The apoplastic continuum,nutrient absorption and plasmatubules in the dwarf mistletoeKorthalsella lindsayi (Viscaceae)

Summary The transfer of nutrients between host and parasite in mistletoes has generally been considered to occur via the xylem to xylem contacts at the host-parasite interface in the haustorial organ of attachment. A few workers, however, have recently begun to question this assumption and have suggested an alternative pathway of transport involving the intervening parenchyma cells which are often abundant in the parasite at the interface. But no morphological experimental evidence has yet been forthcoming in support of an apoplastic continuum across this interface between parasite and host.Our observations on the dwarf mistletoeKorthalsella lindsayi first indicate an absence of plasmodesmata at the interface, with the conclusion that symplastic transport between the two plants is not involved. However, application of apoplastic markers, such as Calcofluor white and lanthanum and uranyl ions, to the stem of the host results in the transfer of these tracers across the interface and into the tissues of the parasite. This demonstrates the existence of an apoplastic continuum between the two plants, and a pathway that is probably used in the normal transfer of water and other nutrients from host to parasite.From the apoplastic continuum provided by the walls of the haustorial parenchyma tissue, nutrients are transferred to the symplast for eventual distribution to other parts of the plant. Evidence for the active uptake of substances from the apoplast by the protoplasts of the parenchyma cells is shown by the convoluted appearance of the plasmalemma and its differentiation often into plasmatubules.     

Two-dimensional standing gradient osmotic flow: a generalization of the “isotonic convection approximation”

A linear two-dimensional model of a flow system for solute and fluid transport in intercellular spaces has been obtained by using the so called isotonic convection approximation, already employed in the one-dimensional case (Segel (1970)). This is equivalent to ignoring the convective components in the relevant differential equation. The solutions found by means of the eigenfunctions of a Sturm-Liouville system have been compared with the numerical solutions of the general non-linear model, in which also the convective terms are present.The results of the linear model agree fairly well with those of the non-linear one, in the range of interest of parameters. This fact shows that the ignorance of the profile of fluid velocity does not introduce significant errors in the evaluation of solute and fluid fluxes.The differences of results between the one-dimensional and the twodimensional, the linear and non-linear models, give a way to evaluate the relative effects of the diffusive and convective terms in the differential equation and to estimate the errors introduced by the one-dimensional approximation.     

A survey of angiosperm species to detect hypodermal Gasparian bands. II. Roots with a multiseriate hypodermis or epidermis

PETERSON, C. A. & PERUMALLA, C. J., 1990. A survey of angiosperm species to detect hypodermal Casparian bands. II. Roots with a multiseriate hypodermis or epidermis.
Roots of 25 species which had either a multiseriate hypodermis or a multiseriate epidermis were tested for the presence of a hypodermal Casparian band. All species save one were in the Liliopsida and six were orchids with both soil and aerial roots. Lignosuberized hypodermal Casparian bands were present in all species tested; those with a biseriate hypodermis had bands in both layers and of those with a multiseriate hypodermis, the three species which were tested had bands in every layer. Although Casparian bands can often be recognized by the presence of sinuous walls in longitudinal views of uniseriate hypodermal layers, these sinuosities were not evident in multiseriate hypodermal layers containing Casparian bands. The lack of air spaces, once thought to be a characteristic feature of the hypodermis, did not hold true for some members of the Liliopsida. All walls of the hypodermis were suberized, indicating that suberin lamellae were probably present in addition to Casparian bands. We recommend using the term 'exodermis' to refer to a hypodermis which has a Casparian band. Epidermal walls of non-orchid roots were suberized whereas those of orchids were lignified. Regardless of their type of modification, all epidermal walls were permeable to the apoplastic dye, Cellufluor.     

Ultrastructural indications for coexistence of symplastic and apoplastic phloem loading in Commelina benghalensis leaves

The ultrastructural ontogeny of Commelina benghalensis minor-vein elements was followed. The mature minor vein has a restricted number of elements: a sheath of six to eight mestome cells encloses one xylem vessel, three to five vascular parenchyma cells, a companion cell, a thin-walled protophloem sieve-tube member and a thick-walled metaphloem sieve-tube member. The protophloem sieve-tube member (diameter 4–5 m; wall thickness 0.12 m) and the companion cell originated from a common mother cell. The metaphloem sieve-tube member (diameter 3 m; wall thickness 0.2 m) developed from the same precursor cell as the phloem parenchyma cells. Counting the plasmodesmatal frequencies demonstrated a symplastic continuum from mesophyll to the minor-vein phloem. The metaphloem sievetube member and the phloem parenchyma cells are the termini of this symplast. The protophloem sieve-tube member and companion cell constitute an insulated symplastic domain. The symplastic route, mesophyll to metaphloem sieve tube, appears to offer a path for symplastic loading; the protophloem sieve tube may be capable of accumulation from the apoplast. A similar two-way system of loading may exist in a number of plant families. Plasmodesmograms (a novel way to depict cell elements, plasmodesmatal frequencies and vein architecture) of some other species also displayed the anatomical requirements for two routes from mesophyll to sieve tube and indicate the potential coexistence of symplastic and apoplastic loading.     

