Summary• CONSTANS is an evolutionarily-conserved central component of the genetic pathway that controls the onset of flowering in response to daylength. However, the specific biochemical mechanism by which the CONSTANS protein regulates the expression of its target genes remains largely unknown.• By using a combination of cell-based expression analysis and in vitro DNA binding studies, we have demonstrated that CONSTANS possesses transcriptional activation potential and is capable of directly binding to DNA.• CONSTANS was found to bind DNA via a unique sequence element containing a consensus TGTG(N2-3)ATG motif. This element is present in tandem within the FLOWERING LOCUS T promoter and is sufficient for CO binding and activity. The conserved CCT (CONSTANS, CONSTANS-like and TOC1) domain of CONSTANS was shown to be required for its recruitment to the DNA motif and other CCTcontaining proteins were also found to have the ability to regulate gene expression via this element.• The CCAAT box, which has been previously hypothesized as a recruitment site for complexes containing the CONSTANS protein, potentiated CONSTANSmediated activation but was not essential for CONSTANS recruitment to a target promoter or for its activity as a transcriptional factor.

Accumulating evidence supports a role for members of the plant Nuclear Factor Y (NF-Y) family of CCAAT-box binding transcription factors in the regulation of flowering time. In this study we have used a genetic approach to show that the homologous proteins NF-YB3 and NF-YB2 have comparable activities and play additive roles in the promotion of flowering, specifically under inductive photoperiodic conditions. We demonstrate that NF-YB2 and NF-YB3 are both essential for the normal induction of flowering by long-days and act through regulation of the expression of FLOWERING LOCUS T (FT). Using an ELISA-based in-vitro assay, we provide a novel demonstration that plant NF-YB subunits are capable of directly binding to a CCAAT-box containing region of the FLOWERING LOCUS T promoter as part of an NF-Y trimer in combination with the yeast HAP2 and HAP5 subunits. These results support an emerging model in which NF-Y complexes provide a component of the DNA target specificity for transcriptional regulators such as CONSTANS.

Summary• The use of the 13 C : 12 C isotopic ratio ( δ 13 C) of leaf-respired CO 2 to trace carbon fluxes in plants and ecosystems is limited by little information on temporal variations in δ 13 C of leaf dark-respired CO 2 ( δ 13 C r ) under field conditions.• Here, we explored variability in δ 13 C r and its relationship to key respiratory substrates from collections of leaf dark-respired CO 2 , carbohydrate extractions and gas exchange measurements over 24-h periods in two Quercus canopies.• Throughout both canopies, δ 13 C r became progressively 13 C-enriched during the photoperiod, by up to 7‰, then 13 C-depleted at night relative to the photoperiod. This cycle could not be reconciled with δ 13 C of soluble sugars ( δ 13 C ss ), starch ( δ 13 C st ), lipids ( δ 13 C l ), cellulose ( δ 13 C c ) or with calculated photosynthetic discrimination ( ∆ ). However, photoperiod progressive enrichment in δ 13 C r was correlated with cumulative carbon assimilation ( r 2 = 0.91).• We concluded that there is considerable short-term variation in δ 13 C r in forest canopies, that it is consistent with current hypotheses for 13 C fractionation during leaf respiration, that leaf carbohydrates cannot be used as surrogates for δ 13 C r , and that diel changes in leaf carbohydrate status could be used to predict changes in δ 13 C r empirically.New Phytologist (2005) 167 : 377-384 © New Phytologist (2005)

Photosynthesis is commonly stimulated in grasslands with experimental increases in atmospheric CO2 concentration ([CO2]), a physiological response that could significantly alter the future carbon cycle if it persists in the long term. Yet an acclimation of photosynthetic capacity suggested by theoretical models and short‐term experiments could completely remove this effect of CO2. Perennial ryegrass (Lolium perenne L. cv. Bastion) was grown under an elevated [CO2] of 600 µmol mol−1 for 10 years using Free Air CO2Enrichment (FACE), with two contrasting nitrogen levels and abrupt changes in the source : sink ratio following periodic harvests. More than 3000 measurements characterized the response of leaf photosynthesis and stomatal conductance to elevated [CO2] across each growing season for the duration of the experiment. Over the 10 years as a whole, growth at elevated [CO2] resulted in a 43% higher rate of light‐saturated leaf photosynthesis and a 36% increase in daily integral of leaf CO2 uptake. Photosynthetic stimulation was maintained despite a 30% decrease in stomatal conductance and significant decreases in both the apparent, maximum carboxylation velocity (Vc,max) and the maximum rate of electron transport (Jmax). Immediately prior to the periodic (every 4–8 weeks) cuts of the L. perenne stands, Vc,max and Jmax, were significantly lower in elevated than in ambient [CO2] in the low‐nitrogen treatment. This difference was smaller after the cut, suggesting a dependence upon the balance between the sources and sinks for carbon. In contrast with theoretical expectations and the results of shorter duration experiments, the present results provide no significant change in photosynthetic stimulation across a 10‐year period, nor greater acclimation in Vc,max and Jmax in the later years in either nitrogen treatment.

