The penalty of a long, hot summer. Photosynthetic acclimation to high CO2 and continuous light in "living fossil" conifers

Longer photoperiods negate the CO2 stimulation of photosynthesis in Betula papyrifera Marsh: Implications to climate change-induced migration

In response to global warming, trees are expected to shift their distribution ranges to higher latitudes. The range shift will expose them to novel environmental conditions, such as new photoperiod regimes. These factors can interact with rising atmospheric CO₂ ([CO₂ ]) to affect trees' physiology and growth. This study simulated future environmental conditions to investigate photosynthetic responses to changes in photoperiod regimes (seed origin [48°N], 52, 55, and 58°N) and [CO₂ ] (ambient 400 vs. elevated 1000 μmol mol^-1 ) in white birch (Betula papyrifera Marsh.) seedlings. Our results show that elevated [CO₂ ] stimulated leaf photosynthesis (P_n ) at the two lower latitudes (48 and 52°N). However, this stimulation by elevated [CO₂ ] was lost in the two higher latitudes (55 and 58°N). Elevated [CO₂ ] led to the downregulation of maximum Rubisco activity (V_cmax ) for the two higher latitudes, and maximum electron transport rate (J_max ) and triose phosphate utilization (TPU) at 58°N, while it enhanced J_max and TPU for the two lower latitudes. Increased instantaneous water-use efficiency (IWUE) for the two lower latitudes was primarily attributed to the CO₂ stimulation of P_n while the higher IWUE under the photoperiod regimes of 55 and 58°N latitudes was explained by reduced water loss. Photoperiod effects varied with [CO₂ ]: P_n increased at the photoperiod regimes of 55 and 58°N in ambient [CO₂ ] while it tended to decline under these photoperiods in elevated [CO₂ ]. Our study suggests that the photosynthesis of white birch will likely respond negatively to northward migration or seed transfer in response to climate change.

Response of photosynthesis, growth and water relations of a savannah-adapted tree and grass grown across high to low CO2

BACKGROUND AND AIMS

By the year 2100, atmospheric CO2 concentration ([CO2]a) could reach 800 ppm, having risen from ~200 ppm since the Neogene, beginning ~24 Myr ago. Changing [CO2]a affects plant carbon-water balance, with implications for growth, drought tolerance and vegetation shifts. The evolution of C4 photosynthesis improved plant hydraulic function under low [CO2]a and preluded the establishment of savannahs, characterized by rapid transitions between open C4-dominated grassland with scattered trees and closed forest. Understanding directional vegetation trends in response to environmental change will require modelling. But models are often parameterized with characteristics observed in plants under current climatic conditions, necessitating experimental quantification of the mechanistic underpinnings of plant acclimation to [CO2]a.

METHODS

We measured growth, photosynthesis and plant-water relations, within wetting-drying cycles, of a C3 tree (Vachellia karroo, an acacia) and a C4 grass (Eragrostis curvula) grown at 200, 400 or 800 ppm [CO2]a. We investigated the mechanistic linkages between trait responses to [CO2]a under moderate soil drying, and photosynthetic characteristics.

KEY RESULTS

For V. karroo, higher [CO2]a increased assimilation, foliar carbon:nitrogen, biomass and leaf starch, but decreased stomatal conductance and root starch. For Eragrostis, higher [CO2]a decreased C:N, did not affect assimilation, biomass or starch, and markedly decreased stomatal conductance. Together, this meant that C4 advantages in efficient water-use over the tree were maintained with rising [CO2]a.

CONCLUSIONS

Acacia and Eragrostis acclimated differently to [CO2]a, with implications for their respective responses to water limitation and environmental change. Our findings question the carbon-centric focus on factors limiting assimilation with changing [CO2]a, how they are predicted and their role in determining productivity. We emphasize the continuing importance of water-conserving strategies in the assimilation response of savannah plants to rising [CO2]a.

Mesophyll conductance in leaves of Japanese white birch (Betula platyphylla var. japonica) seedlings grown under elevated CO2 concentration and low N availability

To test the hypothesis that mesophyll conductance (gm ) would be reduced by leaf starch accumulation in plants grown under elevated CO2 concentration [CO2 ], we investigated gm in seedlings of Japanese white birch grown under ambient and elevated [CO2 ] with an adequate and limited nitrogen supply using simultaneous gas exchange and chlorophyll fluorescence measurements. Both elevated [CO2 ] and limited nitrogen supply decreased area-based leaf N accompanied with a decrease in the maximum rate of Rubisco carboxylation (Vc,max ) on a CO2 concentration at chloroplast stroma (Cc ) basis. Conversely, only seedlings grown at elevated [CO2 ] under limited nitrogen supply had significantly higher leaf starch content with significantly lower gm among the treatment combinations. Based on a leaf anatomical analysis using microscopic photographs, however, there were no significant difference in the area of chloroplast surfaces facing intercellular space per unit leaf area among treatment combinations. Thicker cell walls were suggested in plants grown under limited N by increases in leaf mass per area subtracting non-structural carbohydrates. These results suggest that starch accumulation and/or thicker cell walls in the leaves grown at elevated [CO2 ] under limited N supply might hinder CO2 diffusion in chloroplasts and cell walls, which would be an additional cause of photosynthetic downregulation as well as a reduction in Rubisco activity related to the reduced leaf N under elevated [CO2 ].

