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The influence of irrigation with CO2 -enriched water on plant development and yield is reviewed. The reason for irrigation with CO2 -enriched water was - in most cases - to increase yield. The present evaluation considers results from over a hundred studies performed since the first experiment in 1866. Special emphasis is given to the comparison of 85 experiments made by Mitscherlich in 1910 with 358 irrigation experiments made in the last 80 years. In a statistical analysis of these experiments, the measured plant parameter (often growth and/or gas exchange rates) showed a highly significant mean increase of 2.9 % in plants irrigated with CO2 -enriched water as compared with control. Evidence of five mechanisms was found. The subterranean carbon dioxide concentration influences: (a) the rate of nitrification and hence of nitrogen availability; (b) the rate of weathering and pH, and hence the availability of other plant nutrients; (c) the CO2 uptake via roots into the transpiration stream, contributing to the rate of leaf photosynthesis; (d) the hormone levels in the plant; and (e) the rate of pesticide decomposition in soils. After examining the available evidence we found that (a) and (b) in some experiments are important to plant growth, since they change the physiochemical environment of the roots. On the other hand, while (c) could theoretically contribute up to 5% of plant carbon assimilation, it usually contributes less than 1 %, while (d) contributes most of the observed effects of CO2 -enriched water on plants. In addition, pesticide decomposition in soils can be delayed by supra- or sub-optimal CO2 concentrations. Contents Summary 249 I. Introduction 249 II. Historical background 250 III. Methods for comparing experimental data 251 IV. Analysis of yield ratios 251 V. Temporal and spatial changes in the soil atmosphere during and after irrigation with CO2 -enriched water 252 VI. Mechanisms through which irrigation with CO2 -enriched water influences yield 253 Acknowledgements 256 References 257.

Concerns about global warming and the role of rising atmospheric carbon dioxide (CO2) content seem to have relegated studies of the effects of air pollutants to a position of secondary importance. There are very strong arguments for a continuing high profile for research on the impacts of nitrogenous air pollutants (NOx, NH3), ozone (O3) and non-methane hydrocarbons (NMHCs). Atmospheric concentrations of these pollutants are increasing steadily and it is highly probable that they will continue to do so. Furthermore, these changes in atmospheric composition will be accompanied by increasing levels of UV-B radiation.

Anthropogenic activities have changed the chemical composition of the global atmosphere. Since the middle of the nineteenth century atmospheric CO2 has increased by about 25%, whereas global warming has increased mean surface temperature by only 0.6 °C. Throughout the industrialised regions the air pollutants, O3, SO2, NOx and others, are causing increasing damage to vegetation. The temporal and spatial distribution of air pollutants differs by orders of magnitude between the various managed and unmanaged global ecosystems and hence their effect on vegetation varies greatly. The significance of increasing atmospheric CO2 for plant growth is discussed in terms of its influence on the partitioning of dry matter, leaf growth and development, stem growth and branching, root formation and tillering, growth of the whole plant and flowering. Details are given concerning the influence of CO2 on the growth and development of plants from different ecosystems. Special attention is given to the interaction of CO2 concentration, climate and altitude on vegetation in the unmanaged and usually stressed ecosystems of the Arctic, temperate and sub-tropical zones. Some processes which are likely to enhance plant survival at higher CO2 concentrations are described. Finally, some examples of the ameliorating effects of elevated CO2 on air pollution damage in plants are outlined.

Transpiration rates of potted spray carnation plants Cerise Royalette decreased about 0.04% per vpm CO2 between ambient atmospheric CO2 concentration and 1500 vpm CO2 at several light flux densities and leaf temperatures. Measurements of daily water losses of potted spray carnation plants placed under high solar radiation conditions in two minigreenhouses with 300 and 5000 vpm CO2 demonstrated that elevated CO2 concentrations reduced water losses by 20–30%. The effect of the increase in global CO2 concentration on stomatal closure was calculated to have decreased the yearly transpiration rate of an outdoor crop by 1.6% in this century and is expected to cause a decrease of 10% within the next 50 years if all other factors remain unchanged. From a model of CO2 uptake of carnation plants it was calculated that the expected water use efficiency (net photosynthesis rate/transpiration rate) will increase by about 40–50% over the next 50 years due to the expected increase in global CO2 concentration.

Two-year-old potted sweet chestnut seedlings were grown at 350 ppm CO2 and 700 ppm, day and night in constantly ventilated tunnels during two full growing seasons, near Paris, France (48° N, 2° E). Enrichment with CO2 caused an unusual shoot growth response. After the end of July, stem elongation ceased in 62% of the CO2 enriched plants as compared with 37% in the control. The leaves of CO2-enriched seedlings showed early senescence, indicated by premature yellowing and a decrease in chlorophyll content. This was associated with nutrient dilution brought about by the rapid growth of these trees. The increase in total dry weight of the CO2-enriched seedlings was essentially the result of increase in the root dry weight (69%). Shoot weight decreased by 22% relative to the control. Total leaf area per enriched plant was 25% smaller than the control. This unusual pattern of growth and carbon allocation of the CO2 treated Chestnut trees emphasizes the concept of a response specificity within trees to an increase of atmospheric CO2.

During the 3,400 million years photosynthesizing organisms have existed on Earth, the global atmospheric CO2 concentration has not been constant. Plants have modified and are in turn influenced by atmospheric CO2 concentration. In the last 100 years the global CO2 concentration has increased from a pre-industrial level of about 280 ppm to the present 340 ppm. Further increases are expected to lead to a doubling or even a several-fold increase of atmospheric CO2 during the next one or two centuries. These elevated CO2 concentrations are expected to affect the productivity and modify the global climate. An overview of the plant processes which are likely to be affected are presented. The implication of the changes in CO2 concentration for photosynthesis is discussed. A research technique which can be used to study the influence of elevated atmospheric CO2 on managed and unmanaged plant communities is outlined.