Cotton plants were grown in CO2‐controlled growth chambers in atmospheres of either 35 or 65 Pa CO2. A widely accepted model of C3 leaf photosynthesis was parameterized for leaves from both CO2 treatments using non‐linear least squares regression techniques, but in order to achieve reasonable fits, it was necessary to include a phosphate limitation resulting from inadequate triose phosphate utilization. Despite the accumulation of large amounts of starch (>50 g m−2) in the high CO2 plants, the photosynthetic characteristics of leaves in both treatments were similar, although the maximum rate of Rubisco activity (Vcmax), estimated from A versus Ci response curves measured at 29°C, was ∼10% lower in leaves from plants grown in high CO2. The relationship between key model parameters and total leaf N was linear, the only difference between CO2 treatments being a slight reduction in the slope of the line relating Vcmax to leaf N in plants grown at high CO2. Stomatal conductance of leaves of plants grown and measured at 65 Pa CO2 was approximately 32% lower than that of plants grown and measured at 35 Pa. Because photosynthetic capacity of leaves grown in high CO2 was only slightly less than that of leaves grown in 35 Pa CO2, net photosynthesis measured at the growth CO2, light and temperature conditions was approximately 25% greater in leaves of plants grown in high CO2, despite the reduction in leaf conductance. Greater assimilation rate was one factor allowing plants grown in high CO2 to incorporate 30% more biomass during the first 36 d of growth.

Interactive effects of root restriction and atmospheric CO2 enrichment on plant growth, photosynthetic capacity, and carbohydrate partitioning were studied in cotton seedlings (Gossypium hirsutum L.) grown for 28 days in three atmospheric CO2 partial pressures (270, 350, and Elevated atmospheric CO2 affects plant growth primarily by increasing net photosynthetic rates through an increase in CO2 partial pressure at the site of fixation in the chloroplast (26). Responses of plants to long-term exposure of elevated C02, however, are not well understood. Net photosynthesis of some species after long-term exposure (weeks, months) to elevated CO2 is often lower than net photosynthesis after short-term exposure (days, hours) (6,8,22,23,25,28,29 When photosynthesis was measured at 1000 ,ubar CO2 in Desmodium paniculatum after growth in 1000 Mbar CO2 for 3 to 7 weeks, rates were 33% lower relative to plants grown in 350 Mbar (29). After 3 weeks of growth in 680 Htbar C02, net photosynthetic rates of Eriophorum vaginatum measured at 680 ,ubar decreased 61% relative to plants grown at 340 pbar (25). Reduced photosynthetic capacity in elevated CO2 has been found in cotton growing in pots under nitrogenlimited conditions and under conditions of nonlimiting nitrogen (6, 28). On the other hand, cotton plants grown under field conditions at elevated CO2 maintained higher photosynthetic capacity compared to plants growing at ambient CO2 levels (2 1).It has been established that stomatal conductance of C3 plants typically decreases at elevated CO2 concentrations ( 14).Studies aimed at separating stomatal and biochemical limitations of photosynthesis, however, have concluded that stomatal closure was not responsible for reductions in photosynthetic rates of plants grown under long-term CO2 enrichment (6,8,30). Efforts to understand the physiological nature of the photosynthetic decline in plants exposed to long-term elevated CO2 have focused on chloroplast damage due to excessive carbohydrate accumulation (6, 29), on feedback inhibition associated with low utilization of photosynthate (6,8,22,23), and on changes in Rubisco activity (20,22,30). Starch often accumulates in chloroplasts in response to longterm elevated CO2 (3,6,29). This increase in nonstructural carbohydrate indicates that the plant cannot use photosynthate at the rate at which it is being produced and, when correlated with decreased net photosynthesis, reflects possible feedback effects on the photosynthetic process (1,11,15

To investigate the effects of differences in light and nutrient availability on growth, we planted seven species of shrubs in two genera, Miconia (Melastomataceae) and Piper (Piperaceae), into the centers, edges, and adjacent forest understory of four natural treefall gaps (275—3355 m2) in the tropical premontane rain forest of Costa Rica. We used rooted cutting of species typical of forest understory environments on the one hand and large clearings or disturbed areas on the other. We also compared growth rates of three Miconia species grown in shade houses under 2, 20, and 40% full sunlight. Both light and nutrient availability in newly formed gaps of these sizes were strongly buffered by the canopy and root systems of the surrounding forest. Total incident radiation was higher in gap centers (9—23% full sunlight) than in gap—forest edges (3—11%) or under intact forest canopy (.04—2%), but varied among similar microhabitats from different sites. Relative stem growth rates (RGRs) of all field—grown plants were significantly greater in gap centers than at edges or beneath forest understories. Fertilization did not significantly affect growth rate in any light environment. Light appears to be the most critical resource limiting growth at these gap sizes. In general, shade—tolerant species were less plastic than light—demanding species, but at these gap sizes grew as fast or faster in the gap centers. In shade—houses, the shade—tolerant species grew faster at 20% full sunlight and light—demanding species grew faster at 40% full sunlight. We found no evident of a trade—off between growth and foliar phenolic concentration in these species.

