Some effects of Plasmodiophora brassicae Woron. on the growth of the young cabbage plant

When clubroot galls developed on cabbage inoculated with Plasmodiophorabrassicae Woron., the distribution of dry matter in the plantwas altered. As soon as clubbed roots were visible the stem:root ratio diminished with time until the clubs rotted. Afterclubs appeared the increase in root weight was nearly all fromgrowth of the clubroot gall and the rate of growth of tops correspondinglydiminished. Succulence of the shoot, water content per unitdry weight, was unaffected. Increase in total leaf area closely paralleled that of totaldry weight. It was slower in diseased plants and attained alower limit than in healthy ones. From the 35 th day after inoculation,onwards, diseased plants had fewer and smaller leaves and theirleaves opened more slowly, one new leaf unfolding every 9 dayson diseased plants against one every 4 days on healthy plants.Leaves of diseased plants were both smaller and thinner thanthose in corresponding positions on healthy plants. The relationship between the absorbing system on the one hand,and the assimilating and transpiring system on the other, wasgreatly altered; the dry weight of fibrous root per unit leafarea was decreased after infection from 1.20 to 0.79 mg./cm.²20 days after inoculation and from 1.64 to 0.38 mg./cm.² 82days after inoculation. Though infection consistently lowered net assimilation rate,on the average by about 15 per cent, of the control value, thisdecrease was never statistically significant. Increases in the number of spores in the inoculum progressivelydecreased the dry weights of tops and fibrous roots, and sometimesincreased the dry weight of clubs. Delaying inoculation increased the initial growth rate of thegalls, and sometimes their final size. The extent to which plantswere damaged depended more on how long they had been infectedthan on age at inoculation.     

Effect of Moisture Supply, Root Temperature, and Growth Regulators on Photosynthesis of Isolated Rooted Leaves of Sweet Potato (Ipomoea batatas)

Isolated rooted sweet potato leaves were used to study the effectof carbohydrate use and storage on photosynthesis. Tuberingof the roots was controlled (1) by varying the moisture aroundthe roots, (2) by varying the root temperature, or (3) by treatingthe leaves with growth regulators. When tubering was greatestthe total dry matter formed per unit area of leaf was also greatest.Benzyl adenine applied to the lamina increased the proportionof total dry matter in the tubers. The experiments show that increasing tuber growth increasesnet assimilation rate, supporting the view that rate of photosynthesisdepends on the capacity of sinks to accept photosynthate.     

Photosynthesis of Ears and Flag Leaves of Wheat and Barley

Immediately after anthesis ears of spring wheat absorbed lessthan 0.5 mg CO₂, per hour in daylight and later evolved CO₂,in the light and in the dark. The rate of apparent photosynthesisof the combined flag-leaf lamina and sheath and peduncle (collectivelycalled flag leaf) of two spring wheat varieties, Atle and JufyI, was 3–4 mg per hour; the rates of the flag leaf andthe ear of two spring barleys, Plumage Archer and Proctor, wereeach about 1 mg per hour. The gas exchange of ears and flag leaves between ear emergenceand maturity accounted for most of the final grain dry weight.The CO₂, fixed by the wheat ear was equivalent to between 17and 30 per cent of the grain weight, but more than this waslost by respiration, so assimilation in the flag leaf was equivalentto 110–20 per cent of the final grain weight. In barley,photosynthesis in the flag leaf and the net CO₂ uptake by theear each provided about half of the carbohydrate in the grain. Barley ears photosynthesized more than wheat ears because oftheir greater surface, and flag leaves of wheat photosynthesizedmore than those of barley because they had more surface anda slightly greater rate of photosynthesis per dm².     

