Water and ion fluxes of abscisic acid-treated root systems of pear, Pyrus communis

Water and ion fluxes of intact root systems of Pyrus communis L. cv. Old Home × Farmingdale 97 immersed in a nutrient solution were determined at various pressures and temperatures. Water flux (J_v was normalized on the basis of initial flow rates of a root system after 30 min at 0.50 MPa and 25°C, expressed as the ratio Q_v. Q_v responded linearly to pressures between 0.20 and 0.62 MPa, implying a constant root hydraulic conductivity (L_p) within this range. Similarly Q_v was linearly related to temperatures between 7 and 35°C; however, large, rapid temperature changes resulted in a break of the Arrhenius plot of Q_v versus the reciprocal of temperature, Abscisic acid (ABA) from 2 × 10⁻⁶ to 10⁻⁴ M , applied to intact root systems, increased Q_v within 10–20 min, with the effect leveling off after 1.5 h. At a pressure of 0.50 MPa, ABA at 10_-4 M enhanced Q_v by 28%. The stimulation of Q_v was not due to the ethanol solvent since 0.13 or 1.33% ethanol decreased Q_v-, The osmotic potential of the xylem fluid was determined and was used to calculate total normalized solute flux. The results suggest that ABA-induced or ethano1-induced changes in Q_v were mainly due to changes in L_p and not to changes in ion transport to the xylem.     

The mechanism of water-stress-induced embolism in two species of chaparral shrubs

The mechanism of water-stress-induced embolism of xylem was investigated in Malosma laurina and Heteromeles arbutifolia, two chaparral shrub species of southern California. We tested the hypothesis that the primary cause of xylem dysfunction in these species during dehydration was the pulling of air through the pores in the cell walls of vessels (pores in pit membranes) as a result of high tensions on xylem water. First, we constructed vulnerability-to-embolism curves for (i) excised branches that were increasingly dehydrated in the laboratory and (ii) hydrated branches exposed to increasing levels of external air pressure. Branches of M. laurina that were dehydrated became 50% embolized at a xylem pressure potential of -1.6 MPa, which is equal in magnitude but opposite in sign to the +1.6 MPa of external air pressure that caused 50% embolism in hydrated stems. Dehydrated and pressurized branches of H. arbutifolia reached a 50% level of embolism at -6.0 and +6.4 MPa, respectively. Secondly, polystyrene spheres ranging in diameter from 20 to 149 nm were perfused through hydrated stem segments to estimate the pore size in the vessel cell walls (pit membranes) of the two species. A 50% or greater reduction in hydraulic conductivity occurred in M. laurina at perfusions of 30, 42, 64 and 82 nm spheres and in H. arbutifolia at perfusions of 20 and 30 nm spheres. Application of the capillary equation to these pore diameters predicted 50% embolism at xylem tensions of -2.2 MPa for M. laurina and -6.7 MPa for H. arbutifolia, which are within 0.7 MPa of the actual values. Our results suggest that the size of pores in pit membranes may be a factor in determining both xylem efficiency and vulnerability to embolism in some chaparral species. H. arbutifolia, with smaller pores and narrower vessels, withstands lower water potentials but has lower transport efficiency. M. laurina, with wider pores and wider vessels, has a greater transport efficiency but requires a deeper root system to help avoid catastro-phically low water potentials.     

Structural principles for the propeller assembly of beta-sheets: the preference for seven-fold symmetry.

Twisted beta-sheets, packed face to face, may be arranged in circular formation like blades of a propeller or turbine. This beta-propeller fold has been found in three proteins: that in neuraminidase consists of six beta-sheets while those in methylamine dehydrogenase and galactose oxidase are composed of seven beta-sheets. A model for multisheet packing in the beta-propeller fold is proposed. This model gives both geometrical parameters of the beta-propellers composed of different numbers of sheets and patterns of residue packing at their sheet-to-sheet interfaces. All the known beta-propeller structures have been analyzed, and the observed geometries and residue packing are found to be in good agreement with those predicted by models. It is shown that unusual seven-fold symmetry is preferable to six- or eight-fold symmetry for propeller-like multi-sheet assembly. According to the model, a six-beta-sheet propeller has to have predominantly small residues in the beta-strands closed to its six-fold axis, but no strong sequence constraints are necessary for a seven-fold beta-propeller.     

清代北京会馆园林述略

清代北京外城地区修建了大量的会馆,作为各地同乡或商业行会的聚会、住宿、祭祀场所,其中部分会馆设有专门的园林,园内营造用于宴饮的厅堂,布置亭轩楼台等建筑,并堆叠假山、开辟池沼、种植花木,形成一种独立的园林类型。通过文献考证和现场调查,对清代北京会馆园林的主要实例进行记述,并对其造园意匠做进一步的分析。总体而言,这些园林的格局较为方整,建筑形式以厅堂、亭、轩为主,假山多以青石叠置,水景简单,植物多为易于维护的品种,匾额、楹联富有文化寓意并反映其故乡特色。     

