▪ Abstract The primary effect of the response of plants to rising atmospheric CO2 (Ca) is to increase resource use efficiency. Elevated Ca reduces stomatal conductance and transpiration and improves water use efficiency, and at the same time it stimulates higher rates of photosynthesis and increases light-use efficiency. Acclimation of photosynthesis during long-term exposure to elevated Ca reduces key enzymes of the photosynthetic carbon reduction cycle, and this increases nutrient use efficiency. Improved soil–water balance, increased carbon uptake in the shade, greater carbon to nitrogen ratio, and reduced nutrient quality for insect and animal grazers are all possibilities that have been observed in field studies of the effects of elevated Ca. These effects have major consequences for agriculture and native ecosystems in a world of rising atmospheric Ca and climate change.

More Efficient Plants : a consequence of rising atmospheric CO2

The effect of a doubling in the atmospheric CO2 concentration on the growth of vegetative whole plants was investigated. In a compilation of literature sources, the growth stimulation of 156 plant species was found to be on average 37%. This enhancement is small compared to what could be expected on the basis of CO2-response curves of photosynthesis. The causes for this stimulation being so modest were investigated, partly on the basis of an experiment with 10 wild plant species. Both the source-sink relationship and size constraints on growth can cause the growth-stimulating effect to be transient.

/pdf/interspecific-variation-in-the-growth-response-of-plants-to-mk00u45lug.pdf

Interspecific variation in the growth-response of plants to an elevated ambient co2 concentration

Increased atmospheric CO 2 often but not always leads to large decreases in leaf conductance. Decreased leaf conductance has important implications for a number of components of CO 2 responses, from the plant to the global scale. All of the factors that are sensitive to a change in soil moisture, either amount or timing, may be affected by increased CO 2 . The list of potentially sensitive processes includes soil evaporation, run-off, decomposition, and physiological adjustments of plants, as well as factors such as canopy development and the composition of the plant and microbial communities. Experimental evidence concerning ecosystem-scale consequences of the effects of CO 2 on water use is only beginning to accumulate, but the initial indication is that, in water-limited areas, the effects of CO 2 -induced changes in leaf conductance are comparable in importance to those of CO 2 -induced changes in photosynthesis. Above the leaf scale, a number of processes interact to modulate the response of canopy or regional evapotranspiration to increased CO 2 . While some components of these processes tend to amplify the sensitivity of evapotranspiration to altered leaf conductance, the most likely overall pattern is one in which the responses of canopy and regional evapotranspiration are substantially smaller than the responses of canopy conductance. The effects of increased CO 2 on canopy evapotranspiration are likely to be smallest in aerodynamically smooth canopies with high leaf conductances. Under these circumstances, which are largely restricted to agriculture, decreases in evapotranspiration may be only one-fourth as large as decreases in canopy conductance. Decreased canopy conductances over large regions may lead to altered climate, including increased temperature and decreased precipitation. The simulation experiments to date predict small effects globally, but these could be important regionally, especially in combination with radiative (greenhouse) effects of increased CO 2 .

Stomatal responses to increased CO2: implications from the plant to the global scale

Publisher Summary This chapter discusses the direct effects of increase in the global atmospheric CO 2 concentration on natural and commercial temperate trees and forests. The aim of this chapter is to assess what is known of these relationships in trees, and to predict the consequences of an increase in CO 2 on temperate zone forests. Information concerning the reaction of trees and forests to increase in the atmospheric CO 2 concentration is particularly important. The total amount of carbon stored in terrestrial ecosystems has diminished over recent centuries as a result of anthropogenic actions, especially forestry clearance. On a global scale, this further reduction in area of forest will both exacerbate the rise of CO 2 in the atmosphere through oxidation of wood and wood products, and reduce the sink strength for CO 2 . . Because of the complexity of forest ecosystems, there may be many consequences of long-term changes in the rates of carbon gain and water loss by trees and stands. The four main reasons for being concerned about the rise in CO 2 and its effect on trees and forests given here are: the enhancement of biological knowledge about the functioning of tree species of major ecological and economic importance, the impact on the productivity and value of the economic product, the impact on the ecology and environment of woods and forests, and the downstream, socio-economic consequences.