No abstract

The effect of elevated atmospheric CO2 concentration (Ca) on the aboveground biomass of three oak species, Quercus myrtifolia, Q. geminata, and Q. chapmanii, was estimated nondestructively using allometric relationships between stem diameter and aboveground biomass after four years of experimental treatment in a naturally fire‐regenerated scrub‐oak ecosystem. After burning a stand of scrub‐oak vegetation, re‐growing plants were exposed to either current ambient (379 µL L−1 CO2) or elevated (704 µL L−1 CO2) Ca in 16 open‐top chambers over a four‐year period, and measurements of stem diameter were carried out annually on all oak shoots within each chamber. Elevated Ca significantly increased aboveground biomass, expressed either per unit ground area or per shoot; elevated Ca had no effect on shoot density. The relative effect of elevated Ca on aboveground biomass increased each year of the study from 44% (May 96–Jan 97), to 55% (Jan 97–Jan 98), 66% (Jan 98–Jan 99), and 75% (Jan 99–Jan 00). The effect of elevated Ca was species specific: elevated Ca significantly increased aboveground biomass of the dominant species, Q. myrtifolia, and tended to increase aboveground biomass of Q. chapmanii, but had no effect on aboveground biomass of the subdominant, Q. geminata. These results show that rising atmospheric CO2 has the potential to stimulate aboveground biomass production in ecosystems dominated by woody species, and that species‐specific growth responses could, in the long term, alter the composition of the scrub‐oak community.

Averaged across many previous investigations, doubling the CO 2 concentration ([CO 2 ]) has frequently been reported to cause an instantaneous reduction of leaf dark respiration measured as CO 2 efflux. No known mechanism accounts for this effect, and four recent studies have shown that the measurement of respiratory CO 2 efflux is prone to experimental artifacts that could account for the reported response. Here, these artifacts are avoided by use of a high-resolution dual channel oxygen analyzer within an open gas exchange system to measure respiratory O 2 uptake in normal air. Leaf O 2 uptake was determined in response to instantaneous elevation of [CO 2 ] in nine contrasting species and to long-term elevation in seven species from four field experiments. Over six hundred separate measurements of respiration failed to reveal any decrease in respiratory O 2 uptake with an instantaneous increase in [CO 2 ]. Respiration was found insensitive not only to doubling [CO 2 ], but also to a 5-fold increase and to decrease to zero. Using a wide range of species and conditions, we confirm earlier reports that inhibition of respiration by instantaneous elevation of [CO 2 ] is likely an experimental artifact. Instead of the expected decrease in respiration per unit leaf area in response to long-term growth in the field at elevated [CO 2 ], there was a significant increase of 11% and 7% on an area and mass basis, respectively, averaged across all experiments. The findings suggest that leaf dark respiration will increase not decrease as atmospheric [CO 2 ] rises.A quantitative analysis of prior studies weighted for replication and experimental variation concluded that a doubling of atmospheric CO 2 concentration would decrease respiratory CO 2 efflux by 18% in woody plants (Curtis and Wang, 1998). This decrease is considered the result of a direct instantaneous effect of increased CO 2 concentration and a longer term indirect effect due to changes in leaf composition with long-term growth at elevated CO 2 (for review, see Drake et al., 1999). In an analysis of 45 species, 36 showed an average 15% instantaneous reduction in net respiratory CO 2 efflux per unit leaf area (R d,CO2 ) on transfer to elevated [CO 2 ] (for review, see Amthor, 1997). Terrestrial plant respiration releases 10 times more carbon per annum than fossil fuel combustion (Amthor, 1997). Therefore, a 15% to 20% decrease in foliar respiration might increase the potential of terrestrial vegetation to sequester carbon into biomass, providing a partial amelioration to the rate of increase of atmospheric [CO 2 ] (for review, see Gonzalez-Meler and Siedow, 1999).However, there is considerable variation in the reported instantaneous effects of elevated [CO 2 ] on R d,CO2 with some studies reporting a large decrease and others reporting no change (Bunce, 2002; Bruhn et al., 2002). Furthermore, whereas some biochemical reactions within plant respiratory metabolic pathways are sensitive to [CO 2 ], none of those identified exert sufficient metabolic control to ac...

Increased levels of atmospheric carbon dioxide (CO 2 ) are likely to affect the trophic relationships that exist between plants, their herbivores and the herbivores' natural enemies. This study takes advantage of an open-top CO 2 fertilization experiment in a Florida scrub oak community at Kennedy Space Center, Florida, consisting of eight chambers supplied with ambient CO 2 (360 ppm) and eight chambers supplied with elevated CO 2 (710 ppm). We examined the effects of elevated CO 2 on herbivore densities and levels of leaf consumption, rates of herbivore attack by natural enemies and effects on leaf abscission. Cumulative levels of herbivores and herbivore damage were significantly lower in elevated CO 2 than in ambient CO 2 . This may be because leaf nitrogen levels are lower in elevated CO 2 . More herbivores die of host plant-induced death in elevated CO 2 than in ambient CO 2 . Attack rates of herbivores by parasitoids are also higher in elevated CO 2 , possibly because herbivores need to feed for a longer time in order to accrue sufficient nitrogen (N), thus exposing themselves longer to natural enemies. Insect herbivores cause an increase in abscission rates of leaves throughout the year. Because of the lower insect density in elevated CO 2 , we thought, abscission rates would be lower in these chambers. However, abscission rates were significantly higher in elevated CO 2 . Thus, the direct effects of elevated CO 2 on abscission are greater than the indirect effects on abscission mediated via lower insect densities. A consequence of increased leaf abscission in elevated CO 2 is that nutrient deposition rates to the soil surface are accelerated.