Root Development and Source-Sink Relations in Carrot, Daucus carota L: II. EFFECTS OF ROOT PRUNING ON CARBON ASSIMILATION AND THE PARTITIONING OF ASSIMILATES

Physiological responses to root pruning were investigated bycomparing ¹⁴CO₂ fixation rates, the partitioning of ¹⁴C-labelledassimilate, and soluble and insoluble carbohydrate levels inthe leaves of carrot plants following the removal of some ofthe fibrous roots, or fibrous roots and part of the tap root.Root pruning reduced ¹⁴CO₂ fixation by 28–45% but leafspecific activity (¹⁴C assimilation g-¹ leaf fresh weight) wasunchanged. The proportion of total assimilate exported to theroot system increased following root pruning and this was atthe expense of the developing leaves. In younger plants (wherethe tap root received 10% of the assimilate) the supply of ¹⁴Cto the tap root was maintained in spite of root pruning. However,shortening the tap root to 3 cm in older plants (in which 30%of the fixed 14C was normally exported to the developing storageorgan), reduced its sink capacity and resulted in slightly greaterretention of ¹⁴C in the mature leaves. Greater concentrationsof insoluble carbohydrate were found in the mature leaves followingroot pruning but soluble sugar content was unaffected. Onlysmall differences were observed in the distribution of ¹⁴C betweensoluble and insoluble carbohydrate fractions when plants werefed ¹⁴CO₂ several days after the root pruning operations. Thesephysiological responses were mainly associated with the removalof fibrous roots and support the view that the fibrous rootsystem is more important than the developing storage organ inregulating growth in young carrot plants.     

The Effect of Different Root Temperatures on Dry Matter and Carbohydrate Changes in Rooted Leaves of Phaseolus Spp.

Weekly estimates of dry matter in rooted dwarf-bean leaves (Phaseolusvulgaris) and rooted runner-bean leaves (Phaseolus multiflorus)were made with roots held at 13°, 17°, 21°, or 25°C. Most dry matter accumulated in the lamina of both speciesat the coldest root temperature. Roots of runner bean grew morethan dwarf bean at all temperatures compared, especially at13° and 17° C and the greater net assimilation rate,i.e. dry matter increase per dm1 per week, of runner-bean leavesat cold temperatures, is attributed to faster root growth. Shading lessened accumulation in the lamina, especially at coldroot temperatures, because a greater proportion of assimilatewas transferred to roots. Rates of root growth are mainly influenced by temperature, andno effect could be detected of carbohydrate accumulation inthe lamina at cold temperatures on root growth after the rootswere kept warmer. Water content of the lamina per unit areaincreased as dry matter increased. For any particular dry-mattercontent, the water content was greater with roots at 20°than at 13°, i.e. the leaves were more succulent with warmerroots. When roots first formed on the petioles the lamina lost sugarand subsequently regained it, faster when roots were cold. Whentransferred from warm to cold root temperature, sugars accumulatedin the lamina. Starch changed more than sugars in response tochanging root temperatures. Starch in the lamina progressivelydeclined when roots were warm, but increased when they werecold. The starch content of the lamina was rapidly influencedby root temperature. Sugars and starch together account forless than half of the dry-matter increase in the lamina. Theunidentified fraction is little influenced by changes of roottemperature; it may be largely structural because the palisadecells continued to grow after the leaves were excised, althoughthe lamina was fully expanded before the leaves were rooted.Palisade cells were larger with roots at 20° than at 13°C.     

Estimating the Proportion of Degradable Dry Weight of Kikuyu from Rates of Lamina Senescence

The proportion of degradable dry weight in lamina of kikuyu,was estimated from an analysis of the rate of lamina senescencein dense swards when senescence was initiated by shading. Itwas assumed that laminae in the dark remained green until allenergy reserves, including degradable structural dry weight,were exhausted and then laminae died. The specific maintenancerespiration rate defined the rate of energy use. The estimatedproportion of degradable dry weight increased from 0.10 at 15°C to 0.18 at 30 °C, with corresponding specific maintenancerespiration rates of 10.9 and 37.0 mg d. wt (g d. wt)^–1d^–1 respectively. Similar results were obtained when maintenancerespiration was expressed per unit weight of protein. Degradable dry weight, senescence, respiration, kikuyu, temperature, Pennisetum     

