J. Timothy Ball

Physiological and environmental regulation of stomatal conductance , photosynthesis and transpiration : a model that includes a laminar boundary layer

Abstract This paper presents a system of models for the simulation of gas and energy exchange of a leaf of a C3 plant in free air. The physiological processes are simulated by sub-models that: (a) give net photosynthesis (An) as a function of environmental and leaf parameters and stomatal conductance (gs); (b) give g, as a function of the concentration of CO2 and H2O in air at the leaf surface and the current rate of photosynthesis of the leaf. An energy balance and mass transport sub-model is used to couple the physiological processes through a variable boundary layer to the ambient environment. The models are based on theoretical and empirical analysis of gs, and An measured at the leaf level, and tests with intact attached leaves of soybeans show very good agreement between predicted and measured responses of gs and An over a wide range of leaf temperatures (20–35°C), CO2 concentrations (10–90 Pa), air to leaf water vapor deficits (0.5–3.7 kPa) and light intensities (100–2000 μmol m−2s−1). The combined models were used to simulate the responses of latent heat flux (λE) and gs for a soybean canopy for the course of an idealized summer day, using the ‘big-leaf’ approximation. Appropriate data are not yet available to provide a rigorous test of these simulations, but the response patterns are similar to field observations. These simulations show a pronounced midday depression of λE and gs at low or high values of boundary-layer conductance. Deterioration of plant water relations during midday has often been invoked to explain this common natural phenomenon, but the present models do not consider this possibility. Analysis of the model indicates that the simulated midday depression is, in part, the result of positive feedback mediated by the boundary layer. For example, a change in gs affects An and λE. As a consequence, the temperature, humidity and CO2 concentration of the air in the proximity of the stomata (e.g. the air at the leaf surface) change and these, in turn, affect gs. The simulations illustrate the possible significance of the boundary layer in mediating feedback loops which affect the regulation of stomatal conductance and λE. The simulations also examine the significance of changing the response properties of the photosynthetic component of the model by changing leaf protein content or the CO2 concentration of the atmosphere.

A Model Predicting Stomatal Conductance and its Contribution to the Control of Photosynthesis under Different Environmental Conditions

In the past, stomatal responses have generally been considered in relation to single environmental variables in part because the interactions between factors have appeared difficult to quantify in a simple way. A linear correlation between stomatal conductance (g) and CO2 assimilation rate (A) has been reported when photon fluence was varied and when the photosynthetic capacity of leaves was altered by growth conditions, provided CO2, air humidity and leaf temperature were constant (1). Temperature and humidity are, however, not consistent in nature. Lack of a concise description of stomatal responses to combinations of environmental factors has limited attempts to integrate these responses into quantitative models of leaf energy balance, photosynthesis, and transpiration. Moreover, this lack has hindered progress toward understanding the stomatal mechanism. We have taken a multi-variant approach to the study of stomatal conductance and we show that under many conditions the responses of stornata can be described by a set of linear relationships. This model can be linked to models of leaf carbon metabolism and the environment to predict fluxes of CO2, H2O and energy. In this paper, we show how the model of conductance can be linked to a description of CO2 assimilation as a function of intercellular CO2 (whether empirical or the output of a model) to predict the distribution of flux control between the stornata and leaf “biochemistry” under conditions in a gas-exchange cuvette.

Photosynthesis: principles and field techniques

Among the people who do photosynthesis research at the leaf, plant or canopy level, the devices used to measure photosynthesis are usually referred to as gas-exchange systems or simply as ‘systems’. The concept that photosynthesis is measured with a system, rather than a single instrument, is an important place to start, for two reasons. First, the system concept emphasizes the fact that we have nothing like a discrete photosynthesis sensor. Photosynthesis is always a calculated parameter, determined from measurements of CO2 concentrations, gas flows and sometimes other parameters, depending on the measurement philosophy. Second, the system concept reminds us that gas-exchange systems typically measure more than just photosynthesis, for the reason that photosynthesis data are greatly enhanced by the simultaneous acquisition of other kinds of information. These two aspects of the system concept also frame the material to be presented in this chapter. Developing the idea of a measurement system, we address the design and construction of gas-exchange systems, considering both measurement principles and the devices that form system components. Emphasizing the measurement and control of parameters other than photosynthesis, we discuss some of the trade-offs that influence the choice of gas-exchange systems for different kinds of research questions.

Photosynthetic Control of Electron Transport in Leaves of Phaseolus Vulgaris: Evidence for Regulation of Photosystem 2 by the Proton Gradient

Under physiological conditions, the general distribution of control between different processes (eg. stomata, carbon metabolism, electron transport, photochemisty) may be complex and is always shared to some extent among elements (5). The primary consideration of the analysis reported here is the regulation of chloroplast membrane reactions when carbon metabolism clearly limits the rate of net photosynthesis. This condition will be referred to as ‘photosynthetic control.’ We show how the restriction of carbon metabolism feedsback, regulating the primary photochemisty of both photoreactions to match the rate at which their products can be accepted by the biochemical reactions. A new mechanism is proposed which relates the high-energy quenching of chlorophyll fluorescence (7) to regulation of the quantum yield of PS2 photochemistry.

Effects of elevated CO2 and N fertilization on fine root dynamics and fungal growth in seedling Pinus ponderosa

Abstract The effects of elevated CO 2 and N fertilization on fine root growth of Pinus ponderosa Dougl. ex P. Laws. C. Laws., grown in native soil in open-top field-exposure chambers at Placerville, CA, were monitored for a 2-year period using minirhizotrons. The experimental design was a replicated 3 × 3 factorial with a treatment missing; plants were exposed to ambient (≈ 365 μmol mol −1 ) air or ambient air plus either 175 or 350 μmol mol −1 CO 2 and three levels of N addition (0, 100 and 200 kg ha −1 year −1 ). By the second year, elevated CO 2 increased fine root occurrence and root length while N fertilization had no effect. The CO 2 × N interactions were not significant. Neither elevated CO 2 nor N fertilization altered fine root diameter. Fine root mortality was increased by increasing soil N but was reduced in elevated CO 2 . Highest fine root mortality occurred during summer and was lowest during winter. Elevated CO 2 increased mycorrhizal and fungal occurrence earlier than N fertilization.

