Responses of root length/leaf area ratio and specific root length of an understory herb, Pteridophyllum racemosum, to increases in irradiance

The understory evergreen perennial Pteridophyllum racemosum Sieb. et Zucc. (Papaveraceae) has the ability to increase root mass per unit transpiring leaf area (RMA) if irradiance increases gradually over several years. In this study, we examined how P. racemosum changes its root length/leaf area ratio and specific root length when the species encounters abrupt increases in irradiance, such as sudden and unexpected canopy openings. Plants were transplanted from a low light condition in a subalpine wave-regenerating forest (photon flux density on the forest floor relative to the full sun (RPFD) was 2.7%) to a high light condition in a glasshouse (30% RPFD) (LH treatment). Transplantation from the low light condition in the forest to a low light condition in the glasshouse (LL) and transplantation from a high light condition in the forest (33% RPFD) to a high light condition in the glasshouse (HH) were also conducted as controls. Compared to the LL plants, the LH plants exhibited significant increases in RMA and root length/leaf area ratio from 30 to 70 days after transplantation. On the other hand, the effect of increased irradiance on specific root length (SRL) was weak, and both the LL and LH plants showed increased SRL from 30 to 70 days after transplantation. Increased SRL results from longer root length per unit construction cost. We concluded that increased root length/leaf area ratio of P. racemosum in response to abrupt increases in irradiance was caused by a combination of enhanced carbon allocation to roots with increased SRL.     

Effect of mutual shading on the emergence of nodal roots and the root/shoot ratio of maize

The effect of mutual shading on the root/shoot ratio and on the number of nodal roots of maize was studied. Plants of two varieties (Dea and LG2281) were grown in individual pots of 9 L, at three plant densities: 7.5, 11 and 15 plants m^–2. A control experiment was carried out in order to study if root growth was affected by the small size of the pots. Maize plants (cv Dea) were grown at a low plant density (7.5 plants m^–2) in pots of two different volumes (9 and 25 L respectively). In both experiments plants were watered every two hours with a nutrient solution. Some plants were sampled at five dates in the main experiment and the following data were recorded: foliar stage; root, stem and leaf dry weight; number of root primordia and number of emerged roots per phytomer. The final sampling date occurred at silking.Results of the control experiment showed that the root biomass was lower in small pots but the number of nodal roots per phytomer was not affected.Results of the main experiment showed that the total plant biomass and the root/shoot ratio were lower at high plant density. The number of emerged roots was strongly reduced on the upper phytomer (P₈). This reduction was mainly due to a lower percentage of root primordia which elongated. A proposed interpretation is that the number of roots which emerge on upper phytomers is controlled by carbohydrate availability.     

Large altitudinal increase in tree root/shoot ratio in tropical mountain forests of Ecuador

Tropical rain forests decrease in tree height and aboveground biomass (AGB) with increasing elevation. The causes of this phenomenon remain insufficiently understood despite a number of explanations proposed including direct or indirect effects of low temperature on carbon acquisition and carbon investment, adverse soil conditions and impaired nutrient supply. For analysing altitudinal patterns of aboveground/belowground carbon partitioning, we measured fine (<2 mm in diameter) and coarse root (2–5 mm) biomass and necromass and leaf area index (LAI), and estimated AGB from stand structural parameters in five tropical mountain rain forests at 1050, 1540, 1890, 2380 and 3060 m along an altitudinal transect in the South Ecuadorian Andes. Average tree height and AGB were reduced to less than 50% between 1050 and 3060 m, LAI decreased from 5.1 to 2.9. The leaf area reduction must have resulted in a lowered canopy carbon gain and thus may partly explain the reduced tree growth in the high-elevation stands. In contrast, both fine and coarse root biomass significantly increased with elevation across this transect. The ratio of root biomass (fine and coarse) to AGB increased more than ten-fold from 0.04 at 1050 m to 0.43 at 3060 m. Under the assumption that fine root biomass does reflect root productivity, our data indicate a marked belowground shift in C allocation with increasing elevation. Possible explanations for this allocation shift are discussed including reduced N supply due to low temperatures, water logging or adverse soil chemical conditions. We conclude that the fine root system and its activity may hold the key for understanding the impressive reduction in tree size along tropical mountain slopes in Ecuador. Analyses of fine root turnover and longevity in relation to environmental factors along altitudinal transects in tropical mountains are urgently needed.     