Photosynthesis of Lamina and Sheath of Barley Leaves

Apparent photosynthesis, in mg. CO₂ absorbed per dm.² per hour,of the sheath and enclosed stem of a barley leaf was about 50per cent. of that of the lamina of the same leaf, when the photosynthesizingarea was measured as one side of the lamina and the outer exposedsurface of the sheath. Apparent photosynthesis of a particularlamina or sheath was about 70 per cent. of that of the one aboveon the same stem. Respiration per dm.², though not per g. dry weight, of sheathwith enclosed stem was greater than of lamina in one experimentdone with low-intensity illumination so that true rates of photosynthesisof lamina and sheath were similar. Differences in respirationrates per unit area of laminae and sheaths probably accountedfor most of the greater apparent photosynthesis of the formerin other experiments done with higher intensity illumination. It is suggested that for growth-analysis studies the size ofthe photosynthetic system of cereals should be measured as thatof one side of the leaf laminae plus the outer surface of thecombined leaf sheaths. In the later stages of growth the surfacearea of exposed stem and peduncle should also be included.     

Effects of Edaphic Factors on the Soluble Carbohydrate Content of Roots of Lolium perenneL. and Trifolium repens L.

In a 3³x2² factorial experiment with Lolium perenne L. and Trifoliumrepens L. grown at three soil moisture tensions, three levelsof nitrogen and three levels of phosphorus, there was a widerange of percentage soluble carbohydrate (TSC) in the roots.The significant positive correlations between percentage ofTSC and root weight suggest that root growth-rate was proportionalto the concentration of TSC in the roots. In other experiments with Lolium perenne grown at five constantsoil temperatures and factorial combinations with three soilmoisture treatments with and without manure, there was declinein the percentage of TSC with increasing temperature. This impliedthat the respiration sink strength in the roots did not controlthe partitioning of photosynthate to the roots. No firm conclusions can be drawn from this work, but it is inferredthat there is a dynamic balancing mechanism in the foliage whichcontrols the partitioning of photosynthate between foliage androot growth. This mechanism appears to divert carbohydrate toroots in inverse proportion to root activity.     

Studies on the Expansion of the Leaf Surface: II. THE INFLUENCE OF LIGHT INTENSITY AND DAYLENGTH

The parts played by constant amounts of visible radiation perday and its two components—daylength and intensity—ininfluencing the growth of Cucumis sativus have been investigated.The amount of radiation per day had a far greater influencethan either of its components per se. Nevertheless, small significanteffects of photoperiod were found, leaf expansion and dry weightincrease being greatest at daylengths between 10 and 15 hr.rather than with longer days which, with similar daily totals,would be expected to give the greatest amounts of assimilation. Rates of leaf production and appearance were greatest with thehighest amounts of radiation, but the rates of expansion ofindividual leaves and their maximum areas were greatest withintermediate amounts of radiation. This response resulted inan optimum curve relating the leaf surface and the dry weightattained after a given period to radiation. The amount of radiationgiving the maximum leaf surface and dry weight decreased withage and with external nutrient supply, but at any one age washigher for increase in dry weight than for increase in leafsurface; stem and root tissues responded more to high radiationthan did the leaf surface. The net assimilation rate was a linearfunction of visible radiation over the range of 15–120cal. cm.^-2 day^-1 explored, the highest value of radiation usedrepresenting the intensity at which photosynthesis would beexpected to be maximal over a 12–15 hr. day. The inhibitory effect of high radiation on leaf expansion andthe resultant influence on the growth of the plant are explainedin terms of the number and intensity of ‘sinks’for carbohydrate and mineral nutrients within the plant.     

