Cytokinins: A key player in determining differences in patterns of canopy senescence in Stay-Green and Fast Dry-Down sunflower (Helianthus annuus L.) hybrids

Leaf senescence during grain filling can reduce crop yield. We studied, under field conditions and during grain-filling, the association between leaf cytokinin levels and the onset of leaf senescence in sunflower hybrids of contrasting canopy senescence patterns (Paraiso75, stay-green [SG] and Paraiso65, fast dry down [FDD]). At crop level, dynamics of live root length density (LRLD) and green leaf area index (GLAI) were followed, while at leaf level dynamics of total chlorophyll content, trans-Zeatin content, net photosynthesis and PSII quantum yield, were followed in leaf positions 17, 20, 22 and 24. Responses of these leaf variables to exogenous cytokinin applications to leaves at position 17 were also followed. SG exhibited greater (p < 0.05) LRLD and GLAI values at anthesis. In both hybrids, LRLD began to fall before GLAI. All variables decreased earlier (p < 0.05) in FDD. Initial leaf levels of trans-Zeatin were three times higher (p < 0.05) in SG. Exogenous cytokinin applications maintained leaf-level variables. These are the first results showing associations between LRLD dynamics with the dynamics of leaf cytokinin levels and changes in indicators of leaf functionality. Also, this is the first study in which estimates are made of cytokinin thresholds below which leaf senescence begins in two hybrids of contrasting canopy senescence patterns. These advances in the understanding, at both crop and leaf levels, of the controls and consequences of SG during grain filling, a trait known to improve crop water uptake under drought and increase biomass accumulation during grain filling, provide support for breeding efforts aimed at profiting from this trait to increase crop yields.     

财政分权、社会保障支出与城乡居民收入差距的实证分析

基于1998~2012年各省的面板数据，利用动态广义矩（GMM）和面板门槛模型实证分析财政分权、社会保障支出对城乡收入差距的影响，结果显示：由于中国城乡二元经济的特殊性，财政分权、人均社会保障支出与城乡居民收入差距均成正相关性。但社会保障支出对城乡收入差距的影响存在基于财政分权的双门槛效应，当财政分权水平较低时，社会保障支出的增加加剧了城乡收入差距；当财政分权高于一定水平时，社会保障支出有助于缩小城乡收入差距。     

贫困线视角下农村居民收入对膳食健康的影响研究——基于CHNS数据的微观实证

我国农村居民已基本解决温饱问题,但其膳食健康状况仍缺乏应有的关注,因此分析收入增长对农村居民膳食健康的影响有着重要的现实意义。基于家庭生产理论,利用中国营养与健康调查(CHNS)数据和健康膳食指数(HEI),运用最小二乘虚拟变量回归、门槛回归模型和工具变量回归方法,评估我国农村居民的膳食健康状况,分析收入对农村居民膳食健康的影响,对比分析贫困居民与非贫困居民之间的影响差异,从膳食健康视角评估当前贫困线标准的合理性。结果表明:农村贫困居民与非贫困居民的膳食健康状况存在显著差异,非贫困居民的膳食健康状况明显优于贫困居民;收入对农村居民的膳食健康具有显著的促进作用,贫困居民膳食健康的收入效应明显高于非贫困居民;收入与农村居民的膳食健康存在内生性,不考虑内生性会低估农村居民膳食健康的收入效应;从膳食健康的角度来看,我国的贫困线标准应提升至3 159元,高于当前的贫困线标准(2011年为2 300元)。因此,对于农村居民,收入的持续增长仍是改善农村贫困人口膳食健康的重要途径。如果计划对贫困人口的营养状况进行政策干预,现有的贫困线有必要进行上调。     

基于图像阈值的年轮识别方法

本文主要讲述了年轮识别在林业信息化中的作用和阈值法的依据与闻值法的原理,以及用阈值法处理年轮图像和采用其他方法处理的结果的不同,从而得出用阈值法进行处理的优点.     