The Direct Effects of Increase in the Global Atmospheric CO2 Concentration on Natural and Commercial Temperate Trees and Forests

summary
Because of their prominent role in the global carbon balance and their possible carbon sequestration, trees are very important organisms in relation to global climatic changes. Knowledge of these processes is the key to understanding the functioning of the whole forest ecosystem which can he modelled and predicted based on the physiological process information. This paper reviews the major methods and techniques used to examine the likely effects of elevated CO2 on woody plants, as well as the major physiological responses of trees to elevated CO2. The available exposure techniques and approaches are described. An overview table with all relevant literature data over the period 1989-93 summarizes the percent changes in biomass, root/shoot ratio, photosynthesis, leaf area and water use efficiency under elevated CO2. Interaction between growth, photosynthesis and nutrition is discussed with a special emphasis on downward regulation of photosynthesis. The stimulation or reduction found in the respiratory processes of woody plants are reviewed, as well as the effect of elevated CO2 on stomatal density, conductance and water use efficiency. Changes in plant quality and their consequences are examined. Changes in underground processes under elevated CO2 are especially emphasized and related to the functioning of the ecosystem. Some directions for future research are put forward.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1994.tb03961.x

Tansley Review No. 71 Effects of elevated atmospheric CO2on woody plants

Needles from phosphorus deficient seedlings of Pinus radiata D. Don grown for 8 weeks at either 330 or 660 microliters CO(2) per liter displayed chlorophyll a fluorescence induction kinetics characteristic of structural changes within the thylakoid chloroplast membrane, i.e. constant yield fluorescence (F(O)) was increased and induced fluorescence ([F(P)-F(I)]/F(O)) was reduced. The effect was greatest in the undroughted plants grown at 660 mul CO(2) L(-1). By week 22 at 330 mul CO(2) L(-1) acclimation to P deficiency had occurred as shown by the similarity in the fluorescence characteristics and maximum rates of photosynthesis of the needles from the two P treatments. However, acclimation did not occur in the plants grown at 660 mul CO(2) L(-1). The light saturated rate of photosynthesis of needles with adequate P was higher at 660 mul CO(2) L(-1) than at 330 mul CO(2) L(-1), whereas photosynthesis of P deficient plants showed no increase when grown at the higher CO(2) concentration. The average growth increase due to CO(2) enrichment was 14% in P deficient plants and 32% when P was adequate. In drought stressed plants grown at 330 mul CO(2) L(-1), there was a reduction in the maximal rate of quenching of fluorescence (R(Q)) after the major peak. Constant yield fluorescence was unaffected but induced fluorescence was lower. These results indicate that electron flow subsequent to photosystem II was affected by drought stress. At 660 mul CO(2) L(-1) this response was eliminated showing that CO(2) enrichment improved the ability of the seedlings to acclimate to drought stress. The average growth increase with CO(2) enrichment was 37% in drought stressed plants and 19% in unstressed plants.

/pdf/chlorophyll-a-fluorescence-and-photosynthetic-and-growth-5543f7k0i1.pdf

Chlorophyll a Fluorescence and Photosynthetic and Growth Responses of Pinus radiata to Phosphorus Deficiency, Drought Stress, and High CO2

. Seedlings of Pinus radiata D. Don were grown in growth chambers for 22 weeks with two levels of phosphorus, under either well-watered or water-stressed conditions at CO2 concentrations of either 330 or 660mm3 dm−3. Plant growth, water use efficiency and conductance were measured and the relationship between these and needle photosynthetic capacity, water use efficiency and conductance was determined by gas exchange at week 22. Phosphorus deficiency decreased growth and foliar surface area at both CO2concentrations; however, it only reduced the maximum photosynthetic rates of the needles at 660 mm3 CO2 dm−3 (plants grown and measured at the same CO2 concentration). Water stress reduced growth and foliar surface area at both CO2 concentrations. Increases in needle photosynthetic rates appeared to be partly responsible for the increased growth at high CO2 where phosphorus was adequate. This effect was amplified by accompanying increases in needle production. Phosphorus deficiency inhibited these responses because it severely impaired needle photosynthetic function. The relative increase in growth in response to high CO2 was higher in the periodically water-stressed plants. This was not due to the maintenance of cell volume during drought. Plant water use efficiency was increased by CO2 enrichment due to an increase in dry weight rather than a decrease in shoot conductance and, therefore, transpirational water loss. Changes in needle conductance and water use efficiency in response to high CO2 were generally in the same direction as those at the whole plant level. If the atmospheric CO2 level reaches the predicted concentration of 660 mm3 dm−3 by the end of next Century, then the growth of P. radiata will only be increased in areas where phosphorus nutrition is adequate. Growth will be increased in drought-affected regions but total water use is unlikely to be reduced.

