pH above 6.0 reduces nodulation in Lupinus species

Lupinus angustifolius and Lupinus pilosus differ substantially in root growth in response to high solution pH with L. angustifolius showing much greater sensitivity to pH above 6. This study examined the effect of pH above 6 on nodulation of these two species in buffered solution. Shoot weight and root weight and length in L. pilosus was not significantly affected by pH, whereas the growth of shoots and roots of L. angustifolius was markedly impaired by increasing pH. Total root length, the number of lateral roots, and the length of individual lateral roots were greatly decreased, resulting in decreased uptake of iron and phosphorus. In addition, L. angustifolius had a higher internal requirement for iron than L. pilosus. A solution pH above 6 decreased the number of nodule initials and nodules similarly in both species but decreased nodule mass much more in L. angustifolius. The effect of high pH on nodule formation occurred prior to that on host shoot growth. High pH also decreased nitrogen concentration and content in both species but to a greater extent for L. angustifolius. The results suggested that pH above 6 has a specific effect in the impairment of nodulation in lupins.     

WATER RELATIONS OF WATER-STRESSED,SPLIT-ROOT C4 (SORGHUM BICOLOR; POACEAE) AND C3 (HELIANTHUS ANNUUS; ASTERACEAE) PLANTS

Sorghum [Sorghum bicolor (L.) Moench] and sunflower (Helianthus annuus L.) were grown in a greenhouse with roots divided between sand irrigated with nutrient solution (–0.097 MPa) or nutrient solution containing polyethylene glycol (PEG) (–0.570 MPa) to compare the effect of unequal root zone stress on plant water relations of a C₄ (sorghum) and a C₃ (sunflower) plant. Roots also were divided between two pots of sand irrigated only with nutrient solution (controls) or only with PEG in nutrient solution. In addition to plant water-status measurements, photosynthetic rate, growth (height, root, and shoot dry weights), and evolution of ethylene (a gaseous hormone indicative of stress) were measured. Under all three split-root treatments, sunflower had a lower leaf water potential and produced more ethylene than sorghum. Sunflower was able to survive the PEG stress if half of its root system was under nonstressed conditions. Sunflower with half its root system irrigated with PEG usually had values of leaf water potential, osmotic potential, stomatal resistance, transpiration rate, photosynthetic rate, ethylene evolution, height, and dry weights that were close to those of the control plants. Sunflower with all roots exposed to PEG was wilted severely. Sorghum was little affected by PEG stress applied either to half or all the root system. Growth of sorghum was the same under all treatments. Apparently because stomata of sorghum were more closed in the partial stress test than those of sunflower, sorghum conserved water and had a higher leaf water potential, which might have permitted growth with stress.     

Root proliferation characteristics of seven perennial arid-land grasses in nutrient-enriched microsites

We compared root proliferation in fertilized microsites among seven cultivars of five commonly planted cool-desert perennial grass species that differ in productivity and competitive ability. In a greenhouse experiment on nutrient-limited plants, one soil microsite in each pot received distilled water (control) and a second microsite received a rich, complete nutrient solution (fertilized). Roots in and adjacent to the microsites were mapped on Mylar windows for 22 days after the injections to determine the magnitude and timing of response in root length relative growth rates (RGRs). Because we provided adequate water, used a high level of fertilization in the treatment microsites, and conducted the experiments during rapid vegetative growth, the results provide a measure of the relative capacities and maximal rates of the grasses responses to enriched microsites. Root samples were harvested from control and fertilized microsites at the end of the experiment to determine the morphological basis of the proliferation response. In all seven grasses fine roots proliferated in the fertilized microsites faster than in the control microsites. The grasses did not differ in the timing of their response which showed a peak 7–8 days after injection. Although one species, Pseudoroegneria spicata cv. Goldar, had higher maximum root length RGR and higher RGR ratio (RGR in fertilized to RGR in control microsites) 7–8 days after injection, the seven grasses did not differ significantly in the magnitude of root length RGR response to fertilizer integrated over the 22 day experiment. The grasses also did not differ significantly in root morphological changes in fertilized mocrosites. Compared to roots in control microsites, roots in fertilized microsites had greater specific root length, length of secondary roots per length of main axis, number of lateral and sublateral roots per length of main axis, and mean lateral root length. Root proliferation was mainly the result of increased lateral branching and lateral root growth in all seven grasses. The consistency of root proliferation responses among these seven cultivars suggests that differences in the capacity for, maximum rate, or morphological basis of root proliferation are not directly related to ecological characteristics such as productivity and competitive ability. Other aspects of root response to nutrient enrichment, such as differential responses as a function of microsite nutrient concentration, plant phenology, plant nutrient status, or specific nutrient element(s), may still be important, but further experiments are required to determine whether different responses to enriched soil microsites among species correspond with know species differences in ecological characteristics.     