Summary• Availability of growth limiting resources may alter root dynamics in forest ecosystems, possibly affecting the land-atmosphere exchange of carbon. This was evaluated for a commercially important southern timber species by installing a factorial experiment of fertilization and irrigation treatments in an 8-yr-old loblolly pine ( Pinus taeda ) plantation.• After 3 yr of growth, production and turnover of fine, coarse and mycorrhizal root length was observed using minirhizotrons, and compared with stem growth and foliage development.• Fertilization increased net production of fine roots and mycorrhizal roots, but did not affect coarse roots. Fine roots had average lifespans of 166 d, coarse roots 294 d and mycorrhizal roots 507 d. Foliage growth rate peaked in late spring and declined over the remainder of the growing season, whereas fine roots experienced multiple growth flushes in the spring, summer and fall.• We conclude that increased nutrient availability might increase carbon input to soils through enhanced fine root turnover. However, this will depend on the extent to which mycorrhizal root formation is affected, as these mycorrhizal roots have much longer average lifespans than fine and coarse roots.

Forest trees are major components of the terrestrial biome and their response to rising atmospheric CO2 plays a prominent role in the global carbon cycle. In this study, loblolly pine seedlings were planted in the field in recently disturbed soil of high fertility, and CO2 partial pressures were maintained at ambient CO2 (Amb) and elevated CO2 (Amb + 30 Pa) for 4 years. The objective of the study was to measure seasonal and long-term responses in growth and photosynthesis of loblolly pine exposed to elevated CO2 under ambient field conditions of precipitation, light, temperature and nutrient availability. Loblolly pine trees grown in elevated CO2 produced 90% more biomass after four growing seasons than did trees grown in ambient CO2. This large increase in final biomass was primarily due to a 217% increase in leaf area in the first growing season which resulted in much higher relative growth rates for trees grown in elevated CO2. Although there was not a sustained effect of elevated CO2 on relative growth rate after the first growing season, absolute production of biomass continued to increase each year in trees grown in elevated CO2 as a consequence of the compound interest effect of increased leaf area on the production of more new leaf area and more biomass. Allometric analyses of biomass allocation patterns demonstrated size-dependent shifts in allocation, but no direct effects of elevated CO2 on partitioning of biomass. Leaf photosynthetic rates were always higher in trees grown in elevated CO2, but these difterences were greater in the summer (60-130% increase) than in the winter (14-44% increase), refiecting strong seasonal effects of temperature on photosynthesis. Our results suggest that seasonal variation in the relative photosynthetic response to elevated CO2 will occur in natural ecosystems, but total non-structural carbohydrate (TNC) levels in leaves indicate that this variation may not always be related to sink activity. Despite indications of canopy-level adjustments in carbon assimilation, enhanced levels of leaf photosynthesis coupled with increased total leaf area indicate that net carbon assimilation for the whole tree was greater for trees grown under elevated CO2 compared with ambient CO2. If the large growth enhancement observed in loblolly pine were maintained after canopy closure, then

The effects of long‐term CO2 enhancement and varying nutrient availability on photosynthesis and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (rubisco) were studied on loblolly pine (Pinus taeda L.) seedlings grown in two atmospheric CO2 partial pressures (35 and 65 Pa) and three nutrient treatments (low N, low P, and high N and P). Measurements taken in late autumn (November) after 2 years of CO2 enrichment and nutrient addition showed that photosynthetic rates were higher for plants grown at elevated CO2 only when they received supplemental N. Total rubisco activity and rubisco content decreased at elevated CO2, but there was an increase in activation state. At elevated CO2, proportionately less N was found in rubisco and more N was found in the light reaction components. These results demonstrate acclimation of photosynthetic processes to elevated CO2 through reallocation of N. Loblolly pine grown in nutrient conditions similar to native soils (low N availability) had lower needle N and chlorophyll content, lower total rubisco activity and content, and lower photosynthetic rates than plants grown at high N and P. This suggests that the magnitude of the photosynthetic response to a future, high‐CO2 environment will be dependent on soil fertility in the system.