Comparisons of the Respiratory Effluxes of Nodules and Roots in Six Temperate Legumes

The specific respiration rates of nodulated root systems, ofnodules and of roots were determined during active nitrogenfixation in soya bean, navy bean, pea, lucerne, red clover andwhite clover, by measurements on whole plants before and afterthe removal of nodule populations. Similar measurements weremade on comparable populations of the six legumes, lacking nodulesbut receiving abundant nitrate-nitrogen, to determine the specificrespiration of their roots. All plants were grown in a controlled-environmentclimate which fostered rapid growth. The specific respiration rates of nodulated root systems ofthe three grain and three forage legumes during a 7–14-dayperiod of vegetative growth varied between 10 and 17 mg CO₂g^–1 (dry weight) h^–1. This mean value consistedof two components: a specific root respiration rate of 6–9mg CO₂ g^–1 h^–1 and a specific nodule respirationrate of 22–46 mg CO₂ g^–1 h^–1. Nodule respirationaccounted for 42–70 per cent of nodulated root respiration;nodule weight accounted for 12–40 per cent of nodulatedroot weight. The specific respiration rates of roots lackingnodules and utilizing nitrate nitrogen were generally 20–30per cent greater than the equivalent rates of roots from nodulatedplants. The measured respiratory effluxes are discussed in thecontext of nitrogen nitrogen fixation, nitrate assimilation. Glycine max, Phaseolus vulgaris, Pisum sativum, Medicago sativa, Trifolium pratense, Trifolium repens, soya bean, navy bean, pea, lucerne, red clover, white clover, nodule respiration, root respiration, fixation, nitrate assimilation     

Dinitrocresol, Cyanide, and the Pasteur Effect in Yeast

At pH 5·0 the respiration of yeast is stimulated by lowconcentrations of 3:5-dinitro-o-cresol, reaching a peak levelof 170 per cent, at 10⁵ M. Concentrations above this inhibitoxygen uptake and cause aerobic fermentation to appear, whichin turn reaches a peak value and is then inhibited. The rateof carbohydrate breakdown, or glycolysis, calculated from therates of respiration and aerobic fermentation, increases steadilyup to 3 x 10^–5 M., at which concentration it is 5 timesfaster than the control: higher concentrations depress the rateof glycolysis. The rate of fermentation under nitrogen is abouttwice that of respiration, and it is inhibited over the sameconcentration range as aerobic fermentation. It was found earlier that oxidative assimilation of glucoseby yeast is progressively inhibited by increasing concentrationsof dinitrocresol, and it is now shown that this parallels theincrease in the rate of aerobic glycolysis. It is argued thatdinitrocresol is here acting as an uncoupling agent and thatboth oxidative assimilation and the rate of glycolysis are controlledby the level of energy-rich phosphate. With cyanide there is no stimulation of oxygen uptake, aerobicfermentation only appears when respiration becomes inhibited,and after an initial slight decrease the rate of glycolysisrises to 575 per cent. of the control value at 5 x 10^–4M. It is suggested that the rate of glycolysis only increaseswhen respiration has been inhibited sufficiently to reduce therate of formation of energy-rich phosphate.     

Components of Growth and Dark Respiration of Kikuyu (Pennisetum clandestinum Chiov.) at Various Temperatures

Growth and dark respiration were measured in dense, miniatureswards of kikuyu grass grown at constant temperatures of 15,20, 25 and 30 °C. Total respiration over the first 12 hof darkness was very high and CO² efflux per unit surface areavaried from 2.4 to 3.9 g CO₂ m^–2 h^–1 at 15 and 30°C respectively. Such rates were consistent with the correspondinglyhigh net growth rates of 24 and 63 g d. wt m^–2 d^–1and the heavy yields of herbage. When plants were kept in thedark, CO₂ efflux subsequently declined rapidly to a lower, constantrate which was taken to be the maintenance respiration rate.The half-life of the declining phase of respiration averaged10.9 and 6.0 h at 15 and 30 °C respectively, and was curvilinearlyrelated to the specific maintenance respiration rate (m). Therapid decline in respiration was consistent with the low concentrationsof total soluble carbohydrate and starch in the herbage. Valuesof m for lamina and top growth increased with temperature witha Q₁₀ of 2.6 and 1.42 respectively, but m of stems alone wasnot affected by temperature. Using results from this study forkikuyu and from McCree (1974) for sorghum and white clover,it was noted that all three species have similar m when grownat temperatures which are near their respective optimums forgrowth. Kikuyu, Pennisetum clandestinum, growth, respiration, temperature     