Indicators for green spaces in contrasting urban settings

Urban green spaces (GS) are essential for the well-being of the population. Several works have shown a positive correlation between the amount of GS and the household incomes in both developed and developing countries. Thus, the higher the incomes, the larger the total area covered by GS, the better the quality of these spaces, the higher the amount of private GS. Public policies seek to correct this inequality, but existing indicators, especially the amount of GS per inhabitant, do not provide enough information for effective decision-making. Our aim was to provide tools to evaluate and plan better the location and quality of GS in complex urban areas. For this we applied a set of indicators for GS at two spatial scales city-level and local-level, in order to disclose existing inequalities. The indicators considered (i) the total area of GS in relation to population and urban context, (ii) the quality of GS based on its size, shape and vegetation cover, and (iii) the spatial distribution and accessibility of GS. The proposed indicators were tested in three municipalities, belonging to the Metropolitan Area of Santiago (Chile), with different household incomes. The indicators showed large differences in terms of quantity of GS per inhabitant, vegetation cover and accessibility. The GS proved to be an effective strategy to reduce areas that lack vegetation cover. The sustainability assessments must consider how the diversity of structural attributes of GS has an impact on the well-being of urban inhabitants.     

Effects of root medium pH on root water transport and apoplastic pH in red‐osier dogwood (Cornus sericea) and paper birch (Betula papyrifera) seedlings

• Soil pH is a major factor affecting plant growth. Plant responses to pH conditions widely vary between different species of plants. However, the exact mechanisms of high pH tolerance of plants are largely unknown. In the present study, we compared the pH responses of paper birch (Betula papyrifera) seedlings, a relatively sensitive species to high soil pH, with red‐osier dogwood (Cornus sericea), reported to be relatively tolerant of high pH conditions. We examined the hypotheses that tolerance of plants to high root zone pH is linked to effective control of root apoplastic pH to facilitate nutrient and water transport processes
• In the study, we exposed paper birch and red‐osier dogwood seedlings for six weeks to pH 5, 7 and 9 under controlled‐environment conditions in hydroponic culture. Then, we measured biomass, gas exchange, root hydraulic conductivity, ferric chelate reductase (FCR) activity, xylem sap pH and the relative abundance of major elements in leaf protoplasts and apoplasts.
• The study sheds new light on the rarely studied high pH tolerance mechanisms in plants. We found that compared with paper birch, red‐osier dogwood showed greater growth, higher gas exchange, and maintained higher root hydraulic conductivity as well as lower xylem sap pH under high pH conditions.
• The results suggest that the relatively high pH tolerance of dogwood is associated with greater water uptake ability and maintenance of low apoplastic pH. These traits may have a significant impact on the uptake of Fe and Mn by leaf cells.

     

The vascular plant‐pathogenic bacterium Ralstonia solanacearum produces biofilms required for its virulence on the surfaces of tomato cells adjacent to intercellular spaces

The mechanism of colonization of intercellular spaces by the soil‐borne and vascular plant‐pathogenic bacterium Ralstonia solanacearum strain OE1‐1 after invasion into host plants remains unclear. To analyse the behaviour of OE1‐1 cells in intercellular spaces, tomato leaves with the lower epidermis layers excised after infiltration with OE1‐1 were observed under a scanning electron microscope. OE1‐1 cells formed microcolonies on the surfaces of tomato cells adjacent to intercellular spaces, and then aggregated surrounded by an extracellular matrix, forming mature biofilm structures. Furthermore, OE1‐1 cells produced mushroom‐type biofilms when incubated in fluids of apoplasts including intercellular spaces, but not xylem fluids from tomato plants. This is the first report of biofilm formation by R. solanacearum on host plant cells after invasion into intercellular spaces and mushroom‐type biofilms produced by R. solanacearum in vitro. Sugar application led to enhanced biofilm formation by OE1‐1. Mutation of lecM encoding a lectin, RS‐IIL, which reportedly exhibits affinity for these sugars, led to a significant decrease in biofilm formation. Colonization in intercellular spaces was significantly decreased in the lecM mutant, leading to a loss of virulence on tomato plants. Complementation of the lecM mutant with native lecM resulted in the recovery of mushroom‐type biofilms and virulence on tomato plants. Together, our findings indicate that OE1‐1 produces mature biofilms on the surfaces of tomato cells after invasion into intercellular spaces. RS‐IIL may contribute to biofilm formation by OE1‐1, which is required for OE1‐1 virulence.     

