes in intercellular CO2 concentration (χci) and another from changes in leaf surface water vapour saturation deficit (Dws). The experiments and simulations also show that the boundary layer conductance (gwb) can significantly alter the apparent response of gws to ambient air CO2 mole fraction (χca) and water vapour mole fraction (χwa). Because of the feedback loop involved the responses of gws for χca and χwa each include responses to both χci and Dws. The boundary layer alters the state of the variables sensed by the guard cells—i.e. χci and Dws—and so it is a source of feedback. Thus, when scaling up from responses of stomata to the response of gws for a whole leaf, the effect of the boundary layer must be considered. The results indicate that, for given responses of gws to χci and Dws, the apparent responses of gws to Dwa and χca depend on the size of the leaf and wind speed, showing that this effect of the boundary layer should be considered when comparing data measured under different conditions, or with different methods.

Notes: Seedlings of Eucalyptus grandis were grown at five different rates of nitrogen supply. Once steady-state growth rates were established, a detailed set of CO2 and water vapour exchange measurements were made to investigate the effects of leaf nitrogen content (N), as determined by nitrogen supply rate, on leaf structural, photosynthetic, respiratory and stomatal properties. Gas exchange data were used to parametrize the Farquhar–von Caemmerer photosynthesis model. Leaf mass per area (LMA) was negatively correlated to N. A positive correlation was observed between both day (Rd) and night respiration (Rn) and N when they were expressed on a leaf mass basis, but no correlation was found on a leaf area basis. An Rd/Rn ratio of 0·59 indicated a significant inhibition of dark respiration by light. The maximum net CO2 assimilation rate at ambient CO2 concentration (Amax), the maximum rate of potential electron transport (Jmax) and the maximum rate of carboxylation (Vcmax) significantly increased with N, particularly when expressed on a mass basis. Although the maximum stomatal conductance to CO2 (gscmax) was positively correlated with Amax, there was no relationship between gscmax and N. Leaf N content influenced the allocation of nitrogen to photosynthetic processes, resulting in a decrease of the Jmax/Vcmax ratio with increasing N. It was concluded that leaf nitrogen concentration is a major determinant of photosynthetic capacity in Eucalyptus grandis seedlings and, to a lesser extent, of leaf respiration and nitrogen partitioning among photosynthetic processes, but not of stomatal conductance.

Notes: Abstract. Two experiments are described which test the normal correlations that arise between stomatal conductance, net CO2 assimilation rate, and intercellular CO2 concentration (Ci), using whole shoots of Commelina communis L. In the first, conductance increased with decreasing Ci, at four different quantum flux densities, such that there was no unique relationship between conductance and quantum flux density or Ci, In the second, conductance increased hyperbolically with increasing quantum flux density while Ci was held constant at 466, 302, and 46 μmiolmol−1, and the response differed at each Ci. In neither experiment was conductance consistently related to net CO2 assimilation rate in the mesophyll. In both experiments high Ci suppressed the response of conductance to light, while there was a large response of conductance to light at low Ci, indicating an interaction between the effects of light and CO2 on stomata. The results show that the parallel responses of assimilation and conductance to light result in constant intercellular CO2 concentrations, and not that stomata maintain a ‘constant Ci’.

Notes: Abstract. The response of stomatal conductance to broadband blue and red light was measured in whole shoots of Scots pine and Sitka spruce, two species which have low stomatal sensitivity to CO2. In Scots pine, blue light was more than three times more effective than red light (on an incident quantum basis) in opening stomata, particularly at low quantum flux densities (〈100μmiol m−2 s−1). However, the apparent quantum yield of net CO2 assimilation rate in blue light was only half that in red light. The contrasting effects of red and blue light on conductance and assimilation led to higher intercellular CO2 concentrations (Ci) in blue light (up to 100 μmol mol−1 higher) than in red light. Similar results were obtained with Sitka spruce shoots, though differences in the effectiveness of red and blue light were less marked. In both species, both red and blue light increased conductance in normal and CO2-free air, indicating that neither red nor blue light exert effects through changes in Ci or mesophyll assimilation. However, decreases in Ci caused increases in conductance in both red and blue light, suggesting that these direct effects of light are not wholly independent of CO2.

Notes: Xylem cavitation is a frequent event, but since resistance to flow does not generally increase in vivo, reversal must also occur even under negative potentials. We demonstrated that this can occur in excised wood. Our results suggest that refilling of cavitated tracheids at negative water potentials may result from a change in equilibrium between gas concentrations, water potential and surface tension at the embolism interface. Excised branch-wood specimens from small trees of Pinus sylvestris were dried on the bench to a range of relative water contents and then rehydrated in a permeability apparatus using ultra-filtered, de-aerated water as permeant. Water inflow and outflow were measured gravimetrically by recording the gain or loss from two reservoirs held on balances. Flow was induced through the specimen by holding the balances at different levels, while an overall negative water potential could be imposed by raising the specimen above the inflow/outflow reservoirs. Changes in water content of the specimen were calculated as the difference between inflow and outflow. The time-course data for both relative water content and permeability were fitted to an exponential function to give initial and final estimates and a time constant. Rehydration occurred at all imposed water potentials, but the speed of recovery was affected at lower potentials. Where drying of the specimen was more protracted, permeability was initially lower but also recovered during permeation. Both flow and de-aeration were necessary for complete rehydration. A model requiring new information on gas concentrations and transport coefficients is suggested.