Root elongation rate is accounted for by intercepted PPFD and source-sink relations in field and laboratory-grown sunflower

The existence of relationships between intercepted photo-synthetic photon flux density (PPFD) and growth of individual organs is somewhat controversial. We have tested whether such relationships could account for the natural variability in elongation rates of taproot and secondary roots of sunflower (from 2 to 135 mm d⁻¹), in field and laboratory conditions. Elongation of taproot and secondary roots was recorded daily through windows in the field. A range of PPFD was obtained by following day-to-day natural fluctuation for three contrasting growing periods, and by shading part of the plants under study. A parallel experiment was carried out in a growth chamber with contrasting light intensities and with a ¹⁴CO₂ labelling experiment. After the two-leaf stage, i.e. when the contribution of photosynthetic carbon became appreciable in root growth, daily root elongation rate was closely linked to the PPFD intercepted from 36 to 12 h before the measurement of root elongation. Curvilinear relationships applied to plants grown in the field as well as in a growth chamber, and to shaded plants as well as to plants subjected to day-to-day changes in intercepted PPFD. For a given intercepted PPFD, the taproot elongated faster than secondary roots, and secondary roots originating near the base of the taproot elongated faster than those originating near the apex. The elongation rate of any secondary root apex was accounted for (r= 0.77) by the ratio of intercepted PPFD to the distance between the apex and the base of the taproot. No relationships between intercepted PPFD and elongation rate were observed before the two-leaf stage, when the CO₂ labelling experiment suggests that carbon essentially originates from the seed. Therefore, this study suggests a role for source-sink relations in the distribution of elongation between apices and a role for carbon nutrition in day-to-day variations of root elongation rate. Precise mechanisms explaining this behaviour remain to be investigated.     

Quantitative analysis of the combined effects of temperature, evaporative demand and light on leaf elongation rate in well-watered field and laboratory-grown maize plants

The respective effects of meristem temperature, vapour pressuredeficit (VPD) and photosynthetic photon flux density (PPFD)on leaf elongation rate (LER) of maize, in the absence of waterdeficit in the soil have been quantified. This analysis wascarried out in a series of field experiments in northern andsouthern France over several seasons and years, and in growthchamber experiments. LER was measured with 10 min steps, togetherwith meristem temperature, VPD and PPFD at leaf level in threetypes of experiments: in growth chamber experiments with stepsin PPFD or VPD at constant meristem temperature, in growth chamberexperiments with several combinations of constant, but contrasting,PPFDs, VPDs and meristem temperatures, and in the field withfluctuating conditions, (i) When evaporative demand was low(night or day with low air VPD), LER was only linked to meristemtemperature, regardless of other climatic conditions, (ii) Lighthad no effect per se on LER in the range from 0 to 1500 molm^–2 s^–1 for time-scales longer than 2 h, providedthat its indirect effects on meristem temperature and on evaporativedemand were corrected (in the growth chamber) or taken intoaccount (in the field), and provided that cumulated PPFD overa weekly time-scale was compatible with field conditions, (iii)Evaporative demand sensed by growing leaves, as estimated bymeristem-to-air vapour pressure difference, markedly affectedLER in the range from 1–4 kPa, at all time-scales understudy, with a unique relationship in the growth chamber (constantconditions) and in the field (fluctuating conditions). Thiseffect was only observed when PPFD was high enough for stomatato open. The negative effect of evaporative demand on LER wasprobably not due to long distance root-to-shoot signalling,since soil was wet, calculated root water potential remainedclose to 0 MPa and concentration of ABA in the xylem sap wasvery low. Therefore, it is proposed to model maize LER witha two-step process, involving the calculation of the maximumLER at a given meristem temperature and then the calculationof the reduction in LER due to evaporative demand. Joint analysisof the whole set of data by using the two equations yieldeda r² of 0.75. This two-step process would be more accurate thanthe provision of LER from temperature only in cases where airVPD frequently exceeds 2 kPa. Key words: Leaf growth, light, evaporative demand, temperature, thermal time, water deficit, ABA, Zea mays L.     