Asymmetric cytokinin signaling opposes gravitropism in roots

Plants depend on gravity to provide the constant landmark for downward root growth and upward shoot growth. The phytohormone auxin and its cell‐to‐cell transport machinery are central determinants ensuring gravitropic growth. Statolith sedimentation toward gravity is sensed in specialized cells. This positional cue is translated into the polar distribution of PIN auxin efflux carriers at the plasma membrane, leading to asymmetric auxin distribution and consequently, differential growth and organ bending. While we have started to understand the general principles of how primary organs execute gravitropism, we currently lack basic understanding of how lateral plant organs can defy gravitropic responses. Here we briefly review the establishment of the oblique gravitropic set point angle in lateral roots and particularly discuss the emerging role of asymmetric cytokinin signaling as a central anti‐gravitropic signal. Differential cytokinin signaling is co‐opted in gravitropic lateral and hydrotropic primary roots to counterbalance gravitropic root growth.     

基于景观地形的小流域单元减灾调控评价研究

近年来,中国诸多大中城市内涝灾害频发。武汉市作为城市内涝频发的典型城市,其建成区地表高度（20.0~24.0 m）大多低于外江常年所处的洪水位高度（23.9 m）,在区域性暴雨条件下,极易引发内涝灾害。武汉由于其内涝典型性,2013年被列为全国31个重点防洪城市之一,2015年被列入首批海绵城市试点城市之一。从城市内涝发生机理出发,以水文角度的小流域单元作为内涝风险区划的基本单元,利用小流域单元分析武汉市景观地形要素（地表高程、地表起伏度、地表粗糙度）与城市内涝积水密度的相关性及其影响程度,量算统计出小流域单元的内涝风险等级。基于此,提出武汉市小流域单元减灾地形调控评价。以武汉市作为城市内涝问题的研究案例,以期为武汉市内涝缓解提供新的思考路径,具有典型性和必要性。     

基于文化与政治生态学的社会生态系统演进机制研究 ——以杭州与西湖为例

科学认识社会生态系统演进机制是对其进行有效管理的重要基础。以文化与政治生态学为理论基础,提出适合杭州—西湖社会生态系统的综合研究框架,识别了杭州—西湖社会生态系统的5个演进阶段,并分析出系统演进中的3种潜在状态。自然、政治、经济以及社会驱动力是影响杭州—西湖社会生态系统的主导因素,人类行为影响整个生态系统中扰动的频率、大小和形式并改变西湖生态系统的结构与功能,进而影响西湖为城市提供生态系统服务的潜能。在不同历史时期,基于自然、社会、经济、文化等多层面的需求,西湖在不同系统状态下为城市供给不同类别和质量的生态系统服务,总体而言供给与调节服务比例逐渐下降,文化服务逐渐上升,并且后者逐步成为最主要的生态系统服务类别。杭州与西湖在长期的互馈共生中建立了社会生态系统的自适应性调节机制,其背后的生态智慧可为现代风景园林规划提供重要启示。     

Mutations in MIR396e and MIR396f increase grain size and modulate shoot architecture in rice

Grain size and plant architecture are critical factors determining crop productivity. Here, we performed gene editing of the MIR396 gene family in rice and found that MIR396e and MIR396f are two important regulators of grain size and plant architecture. mir396ef mutations can increase grain yield by increasing grain size. In addition, mir396ef mutations resulted in an altered plant architecture, with lengthened leaves but shortened internodes, especially the uppermost internode. Our research suggests that mir396ef mutations promote leaf elongation by increasing the level of a gibberellin (GA) precursor, mevalonic acid, which subsequently promotes GA biosynthesis. However, internode elongation in mir396ef mutants appears to be suppressed via reduced CYP96B4 expression but not via the GA pathway. This research provides candidate gene‐editing targets to breed elite rice varieties.     

Amazonia trees have limited capacity to acclimate plant hydraulic properties in response to long‐term drought

The fate of tropical forests under future climate change is dependent on the capacity of their trees to adjust to drier conditions. The capacity of trees to withstand drought is likely to be determined by traits associated with their hydraulic systems. However, data on whether tropical trees can adjust hydraulic traits when experiencing drought remain rare. We measured plant hydraulic traits (e.g. hydraulic conductivity and embolism resistance) and plant hydraulic system status (e.g. leaf water potential, native embolism and safety margin) on >150 trees from 12 genera (36 species) and spanning a stem size range from 14 to 68 cm diameter at breast height at the world's only long‐running tropical forest drought experiment. Hydraulic traits showed no adjustment following 15 years of experimentally imposed moisture deficit. This failure to adjust resulted in these drought‐stressed trees experiencing significantly lower leaf water potentials, and higher, but variable, levels of native embolism in the branches. This result suggests that hydraulic damage caused by elevated levels of embolism is likely to be one of the key drivers of drought‐induced mortality following long‐term soil moisture deficit. We demonstrate that some hydraulic traits changed with tree size, however, the direction and magnitude of the change was controlled by taxonomic identity. Our results suggest that Amazonian trees, both small and large, have limited capacity to acclimate their hydraulic systems to future droughts, potentially making them more at risk of drought‐induced mortality.