Jeffrey S. Amthor

Scaling CO2-photosynthesis relationships from the leaf to the canopy.

Responses of individual leaves to short-term changes in CO2 partial pressure have been relatively well studied. Whole-plant and plant community responses to elevated CO2 are less well understood and scaling up from leaves to canopies will be complicated if feedbacks at the small scale differ from feedbacks at the large scale. Mathematical models of leaf, canopy, and ecosystem processes are important tools in the study of effects on plants and ecosystems of global environmental change, and in particular increasing atmospheric CO2, and might be used to scale from leaves to canopies. Models are also important in assessing effects of the biosphere on the atmosphere. Presently, multilayer and big leaf models of canopy photosynthesis and energy exchange exist. Big leaf models — which are advocated here as being applicable to the evaluation of impacts of ‘global change’ on the biosphere — simplify much of the underlying leaf-level physics, physiology, and biochemistry, yet can retain the important features of plant-environment interactions with respect to leaf CO2 exchange processes and are able to make useful, quantitative predictions of canopy and community responses to environmental change. The basis of some big leaf models of photosynthesis, including a new model described herein, is that photosynthetic capacity and activity are scaled vertically within a canopy (by plants themselves) to match approximately the vertical profile of PPFD. The new big leaf model combines physically based models of leaf and canopy level transport processes with a biochemically based model of CO2 assimilation. Predictions made by the model are consistent with canopy CO2 exchange measurements, although a need exists for further testing of this and other canopy physiology models with independent measurements of canopy mass and energy exchange at the time scale of 1 h or less.

Terrestrial Higher Plant Respiration and Net Primary Production

A large fraction of carbon (C) assimilated in higher plant photosynthesis is released into the atmosphere as carbon dioxide during subsequent plant respiration. Thus, plant respiration is a large negative component of the C budget of plants and ecosystems. It contributes to the control of ecosystem net primary production (NPP) because NPP is gross primary production (GPP) minus plant respiration. The relationship between ecosystem NPP and GPP is therefore dictated by respiration. Plant respiration is the metabolic link between GPP and NPP. It is also a large component of a plants C budget; perhaps, typically, 50–70% of carbon assimilated in GPP is released into the atmosphere as carbon dioxide during subsequent plant respiration. Because of great uncertainty concerning in situ measurements of respiration (R a ) and photosynthesis (P), it is hard to quantify more precisely their role in C cycles of various ecosystems. This chapter judges the available data to be too imprecise to assess properly whether the R a /P ratio at present is conservative within or among ecosystems. Moreover, environmental change such as warming and increasing carbon dioxide concentration may affect R a and P differently, so the R a / P ratio may change in the future. In any case, future studies of the relationship between R a and NPP or GPP will be more enlightening than simple measurements of respiration rate.

Higher Plant Respiration and Its Relationships to Photosynthesis

Respiration is the complement of photosynthesis in higher plants.1 The primary function of photosynthesis is to assimilate CO2 and radiant energy in the formation of carbohydrates. A significant portion of those carbohydrates become the main substrates of respiration (James 1953; Krotkov 1960; ap Rees 1980), but often after some period of storage or distance of transport. The function of respiration is to convert photoassimilate into substances usable by growth, maintenance, transport, and nutrient assimilation processes (Beevers 1961). Respiration does this by breaking down sugars into smaller molecules (carbon skeleton intermediates), phosphorylating ADP and other nucleosides, and reducing nucleotides — respiration does not only generate ATP. Some of the carbon skeleton intermediates become the precursors of growth and are diverted away from respiratory metabolism and used in biosynthetic reactions, whereas the ATP and NAD(P)H formed during respiration are used in all heterotrophic energy-requiring processes (Fig. 4.1).

PERSPECTIVE ON THE RELATIVE INSIGNIFICANCE OF INCREASING ATMOSPHERIC CO2 CONCENTRATION TO CROP YIELD

Average yield of most crops in many countries increased significantly during the past 50 to 100 years. Although atmospheric CO2 concentration, [CO2]a, also increased during that time period, and although crop growth and yield can respond positively to [CO2]a increase, yield increases were due mainly to factors other than increasing [CO2]a. Similarly, some yield increases prior to 1900 were also associated primarily with factors other than changes in [CO2]a. In particular, past national average yield increases were the result chiefly of technological advances such as nitrogen fertilization; selection of genotypes with increased harvest index and disease resistance; mechanization of planting, cultivation, and harvesting; and chemical weed and pest control. If technology continues to increase average yields at recent rates, near-future increases in [CO2]a will have only small impacts on yield in comparison to technology in many countries. Conversely, if future increases in [CO2]a are the main drivers of future yield increases, those yield increases will be small. These points are demonstrated through a comparison of (i) long-term records of yield, (ii) data from key controlled-[CO2] experiments, and (iii) records of past [CO2]a. Finally, it is noted that continued [CO2]a increase may bring with it climatic changes that could have negative or positive impacts on future yield.

COTCO2: a cotton growth simulation model for global change

In conjunction with the Free-Air-CO2-Enrichment (FACE) project, a new, physiologically based, mechanistic, modular simulation model of cotton (Gossypium hirsutum L.) physiology, growth, development, yield and water use has been constructed. It is named COTCO2 for cotton response to atmospheric CO2 concentration. The model is capable of predicting cotton crop responses to elevated atmospheric CO2 concentrations and potential concomitant changing climate variables. The major plant processes known to be influenced by CO2 are simulated explicitly, i.e. photosynthesis, photorespiration, and stomatal conductance, and its role in leaf energy balance. The model explicitly simulates the impact of atmospheric CO2 concentration on C3 photosynthesis and photorespiration at the level of carboxylation and oxygenation. Growth is simulated for individual organs, i.e. leaf blade, stem segment, taproot and lateral roots, and fruit which includes squares and bolls. Potential growth is calculated and the carbohydrate and nitrogen required to meet this potential are calculated. Actual growth is based on substrate availability, the potential growth, and water stress. Our intent here is to describe the overall structure of the model, its present status, and future development plans. Further development, documentation, calibration, and validation of the model is in progress. The long range goal of the project is to provide quantitative estimates of global cotton production in a future higher-CO2 world.

2 – Increasing Atmospheric CO2 Concentration, Water Use, and Water Stress: Scaling Up from the Plant to the Landscape

Publisher Summary This chapter discusses the potential effects of atmospheric CO 2 concentration ([CO 2 ]a) increase on ecosystem water use, WUE, and water stress. Environmental stress is defined as an environmental limitation on ecosystem net primary production (NPP). This large divergence in NPP in different ecosystems is primarily because of differences in availability of soil water and therefore degree of water stress, although other environmental differences may also be important. Water stress can be reduced by an increase in precipitation or irrigation, increased access to existing soil water that would otherwise be unavailable to plants, or reduced evapotranspiration. The amount of NPP or plant growth per unit environmental resource used is an important measurement of the relationship between plants and their environment. Efficiency is used to denote phytomass produced per unit resource used, and common examples are radiation-use efficiency, which is plant growth per unit solar radiation absorbed by the plants in an ecosystem and water use efficiency (WUE), which is the amount of growth per unit water evaporated. Thus, if WUE increases, an increase in productivity per unit water added to an ecosystem in precipitation and irrigation can occur.