Periodicity in the development of the root system of young rubber trees (Hevea brasiliensis Müell. Arg.): relationship with shoot development

The growth pattern of the root system of young rubber trees (Hevea brasiliensis) was studied in relation to shoot development over a period of 3 months. Temporal and spatial variations in elongation and branching processes were examined for the different root types, by means of root observation boxes. Shoot growth was typically rhythmic. Root development was periodic and related to leaf expansion. Root elongation was depressed during leaf growth, whereas branching was enhanced. Consequently, highly branched areas with vigorous secondary roots alternated along the taproot with poorly branched areas with shorter roots. Root types were not affected to the same degree by shoot competition: during leaf expansion, taproot growth was just depressed but remained continuous, the emergence and elongation rates of secondary roots were significantly affected and the elongation rates of tertiary roots fell to zero. These results were consistent with the hypothesis that root growth is related to competition for assimilates and to the sink strength of the different root types, whereas root branching appeared to be promoted by leaf development.     

Competition within the root system of rubber seedlings (Hevea brasiliensis) studied by root pruning and blockage

The effects of taproot blocking and pruning on the developmentof the early secondary roots (ESR) of rubber seedlings werestudied in root observation boxes under controlled conditions.During shoot flush, both the mean elongation rate and mean apicaldiameter of the ESR decreased regardless of treatments. Thereafter,the elongation rate of the ESR increased greatly when the taprootwas blocked, slightly for the control and scarcely for the prunedsystems in which fast growing regenerated roots developed. Thedifferences between treatments were related to the proportionof ESR which ceased growing. Following shoot arrest, the apicaldiameter of ESR increased greatly for blocked seedlings andto a lesser extent for pruned seedlings. Branching density ofthe ESR and elongation of tertiary roots were also higher forseedlings without a growing taproot. The dynamics of ESR response was not consistent with activeinhibition of their development by the growing taproot. Moreover,this response was dependent on concurrent development of shootand regenerating roots, hence competition processes were morelikely to be determining. In such an hypothesis, root elongationcan be limited by assimilate availability, but also by eachroot's maximum growth rate in non-limiting conditions, i.e.growth potential. Since the latter is related to apical diameter,a significant acceleration of elongation required a parallelincrease in apical diameter and this may explain the relativeinertia of ESR to taproot alteration. Conversely, regeneratingroots could have a high growth potential because they were initiatedin a favourable context, thus their development competed stronglywith elongation of ESR. Key words: Hevea brasiliensis, root system, development, growth potential, root diameter, competition     

Root Growth of Bulbous Species during Winter

Bulbs of Allium moly, Scilla siberica, Triteleia uniflora, Tulipakaufmanniana and Narcissus lobularis were planted in a rhizotronand root growth was monitored weekly for an 18 week period.All the genera produced roots until the soil temperature droppedto 3 °C. Root outgrowth from the bulbs and growth of emergedroots then ceased or was strongly inhibited in N. lobularis,S. siberica and T. uniflora but not in the other genera. Mostbulbs resumed root elongation when the soil temperature hadwarmed to 4 °C. Root emergence and continued growth occurredfor a longer period during the winter than has generally beenassumed. Allium moly, Scilla siberica, Triteleia uniflora, Tulipa kaufmanniana, Narcissus lobularis, rhizotron, bulb, root growth     

Root elongation rate is correlated with the length of the bare root apex of maize and lupin roots despite contrasting responses of root growth to compact and dry soils

Background

We investigated interacting effects of matric potential and soil strength on root elongation of maize and lupin, and relations between root elongation rates and the length of bare (hairless) root apex.

Methods

Root elongation rates and the length of bare root apex were determined for maize and lupin seedlings in sandy loam soil of various matric potentials (?0.01 to ?1.6 MPa) and bulk densities (0.9 to 1.5 Mg m^?3).