Nutrient and water availability alter belowground patterns of biomass allocation, carbon partitioning, and ectomycorrhizal abundance in Betula nigra

In managed settings, seedlings are often fertilized with the objective of enhancing establishment, growth, and survival. However, responses of seedlings to fertilization can increase their susceptibility to abiotic stresses such as drought. Seedlings acclimate to variation in soil resources by reallocating carbon among different physiological processes and compartments, such as above versus belowground growth, secondary metabolism, and support of ectomycorrhizal fungi (EMF). We examined the effects of nutrient and water availability on carbon allocation to above and belowground growth of river birch (Betula nigra), as well as partitioning among root sugars, starch, phenolics, lignin, and EMF abundance. As nutrient availability increased, total plant biomass and total leaf area increased, while percent root biomass decreased. Root sugars, total root phenolics and EMF abundance responded quadratically to nutrient availability, being lowest at intermediate fertility levels. Decreased water availability reduced total leaf area and root phenolics relative to well-watered controls. No interactions between nutrient and water availability treatments were detected, which may have been due to the moderate degree of drought stress imposed in the low water treatment. Our results indicate that nutrient and water availability significantly alter patterns of carbon allocation and partitioning in roots of Betula nigra seedlings. The potential effects of these responses on stress tolerance are discussed.     

光强对木麻黄幼苗根系形态、解剖结构及其碳氮含量的影响

对光环境的灵敏响应使得森林中常见的光照异质性成为影响植物自我更新的关键因素,然而植物地下根系结构对光照的响应较为难测而缺乏深入研究。为探究不同光强下木麻黄根系响应策略,以一年生木麻黄(Casuarina equisetifolia)幼苗为试验材料,模拟森林幼苗生长的林外(CK)、林缘(L1)、林窗(L2)和林下光环境(L3)设置4种光照强度,测定及分析木麻黄幼苗的生长、根系形态、细根解剖结构及碳氮含量等指标。结果表明:(1) L1下,幼苗采取维持高度,降低横向生长的方式,保证正常累积生物量,随光照强度的下降,株高、地径、叶片生物量及地上部分生物量逐渐下降。(2)在根系表型上,幼苗随光限制的加重呈现抑制纵伸但促进根系的横向生长,其中总根长、根平均直径及根体积达到显著差异。在径级结构上,细根发育程度随光照减弱而下降;而适当的遮光(L1)促进粗根生长但L3时除根尖数较CK上升外,根长度、根表面积、根体积均显著下降。(3)1-3级细根解剖变化较大,相较CK,1级细根皮层细胞面积显著增加,但根半径、维管柱结构、表皮厚度等指标则显著下降,2级细根根半径、皮层细胞面积、表皮厚度明显减少,但维管柱结构仅在L2、L3时显著下降;3级细根根半径、皮层细胞面积和维管柱面积均较CK显著增大,L1时维管柱结构下降,但随光照减弱加重,维管柱面积和中柱占比均明显增加。(4)在碳氮含量上,CK与L1无显著差异,TC在L2时显著下降,TN则在L2时显著上升,TC、TN均在L3达到最大,而C∶N随光强降低逐渐下降。综上所述,光限制时,木麻黄生物量及碳分配稳定根茎部分生长,采取“弱化吸收,强化储存”收缩型生长策略;当限制加重时,光合和呼吸作用失衡导致植物对细根投入养分的浪费,并最终造成林木死亡。研究结果为林下植被的更新提供理论参考。     

Interactions Between High pH and Iron Supply on Nodulation and Iron Nutrition of Lupinus albus L. Genotypes Differing in Sensitivity to Iron Deficiency