高寒草甸主要植物地上地下生物量分布及退化对根冠比和根系表面积的影响

研究高寒草甸主要植物地上地下生物量的分布及其对退化的响应有利于了解高寒草甸的退化过程。该研究首先在西藏那曲生态环境综合观测研究站小嵩草围栏内(2009年围封)选择原生植被较好的地点随机选择小嵩草(Kobresia pygmaea)、矮嵩草(K.humilis)、紫花针茅(Stipa purpurea)、二裂委陵菜(Potentila bifurca)和青藏苔草(Carex moorcroftii)等5种植物斑块,选择退化斑块上(与原生植被相比)的二裂委陵菜和青藏苔草;然后用烘箱烘至恒重并称重,用扫描仪对根系进行扫描用于估算根系表面积;最后利用2因子方差分析检验不同物种个体、不同取样层次对地上和地下生物量的影响,利用物种和退化状态2因子方差分析检验对地上生物量的影响,以及利用物种、取样层次和退化状态3因子方差分析检验对二裂委陵菜和青藏苔草地下生物量、根冠比和根系表面积的影响。结果表明:在未退化条件下,小嵩草、矮嵩草和紫花针茅0~10cml地下生物量占0~30cm地下生物量的70%以上,0~30cm地下生物量占其地上地下总生物量的96%以上;二裂委陵菜(Potentilla bifurca)和青藏苔草(Carex moorcroftii)0~10cm地下生物量占0~30cml地下生物量的50%以上,其中二裂委陵菜0~30cm地下生物量占其地上地下总生物量的57%,青藏苔草0~30cm地下生物量占其地上地下总生物量的87%;对于退化草甸的主要植物,退化显著降低了二裂委陵菜的地上生物量、地下生物量和根冠比,对其根系表面积影响不大,但显著增加了青藏苔草的地上生物量,降低了其根冠比,对其地下生物量和根系表面积影响不大。     

Sapling biomass allocation and growth in the understory of a deciduous hardwood forest

Above- and belowground tissues of co-occurring saplings (0.1-1 m height) of Acer saccharum Marsh. (very shade tolerant), Acer rubrum L. (shade tolerant), Fraxinus americana L. (intermediate shade tolerant), and Prunus serotina Ehrh. (shade intolerant) were harvested from a forest understory to test the hypothesis that the pattern of biomass allocation varied predictably with shade-tolerance rank. The placement and length of branches along the main axis were consistent with the formation of a monolayer of foliage for the tolerant and intermediate species. Other morphological characteristics did not vary predictably with shade-tolerance rank. The maintenance of high specific leaf area (SLA; leaf area/leaf mass) and leaf area ratio (LAR; leaf area/sapling mass) is considered important for growth under extreme shade, yet these traits were not clearly related to the shade-tolerance rank of these species. Fraxinus americana, an intermediate species, had the highest LAR and growth rate in the understory, and with the exception of P. serotina, the very shade-tolerant A. saccharum had the lowest LAR. Prunus serotina maintained a large starch-rich tap root and shoot dieback was common, yielding the largest root/shoot ratio for these species. The observed allocation patterns were not similar to the long-standing expectation for the phenotypic response of juvenile trees to shade, but were consistent with three hypothetical "growth strategies" in the understory: (1) the low SLA and LAR of A. saccharum may provide a measure of defense against herbivores and pathogens and thus promote persistence in the understory, (2) the high SLA for F. americana and high LAR for F. americana and A. rubrum may enable these species to achieve high growth rates in shade, and (3) the large carbohydrate stores of P. serotina may poise this species for opportunistic growth following disturbance. The relative importance of resistance to herbivores and pathogens vs. the maintenance of high growth rates may be important in evaluating the patterns of biomass allocation in the understory.     

