Valery J. Terwilliger

Influences of Heterotrophic and Autotrophic Resource Use on Carbon and Hydrogen Isotopic Compositions of Tropical Tree Leaves

Abstract The δ13C and δD values of newly emerging to senescing tree leaves produced during a rainy season were obtained in dry seasonal and moist forest in Panamá. Newly emerging leaves had less negative δ13C values than older leaves yet instantaneous Pi/Pa was never lowest in the youngest leaves. Furthermore, isotopic enrichment during early growth may have a detectable influence on the δ13C values of mature leaves. The δD values of cellulose nitrate were only related to δD values of leaf water if leaf age was also considered so that, for a given δD of leaf water, δD values were highest in the youngest leaves (R2=98%). There was no correlation between leaf age and δD values of leaf water. Investment of translocated organic carbon is a factor likely to be associated with both 13C and deuterium enrichment effects in new leaves. A coarse, mass balance approach can estimate the proportional heterotrophic investment in leaf growth and improve estimates of integrated Pi/Pa by approximating δ13C for the most autotrophic phase of leaf growth. δ13C values of the predominantly sucrose mobile organic fraction in new leaves were less negative than in older leaves, thereby suggesting that the enrichment did not occur at the original site of production of the substrate for new leaf growth. Although the δ values of early leaf growth must be influenced by inputs of translocated organic carbon, enrichment effects, per se, are apparently caused by other mechanisms such as, for carbon, de novo sucrose synthesis and anaplerotic replenishment. Better recognition of metabolic causes of isotopic

Carbon isotope discrimination correlates with a range of ratios of phosphoenolpyruvate to total carboxylase activities found in two C3species

Summary The most widely used model that relates leaf carbon isotopic ratios to {ie489-1} ratios in C 3 plants suggests that PEPcase contributes approximately 6% of the carbon pool in these species. Variability in this contribution during leaf development is not only possible, but has been shown in studies where PEP-case activity is highest during the heterotrophic (carbon gained from import) stages of leaf development and declines as the leaf attains greater amounts of autotrophy (carbon gained from photosynthesis). We tested the hypothesis that variation in PEPcase: total carboxylase activities associated with carbon sources for leaf growth could affect carbon isotopic discrimination values on mixotrophic (carbon gained from import and photosynthesis) and autotrophic Nicotiana tabacum and Salix interior plants. A range of mean ratios of PEPcase: total carboxylase activities was achieved in these two C 3 species that correlated negatively with mean leaf carbon isotopic discrimination. These results indicate that the proportion of carbon fixed by PEPcase to total carboxylation could vary between different leaf developmental stages within and among different species and that this contribution can exceed 6%.

Deviation Between δ13C and Leaf Intercellular Co2 in Salix Interior Cuttings Developing Under Low Light

Leaf δ13C values of perennial species are sometimes less negative than model‐predicted δ13C values derived from instantaneous measures of pi/pa. It has been hypothesized that the less negative δ13C values could be caused by 13C‐enriched stored carbon imported during the early stages of leaf growth. The δ13C values of newly emerging leaves could thus represent δ13C values of stem‐stored carbohydrates and may also provide integral measures of pi/pa at the end of the past growing season. We tested these hypotheses by sprouting cuttings of Salix interior under wet and dry soil‐moisture conditions in a controlled environmental chamber. Plants were defoliated after 56 d, and watering treatments were then reversed for half of the plants in each treatment. The δ13C values of newly emerging leaves did not correlate with pi/pa ratios of newly emerging leaves or of mature leaves prior to defoliation, thereby indicative that δ13C values of newly emerging leaves are not a simple reflection of prior pi/pa. Also, the δ13C values of newly emerging leaves were more enriched in 13C relative to the δ13C values of stem carbohydrates in the treatments where water regimes were reversed. Newly emerging leaves after defoliation had higher δ13C values despite the lower instantaneous water‐use efficiency and similar values of pi/pa to older photosynthetic leaves. Large differences between observed and model‐predicted pi/pa values also occurred in older, more mature leaves, and this may be because large proportions of their total mass were derived from carbon import.

Variation in δD values of a single, species-specific molecular biomarker: a study of miliacin throughout a field of broomcorn millet (Panicum miliaceum L.).

Compound-specific δD analyses of land plant-derived biomarkers preserved in lake sediments are gaining increasing interest in paleoclimatic studies because of their potential to record essential information on the climatic conditions that prevailed at the time of their synthesis. The accuracy of inferences about climate from these analyses could be better constrained with more study of the variability in the δD values of possible inputs at catchment scales. We measured the δD values of miliacin (olean-18-en-3β-ol methyl ether) extracted from the seeds of millet plants collected in 21 stands spatially distributed in a field with visually heterogeneous soil organic matter contents. The use of a single molecular biomarker extracted from a single plant species eliminates the possibility of variability caused by differences in plant type. The δD values differed between plants by as much as 50‰ and the average δD values per stand differed from one another by a maximum of 30‰. Thus, the δD values of a single, species-specific biomarker can vary markedly among plants even within a similar climate. Differences in δD values within stands could be as high as between stands, suggesting that the δD values are not related to macroscale heterogeneities in soil organic matter content. In addition, δD values were unrelated to factors indicative of differences in environment such as plant height, seed weight or miliacin concentration. The average miliacin δD value was representative of the area sampled, however, since it was normally distributed (p < 0.05).

