Secondary forests cover large areas of the tropics and play an important role in the global carbon cycle. During secondary forest succession, simultaneous changes occur among stand structural attributes, soil properties, and species composition. Most studies classify tree species into categories based on their regeneration requirements. We use a high-resolution secondary forest chronosequence to assign trees to a continuous gradient in species successional status assigned according to their distribution across the chronosequence. Species successional status, not stand age or differences in stand structure or soil properties, was found to be the best predictor of leaf trait variation. Foliar δ13C had a significant positive relationship with species successional status, indicating changes in foliar physiology related to growth and competitive strategy, but was not correlated with stand age, whereas soil δ13C dynamics were largely constrained by plant species composition. Foliar δ15N had a significant negative correlation with both stand age and species successional status, – most likely resulting from a large initial biomass-burning enrichment in soil 15N and 13C and not closure of the nitrogen cycle. Foliar %C was neither correlated with stand age nor species successional status but was found to display significant phylogenetic signal. Results from this study are relevant to understanding the dynamics of tree species growth and competition during forest succession and highlight possibilities of, and potentially confounding signals affecting, the utility of leaf traits to understand community and species dynamics during secondary forest succession.

• Philodendron bipinnatifidum inflorescences heat up to 42 °C and thermoregulate. We investigated whether they generate heat via the cytochrome oxidase pathway uncoupled by uncoupling proteins (pUCPs), or the alternative oxidase (AOX). • Contribution of AOX and pUCPs to heating in fertile (FM) and sterile (SM) male florets was determined using a combination of oxygen isotope discrimination, protein and substrate analyses. • Both FM and SM florets thermoregulated independently for up to 30 h ex planta. In both floret types, AOX contributed > 90% of respiratory flux during peak heating. The AOX protein increased fivefold with the onset of thermogenesis in both floret types, whereas pUCP remained low throughout development. These data indicate that AOX is primarily responsible for heating, despite FM and SM florets potentially using different substrates, carbohydrates or lipids, respectively. Measurements of discrimination between O₂ isotopes in strongly respiring SM florets were affected by diffusion; however, this diffusional limitation was largely overcome using elevated O₂. • The first in vivo respiratory flux measurements in an arum show AOX contributes the bulk of heating in P. bipinnatifidum. Fine-scale regulation of AOX activity is post-translational. We also demonstrate that elevated O₂ can aid measurement of respiratory pathway fluxes in dense tissues.

Salicylic acid (SA) is a signal in systemic acquired resistance and an inducer of the alternative oxidase protein in tobacco (Nicotiana tabacum cv Xanthi nc) cell suspensions and during thermogenesis in aroid spadices. The effects of SA on the levels of alternative oxidase protein and the pathogenesis-related 1a mRNA (a marker for systemic acquired resistance), and on the partitioning of electrons between the Cyt and alternative pathways were investigated in tobacco. Leaves were treated with 1.0 mM SA and mitochondria isolated at times between 1 h and 3 d after treatment. Alternative oxidase protein increased 2.5-fold within 5 h, reached a maximum (9-fold) after 12 h, and remained at twice the level of control plants after 3 d. Measurements of isotope fractionation of 18O by intact leaf tissue gave a value of 23% at all times, identical to that of control plants, indicating a constant 27 to 30% of electron-flow partitioning to the alternative oxidase independent of treatment with SA. Transgenic NahG tobacco plants that express bacterial salicylate hydroxylase and possess very low levels of SA gave a fractionation of 23% and showed control levels of alternative oxidase protein, suggesting that steady-state alternative oxidase accumulates in an SA-independent manner. Infection of plants with tobacco mosaic virus resulted in an increase in alternative oxidase protein in both infected and systemic leaves, but no increase was observed in comparably infected NahG plants. Total respiration rate and partitioning of electrons to the alternative pathway in virus-infected plants was comparable to that in uninfected controls.