Nutrient Supply and the Growth of the Seminal Root System in Barley: I. THE EFFECT OF NITRATE CONCENTRATION ON THE GROWTH OF AXES AND LATERALS

A method was devised by which different zones along a singleseminal axis of an intact plant could be exposed for extendedperiods to contrasting concentrations of nitrate (either 0.01or 1.0 mM) in continuous flow, the supply of all other nutrientsbeing favourable throughout. The concentration of nitrate wasfound to exert a direct and strictly localized effect upon thegrowth of lateral roots which, depending upon the supply ofassimilates from the shoot, resulted in marked modificationsto the form of the root system. Zones receiving 1.0 mM nitrateshowed an increase in the number and extension rate of bothfirst- and second-order laterals, associated with a preferentialaccumulation of dry matter, compared with zones in 0.01 mM nitrate.The average number of laterals (both first and second order)per cm of parent root was 4.4 in the presence of 1.0 mM nitrateand 2.2 in 0.01 mM. The average extension rates of first-orderlaterals were 0.61 and 0.26 cm d^–1 and second-order laterals,0.10 and 0.05 cm d^–1 for nitrate concentrations of 1.0and 0.01 mM respectively. The precise numbers and extensionrates of laterals in any one zone were affected, however, bythe rate of growth of laterals in other parts of the root system.In contrast, the extension rates of axes were little affectedby the concentration of nitrate to which their apical meristemswere exposed and approached 2.0 cm d^–1 provided the plantswere not nitrogen-starved. The significance of these resultsto the physiology of root growth and soil-plant relations isdiscussed.     

Carbon and Nitrogen Assimilation by Vicia faba L. at Low Temperature: the Importance of Concentration and Form of Applied-N

Low temperature (6 C) growth was examined in two cultivarsof Vicia faba L. supplied with 4 and 20 mol m^–3 N as nitrateor urea. Both cultivars showed similar growth responses to increasedapplied-N concentration regardless of N-form. Total leaf areaincreased, as did root, stem and leaf dry weight, total carboncontent and total nitrogen content. In contrast to findingsat higher growth temperatures, 20 mol m^–3 urea-N gavesubstantially greater growth (all parameters measured) than20 mol m^–3 nitrate-N. The increased carbon content per plant associated with increasedapplied nitrate or urea concentration, or with urea in comparisonto nitrate, was due to a greater leaf area per plant for CO₂uptake and not an increased CO₂, uptake per unit area, carbon,chlorophyll or dry weight, all of which either remained constantor decreased. Nitrate reductase activity was substantial inplants given nitrate but negligible in plants given urea. Neitherfree nitrate nor free urea contributed greatly to nitrogen levelsin plant tissues. It is concluded that there is no evidence for a restrictionin nitrate reduction at 6 C, and it is likely that urea givesgreater growth than nitrate because of greater rates of uptake. Vicia faba, broad bean, low temperature growth, carbon assimilation, nitrogen assimilation     

Nutrient inflows into apple roots. II. Nitrate uptake rates measured on intact roots of mature trees under field conditions

Abstract The rates of uptake of nitrate-N per unit length; surface area and volume of root were measured in solution depletion experiments conducted in a root laboratory, using intact roots of two 4.5-year-old apple trees (Discovery/M.9 and Worcester Pearmain/M.9) at two different depths in the soil profile. In Discovery/M.9, NO₃^? uptake rate per unit root was constant over the 20-200 mmol m^?3 range of solution concentration. In Worcester/M.9, the uptake rate per unit root over the 200-150 mmol m^?3 range (corresponding to a ‘lag’ phase) was lower than that over 150-20 mmol m^?3. The uptake rates after the lag phase at depths of 46 and 104 cm were ca. 1.3 and 5.0 times greater than those in Discovery/M.9 at the 46 and 110 cm depths, respectively. The concentration below which net uptake was zero was ca. 1 mmolm^?3. In Discovery/M.9, the uptake rate per unit root at the 46cm depth was about 2.8 times that at 110 cm whereas in Worcester/M.9, the uptake rates at 46cm depth were about 1.8 and 1.4 times lower than those at 104cm over the solution concentration ranges 200-150 and 150-20 mmol m^?3, respectively. Only small differences were observed in uptake rates per unit root between 1400-1700 h, 2400-0400 h, and 0700-1100 h. For successive 5°C-increments in root temperature between 5 and 25° C, the nitrate uptake rate per unit root increased by 130, 10, 30 and 5%, respectively. A major change in the activation energy for nitrate uptake was observed at a transition temperature located between 5°and 10°C.     