黄河源流域单元概念及其提取方法研究

针对黄河源高寒草地生态系统保护的流域属性与差异性，本研究提出了一种准确便捷的流域单元识别方法。以国产高分辨率遥感影像所提取水系矢量数据作为标准参照系，基于数字高程数据（Digital elevation model，DEM）汇流累积计算量阈值设置为30 000时所获取的水系信息与黄河源实际水系状况较为相近。通过合并集水区获取黄河源流域单元878个，其中干流单元1个，一级支流单元219个，二级支流单元420个，三级支流单元194个，四级支流单元21个，断流河流单元23个。受地形地貌影响黄河源流域单元中河流长度与流域面积差距较大。本研究完善了黄河源水系及流域单元特征信息，为黄河源生态治理工作提供参考。     

基于图像阈值的年轮识别方法

本文主要讲述了年轮识别在林业信息化中的作用和阈值法的依据与阈值法的原理，以及用阈值法处理年轮图像和采用其他方法处理的结果的不同，从而得出用阈值法进行处理的优点。     

水稻抽穗期叶色诊断指标与叶面积指数及结实期光强的关系

叶色是氮素营养诊断最常用的指标，获得准确的叶色诊断指标是水稻精确定量施氮的基础。叶色诊断指标实际上就是稻谷产量最高时的最适叶色。已有报告指出，叶色诊断指标受到群体大小和结实期光照条件的影响。研究的目的是，找出叶色诊断指标随群体大小和光照条件而变化的规律，为精确定量施氮提供理论和技术依据。2004—2005年早季和晚季，在广州以两系杂交稻粤杂122为试材，设置8种不同氮肥处理，进行2年4季田间试验，抽穗期测定叶色(SPAD)和叶面积指数(LAI)，成熟期测产。结果表明：(1)不同季节的最适叶色存在明显差异，4季变动于39~45之间。根据产量与叶色的定量关系，可以准确、快速地确定特定条件下的叶色诊断指标。(2)稻谷产量与抽穗期群体指数(SPAD与LAI的乘积)呈开口向下的抛物线关系。抽穗期SPAD、LAI和结实期日照时数，可以解释不同年度、季节和不同氮肥处理的稻谷产量变异的86%。最适群体指数随着结实期日照时数的增加而提高。(3)最适叶色随着日照时数的增加而提高，随着LAI的增加而降低，3者之间存在显著的定量关系。应用这一关系，可根据结实期光照条件，估计出异地异季的叶色诊断指标。     

不同性质农田土壤中铜的可提取性与生物有效性及毒性

以重金属总量为基础的土壤环境质量标准已不适用于当前土壤管理的需求，基于重金属生物有效性的生态风险评估和环境阈值研究对土壤重金属环境质量标准的修订具有重要的意义。本研究选取4种化学提取剂（HNO3、EDTA-Na2、NH4OAc和CaCl2）对我国3种不同性质的模拟Cu污染农田土壤（黑土、潮褐土和脱潜水稻土）进行有效态Cu提取，通过敏感性生态物种生菜和赤子爱胜蚓的Cu暴露实验，推导基于化学提取有效态的土壤Cu生态毒性阈值。研究结果表明，HNO3（41.38%）和EDTA-Na2（56.81%）对3种土壤中Cu的平均提取效率显著高于NH4OAc（0.12%）和CaCl2（8.70%）。CaCl2提取态Cu含量与生菜Cu富集量和毒性效应之间存在显著或极显著相关， HNO3提取态Cu含量则能很好地指示蚯蚓Cu富集量和30 d死亡率。不同生态受体毒性终点对土壤可提取态Cu的敏感性存在差异，选用物种最敏感指标推导了3种土壤中Cu的有效态毒性阈值EC 20和EC 50。基于不同化学提取态Cu含量的生菜毒性阈值EC 20范围分别是90.45~170.10 mg/kg（HNO3），102.78~195.31 mg/kg（EDTA-Na2），3.97~20.06 mg/kg（NH4OAc），和0.21~8.68 mg/kg（CaCl2）；EC 50范围分别是110.48~187.60 mg/kg（HNO3），118.63~230.49 mg/kg（EDTA-Na2），5.69~32.23 mg/kg（NH4OAc）和0.26~9.62 mg/kg（CaCl2）。基于不同化学提取态Cu含量的赤子爱胜蚓死亡率毒性阈值EC 20范围分别是138.26~193.16 mg/kg（HNO3）， 107.80~225.88 mg/kg（EDTA-Na2）， 8.92~11.58 mg/kg（NH4OAc），和0.36~10.57 mg/kg（CaCl2）；EC 50范围分别是183.07~221.23 mg/kg（HNO3），180.38~331.09 mg/kg（EDTA-Na2），13.06~18.30 mg/kg（NH4OAc）和0.54~13.21 mg/kg（CaCl2）。研究结果可为我国农田土壤重金属有效态化学提取方法的比选与优化提供科学依据，同时对基于生物有效性的土壤重金属环境质量基准与标准的研究和制定具有重要意义。     