Results

Root elongation rates slowed with both decreasing matric potential and increasing penetrometer resistance. Root elongation of maize slowed to 10 % of the unimpeded rate when penetrometer resistance increased to 2 MPa, whereas lupin elongated at about 40 % of the unimpeded rate. Maize root elongation rate was more sensitive to changes in matric potential in loosely packed soil (penetrometer resistances <1 MPa) than lupin. Despite these differing responses, root elongation rate of both species was linearly correlated with length of the bare root apex (r² 0.69 to 0.97).

Conclusion

Maize root elongation was more sensitive to changes in matric potential and mechanical impedance than lupin. Robust linear relationships between elongation rate and length of bare apex suggest good potential for estimating root elongation rates for excavated roots.     

Patterns of Root Respiration Associated with the Induction of Aluminium Tolerance in Phaseolus vulgaris L.

Aluminium (Al) tolerance in an Al-tolerant cultivar of Phaseolusvulgaris L. (‘Dade’) was found to be an inducibletrait. Upon exposure to 10 µM Al, the rate of root elongationwas inhibited in comparison to controls. During the following72 h, the rate of elongation returned to levels comparable tocontrols. In contrast, root elongation of an Al-sensitive cultivar(‘Romano’) did not recover after exposure to Al.In Dade, the resumption of root elongation following exposureto Al was accompanied by increased rates of root respiration,whereas respiration rates slowly declined over the 72 h treatmentperiod in Romano. When partitioned into growth and maintenanceexpenditures, a larger proportion of root respiration of Dadeexposed to Al was allocated to maintenance processes, potentiallyreflecting diversion of energy to metabolic pathways that offsetthe adverse effects of Al toxicity. Romano did not show sucha pattern and respiration associated with both growth and maintenancewas reduced after exposure to Al. Root and shoot growth of bothcultivars were also measured to determine the effects of long-term(21 d) exposure to 10 µM Al. Dade plants exposed to Alexhibited enhanced growth in comparison to controls, whereasRomano plants were characterized by reduced shoot and root growth.Modelling the time-course of root respiration and measuringthe long-term growth responses to Al is a valuable method ofelucidating respiratory costs of stress tolerance. Key words: Aluminium, differential tolerance, maintenance respiration, Phaseolus vulgaris, root respiration     

The Carbon Economy of Clonal Plants of Trifolium repens L.

Fluxes of carbon between sources and sinks were quantified forclonal plants of Trifolium repens L. (cv. Blanca) in two glasshouseexperiments. Carbon sources were (a) leaves on the parent (=main)stolon apex, or (b) leaves on either young or old branches,and the major sinks of interest were the parent stolon apex,branches, and the adventitious root arising at the same parentstolon node as a young source branch. Defoliation treatmentswere applied to the parent stolon and/or branches (excludingsource branches). Carbon moved freely from the parent stolon to branches and vice-versa;these bidirectional exchanges of C provided important supplementarysources of carbohydrate for the sinks and buffered them againstthe effects of defoliation. Young branches exported more C tothe parent plant (mean=6.3µmol d^–1) than they importedfrom leaves on the parent stolon (5·2µmol d^–1)which, in turn, exceeded the amount fixed by leaves on the branchand utilized within the branch itself (2·7µmold^–1). In contrast, the C economy of old branches was largelyself-contained with, on average, 25·4µmol d^–1exported to the parent plant, 1·8µmol d^–1imported from the parent, and 63·0µmol d^–1fixed and utilized by the branch itself. Thus the growth ofyoung branches was immediately reduced by removal of parentstolon leaves, but old branches were unaffected. An estimated 42% of the C utilized by the main stolon apex originatedfrom branches, while by far the largest proportion (84%) ofthe C used for growth of young nodal roots originated from theassociated branch and not from leaves on the parent stolon towhich the root was directly attached. Key words: Trifolium repens, clonal growth, carbon economy, physiological integration, defoliation     