Poor growth of white lupin (Lupinus albus L.) on alkaline soils may result from its sensitivity to iron deficiency and poor nodulation. This study examined interactive effects of iron supply and high pH on the growth and nodulation of three genotypes differing in their sensitivity to iron deficiency. Three genotypes (P27486, Ultra and WTD180) were grown for 17 days in buffered solutions with Fe supply of 0.2, 2 and 20 μM. Solution pH was adjusted to 5.2, 6.5 or 7.5. Plant growth, nodulation and nutrient concentrations in plants were measured. Decreasing Fe supply decreased chlorophyll concentration in young leaves by up to 92%. Increasing pH decreased chlorophyll concentration by an average of 40% at pH 6.5 and by 47% at pH 7.5. The decrease of chlorophyll was less obvious in P27485 than in Ultra or WTD180. Shoot biomass was reduced by up to 18% by Fe deficiency, with such decrease being less for P27486. Increasing pH exacerbated the effect of Fe deficiency on shoot biomass only of Ultra. Decreasing Fe supply decreased nodule number by an average of 54%, and increasing pH decreased nodule number by 80%. P27486 formed the greatest number of nodules while WTD180 the least. P27486 had high Fe uptake and low internal requirement. Irrespective of genotype, leaf chlorosis positively correlated with cluster root formation. The results suggest that a combination of Fe deficiency and high pH impaired nodulation in L. albus, and that selection of genotypes for both tolerance of iron deficiency and good nodulation at high pH is important for a successful lupin crop on alkaline soils.     

The formation,morphology and anatomy of cluster root of Lupinus albus L. as dependent on soil type and phosphorus supply

The effect of phosphorus supply on the formation, morphology and anatomy of cluster roots of Lupinus albus L. cv Ultra grown in a loam and two sandy soils was examined relative to its effect on total root length, shoot weight and the phosphorus concentration of the shoots. The loam soil was most conducive to the formation of cluster roots. Cluster roots growing in the sandy soils developed to a lesser extent on plants of an equivalent phosphorus status, suggesting that some biotic or abiotic factors independent of phosphorus supply were also operating. The presence of mature cluster rootlets on a length of lateral root increased the root surface area by 14–22 times of an equal length of lateral roots not bearing cluster rootlets. The application of phosphorus decreased cluster-root length, whereas total root length showed a steady increase. There was an inverse relationship between cluster-root production and phosphorus concentration in shoots ranging from 2 to 8.5 mg g^–1 with the critical phosphorus level for maximum shoot growth being around 2.5 mg g^–1. Cluster roots formed in solution culture were not well developed in comparison with those grown in the loam soil or nutrient solution with added loam soil. The organisation of the cluster rootlet was similar to that of the lateral roots. Mature rootlets lacked an apical meristem and a vascular cambium with a reduced root cap and cortical tissue.     

The Influence of Urea and Other Nitrogen Sources on Growth Rate of Scots Pine Seedlings

The growth rate and water content of urea-fed seedlings of Pinus silvestris L. were compared with those of nitrate-and ammonium-fed seedlings grown in continuously renewed nutrient solutions, in which the hydrolysis of urea to ammonia and carbon dioxide was minimized. The growth rate of seedlings grown in an ammonium nutrient solution, in an urea nutrient solution and in a nitrate nutrient solution was about 90 per cent, 75 per cent and 60 per cent, respectively, of that of seedlings grown in a mixture of ammonium and nitrate. Seedlings with urea as the sole nitrogen source developed very severe chlorosis of the needles, the old roots were dark-coloured, the whole root system was very fragile, and the lateral roots of the third order were missing. Urea-grown seedlings had the highest nitrogen contents, closely followed by the ammonium and the ammonium + nitrate seedlings. The lowest nitrogen level was in nitrate seedlings. The low growth rate and the chlorosis of urea-fed seedlings were suggested to be the result of a hydrolysis of urea inside the root, causing an increase in pH and an accumulation of ammonia in the root.     