The influence of CO2 enrichment, phosphorus deficiency and water stress on the growth, conductance and water use of Pinus radiata D. Don

Pinus radiata D. Don (half-sib families 20010 and 20062) and Pinus caribaea var hondurensis (an open-pollinated family) were grown for 49 weeks at seven levels of phosphorus and at CO2 concentrations of either 340 or 660 microliters per liter, to establish if the phosphorus requirements differed between the CO2 concentrations and if mycorrhizal associations were affected. When soil phosphorus availability was low, phosphorus uptake was increased by elevated CO2. This may have been related to changes in mycorrhizal competition. When the phosphorus concentration in the youngest fully expanded needles was above 600 milligrams per kilogram the shoot weight of all pine families was greater at high CO2 due to increases in rates of photosynthesis. More dry weight was partitioned to the stems of P. radiata family 20010 and P. caribaea. At foliar phosphorus concentrations above 1000 milligrams per kilogram (P. radiata) and 700 milligrams per kilogram (P. caribaea), growth did not increase at 340 microliters of CO2 per liter. Soluble sugar levels in the same needles mirrored the growth response, but the starch concentration declined with increasing phosphorus. At 660 microliters of CO2 per liter, shoot weight and soluble sugar concentrations were still increasing up to a foliar P concentration of 1800 milligrams per kilogram for P. radiata and 1600 milligrams per kilogram for P. caribaea. The starch concentrations did not decline. These results indicate that higher foliar phosphorus concentrations are required to realize the maximum growth potential of pines at elevated CO2.

Increases in Phosphorus Requirements for CO2-Enriched Pine Species

Advanced selections (families 20010 and 20062) of P. radiata D. Don were exposed to either 340 or 660 μmol CO2 mol 1 for 2 years to establish if growth responses to high CO2 would persist during the development of woody tissues. The experiment was carried out in glasshouses and some of the trees at each CO2 concentration were subjected to phosphorus deficiency and to periodic drought. CO2 enrichment increased whole-plant dry matter production irrespective of water availability, but only when phosphorus supply was adequate. The greatest increase occurred during the exponential period of growth and appeared to be tied to increased rates of photosynthesis, which caused accelerated production of leaf area. The increase in whole-plant dry matter production was similar for both families; however, family 20010 partitioned larger amounts of dry weight to the trunks than family 20062. which favoured the roots and branches. Wood density was generally increased by elevated CO2 and for family 20010 this increase was due to thickening of the tracheid walls. Tracheid length was similar at both CO2 levels but differed between families. These results suggest that, as the atmospheric CO2 concentration rises, field-grown P. radiata should produce more dry weight at sites where phosphorus is not acutely deficient, even where drought limits growth; however, increases in wood production are likely only for genotypes which continue to partition at least the same proportion of dry weight to wood in the trunk.

Growth, dry weight partitioning and wood properties of Pinus radiata D. Don after 2 years of CO2 enrichment

Osmotic adjustment occurred during drought in expanded leaves of sunflowers (Helianthus annuus var Hysun 30) which had been continuously exposed to 660 microliters CO2 per liter or had been previously acclimated to drought. The effect was greatest when the treatments were combined and was negligible in nonacclimated plants grown at 340 microliters CO2 per liter. The concentrations of ethanol soluble sugars and potassium increased during drought but they did not account for the osmotic adjustment. The delay in the decline in conductance and relative water content and in the loss of structural integrity with increasing drought was dependent on the degree of osmotic adjustment. Where it was greatest, conductance fell from 5.8 millimeters per second on the first day of drought to 1.3 millimeters per second on the fourth day and was at approximately the same level on the eighth day. The relative water content remained constant at 85% for three days and fell to 36% on the sixth day. There was no evidence of leaf desiccation even on the eighth day. In contrast, the conductance of leaves showing minimal adjustment fell rapidly after the first day of drought and was negligible after the fourth, at which time the relative water content was 36%. By the sixth day of drought, areas near the margins of the leaves were desiccating and the plants did not recover upon rewatering. Despite the differences in the rate of change of conductance and relative water content during drought, photosynthetic electron transport activity, inferred from measurements of chlorophyll a fluorescence in vivo and PSII activity of isolated thylakoids, remained functional until desiccation occurred.

/pdf/influence-of-drought-acclimation-and-co2-enrichment-on-5ehd0f8p1h.pdf

Jann P. Conroy

Papers

Chlorophyll a Fluorescence and Photosynthetic and Growth Responses of Pinus radiata to Phosphorus Deficiency, Drought Stress, and High CO2

The influence of CO2 enrichment, phosphorus deficiency and water stress on the growth, conductance and water use of Pinus radiata D. Don

Increases in Phosphorus Requirements for CO2-Enriched Pine Species

Growth, dry weight partitioning and wood properties of Pinus radiata D. Don after 2 years of CO2 enrichment

Influence of Drought Acclimation and CO2 Enrichment on Osmotic Adjustment and Chlorophyll a Fluorescence of Sunflower during Drought