Landscape-level thresholds of habitat cover for woodland-dependent birds

Theory suggests that a disproportionate loss of species occurs when total habitat cover decreases to 10-30% of the landscape. To date, little empirical evidence has been collected to test for such thresholds in habitat cover, especially at the landscape scale. Here, we present empirical data on the species richness of woodland-dependent birds collected systematically from 24 landscapes (each 100 km²) that sample a gradient in habitat cover from <2% to 60%. To compare the relative effects of habitat cover and habitat configuration, landscapes with similar amounts of habitat but contrasting configuration (i.e., aggregated versus dispersed) were surveyed and the richness of woodland-dependent birds collated for each landscape. The relationship between species richness, habitat cover and habitat configuration was examined using analysis of co-variance (ANCOVA), multiple linear regression and univariate non-linear modelling. There was a significant effect of habitat cover (co-variate) in the ANCOVA, but the main treatment effect of configuration was not significant. However, comparison of non-linear models indicated that the shape of the response curve of species loss with decreasing habitat cover differed between aggregated and dispersed landscapes. Species richness was significantly related to habitat cover in all analyses, explaining between 55% and 60% of the variance in regression models. Mean patch shape complexity and the extent of habitat aggregation were also significant explanatory variables, but explained less than 10% of the variance in richness of woodland birds. Biogeographic variables (range in elevation and geographic location) explained up to 14% of the variance in species richness. There was strong evidence for a threshold response in species richness: non-linear models (broken-stick, exponential, inverse) exhibiting a sharp decline in species richness in landscapes with less than 10% habitat cover provided a better fit to the observed data than linear models. To our knowledge, this is the first empirical demonstration of landscape-level thresholds in species richness. We emphasise that thresholds in species richness denote multiple species’ extinction events, the end point of the process of species decline. For viable populations, habitat cover must be maintained well above the threshold level. Finally, thresholds of assemblage measures, such as species richness, potentially mask compositional changes in the avifauna community and may also conceal the loss of species with greater sensitivity to landscape change.     

Modelling start of oak pollen season in different climatic zones in Spain

Quercus pollen and meteorological data for several years from eight sites in Spain have been statistically analysed to select the threshold temperature and calculate the mean heat accumulation for predicting the Quercus pollination start in different climatic areas. The growing degree days method, which assumes the daily temperature varies as a sine wave, was used for heat accumulation calculations. Threshold temperatures between 4 and 12 °C were chosen using linear regression equations forced through the origin and their root mean square error (RMSE) of predicted against the observed dates for each observation site. Above the threshold, the average growing degree days (up to 1999) for the studied years was taken as the predictor value. Results showed a relationship between the selected threshold and elevation and a stronger and statistically significant correlation between threshold and yearly mean temperature, for each site. Regression analysis indicated that the selected threshold and the calculated heat accumulation were optimum for most of the localities. The validity of the results was tested using the meteorological data for the year 2000 as independent variable and this confirmed that there were only a few days difference between the predicted and observed day of the first pollen release for most of the studied localities.