Morphology and anatomy of shoot, root, and propagation systems in Hoffmannseggia glauca

Abstract: Hoffmannseggia glauca is a perennial weed that has tubers and root-borne buds. Some authors only consider root tubers without mentioning root-borne buds, while others consider that more anatomic studies become necessary to determine the origin of these structures and to interpret their behaviour. The objectives are: to study the growth form of the plant in order to analyze the ontogeny of its propagation organs, and to study its shoot and root anatomical characters that affect water conductivity. Hoffmannseggia glauca was collected in Argentina. Development of its shoot and root systems was observed. Shoots and roots were processed to obtain histological slides. Macerations were prepared to study vessel members. Primary and lateral roots originate buds that develop shoots at the end of the first year. In winter, aerial parts die and only latent buds at soil surface level and subterranean organs remain. In the following spring, they develop innovation shoots. Roots show localized swellings (tuberous roots), due to a pronounced increase of ray thickness and parenchymatous proliferation in the root center. Root vessel members are wider than those of aerial and subterranean shoots. Early development of an extensive root system, presence of root borne buds, anatomic and physiological specialization of innovation shoots, capability of parenchymatous rays to originate buds and tuberous roots, and high water transport efficiency in subterranean organs lead Hoffmannseggia glauca to display higher colonization potential than other species.     

Signaling mechanisms integrating root and shoot responses to changes in the nitrogen supply

During their life cycle, plants must be able to adapt to wide variations in the supply of soil nitrogen (N). Changes in N availability, and in the relative concentrations of NO₃ ⁻and NH₄ ⁺, are known to have profound regulatory effects on the N uptake systems in the root, on C and N metabolism throughout the plant, and on root and shoot morphology. Optimising the plant’s responses to fluctuations in the N supply requires co-ordination of the pathways of C and N assimilation, as well as establishment of the appropriate allocation of resources between root and shoot growth. Achieving this integration of responses at the whole plant level implies long-distance signaling mechanisms that can communicate information about the current availability of N from root-to-shoot, and information about the C/N status of the shoot in the reverse direction. In this review we will discuss recent advances which have contributed to our understanding of these long-range signaling pathways.     

Regulation of shoot/root ratio by cytokinins from roots inUrtica dioica: Opinion

According to current knowledge, cytokinins are predominantly root-born phytohormones which are transported into the shoot by the transpiration stream. In the hormone message concept they are considered the root signals, which mediate the flux of the photosynthates to the various sinks of the plant. In this review, experiments are assessed, in which changes of the shoot to root ratio of biomass, caused by different levels of nitrogen supply to a model plant,Urtica dioica, could be traced to the natural cytokinin relations of the plant. Disturbance of the internal cytokinin balance of the plant resulted in a disproportionate distribution of the assimilates in favour of the cytokinin-enriched shoot. Inspite of some shortcomings of the hormone message concept, the presented work corroborates the significance of root-sourced cytokinins in the regulation of biomass partitioning between shoot and root.     

Annual and spatial variation in shoot demography associated with masting in Betula grossa: comparison between mature trees and saplings

Backgrounds and Aims

Shoot demography affects the growth of the tree crown and the number of leaves on a tree. Masting may cause inter-annual and spatial variation in shoot demography of mature trees, which may in turn affect the resource budget of the tree. The aim of this study was to evaluate the effect of masting on the temporal and spatial variations in shoot demography of mature Betula grossa.

Methods

The shoot demography was analysed in the upper and lower parts of the tree crown in mature trees and saplings over 7 years. Mature trees and saplings were compared to differentiate the effect of masting from the effect of exogenous environment on shoot demography. The fate of different shoot types (reproductive, vegetative, short, long), shoot length and leaf area were investigated by monitoring and by retrospective survey using morphological markers on branches. The effects of year and branch position on demographic parameters were evaluated.

Key Results

Shoot increase rate, production of long shoots, bud mortality, length of long shoots and leaf area of a branch fluctuated periodically from year to year in mature trees over 7 years, in which two masting events occurred. Branches within a crown showed synchronized annual variation, and the extent of fluctuation was larger in the upper branches than the lower branches. Vegetative shoots varied in their bud differentiation each year and contributed to the dynamic shoot demography as much as did reproductive shoots, suggesting physiological integration in shoot demography through hormonal regulation and resource allocation.