Growth, water relations and photosynthesis of seedlings and resprouts after fire

Seasonal patterns of growth, water relations, photosynthesis and leaf characteristics were compared between obligate seeders (Cistus monspeliensis and Cistus ladanifer) and resprouters (Arbutus unedo and Pistacia lentiscus) from the first to the second year after fire. We hypothesized that seedlings would be more water-limited than resprouts due to their shallower root systems. Regarding water use strategies, Cistus species are drought semi-deciduous and A. unedo and P. lentiscus are evergreen sclerophylls, therefore, comparisons were based on the relative deviation from mature conspecific plants. Seedlings and resprouts had higher shoot elongation and leaf production than mature plants, and over an extended period. Differences from mature plants were larger in resprouts, with two-fold transpiration, leaf conductance and photosynthesis in late spring/early summer. Seedlings of C. monspeliensis exhibited higher transpiration and leaf conductance than mature plants, while those of C. ladanifer only exhibited higher water potential. Growth increments and ameliorated water relations and photosynthesis after fire were attributed to an increase in water and nutrient availability. The small differences in water relations and photosynthesis between seedlings and mature conspecifics are in accordance with the prediction of seedlings experiencing higher water limitation than resprouts. We attribute these results to differences in root systems: resprouters benefited from an increase in root/shoot ratios and the presence of deep roots whereas Cistus seedlings relied on very shallow roots, which cannot provide assess to deep water during summer. Nevertheless, seedlings did not show evidence of experiencing a more severe water limitation than mature conspecifics, which we attributed to the presence of efficient mechanisms of avoiding and tolerating water stress. The results are discussed in relation to post-fire demography of seeders and resprouters in Mediterranean communities.     

Development correlations between roots in heterogeneous environments

Roots are known to respond to favourable nutrient conditions by increased initiation and growth of lateral roots. The problem studied here was to what extent does this local developmental response depend on the environments of other roots on the same plant. Such dependence could allow for an optimal allocation of resources required for root growth in unpredictable, heterogeneous soils. Pea seedlings (Pisum sativum var. arvense cv. Dun) were pruned and grown to have two equal root systems, each in an individual container. As expected, these roots responded by increased development to a wide range of nutrient solution concentrations. The local development of these roots, expressed by their dry weight, was a function of the relative rather than the absolute conditions in which they were grown: roots in a given environment developed more rapidly if other roots on the same plant were in poorer than if they were in richer nutrient conditions. The number of lateral initials doubled within 3d after the roots were exposed to optimal nutrient conditions, before any dry weight differences could be detected. This rapid root initiation was also a function of the conditions other roots of the same plant were in. These results mean that root development, and especially lateral root initiation, depends on the integrated effects of the local environment and the internal correlative relations between the roots.     

Architectural Tradeoffs between Adventitious and Basal Roots for Phosphorus Acquisition

Adventitious rooting contributes to efficient phosphorus acquisition by enhancing topsoil foraging. However, metabolic investment in adventitious roots may retard the development of other root classes such as basal roots, which are also important for phosphorus acquisition. In this study we quantitatively assessed the potential effects of adventitious rooting on basal root growth and whole plant phosphorus acquisition in young bean plants. The geometric simulation model SimRoot was used to dynamically model root systems with varying architecture and C availability growing for 21 days at 3 planting depths in 3 soil types with contrasting nutrient mobility. Simulated root architectures, tradeoffs between adventitious and basal root growth, and phosphorus acquisition were validated with empirical measurements. Phosphorus acquisition and phosphorus acquisition efficiency (defined as mol phosphorus acquired per mol C allocated to roots) were estimated for plants growing in soil in which phosphorus availability was uniform with depth or was greatest in the topsoil, as occurs in most natural soils. Phosphorus acquisition and acquisition efficiency increased with increasing allocation to adventitious roots in stratified soil, due to increased phosphorus depletion of surface soil. In uniform soil, increased adventitious rooting decreased phosphorus acquisition by reducing the growth of lateral roots arising from the tap root and basal roots. The benefit of adventitious roots for phosphorus acquisition was dependent on the specific respiration rate of adventitious roots as well as on whether overall C allocation to root growth was increased, as occurs in plants under phosphorus stress, or was lower, as observed in unstressed plants. In stratified soil, adventitious rooting reduced the growth of tap and basal lateral roots, yet phosphorus acquisition increased by up to 10% when total C allocation to roots was high and adventitious root respiration was similar to that in basal roots. With C allocation to roots decreased by 38%, adventitious roots still increased phosphorus acquisition by 5%. Allocation to adventitious roots enhanced phosphorus acquisition and efficiency as long as the specific respiration of adventitious roots was similar to that of basal roots and less than twice that of tap roots. When adventitious roots were assigned greater specific respiration rates, increased adventitious rooting reduced phosphorus acquisition and efficiency by diverting carbohydrate from other root types. Varying the phosphorus diffusion coefficient to reflect varying mobilities in different soil types had little effect on the value of adventitious rooting for phosphorus acquisition. Adventitious roots benefited plants regardless of basal root growth angle. Seed planting depth only affected phosphorus uptake and efficiency when seed was planted below the high phosphorus surface stratum. Our results confirm the importance of root respiration in nutrient foraging strategies, and demonstrate functional tradeoffs among distinct components of the root system. These results will be useful in developing ideotypes for more nutrient efficient crops.     

