SummaryLeaf dark respiration (R dark ) is an important yet poorly quantified component of the global carbon cycle. Given this, we analyzed a new global database of R dark and associated leaf traits. Data for 899 species were compiled from 100 sites (from the Arctic to the tropics). Several woody and nonwoody plant functional types (PFTs) were represented. Mixed-effects models were used to disentangle sources of variation in R dark .Area-based R dark at the prevailing average daily growth temperature (T) of each site increased only twofold from the Arctic to the tropics, despite a 20°C increase in growing T (8-28°C). By contrast, R dark at a standard T (25°C, R dark 25 ) was threefold higher in the Arctic than in the tropics, and twofold higher at arid than at mesic sites. Species and PFTs at cold sites exhibited higher R dark 25 at a given photosynthetic capacity (V cmax 25 ) or leaf nitrogen concentration ([N]) than species at warmer sites. R dark 25 values at any given V cmax 25 or [N] were higher in herbs than in woody plants.The results highlight variation in R dark among species and across global gradients in T and aridity. In addition to their ecological significance, the results provide a framework for improving representation of R dark in terrestrial biosphere models (TBMs) and associated land-surface components of Earth system models (ESMs).

Summary We examined whether variations in photosynthetic capacity are linked to variations in the environment and/or associated leaf traits for tropical moist forests (TMFs) in the Andes/western Amazon regions of Peru. We compared photosynthetic capacity (maximal rate of carboxylation of Rubisco (Vcmax), and the maximum rate of electron transport (Jmax)), leaf mass, nitrogen (N) and phosphorus (P) per unit leaf area (Ma, Na and Pa, respectively), and chlorophyll from 210 species at 18 field sites along a 3300‐m elevation gradient. Western blots were used to quantify the abundance of the CO2‐fixing enzyme Rubisco. Area‐ and N‐based rates of photosynthetic capacity at 25°C were higher in upland than lowland TMFs, underpinned by greater investment of N in photosynthesis in high‐elevation trees. Soil [P] and leaf Pa were key explanatory factors for models of area‐based Vcmax and Jmax but did not account for variations in photosynthetic N‐use efficiency. At any given Na and Pa, the fraction of N allocated to photosynthesis was higher in upland than lowland species. For a small subset of lowland TMF trees examined, a substantial fraction of Rubisco was inactive. These results highlight the importance of soil‐ and leaf‐P in defining the photosynthetic capacity of TMFs, with variations in N allocation and Rubisco activation state further influencing photosynthetic rates and N‐use efficiency of these critically important forests.

SummaryLeaf aging is a fundamental driver of changes in leaf traits, thereby regulating ecosystem processes and remotely sensed canopy dynamics.We explore leaf reflectance as a tool to monitor leaf age and develop a spectra-based partial least squares regression (PLSR) model to predict age using data from a phenological study of 1099 leaves from 12 lowland Amazonian canopy trees in southern Peru.Results demonstrated monotonic decreases in leaf water (LWC) and phosphorus (P mass ) contents and an increase in leaf mass per unit area (LMA) with age across trees; leaf nitrogen (N mass ) and carbon (C mass ) contents showed monotonic but tree-specific age responses. We observed large age-related variation in leaf spectra across trees. A spectra-based model was more accurate in predicting leaf age (R 2 = 0.86; percent root mean square error (%RMSE) = 33) compared with trait-based models using single (R 2 = 0.07-0.73; %RMSE = 7-38) and multiple (R 2 = 0.76; %RMSE = 28) predictors. Spectra-and trait-based models established a physiochemical basis for the spectral age model. Vegetation indices (VIs) including the normalized difference vegetation index (NDVI), enhanced vegetation index 2 (EVI2), normalized difference water index (NDWI) and photosynthetic reflectance index (PRI) were all age-dependent. This study highlights the importance of leaf age as a mediator of leaf traits, provides evidence of age-related leaf reflectance changes that have important impacts on VIs used to monitor canopy dynamics and productivity and proposes a new approach to predicting and monitoring leaf age with important implications for remote sensing.

Maca, Lepidium meyenii Walpers (Brassicaceae), is an annual herbaceous plant native to the high plateaus of the Peruvian central Andes. Its underground storage hypocotyls have been a traditional medicinal agent and dietary staple since pre-Columbian times. Reported properties include energizing and fertility-enhancing effects. Published reports have focused on the benzylalkamides (macamides) present in dry hypocotyls as one of the main bioactive components. Macamides are secondary amides formed by benzylamine and a fatty acid moiety, with varying hydrocarbon chain lengths and degree of unsaturation. Although it has been assumed that they are usually present in fresh undamaged tissues, analyses show them to be essentially absent from them. However, hypocotyls dried by traditional Andean postharvest practices or industrial oven drying contain up to 800μgg(-1) dry wt (2.3μmolg(-1) dry wt) of macamides. In this study, the generation of macamides and their putative precursors were studied during nine-week traditional drying trials at 4200m altitude and in ovens under laboratory conditions. Freeze-thaw cycles in the open field during drying result in tissue maceration and release of free fatty acids from storage and membrane lipids up to levels of 1200μgg(-1) dry wt (4.3μmolg(-1) dry wt). Endogenous metabolism of the isothiocyanates generated from glucosinolate hydrolysis during drying results in maximal benzylamine values of 4300μgg(-1) dry wt (40.2μmolg(-1) dry wt). Pearson correlation coefficients of the accumulation profiles of benzylamine and free fatty acid to that of macamides showed good values of 0.898 and 0.934, respectively, suggesting that both provide sufficient substrate for amide synthesis during the drying process.