Mathematical Growth Analysis of Young Terminalia superba Plants in a Controlled Environment: Comparison of Growth Rhythms of the Principal Axis and of Axillary Branches

A Fourier analysis was used to study the marked variations inthe rate of shoot elongation and branch formation of young Terminaliasuperba plants. The growth of aerial parts of 3-month-old Terminaliaseedlings was measured daily during the following 7 months ofgrowth, and then weekly over a 15-month period of study. Measurementswere carried out in a controlled glasshouse at 27 °C and16 h photoperiod. The Fourier method clearly demonstrated thatthe main axis of young T. superba showed periodical regularvariations, with rest periods. This study revealed that theplagiotropic branch formation by apposition and elongation ofaxillary units was equally rhythmic and directly related tothe main shoot growth rhythm. However, the dates of outbreakof these new units on branches seemed relatively independentfrom variations in shoot growth rate. This developmental patternended in a characteristic and particular stratified branch systemin young Terminalia superba, comparable to the system shownby adult plants in their natural environment. Terminalia superba, Fourier analysis, growth rhythms     

Response to Temperature in a Stand of Pearl Millet (Pennisetum typhoides S. & H.): VIII. ROOT GROWTH

P J. Gregory. 1986. Response to temperature in a stand of pearlmillet (Pennisetum typhoides S. & H.). VIII. Root growth—J.exp. Bot. 37: 379–388. Two experiments were made in controlled glasshouscs to investigatethe growth of roots of pearl millet at different air and soiltemperatures. The experimental plants were grown in columnsof soil within stands of millet for 3 to 4 weeks and destructivelysampled at regular intervals to estimate the length of individualroot axes and of the root system. The length of individual rootaxes increased exponentially with time and at any particulartime the rate of extension was faster the higher the soil temperature.Clear ontogenctic effects on the rates of elongation were detected,with each succeeding axis elongating faster than its predecessor.Total root length was longer the higher the soil temperature(at a particular air temperature) and increased exponentiallywith time and with thermal time assessed from temperatures measuredat 2·0 cm depth. Whereas length at a particular timehad a 10-fold range, length at a particular thermal time hadonly a 3-fold range. Mean irradiance differed between the twoexperiments and as a means of exploring the importance of carbohydrateresources for root extension, relations between root length,leaf area and the amount of radiation intercepted were sought. Root length and leaf area were linearly related for all temperaturetreatments in both years as were root length and interceptedradiation. However, whilst the former relation was the samein both years, the latter was different. Root dry weight andintercepted radiation were also linearly related with the samerelation for both years so that the root length: weight ratiosdiffered between years because of factors not controlled inthese experiments. The results show the close relation between root and shoot growthand that thermal time together with the amount of radiationintercepted by the leaves might be used as the basis for quantifyingthe effects of temperature on root growth. Key words: Pearl millet, temperature, thermal time, root extension, root growth     

How periodic growth pattern and source/sink relations affect root growth in oak tree seedlings

Seedlings of Quercus pubescens were grown in root boxes to study the growth pattern of the root system in relation to shoot development. Shoot growth was typically rhythmic. Root elongation was also periodic, in contrast to several previous reports on other Quercus species. Both taproot and lateral root elongation were depressed during expansion of the second leaf flush, with a more pronounced response of lateral root growth. Apical diameter of the taproot followed comparable but less prominent trends than taproot elongation. Modifying source/sink relationships through various defoliation treatments altered the root growth pattern. Ablation of source organs (mature leaves or cotyledons) amplified the decrease in root growth concomitant with leaf expansion. Root growth recovery was even more difficult when both cotyledons and mature leaves had been removed. Ablation of sink aerial organs (young leaves) initially suppressed competition for growth between the shoot and the root, and then caused a gradual decrease in lateral root growth. Antagonism between maximum leaf expansion and root growth reduction during the second flush, and various responses of seedlings with modified source/sink relationships, raise an hypothesis of mutual competition for carbohydrates. The gradual decrease in lateral root growth after ablation of young leaves suggests a long-term carbohydrate limitation, or auxin limitation as auxin sources have been removed.     