Lessons from Lysimeters: Soil N Release from Disturbance Compromises Controlled Environment Study

A controlled environment study of the effects of carbon dioxide (CO2) and nitrogen (N) on growth of ponderosa pine seedlings produced results contradictory to those obtained in the field with the same species, soil, and treatments. In the controlled envi- ronment study, there was a significant negative growth response to N fertilization, whereas in the field there was a significant positive response to N. The difference was due to high rates of native N mineralization after soil disturbance during potting. This was evident from soil solution N03- concentrations that peaked at -5000 Kmol/L in the unfertilized pots and 20000 Kmol/L in the fertilized pots. These concentrations are orders of magnitude greater than those typically observed in the field. The effects of soil disturbance on N mineralization and nitrification need to be carefully considered before initiating controlled environment studies. The results of this study show that excessive N mineralization caused by soil disturbance can seriously compromise the results of controlled environment studies

A General Expression for the Control of the Rate of Photosynthetic CO2 Fixation by Stomata, the Boundary Layer and Radiation Exchange

Previous descriptions of the “limitation” to the rate of photosynthetic CO2 assimilation imposed by the stornata and boundary layer have focused on the basic mechanism of CO2 diffusion and have made use of several simplifying assumptions regarding leaf temperature and transpiration rate. These descriptions are consequently useful under controlled conditions but, in the field, it is impossible to consider the role of the stornata and boundary layer in regulating CO2 uptake without considering their roles in controlling water and heat exchange and leaf temperature. In the following, we examine a complex leaf system that includes CO2, water and heat exchange and present expressions quantifying the degree to which each element of the system “limits” the rate of CO2 assimilation.

The photosynthesis - leaf nitrogen relationship at ambient and elevated atmospheric carbon dioxide: a meta-analysis

Estimation of leaf photosynthetic rate (A) from leaf nitrogen content (N) is both conceptually and numerically important in models of plant, ecosystem and biosphere responses to global change. The relationship between A and N has been studied extensively at ambient CO{sub 2} but much less at elevated CO{sub 2}. This study was designed to (1) assess whether the A-N relationship was more similar for species within than between community and vegetation types, and (2) examine how growth at elevated CO{sub 2} affects the A-N relationship. Data were obtained for 39 C{sub 3} species grown at ambient CO{sub 2} and 10 C{sub 3} species grown at ambient and elevated CO{sub 2}. A regression model was applied to each species as well as to species pooled within different community and vegetation types. Cluster analysis of the regression coefficients indicated that species measured at ambient CO{sub 2} did not separate into distinct groups matching community or vegetation type. Instead, most community and vegetation types shared the same general parameter space for regression coefficients. Growth at elevated CO{sub 2} increased photosynthetic nitrogen use efficiency for pines and deciduous trees. When species were pooled by vegetation type, the A-N relationship for deciduous trees expressed on a leaf-mass bask was not altered by elevated CO{sub 2}, while the intercept increased for pines. When regression coefficients were averaged to give mean responses for different vegetation types, elevated CO{sub 2} increased the intercept and the slope for deciduous trees but increased only the intercept for pines. There were no statistical differences between the pines and deciduous trees for the effect of CO{sub 2}. Generalizations about the effect of elevated CO{sub 2} on the A-N relationship, and differences between pines and deciduous trees will be enhanced as more data become available.

Climate change and wetland processes in the Southwest United States: Response of riparian communities to rising CO{sub 2} levels. Final report

The current impact of Salt Cedar on the riparian areas of the southwestern US are recognized as being negative. If atmospheric levels of CO{sub 2} continue to rise--as seems likely--the results of this study indicate that the Salt Cedar--Cottonwood competitive interaction maybe moved further in the direction of favoring Salt Cedar. Further study confirming these results and elucidating the basis for competitive resource use by Salt Cedar and other riparian species would be prudent.

Whole System Carbon Exchange of Small Stands of Pinus Ponderosa Growing at Different CO{sub 2} concentrations in open top chambers

Functional understanding of the carbon cycle from the molecular to the global level is a high scientific priority requiring explanation of the relationship between fluxes at different spatial and temporal scales. We describe methods used to convert an open top chamber into both closed and open flow gas exchange systems utilized to measure such fluxes. The systems described consist of temporary modifications to an open top chamber, and are put in place for several days on one or several open top chambers. In the closed system approach, a chamber is quickly sealed for a short, predetermined time interval, the change in gas concentrations is measured, then the chamber is unsealed and ventilated. In the open flow system approach, airflow into the open top chamber is measured by trace gas injection, and the air stream concentration of CO{sub 2} and water vapor is measured before and after injection into the chamber. The closed chamber approach can resolve smaller fluxes, but causes transient increases in chamber air temperature, and has a high labor requirement. The open flow approach reduces the deviation of measuring conditions from ambient, may be semi-automated (requiring less labor), allows a more frequent sampling interval, but cannot resolve low fluxes well. Data demonstrating the capabilities of these systems show that, in open canopies of ponderosa pine, scaling fluxes from leaves to whole canopies is well approximated from summation of leaf P{sub s} rates. Flux measurements obtained from these systems can be a valuable contribution to our understanding whole system material fluxes, and challenge our understanding of ecosystem carbon budgets.