Reproductive organs regulate leaf nitrogen metabolism mediated by cytokinin signal

The metabolism of vegetative organs in plants changes during the development of the reproductive organs. The regulation of this metabolism is important in the control of crop productivity. However, the complexity of the regulatory systems makes it difficult to elucidate their mechanisms. To examine these mechanisms, we constructed model experiments using Arabidopsis to analyze metabolic and gene expression changes during leaf-stage progression and after removal of the reproductive organs. Leaf gene expression levels and content of major amino acids, both of which decreased during leaf-stage progression, increased after removal of the reproductive organs. In particular, the levels of expression of cytokinin biosynthesis genes and cytokinin-responsive genes and the cytokinin content increased after removal of the reproductive organs. Analysis of plants with knockout of a cytokinin-biosynthetic gene (AtIPT3) and a cytokinin receptor gene (AHK3) indicated that glutamate dehydrogenase genes (GDH3) were regulated by cytokinin signaling. These data suggest that cytokinins regulate communication between reproductive and vegetative organs, and that GDH3 is one target of the cytokinin-mediated regulation of nitrogen metabolism. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Ecological ramifications of the direct foliar uptake of nitrogen

The foliar incorporation of various reactive forms of nitrogen (N) has been identified and studied for nearly 30 years. However, the ecosystem-level ramifications of this uptake pathway have only recently been considered by the scientific community. In this review, I present our current understanding of the foliar uptake process and then discuss why this pathway of N addition to ecosystems should be considered separately from the bulk deposition of N to the soil surface. Direct foliar uptake is a direct addition of N to plant metabolism and could potentially more readily influence plant growth compared to soil-deposited N. Current ecosystem process models do not partition reactive N between foliar and soil entry pathways and the influence of N deposition on ecosystem C sequestration is likely inadequately represented in most models. I also outline several research priorities for the future understanding of the ecological consequences of foliar uptake of reactive N.     

Conservation of nitrogen increases with precipitation across a major grassland gradient in the Central Great Plains of North America

Regional analyses and biogeochemical models predict that ecosystem N pools and N cycling rates must increase from the semi-arid shortgrass steppe to the sub-humid tallgrass prairie of the Central Great Plains, yet few field data exist to evaluate these predictions. In this paper, we measured rates of net N mineralization, N in above- and belowground primary production, total soil organic matter N pools, soil inorganic N pools and capture in resin bags, decomposition rates, foliar ¹⁵N, and N use efficiency (NUE) across a precipitation gradient. We found that net N mineralization did not increase across the gradient, despite more N generally being found in plant production, suggesting higher N uptake, in the wetter areas. NUE of plants increased with precipitation, and δ¹⁵N foliar values and resin-captured N in soils decreased, all of which are consistent with the hypothesis that N cycling is tighter at the wet end of the gradient. Litter decomposition appeared to play a role in maintaining this regional N cycling trend: litter decomposed more slowly and released less N at the wet end of the gradient. These results suggest that immobilization of N within the plant–soil system increases from semi-arid shortgrass steppe to sub-humid tallgrass prairie. Despite the fact that N pools increase along a bio-climatic gradient from shortgrass steppe to mixed grass and tallgrass prairie, this element becomes relatively more limiting and is therefore more tightly conserved at the wettest end of the gradient. Similar to findings from forested systems, our results suggest that grassland N cycling becomes more open to N loss with increasing aridity.     

长白山地区散射辐射与云的关系

根据中国科学院长白山森林生态系统定位站1982—2001年太阳辐射和云的观测资料对散射辐射及散射比进行了分析,探讨了散射辐射及散射比的日、季节、年际变化特征以及云量、云状、太阳高度角与散射辐射及散射比的关系。结果表明：1)散射辐射日变化特点是,中午最大,早晚较小;季节变化特点是,7月最大,年初和年末较小;年际变化特点是从1982—2001年呈减少趋势;散射比1 d中中午最小,7月达到最大值,年际变化为0.38～0.55;2)散射辐射及散射比均随高云量或低云量增加而呈增加趋势;散射辐射日平均量及散射比与总云量和低云量的日平均值均呈线性正相关关系,且与总云量的关系比与低云量的关系更密切;3)不同云相比较,散射辐射和散射比最大的是高积云（Ac）,卷云（Ci）次之,较小为对流性积云(积雨云Cb、积云Cu),碧空时散射辐射及散射比最小;4)散射辐射随太阳高度角增加而增大,呈二次函数关系;散射比随太阳高度角增加而减小且呈二次函数关系。     

The effects of elevated atmospheric CO<Subscript>2</Subscript> and nitrogen amendments on subsurface CO<Subscript>2</Subscript> production and concentration dynamics in a maturing pine forest

Profiles of subsurface soil CO₂ concentration, soil temperature, and soil moisture, and throughfall were measured continuously during the years 2005 and 2006 in 16 locations at the free air CO₂ enrichment facility situated within a temperate loblolly pine (Pinus taeda L.) stand. Sampling at these locations followed a 4 by 4 replicated experimental design comprised of two atmospheric CO₂ concentration levels (ambient [CO₂]^a, ambient + 200 ppmv, [CO₂]^e) and two soil nitrogen (N) deposition levels (ambient, ambient + fertilization at 11.2 g_N m⁻² year⁻¹). The combination of these measurements permitted indirect estimation of belowground CO₂ production and flux profiles in the mineral soil. Adjacent to the soil CO₂ profiles, direct (chamber-based) measurements of CO₂ fluxes from the soil–litter complex were simultaneously conducted using the automated carbon efflux system. Based on the measured soil CO₂ profiles, neither [CO₂]^e nor N fertilization had a statistically significant effect on seasonal soil CO₂, CO₂ production, and effluxes from the mineral soil over the study period. Soil moisture and temperature had different effects on CO₂ concentration depending on the depth. Variations in CO₂ were mostly explained by soil temperature at deeper soil layers, while water content was an important driver at the surface (within the first 10 cm), where CO₂ pulses were induced by rainfall events. The soil effluxes were equal to the CO₂ production for most of the time, suggesting that the site reached near steady-state conditions. The fluxes estimated from the CO₂ profiles were highly correlated to the direct measurements when the soil was neither very dry nor very wet. This suggests that a better parameterization of the soil CO₂ diffusivity is required for these soil moisture extremes.     