The Cellular Pathway of Short-distance Transfer of Photosynthates and Potassium in the Elongating Stem of Phaseolus vulgaris L. Stem Anatomy, Solute Transport and Pool Sizes

Based on an uniform elongation growth pattern and cellular structure,the apical 0·5-2·5 cm elongation zone of internode2 of Phaseolus vulgaris L. seedlings was selected as an experimentalsystem to study the radial pathway of photosynthate and potassiumtransfer from the phloem. An histological examination of thephloem within the elongation zone of internode 2 showed thatboth proto- and meta- phloem sieve elements were present. Theformer were fully differentiated at the commencement of elongationand became crushed as elongation proceeded. In contrast, about50% of the final number of metaphloem sieve element-companioncell complexes differentiated during the same period. The phloemdelivered some 99% of the sucrose and 72-82% of the potassiumaccumulated by the elongation zone. Solute budgets showed that,of the photosynthates and potassium entering the elongationzone, approximately 40% were retained and 60% transferred tothe shoot apex. Thus, the elongating stem acts not only as asignificant sink for photosynthates and potassium, but alsoas an axial phloem transport system to supply the shoot apex.Within the elongation zone, the principal tissue sinks weredetermined by the cellular localisation of [¹⁴C] photosynthatesand potassium by microautoradiography and ion electron microprobeanalysis respectively. About 80% of the photosynthates and potassiumwere located outside the phloem. The cortex and pith exhibitedthe greatest accumulation for photosynthates and the pith forpotassium.Copyright 1994, 1999 Academic Press Phaseolus vulgaris, elongating stem, French bean, photosynthates, potassium, radial transfer, stem anatomy, transport     

Influence of temperature and water potential on root growth of white oak

Root growth of white oak ( Quercus alba L.) was observed under field conditions using a rhizotron. The effects of temperature, soil water potential, and leaf water potential were evaluated on three measures of root growth and development: root elongation rate, number of growing roots, and root growth intensity (sum of projected root area compared to the total root viewing area). Root elongation rate was linearly related to changes in soil temperature and soil water potential. At soil temperatures less than 17deg;C, temperature was the dominant factor affecting rate of growth, bat at temperatures greater than 17°C soil water potential became the important factor. Unlike root elongation rate, the number of growing roots and root growth intensity increased at cold soil temperatures (8°C) and at soil water potentials of-0.3 to -0.8 MPa. At high soil water potentials (-0.1 MPa) root elongation rate reached a maximum while the number of growing roots and root growth intensity were low. These differences showed that root growth and development were not exclusively affected by the soil environment. In addition, the relationship between root growth and predawn leaf water potential suggested that root growth was a contributing factor to the drought resistance of white oak.     

Net CO2 Exchange Rate and Shoot Growth of Young Poplar (Populus) Clones

Eighteen poplar (Populus) clones, varying in growth capacitywere grown in plastic containers under outdoor conditions. Duringthe first year of growth their net CO₂ exchange rate (NCER)was studied by infra-red gas analysis as a function of photosyntheticphoton flux density (PPFD) under controlled environment conditions.Maximal NCER (under saturating PPFD) and the PPFD compensationpoint were significantly correlated with the first year's shootlength and hence with the above ground biomass production ofthe various clones. Key words: Poplar, Shoot growth, CO₂ exchange rate     

The Initiation and Development of Secondary Xylem in Axillary Branches of Populus deltoides

