Changes in specific leaf area (SLA, projected leaf area per unit leaf dry mass) and nitrogen partitioning between proteins within leaves occur during the acclimation of plants to their growth irradiance. In this paper, the relative importance of both of these changes in maximizing carbon gain is quantified. Photosynthesis, SLA and nitrogen partitioning within leaves was determined from 10 dicotyledonous C 3 species grown in photon irradiances of 200 and 1000 μ mol m - 2 s - 1 . Photosynthetic rate per unit leaf area measured under the growth irradiance was, on average, three times higher for high-light-grown plants than for those grown under low light, and two times higher when measured near light saturation. However, light-saturated photosynthetic rate per unit leaf dry mass was unaltered by growth irradiance because low-light plants had double the SLA. Nitrogen concentrations per unit leaf mass were constant between the two light treatments, but plants grown in low light partitioned a larger fraction of leaf nitrogen into light harvesting. Leaf absorptance was curvilinearly related to chlorophyll content and independent of SLA. Daily photosynthesis per unit leaf dry mass under low-light conditions was much more responsive to changes in SLA than to nitrogen partitioning. Under high light, sensitivity to nitrogen partitioning increased, but changes in SLA were still more important.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.2001.00724.x

Photosynthetic acclimation of plants to growth irradiance: the relative importance of specific leaf area and nitrogen partitioning in maximizing carbon gain

differences in the assimilation of atmospheric CO2 depends upon differences in the capacities for the biochemical reactions that regulate the gas-exchange process. Quantifying these differences for more than a few species, however, has proven difficult. Therefore, to understand better how species differ in their capacity for CO2 assimilation, a widely used model, capable of partitioning limitations to the activity of ribulose-l,5-W.sphosphate carboxylase-oxygenase, to the rate of ribulose 1,5-tophosphate regeneration via electron transport, and to the rate of triose phosphate utilization was used to analyse 164 previously published A/C, curves for 109 C3 plant species. Based on this analysis, the maximum rate of carboxylation, Vcmax, ranged from 6/umol m~2 s"1 for the coniferous species Picea abies to 194jj,mol m" 2 s"1 for the agricultural species Beta vulgaris, and averaged 64^mol m" 2 s"1 across all species. The maximum rate of electron transport, Jmx, ranged from 17/^mol m~2 s"1 again for Picea abies to 372/j.mol m~2 s"1 for the desert annual Mahastrum rotundifolium, and averaged 134fxmol m~2 s"1 across all species. A strong positive correlation between Vc^x and Jmax indicated that the assimilation of CO2 was regulated in a co-ordinated manner by these two component processes. Of the AjC{ curves analysed, 23 showed either an insensitivity or reversed-sensitivity to increasing CO2 concentration, indicating that CO2 assimilation was limited by the utilization of triose phosphates. The rate of triose phosphate utilization ranged from 4-9/xtnol m" 2 s"1 for the tropical perennial Tabebuia rosea to 20-1 /xmol m~2 s"1 for the weedy annual Xanthium strumarium, and averaged 101 ftmol m" 2 s"1 across all species. Despite what at first glance would appear to be a wide range of estimates for the biochemical capacities that regulate CO2 assimilation, separating these species-specific results into those of broad plant categories revealed that Vcmax and Jmax were in general higher for herbaceous annuals than they were for woody perennials. For annuals, Vc^^ and Jmax averaged 75 and 154ftmol m~2 s"1, while for perennials these same two parameters averaged only 44 and 97/xmol m~2 s"1, respectively. Although these differences between groups may be coincidental, such an observation points to differences between annuals and perennials in either the availability or allocation of resources to the gas-exchange process.

Biochemical Limitations to Carbon Assimilation in C3 Plants—A Retrospective Analysis of the A/Ci Curves from 109 Species

Light is widely considered to be the most important factor limiting the performance of plants on the floors of forests and woodlands, but the roles of nutrient availability and water supply remain poorly defined. We seek to predict the types of forest in which root competition affects seedling performance, and the types of plants that respond most strongly to release from root competition. We then test our predictions by reviewing experiments in which tree seedlings and forest herbs are released from belowground competition, usually by cutting trenches to sever the roots of surrounding trees. First, we provide a worldwide review of changes in canopy form and fine-root mass along gradients of soil fertility and seasonal drought, keeping in mind the stages of forest development. Our review shows that penetration of light is least in forests on moist soils providing large amounts of major nutrients. The changes are far more complex than those considered by allocation models. Dry woodlands typically allow 20 ...