Water relations of rough lemon (Citrus jambhiri Lush.) citrus seedlings infected withFusarium solani

Summary Rough lemon citrus seedlings were inoculated withFusarium solani and evaluated for changes in water relations of leaves, stems, and roots. Inoculated seedlings had decreased leaf stomatal conductance, lower leaf water potential, lower water content, and higher leaf osmotic values compared to healthy plants. Visible wilt symptoms occurred as early as 24 h after inoculation. Transpiration and root conductivity were lower in diseased plants but stem conductivity in diseased plants did not differ from the control. Thus, wilting appears to be due to the inability of roots to supply water to the leaves.     

Does high bicarbonate supply to roots change availability of iron in the leaf apoplast?

The role of the leaf apoplast in iron (Fe) uptake into the leaf symplast is insufficiently understood, particularly in relation to the supposed inactivation of Fe in leaves caused by elevated bicarbonate in calcareous soils. It has been supposed that high bicarbonate supply to roots increases the pH of the leaf apoplast which decreases the physiological availability of Fe in leaf tissues. The study reported here has been carried out with sunflower plants grown in nutrient solution and with grapevine plants grown on calcareous soil under field conditions. The data obtained clearly show that the pH of the leaf apoplastic fluid was not affected by high bicarbonate supply in the root medium (nutrient solution and field experiments). The concentrations of total, symplastic and apoplastic Fe were decreased in chlorotic leaves of both sunflower (nutrient solution experiment) and grapevine plants in which leaf expansion was slightly inhibited (field experiment). However, in grapevine showing severe inhibition of leaf growth, total Fe concentration in chlorotic leaves was the same or even higher than in green ones, indicative to the so-called `chlorosis paradox'. The findings do not support the hypothesis of Fe inactivation in the leaf apoplast as the cause of Fe deficiency chlorosis since no increase was found in the relative amount of apoplastic Fe (% of total leaf Fe) either in the leaves of sunflower or grapevine plants. It is concluded that high bicarbonate concentration in the soil solution does not decrease Fe availability in the leaf apoplast.     

不同培育方式对橡胶树实生苗生理指标、养分及长势的影响

橡胶树(Heveabrasiliensis)种子催芽生长一般使用沙床培育,沙子是不可再生资源,为了选择一种适合橡胶树种子培育方式来替代对沙子的依赖,该研究通过水培、悬空培育和传统的沙培比较橡胶树实生苗第1蓬叶稳定时,苗木的生长势、生理指标及养分含量。结果表明,水培实生苗地上部株高、茎粗、叶面积的长势最佳,壮苗指数和生物量的含量最高,但其根太长,根相对较细。水培的叶、茎、根的可溶性糖、丙二醛、游离脯氨酸、超氧化物歧化酶的含量均较低;水培和悬空培育的叶片和茎的叶绿素、类胡萝卜素及根系活力的含量没有显著性差异,均高于沙培。水培的叶、茎、根中的氮和磷含量最低,沙培的最高;而水培实生苗根和茎中钾的含量较高,叶片中含量与悬空培育、沙培均没有显著性差异;悬空培育在叶、茎、根中钾的含量最低。水培促进了苗木的生长,降低干旱胁迫,提高养分利用率,但后续还需调控根系,建设良好根团。悬空培育的苗木长势较弱,还需进一步完善方法。     