Conclusions

Masting caused periodic annual variation in shoot demography of the mature trees and the effect was spatially variable within a tree crown. Since masting is a common phenomenon among tree species, annual variation in shoot demography and leaf area should be incorporated into resource allocation models of mature masting trees.     

The effects of environmental heterogeneity on root growth and root/shoot partitioning

AIMS: The purpose of this Botanical Briefing is to stimulate reappraisal of root growth, root/shoot partitioning, and analysis of other aspects of plant growth under heterogeneous conditions. SCOPE: Until recently, most knowledge of plant growth was based upon experimental studies carried out under homogeneous conditions. Natural environments are heterogeneous at scales relevant to plants and in forms to which they can respond. Responses to environmental heterogeneity are often localized rather than plant-wide, and not always predictable from traditional optimization arguments or from knowledge of the ontogenetic trends of plants growing under homogeneous conditions. These responses can have substantial impacts, both locally and plant-wide, on patterns of resource allocation, and significant effects on whole-plant growth. Results from recent studies are presented to illustrate responses of plants, plant populations and plant communities to nutritionally heterogeneous conditions. CONCLUSIONS: Environmental heterogeneity is a constant presence in the natural world that significantly influences plant behaviour at a variety of levels of complexity. Failure to understand its effects on plants prevents us from fully exploiting aspects of plant behaviour that are only revealed under patchy conditions. More effort should be invested into analysis of the behaviour of plants under heterogeneous conditions.     

Root/Shoot Allocation and Root Architecture in Seedlings: Variation among Forest Sites, Microhabitats, and Ecological Groups1

I analyzed patterns of variation in root mass allocation and root morphology among seedlings of woody species in relation to environmental factors in four Neotropical forests. Among forests, I explored the response of root traits to sites varying in water or nutrient availability. Within each forest, I explored the plastic response of species to different microhabitats: gaps and understory. Additionally, I explored evidence for life history correlation of root and shoot traits by comparing species differing in their successional group (light‐demanding [22 spp.] or shade tolerant [27 spp.]) and germination type (species with photosynthetic cotyledons or species with reserve cotyledons). At each forest site, young seedlings from 10 to 20 species were excavated. A total of 55 species was collected in understory conditions and 31 of them were also collected in gaps. From each seedling, six morphological ratios were determined. Allocation to roots was higher in forest sites with the lowest soil resources. Roots were finer and longer in the most infertile site, while roots were deeper in the site with the longest dry season. Seedling traits did not differ between germination types. Shade tolerant species allocated more to roots and developed thicker roots than light‐demanding species. Light‐demanding species showed stronger plastic responses to habitat than shade tolerant species, and species with photo‐synthetic cotyledons showed lower plasticity than species with reserve cotyledons. Overall, these results suggest that among Neotropical species, root allocation and root morphology of seedlings reflect plant adjustments to water or nutrient availability at geographic and microhabitat scales. In addition, life history specialization to light environments is suggested by differences among groups of species in their allocation to roots and in their root morphology.     

Morphological differentiation of current-year shoots of deciduous and evergreen lianas in temperate forests in Japan

Morphological variation in current-year shoots within plants was examined in five deciduous and four evergreen liana species from temperate forests in Japan to elucidate the role differentiation in shoots. All lianas had both shoots that twined or developed adventitious roots to gain support on host materials (searcher shoots) and self-supporting shoots with no climbing structures (ordinary shoots). Searcher shoots were 20–295 times longer than ordinary shoots. The allometric relationships between stem length and leaf area differed between searcher and ordinary shoots, and the stem length for a given leaf area was greater in searcher shoots. Leaf area per shoot mass was 1.4–4.3 times higher in ordinary shoots because of the greater allocation to leaf biomass. Searcher shoots comprised only 1–6% of total shoots but 30–85% of total shoot length in deciduous lianas. Ordinary shoots accounted for 70–95% of the total leaf area in these liana species. These results suggest that the exploration of new space was primarily achieved by searcher shoots, whereas a large proportion of current photosynthetic production was achieved by ordinary shoots. The range of stem length and leaf mass ratio of ordinary shoots was similar to that in shoots of tree species. Specialization of shoots in lianas is discussed.     