Impacts of root competition in forests and woodlands: a theoretical framework and review of experiments

Estimates of the extent of the discrimination against13CO2 during photosynthesis (ΔA) on a global basis were made using gridded data sets of temperature, precipitation, elevation, humidity and vegetation type. Stomatal responses to leaf-to-air vapour mole fraction difference (D, leaf-to-air vapour pressure difference divided by atmospheric pressure) were first determined by a literature review and by assuming that stomatal behaviour results in the optimisation of plant water use in relation to carbon gain. Using monthly time steps, modelled stomatal responses toD were used to calculate the ratio of stomatal cavity to ambient CO2 mole fractions and then, in association with leaf internal conductances, to calculate ΔA. Weighted according to gross primary productivity (GPP, annual net CO2 asimilation per unit ground area), estimated ΔA for C3 biomes ranged from 12.9‰ for xerophytic woods and shrub to 19.6‰ for cool/cold deciduous forest, with an average value from C3 plants of 17.8‰. This is slightly less than the commonly used values of 18–20‰. For C4 plants the average modelled discrimination was 3.6‰, again slightly less than would be calculated from C4 plant dry matter carbon isotopic composition (yielding around 5‰). From our model we estimate that, on a global basis, 21% of GPP is by C4 plants and for the terrestrial biosphere as a whole we calculate an average isotope discrimination during photosynthesis of 14.8‰. There are large variations in ΔA across the globe, the largest of which are associated with the precence or absence of C4 plants. Due to longitudinal variations in ΔA, there are problems in using latitudinally averaged terrestrial carbon isotope discriminations to calculate the ratio of net oceanic to net terrestrial carbon fluxes.

13C discrimination during CO2 assimilation by the terrestrial biosphere

Factors that contribute to interspecific varia- tion in photosynthetic nitrogen-use eAciency (PNUE, the ratio of CO2 assimilation rate to leaf organic nitro- gen content) were investigated, comparing ten dicoty- ledonous species that diAer inherently in specific leaf area (SLA, leaf area:leaf dry mass). Plants were grown hydroponically in controlled environment cabinets at two irradiances (200 and 1000 lmol m -2 s -1 ). CO2 and irradiance response curves of photosynthesis were mea- sured followed by analysis of the chlorophyll, Rubisco, nitrate and total nitrogen contents of the leaves. At both irradiances, SLA ranged more than twofold across spe- cies. High-SLA species had higher in situ rates of pho- tosynthesis per unit leaf mass, but similar rates on an area basis. The organic N content per unit leaf area was lower for the high-SLA species and consequently PNUE at ambient light conditions (PNUEamb) was higher in those plants. DiAerences were somewhat smaller, but still present, when PNUE was determined at saturating irradiances (PNUEmax). An assessment was made of the relative importance of the various factors that underlay interspecific variation in PNUE. For plants grown under low irradiance, PNUEamb of high-SLA species was higher primarily due to their lower N content per unit leaf area. Low-SLA species clearly had an overinvest- ment in photosynthetic N under these conditions. In addition, high SLA-species allocated a larger fraction of organic nitrogen to thylakoids and Rubisco, which fur- ther increased PNUEamb. High-SLA species grown un- der high irradiance showed higher PNUEamb mainly due to a higher Rubisco specific activity. Other factors that contributed were again their lower contents of Norg per unit leaf area and a higher fraction of photosynthetic N in electron transport and Rubisco. For PNUEmax, dif- ferences between species in organic leaf nitrogen content per se were no longer important and higher PNUEmax of the high SLA species was due to a higher fraction of N in photosynthetic compounds (for low-light plants) and a higher Rubisco specific activity (for high-light grown plants).

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Photosynthetic nitrogen-use efficiency of species that differ inherently in specific leaf area

Species with contrasting shade tolerance were grown under three light by two nutrient treatments. Gas exchange by intact leaves, leaf disk O2 evolution and chlorophyll fluorescence were measured. In shade-tolerant evergreen species (Argyrodendron sp., A. trifoliolatum and Flindersia brayleyana) photosynthetic activity of seedlings in air at light saturation (A) was lower under weak (30 pmol quanta m-2 day-1 ), compared with medium (130) or strong light (535). In Toona australis, a shade-intolerant and deciduous tree, A was reduced 44% from strong to weak light treatment on high nutrients (71 mg N L-1 nutrient solution). Nevertheless, nitrogen-use efficiency for leaf photosynthesis was highest in Toona under all growing conditions and, with higher specific leaf area, probably contributes towards fast occupancy of sites which underlies early succession in this species. All species made photosynthetic and respiratory adjustments from strong to medium to weak light, which resulted in a lower light compensation point (Q0). Such adjustments were accentuated by low nutrient supply (1.0 mg N L-1 nutrient solution) and were especially pronounced for shade-intolerant Toona. Reduced Q0 in Toona was accompanied by lower A and light saturation point (QA). Both species of Argyrodendron showed no decrease in QA despite reduction in Q0 under weak light.Contrary to expectation, photosynthetic responses to light × nutrient treatments did not correlate with degree of shade tolerance accorded each species by rainforest ecologists.

Photosynthetic Response to Light and Nutrients in Sun-Tolerant and Shade-Tolerant Rainforest Trees. II. Leaf Gas Exchange and Component Processes of Photosynthesis

WA Thompson

Papers

Photosynthetic Response to Light and Nutrients in Sun-Tolerant and Shade-Tolerant Rainforest Trees. II. Leaf Gas Exchange and Component Processes of Photosynthesis