干旱胁迫对欧李幼苗表型可塑性的影响

该研究以欧李为材料,探讨了干旱胁迫对欧李表型可塑性的影响。结果表明:(1)随着干旱胁迫的加剧,欧李根生物量、枝叶生物量、植株总生物量积累、根冠比和根冠比胁迫指数均呈现先升高后降低的趋势,在T1处理下达到最大值,并显著高于其他处理(α=0.05)。(2)随土壤含水量的降低,欧李根的生物量分配指数呈现先增加后降低的趋势,叶生物量与之相反,在T1处理下根的生物量分配指数最大,枝叶的最小(α=0.05)。(3)在水分供应为60%～80%时,欧李的株高、冠幅、基径、二级分枝数、主根长、主根直径及侧根数量均达到最大值(α=0.05),对一级分枝数的生长没有显著影响。(4)随着水分胁迫的加剧,叶片长从T2处理开始下降,叶片宽、单片叶面积及比叶面积均呈现先增加后减少的趋势(α=0.05)。综上可得,欧李通过调整形态特性和各器官生物量积累及其分配对不同干旱胁迫条件产生了较强的可塑性。     

Lateral root formation in pine seedlings

The role of assimilates in lateral root development was studied in Pinus pinea seedlings grown in a nutrient solution. Seedlings were treated with ¹⁴CO₂ for 2 h following removal of the tap root tip at various times prior to the application of ¹⁴CO₂ or removal of a different number of cotyledons at one time. In seedlings with intact root systems most of the radioactivity accumulated in the lower section of the root containing the tap root apex. When the tap root tip was removed, the pattern of radioactivity accumulation along the root was affected by the presence and the stage of lateral root development. Removing the tap root tip of young seedlings (with no lateral roots) resulted in an almost equal distribution of radioactivity along the root. About 50% of the total radioactivity was found in the section showing the highest lateral root growth. Removing the tap root tip of mature seedlings (with lateral roots in the upper section) resulted in an immediate increase in the radioactivity accumulation in the upper section. When lateral roots appeared in the middle section, the pattern of radioactivity distribution was similar to that found in root decapitated young seedlings. Removal of cotyledons of mature seedlings somewhat increased the transport of radioactivity to the lower root section at the expense of the radioactivity in the lateral roots of the upper section. The present study suggests that competition within the root system between the tap root apex and the lateral roots may play an important role in determining the morphology of the root system.     

Increased nutrient supply facilitates acclimation to high-water level in the marsh plant Deyeuxia angustifolia: The response of root morphology

Both water level and nutrient availability are important factors influencing the growth of wetland plants. Increased nutrient supply might counteract the negative effects of flooding on the growth of the fast-growing species. Experimental evidence is scarce and the mechanism is far from clear. The aim of this study is to identify the role of nutrient availability in acclimation to high-water level by investigating the growth and root morphology of the marsh plant Deyeuxia angustifolia, one of the dominant species in the Sanjiang Plain, China. Experimental treatments included two water levels (0 and 10 cm, relative to soil surface) and three levels of nutrient supply (0, 0.5 and 1 g fertilizer per container). High-water level usually led to decreased biomass accumulation, shoot mass and root mass, whereas biomass accumulation was unaffected by water level at the highest nutrient level, indicating that high-nutrient availability played a role in compensating for the growth loss induced by the high-water level. Increased nutrient supply led to decreased root length in 0 cm water-level treatments, but root length increased with nutrient supply in the 10 cm water-level treatments. High-water level usually led to a lower lateral root density, lateral root:main root length ratio and the diameter of main roots and laterals, whereas increased nutrient supply resulted in thicker main roots or laterals, and a higher total root length, lateral root density and lateral root:main root length ratio. These data indicate that the growth of D. angustifolia is restrained by high-water level, and that increased nutrient supply not only ameliorates root characteristics to acclimate to high-water level but also results in a high-total root length to facilitate nutrient acquisition.     