The goal of this investigation was to determine the effects of allelochemicals on plant respiration that thereby may be responsible for their role in growth inhibition. We have tested the effects of juglone, quercetin, cinnamic acid, andα-pinene on respiration rates, and electron partitioning through the cytochrome and alternative respiratory pathways, by measuring on-line oxygen consumption and oxygen isotope fractionation in soybean cotyledon tissue. Cinnamic acid andα-pinene decreased the oxygen consumption rate and increased the relative partitioning of electron transport to the alternative pathway. Possible biochemical mechanisms of these effects are discussed.

Two direct but independent approaches were developed to identify the average delta(18)O value of the water fraction in the chloroplasts of transpiring leaves. In the first approach, we used the delta(18)O value of CO2 in isotopic equilibrium with leaf water to reconstruct the delta(18)O value of water in the chloroplasts. This method was based on the idea that the enzyme carbonic anhydrase facilitates isotopic equilibrium between CO2 and H2O predominantly in the chloroplasts, at a rate that is several orders of magnitude faster than the non-catalysed exchange in other leaf water fractions. In the second approach, we measured the delta(18)O value of O-2 from photosynthetic water oxidation in the chloroplasts of intact leaves. Since O-2 is produced from chloroplast water irreversibly and without discrimination, the delta(18)O value of the O-2 Should be identical to that of chloroplast water. In intact, transpiring leaves of sunflower (Helianthus annuus cv. giant mammoth) under the experimental conditions used, the average delta(18)O value of chloroplasts water was displaced by 3-10 parts per thousand (depending on relative humidity and atmospheric composition) below the value predicted by the conventional Craig and Gordon model. Furthermore, this delta(18)O value was always lower than the delta(18)O value that was measured for bulk leaf water. Our results have implications for a variety of environmental studies since it is the delta(18)O value of water in the chloroplasts that is the relevant quantity in considering terrestrial plants influence on the delta(18)O values of atmospheric CO2 and O-2, as well as in influencing the delta(18)O of plant organic matter.

In Ceropegia dichotoma, Crassula argentea, Esheveria colorata, Kalanchoë beharensis, Opuntia ficus-indica, Sansveria stuckyi and Opuntia inermis the carbon-isotope ratio (δ13C) of tissues close to the epidermis is 2–4.3‰ more negative than those in the centre of the leaf or cladode. The greatest change in δ13C value occurs between the epidermal layer and the layer of mesophyll tissue immediately underneath. Analysis of major metabolic and structural components in successive layers of Crassula argentea grown under controlled environmental conditions conducive to Crassulacean acid metabolism confirmed that much of the variation in δ13C values of bulk carbon is caused by differences in chemical composition. Thus the steep gradient in δ13C value at the epidermis reflects, in part, the contribution of more-negative δ13C values of lipids in these tissues. Moreover, during nocturnal CO2 fixation the amount of malic acid synthesised decreases with depth and the δ13C value of the methanol-soluble fraction is less negative with distance away from the upper epidermis. These results are consistent with diffusion limitation to CO2 uptake in these thick leaf tissues, which also contributes to the observed gradients in δ13C value.

Discrimination against 180during darkrespiration intissues of Kalanchoe daigremontiana, Medicago sativa, andGlycine maxwas measured using anon-line system thatenabled direct measure- mentsoftheoxygen fractionation ofsamples inagas-phase leaf disk electrode unit. Discrimination factors forcytochrome pathway respiration were18.6to19.8%o forall tissues. However, discrim- ination incyanide-resistant respiration wassignificantly higher in greentissues (30.4-31.2%o) compared withnongreen tissues (25.3-25.9%o). Using these discrimination factors, thepartitioning ofelectron transport tothese pathways wascalculated frommeas- urements ofdiscrimination intheabsence ofinhibitors. Changes influx through thealternative pathway weremeasured during the light anddarkphases ofCrassulacean acid metabolism inleaf disks ofK.daigremontiana. Theflux ofelectrons through thealternative pathway washigher during deacidification thanduring theother phases ofCrassulacean acidmetabolism. Theincrease inalterna- tivepathway electron fluxaccounted foralloftheincreased respiration inthelight phase. Despite this increase, simultaneous measurements ofmalate concentration andrespiratory flux con- firm that only asmall proportion ofthetotal malate decarboxyla- tion occurs inthemitochondria.