A model of water flow through plants incorporating shoot/root 'message' control of stomatal conductance

Abstract. A model of water flow from the soil into the plant, and from the plant to the atmosphere is described. There are three state variables in the model: the soil, root and shoot water contents. The flow rate of water from the soil to the root is calculated by dividing the gradient in water potential by a resistance, comprising the resistance from the bulk soil to the root surface, and that from the root surface to the root interior. The resistance in the soil depends on the soil hydraulic conductivity, which in turn depends on the soil water potential. The flow rate from the root to the shoot is given by the gradient in water potential divided by a resistance, which depends on the structural dry mass of the plant. Transpiration is described by the Penman-Monteith equation. The plant water characteristics can be modified to take account of osmotic and cell wall rigidity parameters. The model incorporates the concept of shoot/root ‘messages’ of water stress, which influence stomatal conductance. The message works through the generation of a hormone as the pressure potential in the shoot (mesophyll) or root falls. This hormone induces a shift of osmoticum from the guard cells to the surrounding mesophyll cells, which causes an increase (i.e. closer to zero) in the osmotic potential in these cells. This, in turn, causes a decrease in their pressure potential, and so reduces stomatal conductance. The model is used as a framework to address some of the issues that have recently been raised concerning the role of water potential in describing water flow through plants. We conclude that, with the hormone present, there is unlikely to be a unique relationship between stomatal conductance and shoot total water potential, since stomatal conductance depends on the pressure potential in the guard cells, which may differ from that in other cells. Nevertheless, this does not imply that water potential is not an important, and indeed fundamental, component for describing water flow through plants. Other aspects of water flow through plants are also considered, such as diurnal patterns of shoot, root and soil water potential components. It is seen that these may differ from the commonly held view that, as the soil dries down, they all attain the same values during the dark period, and which, as we show, is largely unsubstantiated either theoretically or experimentally.     

Growth and shoot: root ratio of seedlings in relation to nutrient availability

The influence of mineral nutrient availability, light intensity and CO₂ on growth and shoot:root ratio in young plants is reviewed. Special emphasis in this evaluation is given to data from laboratory experiments with small Betula pendula plants, in which the concept of steady-state nutrition has been applied.Three distinctly different dry matter allocation patterns were observed when growth was limited by the availability of mineral nutrients: 1, Root growth was favoured when N, P or S were the major growth constraints. 2, The opposite pattern obtained when K, Mg and Mn restricted growth. 3, Shortage of Ca, Fe and Zn had almost no effect on the shoot:root ratio. The light regime had no effect on dry matter allocation except at very low photon flux densities (< 6.5 mol m^-2 day^-1), in which a small decrease in the root fraction was observed. Shortage of CO₂, on the other hand, strongly decreased root development, while an increase of the atmospheric CO₂ concentration had no influence on dry matter partitioning. An increased allocation of dry matter to below-ground parts was associated with an increased amount of starch in the tissues. Depletion of the carbohydrate stores occurred under all conditions in which root development was inhibited. It is concluded that the internal balance between labile nitrogen and carbon in the root and the shoot system determines how dry matter is being partitioned in the plant. The consistency of this statement with literature data and existing models for shoot:root regulation is examined.     