Carbon isotopic ratios of atmospheric CO2 affect the δ13C values of heterotrophic growth in Nicotiana tabacum

Abstract Heterotrophic Nicotiana tabacum (L. CV. Wisconsin 38) plants are enriched in 13 C relative to the carbon sources in their growth medium. We examined whether carboxylation via phosphoenolpyruvate carboxylase contributes to the enrichment. Achlorophyllous plants were produced using an inhibitor of carotenoid synthesis and were grown on sucrose with known δ 13 C values. Groups of plants were exposed to air with different δ 13 C values as well as to CO 2 -free air. The δ 13 C values of heterotrophic plants were greater than the sucrose source in all treatments and this enrichment increased as 13 CO 2 / 12 CO 2 ratios increased in the source air. Rubisco activity was ruled out as a cause for the enrichment observed as 13 CO 2 / 12 CO 2 ratios increased because the δ 13 C values of heterotrophic plants were similar when exposed to high 13 CO 2 while grown in the light or dark. Neither was enrichment due to the adsorption of 13 CO 2 in the high 13 CO 2 treatment because dead plants did not exhibit this effect when subjected to the same atmospheric treatments. Carboxylation by PEP carboxylase is a likely mechanism causing the 13 C-enriched values of living white tissues relative to their organic carbon sources. These results experimentally support suggestions that the anaplerotic activity of PEP is responsible for the 13 C-enrichment commonly observed where heterotrophic inputs to growth are large such as in very young leaves.

Influence of Phenotypic Plasticity in Photosynthetic Functions on the Spatial Distributions of Tropical Trees

The services that tropical rain forests provide and the roles they play in harboring much of the worlds terrestrial biodiversity are widely appreciated. There is worldwide concern about the consequences of human activities on these services. Knowledge of the role of plant function in shaping biogeographical distributions of trees contributes to efforts to preserve, restore, and manage these forests. Recognition is growing that plasticity in the expression of functions by a given genotype and not just genotypic specialization may play an adaptive role in shaping spatial distributions of species. Their structure and life history characteristics make trees among the most likely candidates for higher levels of plasticity in functional phenotype of all plant forms. Moreover, many tropical areas meet theoretical expectations for the environmental heterogeneity that may favor selection for phenotypic plasticity. Trees are, however, generally avoided in tests of the adaptiveness of levels of phenotypic plasticity for particular ranges of environments. I emphasize a need to quantify specific photosynthetic and water use functions that are fundamental bases for fitness and outline some avenues to achieve these measurements on tree species. Tests of the adaptiveness of levels of plasticity in phenotype remain more elusive. I present the case, however, that tests of correlation between levels of plasticity in functions of species and quantified parameters of their geographies can generate knowledge of what combinations of species can co-occur in a given landscape.

Gas exchange of a desert shrub (Zygophyllum dumosum Boiss.) under different soil moisture regimes during summer drought

The effects of soil water potential on photosynthesis and transpiration of whole Zygophyllum dumosum Boiss. shrubs were examined with a field IRGA system during a rainless summer. Daily photosynthesis and transpiration activities were not notably different on a unit phyllode area basis among shrubs at naturally differing soil water potentials. Irrigation of shrubs caused phyllodes to increase significantly in water content and new leaflets to appear. Leaflets had three times as many stomata per unit area (23000 stomata cm-2) as phyllodes (7100 stomata cm-2) but photosynthesis and transpiration rates were not measurably different between irrigated and non-irrigated shrubs on a unit area basis. This finding suggests that sufficient soil moisture will lead to increased carbon uptake of the entire shrub simply because the total area of photosynthesizing tissue increases. Gas exchange rates appear to be controlled solely by atmospheric conditions under the stresses of summer.

Foundations of Biogeography: Classic Papers with Commentaries

on the larger community, and has, as a result, started to take on the appearance of a socially constructed phenomenon. However, these and other overarching issues are barely touched upon. An appreciation of how the practical applications of geographical knowledge are enhanced by a critical understanding of their wider environmental and social contexts also is lacking. Consider, for example, the dilemma of whether more attention should be directed toward solving the problems that accrue from the combustion of fossil fuels or those relating to poverty and development, and the inherently interdisciplinary nature of these complex global issues. I say ‘‘purports to explore the relationship between philosophy, science, and physical geography’’ because, as a geomorphologist, I was impressed with the way Inkpen integrates theoretical knowledge with empirical approaches to a range of Earth-surface phenomena, and demonstrates how this information could be applied to problem-solving. Inkpen also has succeeded in showing how the development of appropriate field and laboratory analytical techniques is complemented by enhancements to theoretical understanding of geomorphological systems. However, students and teachers of biogeography, climatology, and hydrology will be disappointed by what amounts to a theory impasse. Inkpen excuses this omission (in the preface) by stating that his book is not meant to be a complete review of the existing literature, that it has not been possible to cover physical geography in anything approaching satisfactory depth, and that by default he has tended to fall back on examples and ideas drawn from his own particular subdiscipline (geomorphology). Secondand third-year undergraduates (the audience to which the book is directed), who are practitioners in one of the neglected geospheres and who, like geomorphologists, also utilize observation, experience, and experimental research to interpret Earth’s natural phenomena and features, will draw little comfort from this, or from the lack of examples they can relate to in an eminently approachable book that provides important insights into how the current development of geomorphology relates to the philosophy of science. This is unfortunate because the key questions Inkpen poses (what is the reality we study? what are the things we study? and why do we study them? what counts as valid explanation? and how do we engage with reality to derive information from it?) clearly are vital for physical geography as a whole. Not least because the answers that emerge may help to connect natural systems with human decisions, actions, and experiences, and thereby facilitate better communication between physical and human geographers. In summary, this book, although mistitled, nevertheless provides an insightful perspective on how understandings that have emerged from geomorphology (which, it should be noted, also is practiced outside geography) are influenced by what its adherents believe and the way they comprehend reality. It is in this context that Science, Philosophy and Physical Geography should find an appreciative audience, albeit one that is perhaps not as diverse as was initially envisioned.