The initiation of secondary xylem in elongating axillary branchesof Populus deltoides Bartr. ex Marsh. is independent of thatin the main stem. Although secondary xylem differentiates acropetallyin the main stem, it does not differentiate from the stem intothe axillary branch. Secondary xylem is usually initiated ininternode 4 (occasionally 3) of the axillary branch, and fromthis site it develops both acropetally in the elongating branchand basipetally toward the main stem. Secondary vessel differentiationalways precedes fibre differentiation. Although secondary xylemdifferentiates in internodes that have ceased elongation, itdifferentiates first in traces of the vascular cylinder servingrapidly expanding and maturing foliage leaves. As younger leaveson the branch expand and mature, secondary xylem differentiatesin their traces eventually producing a complete secondary vascularcylinder. Scale leaves do not initiate secondary xylem independentlyin their traces; they are activated by adjacent traces in thevascular cylinder serving foliage leaves. Once established,the primary-secondary vascular transition zone advances acropetallyin a branch just as it does in the main stem. Populus deltoides Bartr. ex Marsh., cottonwood, axillary branches, secondary xylem, plastochron index, post-dormancy development, xylem.     

Influence of Position and Number of Nodal Roots on Outgrowth of Axillary Buds and Development of Branches in Trifolium repens (L.)

The implications of the presence of a root, either at the parentnode or at neighbour nodes, on branch formation of Trifoliumrepens (white clover) was investigated. Plants were freely rootedor rooting was restricted to every sixth or every twelfth nodealong the parent axis. The absence of a root at the parent nodehad little influence on the probability of the subtending axillarybud forming a branch but, on average, delayed the outgrowthof the bud. The probability that an axillary bud, emerging froma non-rooted parent node, developed to a lateral branch (branchwith elongated internodes) decreased with decreasing proximityof the parent node to a rooted node. Lateral branches emergingfrom non-rooted parent nodes which were two nodes distal toa rooted node had a higher rate of node appearance, a greatermean internode length and area per leaf, and were more branchedthan lateral branches emerging from other non-rooted parentnodes. The dry mass of each single root and of branches grownat rooted parent nodes were significantly higher in plants withrestricted rooting than in freely rooted plants. Restrictionin the number of rooted nodes per plant increased the numberof inflorescences. It is concluded that the whole plant responseto restricted root formation was continuous growth of the parentaxis and compensatory growth of the branch at the rooted node.In general, growth was slow for axillary buds whose developmentwas dependent on the basipetal movement or cross-transport withinthe stolons of resources exported from roots. Trifolium repens (L.); white clover; axillary bud outgrowth; branch development; clonal growth; nodal root     

Assessing variability in root traits of wild Lupinus angustifolius germplasm: basis for modelling root system structure

Background and aims

Intra-specific variation in root system architecture and consequent efficiency of resource capture by major crops has received recent attention. The aim of this study was to assess variability in a number of root traits among wild genotypes of narrow-leafed lupin (Lupinus angustifolius L.), to provide a basis for modelling of root structure.

Methods

A subset of 111 genotypes of L. angustifolius was selected from a large germplasm pool based on similarity matrices calculated using Diversity Array Technology markers. Plants were grown for 6?weeks in the established semi-hydroponic phenotyping systems to measure the fine-scale features of the root systems.

Results

Root morphology of wild L. angustifolius was primarily dominated by the taproot and first-order branches, with the presence of densely or sparsely distributed second-order branches in the late growth stage. Large variation in most root traits was identified among the tested genotypes. Total root length, branch length and branch number in the entire root system and in the upper roots were the most varied traits (coefficient of variation CV >0.50). Over 94% of the root system architectural variation determined from the principal components analysis was captured by six components (eigenvalue >1). Five relatively homogeneous groups of genotypes with distinguished patterns of root architecture were separated by k-means clustering analysis.

Conclusions

Variability in the fine-scale features of root systems such as branching behaviour and taproot growth rates provides a basis for modelling root system structure, which is a promising path for selecting desirable root traits in breeding and domestication of wild and exotic resources of L. angustifolius for stressful or poor soil environments.     