The Effect of Nitrogen on the Growth and Sugar Content of Sugar-beet

Nitrogen fertilizer applied to sugar-beet increased plant androot dry weight and leaf area, and decreased the sugar contentof the roots per cent of both fresh and dry weight. Change inleaf area accounted wholly for the increase in plant dry weightproduced by nitrogen, because net assimilation rate was unaffected.Nitrogen did not alter the partition of the total assimilatebetween roots and shoots, but increased the fraction of totalassimilate entering the roots that was used in growth, at theexpense of that stored as sugar. Thus, plants with more nitrogenhad a smaller proportion of their root dry weight as sugar becausemore was metabolized in growth of the roots, and not becauseless entered the roots. The heavier roots of plants given more nitrogen were largerin cross-sectional area because the areas of both parenchymaand vascular zones of each peripheral ring within the root werelarger; the number of rings was not increased. Nitrogen increasedthe areas of the tissues in these zones by enlarging cell volumes,not by increasing the number of cells within the tissues. Increasein cell volume was accompanied by proportional increases inthe weights of non-sugar dry matter per cell and water per cell,but the amount of sugar per cell was proportional to cell volumeonly during the initial stage of cell expansion up to cell volumesof about 15x10^–8 cm²; thereafter it was less than proportional,so that sugar per cent of both fresh and dry weight decreasedas cell size increased beyond 15x10^–8 cm². The relationof sugar per cell to cell volume was the same with both amountsof nitrogen given. This implies that increase in nitrogen supplymade the sugar concentration of the root less by increasingthe size of the root cells and not by a specific effect on sugarstorage.     

Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 1. Development and Application of 13CO2 Assimilation System at a Constant 13C Abundance

A long-term, steady-state ¹³CO₂ assimilation system at a constantCO₂ concentration with a constant ¹³C abundance was designedand applied to quantitative investigations on the allocationof photoassimilated carbon in nodulated soya bean (Glycine maxL.) plants. The CO₂ concentration in the assimilation chamberand its ¹³C abundance were maintained constant with relativevariances of less than ±0.5 per cent during an 8-h assimilationperiod. At the termination of 8-h ¹³CO₂ assimilation by plantsat early flowering stage, the currently assimilated carbon relativeto total tissue carbon (measured by the degree of isotopic saturation)were for young leaves (including flower buds), 13.9 per cent;mature leaves, 15.7 per cent; stems+petioles, 5.9 per cent;roots, 5.4 per cent and nodules, 6.9 per cent, 48 h after theend of the ¹³CO₂ assimilation period, they were 12.3, 7.5, 7.4,6.8 and 6.1 per cent, respectively. The treatment with a highconcentration of nitrate in the nutrient media significantlydecreased the allocation of ¹³C into nodules. Experiments on¹³CO₂ assimilation by plants at the pod-filling stage were alsoconducted. Labelling by ¹³C was weaker than at the early floweringstage, but an intense accumulation of ¹³C into reproductiveorgans was observed. Glycine max L., nodulated soya bean plants, ¹³CO₂ assimilation, carbon dynamics     

The Uptake of Phosphate by Plants from Flowing Nutrient Solution: III. EFFECT OF CHANGED PHOSPHATE CONCENTRATIONS ON THE GROWTH AND DISTRIBUTION OF PHOSPHATE WITHIN PLANTS OF LOLIUM PERENNE L.