Water-use responses of 'living fossil' conifers to CO2 enrichment in a simulated Cretaceous polar environment

BACKGROUND AND AIMS

During the Mesozoic, the polar regions supported coniferous forests that experienced warm climates, a CO(2)-rich atmosphere and extreme seasonal variations in daylight. How the interaction between the last two factors might have influenced water use of these conifers was investigated. An experimental approach was used to test the following hypotheses: (1) the expected beneficial effects of elevated [CO(2)] on water-use efficiency (WUE) are reduced or lost during the 24-h light of the high-latitude summer; and (2) elevated [CO(2)] reduces plant water use over the growing season.

METHODS

Measurements of leaf and whole-plant gas exchange, and leaf-stable carbon isotope composition were made on one evergreen (Sequoia sempervirens) and two deciduous (Metasequoia glyptostroboides and Taxodium distichum) 'living fossil' coniferous species after 3 years' growth in controlled-environment simulated Cretaceous Arctic (69 degrees N) conditions at either ambient (400 micromol mol(-1)) or elevated (800 micromol mol(-1)) [CO(2)].

KEY RESULTS

Stimulation of whole-plant WUE (WUE(P)) by CO(2) enrichment was maintained over the growing season for the three studied species but this pattern was not reflected in patterns of WUE inferred from leaf-scale gas exchange measurements (iWUE(L)) and delta(13)C of foliage (tWUE(L)). This response was driven largely by increased rates of carbon uptake, because there was no overall CO(2) effect on daily whole-plant transpiration or whole-plant water loss integrated over the study period. Seasonal patterns of tWUE(L) differed from those measured for iWUE(L). The results suggest caution against over simplistic interpretations of WUE(P) based on leaf isotopic composition.

CONCLUSIONS

The data suggest that the efficiency of whole-tree water use may be improved by CO(2) enrichment in a simulated high-latitude environment, but that transpiration is relatively insensitive to atmospheric CO(2) in the living fossil species investigated.

Photosynthetic downregulation in the conifer Metasequoia glyptostroboides growing under continuous light: the significance of carbohydrate sinks and paleoecophysiological implications

During the Eocene (ca. 45 Ma) a temperate climate at high northern latitudes provided an environment unlike any that currently exists on Earth. The growing season was characterized by long (up to 4 months) periods of continuous, low- to moderate-intensity illumination. While this remarkable light regime offered opportunities for substantial growth, it also imposed physiological challenges consequential to potential carbon sink–source imbalance and resulting downregulation of photosynthetic capacity. To better understand the physiology of adaptation to a continuous-light (CL) environment, we experimentally investigated the effects of CL and carbon sink–source relationships in the deciduous conifer Metasequoia glyptostroboides Hu et Cheng, an extant representative of a genus that was the dominant tree component of many Eocene high-latitude forests. We tested the importance of branch-level and whole-plant sinks in curtailing feedback inhibition and the specific roles of starch and sugars in that process using manipulative experiments. Trees growing under either normal day–night cycles or continuous illumination were subjected to reduction of local, branch-level sinks or both local and whole-tree sinks. Reduction in sink strength led to downregulation of photosynthetic capacity, as evidenced by reduction of photosynthetic rates, carboxylation capacity, and electron transfer capacity. Our results suggest that photosynthetic downregulation is minimized by maintenance of both whole-tree sinks and local sinks. downregulation showed a greater correlation with starch than with sugar content, and ultrastructural evidence indicated that foliar starch accumulated only in chloroplasts, and was accompanied by reduction in functional chloroplast grana, but showed no evidence of physical disruption of thylakoids.

Climatic and ecological determinants of leaf lifespan in polar forests of the high CO2 Cretaceous 'greenhouse' world

Polar forests once extended across the high-latitude landmasses during ice-free 'greenhouse' intervals in Earth history. In the Cretaceous 'greenhouse' world, Arctic conifer forests were considered predominantly deciduous, while those on Antarctica contained a significantly greater proportion of evergreens. To investigate the causes of this distinctive biogeographical pattern, we developed a coupled model of conifer growth, soil biogeochemistry and forest dynamics. Our approach emphasized general relationships between leaf lifespan (LL) and function, and incorporated the feedback of LL on soil nutrient status. The model was forced with a mid-Cretaceous 'greenhouse' climate simulated by the Hadley Centre GCM. Simulated polar forests contained mixtures of dominant LLs, which reproduced observed biogeographical patterns of deciduous, mixed and evergreen biomes. It emerged that disturbance by fire was a critical factor. Frequent fires in simulated Arctic ecosystems promoted the dominance of trees with short LLs that were characterized by the rapid growth and colonization rates typical of today's boreal pioneer species. In Antarctica, however, infrequent fires allowed trees with longer LLs to dominate because they attained greater height, despite slower growth rates. A direct test of the approach was successfully achieved by comparing modelled LLs with quantitative estimates using Cretaceous fossil woods from Svalbard in the European Arctic and Alexander Island, Antarctica. Observations and the model both revealed mixed Arctic and evergreen Antarctic communities with peak dominance of trees with the same LLs. Our study represents a significant departure from the long-held belief that leaf habit was an adaptation to warm, dark winter climates, and highlights a previously unrecognized role for disturbance (in whatever guise) in polar forest ecology.