Soybean membranes possess high‐affinity binding sites for fungal β‐glucans that elicit phytoalexin synthesis. The ability of 1,3‐1,6‐β‐glucans, released by acid hydrolysis from mycelial walls of Phytophthora megasperma f.sp. glycinea, to compete for the putative phytoalexin elicitor receptors increases with their average degree of polymerization (DP). The results suggest a function where the probability for glucan fragments of containing a structural determinant that is optimal for binding approaches 1 as the DP tends to infinity. Ligand displacement data obtained against a 125I‐labeled glucan elicitor (average DP= 18) provided a theoretical minimum IC50 (50% inhibitory concentration) for 1,3‐1,6‐β‐glucans of 3 nM. The IC50 value obtained for a synthetic hepta‐β‐glucoside having a known elicitor‐active structure was 8 nM, remarkably close to the predicted value. Displacement of the 125I‐glucan of large DP was uniform and complete showing that the heptaglucoside had access, with similar affinity, to all sites available to the radioligand. Further analysis using a 125I‐labeled aminophenethylamine derivative of the heptaglucoside suggested that the putative glucan‐elicitor receptors bind a basic structural determinant present in all elicitor‐active glucans from the soybean pathogen P. megasperma.

We have recently reported the existence of binding sites in soybean membranes for a beta-glucan fraction derived from the fungal pathogen Phytophthora megasperma f. sp. glycinea, which may play a role in the elicitor-mediated phytoalexin response of this plant [Schmidt, W. E. & Ebel, J. (1987) Proc. Natl Acad. Sci. USA 84, 4117-4121]. The specificity of beta-glucan binding to soybean membranes has now been investigated using a variety of competing polyglucans and oligoglucans of fungal origin. P. megasperma beta-glucan binding showed high apparent affinity for branched glucans with degrees of polymerization greater than 12. Binding affinity showed good correlation with elicitor activity as measured in a soybean cotyledon bioassay. Modification of the glucans at the reducing end with phenylalkylamine reagents had no effect on binding affinity. This characteristic was used to synthesize an oligoglucosyl tyramine derivative suitable for radioiodination. The 125I-glucan (15-30 Ci/mmol) provided higher sensitivity and lower detection limits for the binding assays while behaving in a manner identical to the [3H]glucan used previously. More accurate determinations of the Kd value for glucan binding indicated a higher affinity than previously shown (37 nM versus 200 nM). The 125I-glucan was used to provide the first reported evidence of specific binding of a fungal beta-glucan fraction in vivo to soybean protoplasts. The binding affinity to protoplasts proved identical to that found in microsomal fractions.

A putative receptor protein for a hepta-B-glucoside phytoalexin elicitor was identified by photoaffinity labeling of detergent-solubilized proteins from soybean root membranes. Incubation of partially purified P-glucan-binding proteins with a photolabile '251-labeled 2-(4-azidophenyl)ethylamino conjugate of the heptaglucoside elicitor, followed by irradiation with ultraviolet light (366 nm) resulted in specific labeling of a 70-kDa band in SDS/PAGE. Half-maximal inhibition of the 1251-labeling of the protein band by underivatized hepta-fi-glucoside was achieved by 15 nM heptaglucoside. Analysis of the affinity of radiolabel incorporation into the protein by ligand-saturation experiments, gave an apparent Kd value of 3 nM, in full agreement with the results from radioligandbinding studies. Good correlation was also observed between the amount of radiolabel incorporated into the protein and the binding activity of the fractions obtained at different stages in the purification of heptaglucoside-binding activity. Photoaffinity labeling of proteins purified by glucan-affinity chromatography showed the 70-kDa band as the main component along with weak '251-labeling of a 100-kDa band. The 70-kDa band was also the major protein visualized by silver staining after SDS/ PAGE of this fraction, suggesting that it is the predominant form of the heptaglucoside-binding proteins in detergent-solubilized soybean membranes.The perception mechanisms that signal the presence of potential pathogens and lead to the activation of defense responses in plant cells are still poorly understood. A representative case is that of the production of pterocarpan phytoalexins by soybean cells when infected by the pathogenic fungus Phytuphthora megasperma f.sp. glycineu. The phytoalexin response in soybean has been the object of detailed study (Ebel, 1986;Ebel and Grisebach, 1988). The accumulated information on the minimal structural requirements for phytoalexin-elicitor activity of P . megasperma mycelial-wall glucans (Sharp et al., 1984a, b; A commonly proposed model for signal recognition in the phytoalexin response involves a receptor located on the surface of plant cells that specifically binds phytoalexin elicitor(s) (Ebel, 1986;Dixon and Lamb, 1990). The best charac- 3) and (1,6)-linked-P-glucoside with high elicitor activity from mycelial-wall hydrolysates provided initial insight into the minimum structural requirements for elicitor activity in soybean. Additional information was obtained with the chemical synthesis of the hepta-fi-glucoside and comparison of its biological activity with that of a number of oligoglucosides of related structure (Sharp et al., 1984b;Ossowski et al., 1984;. Evidence for the existence of high-affinity-binding sites for P. megasperma (1,3) and (1,6)-linked P-glucan fragments in soybean membranes was provided initially by Schmidt and Ebel (1987) Additional work by our group defined optimal conditions for the solubilization, using detergents, of the glucan-binding proteins and for a partial purification of ...

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