Decades of atmospheric deposition have not resulted in widespread phosphorus limitation or saturation of tree demand for nitrogen in southern New England

It is commonly assumed that nitrogen (N) is the primary mineral resource limiting the productivity of temperate forests. Sustained inputs of N via atmospheric deposition are altering the N status of temperate forests raising the possibility that nutrients such as phosphorus (P) are increasingly limiting productivity. The objective of this study was to determine whether P availability limits tree growth alone or in combination with N. This study was conducted in two forest types common throughout the New England landscape of the northeastern United States; in sugar maple and white ash dominated stands growing on base rich parent material characterized by rapid rates of N cycling and high N availability, and in red oak–beech–hemlock dominated stands growing on base-poor parent material characterized by slow rates of N cycling and low N availability. Starting in 2004, N and P were added to replicate plots in each forest type in factorial combination at a rate of 150 and 50 kg ha⁻¹ year⁻¹, respectively. Diameter growth rates of all trees >10 cm DBH were measured in 2005 and 2006 using dendrometer bands and converted into units of basal area increment (BAI) and wood production. Following 2 years of fertilization, basal area increment in the sugar maple–white ash forests remained strongly N limited. Fertilization with P did not significantly increase BAI alone, although both N and P fertilization tended (P < 0.10) to increase diameter growth in white ash. Wood production in the N-fertilized plots increased by 100 g C m⁻² year⁻¹, roughly doubling production in the non-fertilized plots. In the red oak–beech–hemlock stands, there was no overall effect of N or P fertilization on BAI or wood production because BAI in some species was stimulated by fertilization with N alone (e.g., black cherry, red oak), while in other species BAI was unaffected (e.g., red maple, beech) or negatively affected by fertilization with N or P (e.g., eastern hemlock). Given that BAI in several tree species responded to fertilization with N alone and that only one species responded to P fertilization once N was added, this study suggests that decades of atmospheric N deposition have not (yet) resulted in widespread P limitation or saturation of tree demand for N.     

Soil amino-acid availability across a temperate-forest fertility gradient

Despite increasing recognition that free amino acids can be an important source of N for plant uptake, we have a poor understanding of environmental variation in the availability of amino-acid N in soils outside of arctic, alpine and boreal regions. I investigated patterns of amino-acid availability along a temperate forest fertility gradient ranging from low mineral N availability, oak-dominated forests to high mineral N availability, maple-basswood forests (5 sites). I measured standing pools of free amino acids, soluble peptides, ammonium and nitrate, rates of amino acid production (native proteolysis activity) and rates of consumption of a ¹⁵N-labeled leucine tracer. Standing pools of amino acid N decreased consistently along the fertility gradient from the low fertility black oak/white oak system to the high fertility sugar maple/basswood system, with a 25-fold difference in pool sizes between the poorest and richest sites. Standing pools of soluble peptides varied little among sites, instead, the relationship between free amino acids and peptides changed markedly across the gradient. At low fertility sites free amino acids were positively correlated with soluble peptides, whereas free amino acid pools were universally low at high fertility sites, regardless of peptide pools. Assays for native proteolysis activity indicated that amino acid production did not vary significantly among sites. Recovery of leucine tracer in inorganic (NH₄ ⁺ and NO₃ ⁻) pools and in residual soil organic matter both increased with increasing soil fertility; however, total consumption of the added amino-acid tracer did not vary among sites. Results from this study demonstrate that free amino acids can make an important contribution to potentially plant-available N pools in temperate forest soils, particularly at low fertility sites.     

Effects of indole-3-acetic acid on <Emphasis Type="Italic">Sinorhizobium meliloti</Emphasis> survival and on symbiotic nitrogen fixation and stem dry weight production

We evaluated the effects of the main auxin phytohormone, indole-3-acetic acid (IAA), on the central metabolism of Sinorhizobium meliloti 1021. We either treated S. meliloti 1021 wild-type cells with 0.5 mM IAA, 1021+, or use a derivative, RD64, of the same strain harboring an additional pathway for IAA biosynthesis (converting tryptophan into IAA via indoleacetamide). We assayed the activity of tricarboxylic acid cycle (TCA) key enzymes and found that activity of citrate synthase and α-ketoglutarate dehydrogenase were increased in both 1021+ and RD64 as compared to the wild-type strain. We also showed that the intracellular acetyl-CoA content was enhanced in both RD64 and 1021+ strains when compared to the control strain. The activity of key enzymes, utilizing acetyl-CoA for poly-β-hydroxybutyrate (PHB) biosynthesis, was also induced. The PHB level measured in these cells were significantly higher than that found in control cells. Moreover, 4-week-long survival experiments showed that 80% of 1021 cells died, whereas 50% of RD64 cells were viable. Medicago truncatula plants nodulated by RD64 (Mt-RD64) showed an induction of both acetylene reduction activity and stem dry weight production.