Analysis of Branching in Spring-sown White Lupins (Lupinus albusL.): The Significance of the Number of Axillary Buds

Plant density and sowing date were shown to affect branchingin spring-sown white lupin (Lupinus albusL.), but the responsevaried among environments. The patterns of primary and secondarybranching in the cv. Lublanc were studied as a function of boththe number of axillary buds and the plant growth rate. Fieldexperiments that used a wide range of sowing dates and plantdensities to alter plant architecture were conducted over 5years, and these were supplemented with data from additionalglasshouse and growth cabinet experiments. The number of axillary buds on the main stem or primary branches,which determined the potential number of branches, increasedlinearly with the number of nodes. In situations where all axillarybuds did not produce branches, it was found that the numberof primary and secondary branches produced was related to theplant growth rate at the beginning of branch elongation. Knowledgeof the number of axillary buds improved the analysis of theinteraction between cultural practices and environmental conditionson plant architecture. The variability of branching potentialamong genotypes was discussed. Lupinus albus; branches; axillary buds; growth; sowing date; plant density     

Kinetics of Leaf Growth in Lolium temulentum at Optimal and Chilling Temperatures

Lolium temulentum plants were grown at 20 °C, under an 8-hdaylength, in a controlled-environment chamber, and the kineticsof leaf expansion were observed by measuring the movement ofan optical grid attached to the fourth leaf. The leaf emerged23–24 d after sowing and was fully expanded 9–10d later. Extension rate was maximal between the second and fifthdays after emergence and declined markedly thereafter. Duringthe rapid growth phase the rate of elongation exhibited a distinctdiurnal rhythm, fluctuating between 1.9 to 2.3 mm h^–1in the light period, and 1.3 to 1.7 mm h^–1 in the dark.A circadian oscillation with a period of about 27 h was observedin leaves elongating in continuous darkness. When plants weretransferred to 5 °C soon after emergence of the fourth leafthere was an immediate reduction in rate of growth to about22 per cent of the rate at 20 °C: the Q₁₀ for the mean elongationrate in the range 20–5 °C was 3.7. When plants weretransferred from 20 to 2 °C at fourth leaf emergence, meanextension rate declined to less than 5 per cent, correspondingto a Q₁₀ in the range 5–2 °C of more than 300. Furthermore,growth at 2 °C was confined almost entirely to the darkphase of the photoperiod cycle. The responsive tissue was shownto be a small area of expanding leafless than 1.5 cm above theshoot apex and the possible mechanisms underlying low temperatureeffects in this region are discussed. Lolium temulentum L., leaf growth, auxanometer, low temperature, diurnal rhythm     

The Relationship between Cyanide-Resistant Root Respiration and the Storage of Sugars in the Taproot in Daucus carota L.

Dry weight increase, root respiration, photosynthesis, and shootdark respiration were measured prior to and during the developmentof the taproot of carrot (Daucus carota L., var. AmsterdamseBak) which stores sucrose and reducing sugars. Before storageof sugars in the taproot started, root respiration was highand up to 50% inhibited by salicyl hydroxamic acid (SHAM), indicatinga high activity of the alternative pathway. The onset of sugar storage in the taproot coincided with a sharpdecrease in alternative pathway activity. When sugars were storedin the taproot, the alternative pathway in the taproot was notoperative, although present. It was concluded that the alternative pathway only consumessugars that are not used for energy production in growth ormaintenance processes, for carbon skeletons, osmoregulationor storage, in accordance with the energy overflow model. Apositive quantitative relationship exists between the storageof sugars in the taproot and the efficiency of root respiration.After day 32 the amount of daily produced carbohydrates transportedto the root system increased and these sugars were stored inthe taproot. In this period less sugars were invested in structuralshoot growth. In the present investigation no positive influence from thedeveloping sink is found on photosynthetic rate or net assimilationrate. Both rates decrease with increasing age of the plants,although increasing amounts of sugars were stored in the taproot.