Shade avoidance and reproductive strategies of an early successional species Penthorum chinense in relation to shade treatments

• Shade avoidance is expected to be favoured under moderate light. However, in previous studies, shade avoidance was highest in the deepest shade, despite the fact that the plants incur the costs of shade avoidance without the benefits of being exposed to increased light.
• We performed shading experiments under different light intensities to understand: (i) how shade avoidance traits of Penthorum chinense could peak in moderate light, and (ii) if there was a trade‐off between plant height and allocation of seeds along the light gradients.
• Penthorum chinense increased shade avoidance traits such as height per total dry mass as the amount of light decreased. Side stem number per total dry mass of P. chinense decreased as shade became deeper, from full light to low light. Regressions on seed mass fraction and height were significant with a linear model (y = ?0.0006x + 0.1338). There were more resources allocated to seeds under low light than under moderate light.
• Penthorum chinense increased shade avoidance traits with the decrease in light amount, as found in previously studied species. There was a trade‐off between height and production of more seeds. The reproductive strategy of P. chinense was to increase seed mass fraction under low light more than under moderate light. This species might be able to expand established populations by both rhizomes and seeds under low light environments.

     

长江三角洲城市群“三生”功能耦合协调时空分异及其影响因素

长江三角洲城市群是中国城市化水平和经济发展水平最高的城市群，其"三生"功能问题和矛盾日益凸显，"三生"功能耦合协调分析是国土空间规划及区域可持续发展的基础。以长江三角洲城市群为研究区，从农业生产、非农业生产、生活水平、生活保障、生态压力和生态保障六个方面筛选31项指标构建"三生"功能评价指标体系，采用熵权法和AHP法组合确定评价指标权重。并引入耦合协调度模型，以2000-2019年为研究时段进行"三生"功能评价及其耦合协调度分析，并运用面板数据模型和Robust回归分析影响研究区"三生"功能的主要因素。结果表明：（1）研究区2000-2019年间生产、生活、生态功能整体上稳步提升，其中，生产功能东西部发展优于中部，低值区缩减；生活功能东部优于西部，南部优于北部；生态功能随时间呈现由西南部到东南部最优的变化特征。"三生"功能总体表现为"东部优于西部，南部优于北部，东南部发展最优"格局，空间差异明显；（2）生产-生活严重失调城市占比由23.08%缩减为0，基本协调城市占比由11.54%提高到46.15%，中度协调城市占比由0提高到53.85%，耦合协调性提高显著；生产-生态中度失调城市占比由57.69%降为4.8%，基本协调城市占比由19.23%提升为88.46%；生活-生态耦合严重和中度失调城市占比由有到无，中度协调城市占比由0提升为88.46%，上海市升为高度协调城市；生产-生活-生态功能耦合协调度逐步提高，在空间上呈现东部、东南部为主要高值区，沿海优于内陆的分布格局；（3）地均农业机械总动力、地均固定资产投资、城镇常住居民人均可支配收入、草地面积、绿地覆盖率等因素对研究区"三生"功能耦合协调发展起到了巨大的推动作用，而农村常住居民人均可支配收入对"三生"功能耦合协调起到了限制作用。研究结果能为其社会经济稳定发展与国土空间资源利用可持续发展提供参考依据。     

Structure and permeability of the fungal sheath in thePisonia mycorrhiza

Summary The tracer Cellufluor has been used to test the apoplastic permeability of the fungal sheath inPisonia grandis R. Br. mycorrhizas. In the tip region in the immediate vicinity of the root cap, where the sheath is not yet fully differentiated, Celluflor penetrates as far as the root epidermal cells. Behind this (i.e. just proximal to it) in differentiated regions, where the ultrastructure of both the root and fungal cells indicates that the mycorrhiza is likely to be functionally active, the sheath is impermeable to Cellufluor. During the development and differentiation of the sheath, the interhyphal spaces become filled with extracellular material. In the outer and middle regions this becomes electron opaque after fixation and staining. It is proposed that the dramatic decrease in apoplastic permeability over a short distance back from the root apex as the fungal sheath differentiates results from secretion of extracellular material by the fungus and its modification by deposition of phenolic substances. The symplastic pathway within the fungus may be very important for radial transfer of materials across the sheath. Blockage of the sheath apoplast could provide a sealed apoplastic compartment at the fungus-root interface, with resulting increase in efficiency of transfer between partners. The implications of these observations are discussed in relation to radial transfer across the sheath and transfer between partners in sheathing mycorrhizas in general.