Our objective was to assess the photosynthetic responses of loblolly pine trees (Pinus taeda L.) during the first full growth season (1997) at the Brookhaven National Lab/Duke University Free Air CO 2 Enrichment (FACE) experiment. Gas exchange, fluorescence characteristics, and leaf biochemistry of ambient CO 2 (control) needles and ambient + 20 Pa CO 2 (elevated) needles were examined five times during the year. The enhancement of photosynthesis by elevated CO 2 in mature loblolly pine trees varied across the season and was influenced by abiotic and biotic factors. Photosynthetic enhancement by elevated CO 2 was strongly correlated with leaf temperature. The magnitude of photosynthetic enhancement was zero in March but was as great as 52% later in the season. In March, reduced sink demand and lower temperatures resulted in lower net photosynthesis, lower carboxylation rates and higher excess energy dissipation from the elevated CO 2 needles than from control needles. The greatest photosynthetic enhancement by CO 2 enrichment was observed in July during a period of high temperature and low precipitation, and in September during recovery from this period of low precipitation. In July, loblolly pine trees in the control rings exhibited lower net photosynthetic rates, lower maximum rates of photosynthesis at saturating CO 2 and light, lower values of carboxylation and electron transport rates (modelled from A-C i curves), lower total Rubisco activity, and lower photochemical quenching of fluorescence in comparison to other measurement periods. During this period of low precipitation trees in the elevated CO 2 rings exhibited reduced net photosynthesis and photochemical quenching of fluorescence, but there was little effect on light-and CO 2 -saturated rates of photosynthesis, modelled rates of carboxylation or electron transport, or Rubisco activity. These first-year data will be used to compare with similar measurements from subsequent years of the FACE experiment in order to determine whether photosynthetic acclimation to CO 2 occurs in these canopy loblolly pine trees growing in a forest ecosystem.

Sink strength in loblolly pine (Pinus taeda L.) was experimentally manipulated on two sun-exposed branches on each of two neighboring trees by excising the emerging terminal cohort (second flush of 1996) during a period of rapid needle expansion. In addition, export of photosynthate was restricted on one of these branches from each tree by removal of bark and phloem just below the second flush of 1995. Treatment-induced changes in needle biochemistry were measured in 3-month-old (first flush of 1996) and 1-year-old (final flush of 1995) needles collected 1, 5 and 8 days after treatment. In 3-month-old needles, sugar concentration increased by 24% one day after leader excision, and increased by 86% on Day 8 after leader excision and girdling. Starch concentration increased by 33% in 3-month-old needles on Day 1 after leader excision, and by 400% in 1-year-old needles on Day 8 after leader excision and girdling. Physiological changes in 3-month-old and 1-year-old needles were measured by open-flow gas exchange and chlorophyll fluorescence on Day 8 after leader excision and girdling. Light- and CO(2)-saturated net photosynthesis decreased following treatment in both 3-month-old and 1-year-old needles (23 and 17%, respectively). Maximum rate of carboxylation (V(cmax)) decreased by 25% in 3-month-old needles and by 31% in 1-year-old needles in response to leader excision and girdling. The combined treatment resulted in a 38% decrease in maximum rate of electron transport (J(max)) in 3-month-old needles and a 37% decrease in J(max) in 1-year-old needles. Before leader excision and girdling, 2% oxygen in air stimulated photosynthesis by 17 to 19%, but this stimulation was only 3 to 4% at 9 days after treatment. These physiological responses indicate that experimentally lowered sink strength resulted in rapid feedback inhibition of leaf-level photosynthetic capacity in loblolly pine.

lmage analysis was used to quantify the lateral heterogeneity of the radiation field within the palisade of Oxialis acetosella L. leaves. Oxalis acetosella epidermal cells focus light up to four times incident irradiance, resulting in regions of high and low internal fluenec rate within the palisade. Chlorophyll fluorescence from leaves irradiated with directional light was found to originate primarily from palisade cell chloroplasts located within focal zones. When the internal radiation field was made more homogeneous by using diffuse light or by coating the leaf with a layer of mineral oil to eliminate epidermal focussing, the characteristics of the chlorophyll fluorescence signal were altered: non‐photochemical quenching (qN) increased, while relaxation of qN was slowed. This indicates that upper palisade chloroplasts may fine‐tune their light utilization to the intra leaf light microenvironment.

