AMAX

Abstract: 1. Net photosynthetic capacity (Amax, defined as light-saturated net photosynthesis under near optimal ambient environmental conditions) of mature leaves often depends on the level of leaf nitrogen (N), but an assortment of relationships between these variables has been observed in studies of diverse plant species. Variation in leaf structure has been identified as an important factor associated with differences between the area- and mass-based expressions of the Amax–N relationship. In this paper we test the hypothesis that leaf structure, quantified using a measure of leaf area displayed per unit dry mass invested [specific leaf area (SLA)], is more than just a conversion factor, but itself can influence Amax–N relationships. We test this using several kinds of comparisons, based on field data for 107 species from sites representing six biomes and on literature data for 162 species from an equally diverse set of biomes. 2. Species and genera with thicker and/or denser leaves (lower SLA) consistently have flatter slopes of the Amax–N (mass-based) relationship than those with higher SLA. These and all other contrasts usually applied as well using area-based expressions, although such relationships were less consistent and weaker overall. A steeper slope indicates greater incremental change in Amax per unit variation in N. 3. Functional groups (e.g. needle-leafed evergreen trees, broad-leafed trees or shrubs, forbs) show the same patterns: groups with lower SLA have lower Amax–N slopes. Functional groups differ in mean leaf traits as well as in Amax–N relationships. Forbs have the highest SLA and mass-based N and Amax, followed by deciduous species (whether needle-leafed or broad-leafed, shrub or tree), with lowest values in evergreen species (again regardless of leaf type or functional group). 4. Interspecific variation in mass-based Amax is highly significantly related to the combination of leaf N and SLA (r2 = 0·86). At any value of leaf N, Amax increases with increasing SLA and at any value of SLA, Amax increases with increasing leaf N. Because this relationship, between Amax and the combination of N and SLA, is similar in two independent data sets, and as well, across broad taxonomic and geographic gradients, we hypothesize that it is universal in nature. Therefore, for broad interspecific contrasts among dicotyledons in any biome, we can reasonably well predict Amax based on the combination of SLA and leaf N. These findings have important implications for convergent evolution of leaf adaptation and great potential utility in models of global vegetation functioning.

Abstract: The effect of soil thawing and soil temperature on postwinter recovery of photosynthetic capacity was studied, during late spring and early summer, in Norway spruce stands in northern Sweden. Soil temperature was manipulated by means of buried heating cables. The warming treatment was applied to stands with low (natural) and high (fertilized) availability of nutrients. Soil thawing, expressed as water availability, was followed by means of sapflow in stems, and shoot water potentials. The recovery of photosynthetic capacity was assessed by measuring the rate of light-saturated photosynthesis (Amax), and maximum photochemical efficiency of photosystem II in detached shoots, and chlorophyll a fluorescence. Accumulation of starch reserves in the needles was followed as an independent indicator of photosynthetic performance in situ. Snowmelt and soil thawing occurred more than one month earlier in heated than in unheated plots. This was expressed both as sapflow and as differences in shoot water potential between treatments. During May, the rates of Amax were significantly higher on heated than on control plots. The effect of soil warming on Amax was, however, not reflected in chlorophyll fluorescence or needle starch content. The time course of the recovery of photosynthetic capacity was mainly controlled by mean air temperature and by the frequency of severe night frosts, and to a lesser extent by earlier soil thawing and higher soil temperatures.

Abstract: We calculate the physical structure of protoplanetary disks by evaluating the gas density and temperature selfconsistently and solving separately for the dust temperature. The effect of grain growth is taken into account by assuming a power-law size distribution and varying the maximum radius of grains amax. In our fiducial model with amax ¼ 10 � m, the gas is warmer than the dust in the surface layer of the disk, while the gas and dust have the same temperature in deeper layers. In the modelswith largeramax, the gastemperature in the surface layer islowerthanin the fiducial model because of reduced photoelectric heating rates from small grains, while the deeper penetration of stellar radiation warms the gas at intermediate height. A detailed chemical reaction network is solved at outer radii (r � 50 AU). Vertical distributions of some molecular species at different radii are similar when plotted as a function of hydrogen column density H from the disk surface. Consequently, molecular column densities do not much depend on disk radius. In the models with larger amax, the lower temperature in the surface layer makes the geometrical thickness of the disk smaller, and the gaseous molecules are confined to smaller heights. However, if we plot the vertical distributions of molecules as a function of H, they do not significantly depend on amax .T he dependence of the molecular column densities on amax is not significant either. Notable exceptions are HCO þ ,H þ , and H2D þ , which have smaller column densities in the models with larger amax. Subject headingg ISM: molecules — planetary systems: protoplanetary disks — stars: pre‐main-sequence

Abstract: In white pine (Pinus strobus) seedlings grown in five forest soils from New York State, net photosynthetic capacity (Amax) plant-1 was correlated with total foliar N plant-1 (r2=0.57), but was more highly correlated with total foliar P plant-1 (r2=0.82). There was no relationship (r2<0.01) between Amax [g leaf]-1 and foliar N [g leaf]-1 for the pooled data set, but there was a significant (P<0.001), but weak (r2=0.20) positive relationship between Amax [g leaf]-1 and foliar P [g leaf]-1 across all soils. However, within two of the five soils leaf N concentration was a significant (P<0.05) determinant of photosynthetic capacity. Due to differences in soil nutrient availabilities a large range in foliar P:N ratio (0.02–0.15) was observed, and the proportion of leaf P:N appeared to control Amax [g leaf N]-1. Whole plant nitrogen (NUE) and phosphorus (PUE) use efficiencies were well correlated with whole plant P:N ratio. In addition, NUE was well correlated with Amax [g leaf N]-1 and PUE was well correlated with Amax [g leaf P]-1. However, NUE was not well correlated with PUE, and Amax [g leaf N]-1 was not well correlated with Amax [g leaf P]-1. These results indicated that P and/or N limitations were important components of photosynthetic nutrient relations in white pine grown in these five soils and suggest that both P and N and their proportions should be considered in analyses of photosynthesis-nutrient relations.