Factors affecting formation of cluster roots in Myrica gale seedlings in water culture

The effect of nutrient deficiency, aeration, phosphorus supply, and nitrogen source on the formation of cluster (proteoid) roots was examined in Myrica gale seedlings growing in water culture. Only the omission of phosphorus resulted in the formation of significant numbers to cluster roots when plants were grown in a number of 1/4 strength Hoagland's solutions, each lacking one mineral nutrient. Aeration shortened the time required for cluster root formation and increased the percentage of plants forming cluster roots. The proportion of the root system comprised of cluster roots decreased as the phosphorus concentration in the solution increased and no cluster roots formed in solutions containing 8 mg P/L. Phosphorus supply also affected total plant biomass, proportion of biomass comprising nitrogen-fixing nodules, shoot:root ratio, phosphorus concentration in the leaves and phosphorus content of the plants. The plants showed luxury consumption of phosphorus and were able to produce large amounts of biomass utilizing only stored phosphorus.Nitrogen source also affected cluster root formation. Urea-fed plants produced cluster roots more quickly and devoted a substantially larger proportion of root growth to cluster roots than did nitrate-fed plants. The longest cluster root axes were produced in nitrate-fed plants supplied with no phosphorus and the shortest were in urea-fed plants at 4 mg P L^–1.Four methods for expressing the extent of cluster root formation were examined and it was concluded that cluster roots as a proportion of total fine root dry weight is preferable in many cases. Formation of cluster roots in response to phosphorus deficiency coupled with previously demonstrated traits allows Myrica gale to adapt to a wide range of soil conditions.     

The effect of carbohydrate on the development of a Cattleya hybrid in association with its mycorrhizal fungus

To investigate beneficial effects of mycorrhizal fungi to advanced leafy orchids, growth studies on the development of symbiotic seedlings of the orchid Cattleya (aclandiae x schoeffeldiana) x aclandiae were conducted in vitro over a period of 18 months using split plates with minerals and carbohydrates on one side and water agar on the other. Mycorrhizal infection and shoot and root growth of seedlings on the nutrient side were compared to growth on the water agar side with nutrient uptake by the orchid only possible via external mycorrhizal hyphae. Seed germination was followed by mycorrhizal infection and rapid development of protocorms on both nutrient and non-nutrient sides of the plates. With 0.5% starch, development of protocorms was sustained for a least 12 weeks, compared to only 6 weeks with 0.1% starch. Advanced protocorms with two small leaves and a smoll root were transferred at week 22 to new fungal plates. When harvested at week 43, plantlets on 0.5% starch (both nutrient and water agar sides) had 2.7 times the dry weight of plantlets on 0.1% starch. Shoot-root ratios were higher on the lower level of carbon. In all plantlets, mycorrhizal infection involved less than 5% of the root length. With zero, 0.1% or 0.5% starch, the roots were re-infected on transfer to fresh fungal plates but young roots that developed following the transfer stayed free of infection, Plantlets on 0.5% starch (nutrient and water agar side) after 18 months had longer roots than plantlets grown in the absence of starch or on 0.1% starch. Shoots were small but significantly larger on the nutrient side than on the water agar side, independent of the carbohydrate level. The shoot-root ratio was highest on the nutrient side with no starch present. In this latter case, plantlet development was steady but plantlets on the non-nutrient side developed slowly; thus there was little evidence of nutrient translocation by the mycorrhizal fungus from the nutrient to the non-nutrient side in the absence of carbohydrates. Mycorrhizal infection is discussed as a mechanism for heterotrophic carbon assimilation. In advanced leafy orchids of Cattleya, external carbon resulted in increased root growth, decreased shoot/root ratio and sometimes yellowish-green plantlets.     

闽西南红壤侵蚀区黑莎草功能性状特征及其与土壤因子的关系

为了解黑莎草(Gahnia tristis)在南方红壤侵蚀区的适应状况,测定了长汀县红壤侵蚀区的黑莎草叶片、根系的功能性状及土壤理化性质,并应用数理统计方法分析了黑莎草叶片与根系功能性状之间的相关性,及其对土壤因子的响应。结果表明,黑莎草叶片表型性状在冬夏季间存在显著差异,叶长、叶宽、叶组织密度和叶绿素表现为夏季显著大于冬季,根系表型性状则更具稳定性,冬季的根系养分含量均高于夏季,养分的分配上叶片养分高于根系养分。叶组织密度与叶绿素含量呈显著正相关,与比叶面积呈显著负相关;根组织密度与比根长和比根面积均呈显著负相关,叶片和根系养分间均呈显著正相关,土壤碳、氮、磷含量是影响黑莎草功能性状主要因子。因此,黑莎草可通过调节功能性状以适应环境变化,可作为地带性植物应用于南方红壤侵蚀区的植被恢复和水土流失治理。