Perennial ryegrass (Lolium perenne L.) was grown from seed for29 d in flowing solution culture containing 0.1, 0.4 or 6.4mmol m^–3 P before the concentrations were changed (0.1and 0.4 raised to 6.4; 6.4 lowered to 0.4; controls unchanged)for an experimental period of two weeks to test the hypothesisthat after the seedling stage, the maximum rate of plant growthcould be sustained by a lower concentration of phosphate atthe root/solution interface than was necessary for the maximumrate of seedling growth. During the 29 d seedling period growthwas greatest on 6.4 mmol m^–3 P achieving 179 mg per plantdry weight compared with 122 and 26 mg on 0.4 and 0.1 mmol m^–3P respectively. During the experimental period growth on thetreatment 6.4 lowered to 0.4 mmol m^–3 P continued at thesame rate as the 6.4 control achieving 981 and 983 mg per plantdry weight respectively. Similarly growth of the treatment 0.4raised to 6.4 mol m^–3 P was unaffected by the change inconcentration and was comparable with the 0.4 control. Bothresults support the hypothesis for seedlings exceeding about100 mg per plant dry weight. In contrast the small plants ofthe treatment 0.1 raised to 6.4 mmol m^–3 P behaved similarlyto seedlings and responded rapidly to the increased concentrationof phosphate in solution, achieving high rates of phosphateuptake and increasing the growth of shoot more than the growthof root so that the ratio of root: shoot declined from 065 to0.34, a value similar to that for the seedlings grown on 6.4mmol m^–3 P. Key words: Lolium perenne L, Phosphate concentration, Seedling growth     

The Position and Characteristics of 'Leaky' Cells in Root Tip Meristems of Helianthus annuus

Sunflower root meristems are composed of two populations ofcells which respond differently to stress. One population becomesarrested in G₁ and G₂, while the second ‘leaky’population (0.25–1.0 per cent) is able to pass throughS even during carbohydrate starvation. Leaky cells enter S ata rate of 0.06 per cent cells h^–1 after 48 h of starvation.The character of leakiness is retained by roots starved fortwo successive 48 h starvation periods separated by an 8 h sucrosepulse. Single and double layer autoradiograph experiments demonstratedthat leaky cell progeny maintain their leaky character throughat least two cell generations. Leaky cells are located at randomin the root cap, promeristem, ground meristem, protoderm, cortex,and pericycle. The presence of leaky cells may indicate a stressresponse mechanism to repopulate the root meristem.     

土壤水分对黄土高原主要造林树种细根表面积季节动态的影响

在黄土丘陵沟壑区陕西省安塞县,于2007年生长季内,采用根钻法对刺槐(Robinia pseudoacacia)、侧柏(Platycladus orientalis)、油松(Pinus tabulaeformis)林地的细根和土壤水分进行了动态调查。结果表明:生长季内,刺槐、侧柏、油松林地0-200cm土层的土壤含水量变动较大,此土层是树木细根表面积的主要分布层,分别有82.4%(侧柏)、86.5%(刺槐)和87.5%(油松)的细根表面积分布。侧柏、刺槐、油松细根表面积垂直分布与剖面土壤水分间呈显著的正相关关系(p0.05)。模型S=AhB(C+Dh+Eh2+Fh3)可以较好地拟合不同树种细根表面积的垂直分布,拟合决定系数R2均在0.85以上。刺槐、侧柏、油松林地土壤含水量的动态变化均表现为10月4月6月8月。刺槐、油松细根表面积在6月出现1个高峰,侧柏在6月和10月各出现1个高峰。树木细根表面积动态与土壤含水量的季节动态不完全一致。侧柏、刺槐、油松生长所需的水分约87%来自降水的补给。但是,总体上侧柏、刺槐、油松细根表面积与林地土壤含水量的相关性不显著(p0.05)。全面了解树木细根季节动态的机理,还需对水分、温度、养分和树种本身遗传特性等影响因子进行综合研究。