Atmospheric CO partial pressure (pCO) was as low as 18 Pa during the Pleistocene and is projected to increase from 36 to 70 Pa CO before the end of the 21st century. High pCO often increases the growth and reproduction of C annuals, whereas low pCO decreases growth and may reduce or prevent reproduction. Previous predictions regarding the effects of high and low pCO on C plants have rarely considered the effects of evolution. Knowledge of the potential for evolution of C plants in response to CO is important for predicting the degree to which plants may sequester atmospheric CO in the future, and for understanding how plants may have functioned in response to low pCO during the Pleistocene. Therefore, three studies using Arabidopsis thaliana as a model system for C annuals were conducted: (1) a selection experiment to measure responses to selection for high seed number (a major component of fitness) at Pleistocene (20 Pa) and future (70 Pa) pCO and to determine changes in development rate and biomass production during selection, (2) a growth experiment to determine if the effects of selection on final biomass were evident prior to reproduction, and (3) a reciprocal transplant experiment to test if pCO was a selective agent on Arabidopsis. Arabidopsis showed significant positive responses to selection for high seed number at both 20 and 70 Pa CO during the selection process. Furthermore, plants selected at 20 Pa CO performed better than plants selected at 70 Pa CO under low CO conditions, indicating that low CO acted as a selective agent on these annuals. However, plants selected at 70 Pa CO did not have significantly higher seed production than plants selected at 20 Pa CO when grown at high pCO. Nevertheless, there was some evidence that high CO may also be a selective agent because changes in development rate and biomass production during selection occurred in opposite directions at low and high pCO. Plants selected at high pCO showed no change or reductions in biomass relative to control plants due to a decrease in the length of the life cycle, as indicated by earlier initiation of flowering and senescence. In contrast, selection at low CO resulted in an average 35% increase in biomass production, due to an increase in the length of the life cycle that resulted in a longer period for biomass accumulation before senescence. From the Arabidopsis model system we conclude that some C annuals may have produced greater biomass in response to low pCO during the Pleistocene relative to what has been predicted from studies exposing a single generation of C plants to low pCO. Furthermore, C annuals may exhibit evolutionary responses to high pCO in the future that may result in developmental changes, but these are unlikely to increase biomass production. This series of studies shows that CO may potentially act as a selective agent on C annuals, producing changes in development rate and carbon accumulation that could not have been predicted from single-generation studies.

Summary Interactive effects of CO2 and water availability have been predicted to alter the competitive relationships between C3 and C4 species over geological and contemporary time scales. We tested the effects of drought and CO2 partial pressures (pCO2) ranging from values of the Pleistocene to those predicted for the future on the physiology and growth of model C3 and C4 species. We grew co‐occurring Abutilon theophrasti (C3) and Amaranthus retroflexus (C4) in monoculture at 18 (Pleistocene), 27 (preindustrial), 35 (current), and 70 (future) Pa CO2 under conditions of high light and nutrient availability. After 27 days of growth, water was withheld from randomly chosen plants of each species until visible wilting occurred. Under well‐watered conditions, low pCO2 that occurred during the Pleistocene was highly limiting to C3 photosynthesis and growth, and C3 plants showed increased photosynthesis and growth with increasing pCO2 between the Pleistocene and future CO2 values. Well‐watered C4 plants exhibited increased photosynthesis in response to increasing pCO2, but total mass and leaf area were unaffected by pCO2. In response to drought, C3 plants dropped a large amount of leaf area and maintained relatively high leaf water potential in remaining leaves, whereas C4 plants retained greater leaf area, but at a lower leaf water potential. Furthermore, drought‐treated C3 plants grown at 18 Pa CO2 retained relatively greater leaf area than C3 plants grown at higher pCO2 and exhibited a delay in the reduction of stomatal conductance that may have occurred in response to severe carbon limitations. The C4 plants grown at 70 Pa CO2 showed lower relative reductions in net photosynthesis by the end of the drought compared to plants at lower pCO2, indicating that CO2 enrichment may alleviate drought effects in C4 plants. At the Pleistocene pCO2, C3 and C4 plants showed similar relative recovery from drought for leaf area and biomass production, whereas C4 plants showed higher recovery than C3 plants at current and elevated pCO2. Based on these model systems, we conclude that C3 species may not have been at a disadvantage relative to C4 species in response to low CO2 and severe drought during the Pleistocene. Furthermore, C4 species may have an advantage over C3 species in response to increasing atmospheric CO2 and more frequent and severe droughts.