亚热带6种树种细根序级结构和形态特征

以福建省建瓯市万木林自然保护区内占优势的6种天然林树种(沉水樟Cinnamomum micranthum,CIM;观光木Tsoongiodendron odorum Chun,TOC;浙江桂Cinnamomum chekiangense,CIC;罗浮栲Castanopsis fabri,CAF;细柄阿丁枫Altingiagracilipes,ALG;米槠Castanopsis carlesii,CAC)为研究对象,对其1—5级细根的结构,形态特征及生物量进行了分析。结果表明:沉水樟,细柄阿丁枫和米槠细根分支比表现出在1,2级(4倍以上)明显大于其它序级(3倍左右);其余3种树种则是在3,4级的细根分支比最大,其中浙江桂达到8.65倍,其它序级则大致为3倍左右。6种树种1,2级细根数量占到总数的70%—90%。6种树种细根直径,根长,组织密度随序级升高逐渐增大,比根长减小,生物量未表现出一致的变化规律,6种树种生物量主要集中在高级根部分。方差分析表明,树种对细根分支比例有显著影响(P<0.05),浙江桂和米槠细根分支水平对分支比例有极显著影响(P<0.01),其余4种树种分支水平对分支比例有显著影响(P<0.05),树种和分支水平的交互作用对6种树种细根分支比均有极显著的影响(P<0.01);树种对细根根长,直径以及生物量均有极显著影响(P<0.01),对比根长有显著影响(P<0.05),而对组织密度的影响则不显著(P>0.05);树种和序级的交互作用对细根根长,直径以及生物量均有极显著影响(P<0.01),对组织密度有显著影响(P<0.05),对比根长影响不显著(P>0.05)。序级对6种树种细根根长,直径,比根长以及生物量的影响并未达到一致,对6种树种细根组织密度有极显著影响(P<0.01)。树种间1—4级根的比根长变异主要由组织密度引起,而5级根的比根长变异则由直径引起,同时在1级根中组织密度与直径呈现出权衡的关系。6种树种细根数量,直径,根长,比根长,组织密度以及生物量与序级之间回归分析发现它们与序级之间具有指数函数,线性函数,二次函数,三次函数或者幂函数关系。     

Seasonal patterns of fine root demography in a cool-temperate deciduous forest in central Japan

In forest ecosystems, fine roots have a considerable role in carbon cycling. To investigate the seasonal pattern of fine root demography, we observed the fine root production and decomposition processes using a minirhizotron system in a Betula-dominated forest with understory evergreen dwarf bamboo. The length density of fine roots decreased with increasing soil depth. The seasonal patterns of each fine root demographic parameter (length density of visible roots, rates of stand-total fine root production and decomposition) were almost the same at different soil depths. The peak seasons of the fine root demographic parameters were observed in the order: stand-total fine root production rate (late summer) > length density of the visible roots (early autumn) > stand-total fine root decomposition rate (autumn, and a second small peak in spring). The fine root production rate was high in the latter part of the plant growing season. Fine root production peaked in late summer and remained high until the end of the tree defoliation season. The higher stand-total fine root production rate in autumn suggests the effect of understory evergreen bamboo on the stand-total fine root demography. The stand-total fine root decomposition rate was high in late autumn. In the snow-cover period, the rates of both fine root production and decomposition were low. The fine root demographic parameters appeared to show seasonal patterns. The fine root production rate had a clearer seasonality than the fine root decomposition rate. The seasonal pattern of stand-total fine root production rate could be explained by both overstory and understory above-ground productivities.     

Shoot structure and dynamics of saplings and canopies of three deciduous broad-leaved trees of a coppice forest in central Japan

Three deciduous broad-leaved trees, Quercus serrata, Castanea crenata and Carpinus laxiflora, were the main constituents of a coppice forest in central Japan. The shoot elongation and leaf emergence modes of both saplings and the canopy of the three species were investigated. The shoot elongation modes of Q. serrata and C. crenata were the same in saplings and the upper layer of the canopy. The second shoots of these two species were formed after the first shoots were elongated. C. laxiflora was different between saplings and the upper layer of the canopy. In saplings, only the first shoots took a long time to elongate. In the upper canopy layer, higher order shoots were formed in the same way as in the other two species. In the lower layer of the canopy, all three species showed the same shoot elongation mode, in which only the first shoot and its duration of elongation was short. Leaf longevity, individual leaf area, leaf mass per unit leaf area and the stem mass per unit stem length of C. laxiflora were significantly shorter or significantly smaller than those of Q. serrata and C. crenata. The length of the stem per unit leaf area of C. laxiflora was three times that of Q. serrata and five times that of C. crenata. The elongation growth of C. laxiflora was highly efficient as it occurred with a small leaf area. The shoot dynamics and the shoot structure of C. laxiflora are more suitable for elongation growth than in Q. serrata and C. crenata. Furthermore, the shoot structures of the three species were compared and ecological characteristics of the three species are discussed. Received: 29 September 1998 / Accepted: 17 September 1999