1997. Inclination of sun and shade leaves influences chloropiast light harvesting and utilization. -Physiol. Plant. 99; 395-404.Light harvesting and utilization by chloroplasts located near the adaxial vs the abaxial surface of sun and shade leaves were examined by fluorometry in two herbaceous perennials that differed in their anatomy atid leaf inclination. Leaves of Thermopsis montana had well-developed palisade and spongy mesophyl! whereas the photosynthetic tissue of Smiladna stellata consisted of spongy mesophyll only. Leaf orientation depended upon the irradiance during leaf development. When grown under low-light levels, leaves of S. stellata and T. montana were nearly horiiontal, whereas under high-light levels, S. stellata leaves and T. montana leaves were inclined 60° and 30°, respectively. Leaf inclination increased the amount of light that was intercepted by the lower leaf surfaces and affected the photosynthetic properties of the chloroplasts located near the abaxial leaf surface. The slowest rates of quinone pool reduction and reoxidation were found in chloroplasts located near the adaxial le^ surface of T. montana plants grown tmder high light, indicating large quinone pools in these chloroplasts. Chloroplasts near the abaxia! surface of low-light leaves had lower light utilization capacities as shown by photochemical quenching measurements. The amount of photosystem D (PSII) down regulation, measured bom each leaf surface, was also found to be influenced by irradiance and leaf inclination. The greatest difference between down regulation monitored from the adaxial vs abaxial surfaces was fotind in plants with horizontal leaves. Different energy dissipation mechanisms may be employed by the two species. Values for down regulation in S. stellata were 2-3 times higher than those in T. montana, while the portion of the PSII populadoB which was found to be QB nonreducing was 4-6 times lower in high light S. stellata leaves than in T. montana. All values of Stem-Volmer type nonphotochemical quenching (NPQ) from S. stellata leaves were similar when quenching analysis was performed at actinic irradiances that were higher thaji the iiradiance to which the leaf surface was exposed during growth. In contrast, with T. montana, NPQ values from the abaxial leaf surface were up to 45% higher than those from the adaxial leaf surface regardless of growth conditions. The observed differences in chloroplast properties between species and between the adaxia! and abaxia! leaf surfaces may depend upon a complex interaction among light, leaf anatomy and leaf isclination.

exhibited a large growth advantage over the C 3 species at low [CO 2 ]. However, this advantage was reduced at low temperature, where the C 4 species produced 5× the total mass of the C 3 species versus 14× at the high temperature. This difference was due to a reduction in C 4 growth at low temperature, since the C 3 species exhibited similar growth between temperatures. Physiological differences between temperatures were not detected for either species, although photorespiration/net photosynthesis was reduced in the C 3 species grown at low temperature, suggesting evidence of improved carbon balance at this treatment. This system suggests that C 4 species had a growth advantage over C 3 species during low [ Studying plant responses to global changes of the past provides valuable insights for predicting future responses to a rapidly changing environment (Ward et al. 2005; Edwards et al. 2007; Jackson 2007). In addition, studies involving treatments that simulate past climates provide a baseline for understanding the physiological functioning of plants prior to anthropogenic influences (Polley et al. 1993a,b; Anderson et al. 2001 In addition to physiological models, empirical studies examining the growth and development of C 3 and C 4 species at reduced temperatures and low [CO 2 ] are also needed to better Table 1. The effects of modern and glacial temperatures on total mass (n = 13-14), leaf area (LA, n = 13-14), stomatal conductance (g s , n = 9-12), and specific leaf mass (SLM, n = 13-14) for Abutilon theophrasti (C 3 ) and Amaranthus retroflexus (C 4 ) grown at low CO 2 (200 μmol/mol) for 22 d Values are means ± 1 SE. Data from different chambers within the same treatment were combined because a chamber effect was not detected.Different superscript letters between temperatures and species indicate significant differences at the P < 0.05 level according to ANOVA. ResultsOn an absolute basis, the C 3 (Abutilon) and C 4 species (Amaranthus) exhibited large differences in total mass when grown at low [CO 2 ] (200 μmol/mol) for 22 d. More specifically, the C 4 species had five times the total mass of the C 3 species when grown at the low temperature (22 light/16 dark • C), and almost 14 times the total mass of the C 3 species when grown at the high temperature (30/24 • C; Table 1). In this case, all plants were grown from seed for a total of 22 d, and therefore differences in final biomass mainly reflect differences in relative growth rate (change in biomass per unit biomass per unit time; hereafter RGR). Therefore, the RGR of the C 4 species greatly exceeded that of the C 3 species at low [CO 2 ]. In addition, Amaranthus has inherently small seeds compared with Abutilon, and therefore initial seed size would not have been a factor in producing a relative growth advantage of the C 4 species over the C 3 species when grown at low [CO 2 ]. Furthermore, the C 3 and C 4 species exhibited relative differences in their responses to temperature for total mass and LA (leaf area) (significant species X temperature inte...