The use of internal nitrogen stores in the rhizomatous grass Calamagrostis epigejos during regrowth after defoliation

BACKGROUND AND AIMS: The regrowth dynamics after defoliation of the invasive grass Calamagrostis epigejos were studied. As nitrogen (N) reserves have been shown to play an important role during plant regrowth, the identity, location and relative importance for regrowth of N stores were determined in this rhizomatous grass. METHODS: Plant growth, nitrate uptake and root respiration were followed during recovery from defoliation. Water soluble carbohydrates, nitrate, free amino acids and soluble proteins were analysed in the remaining organs. KEY RESULTS: Nitrate uptake and root respiration were severely reduced during the first days of regrowth. Roots were the main net source of mobilized N. The quantitatively dominant N storage compounds were free amino acids. Free amino acids and soluble proteins in the roots decreased by 55 and 50%, respectively, and a substantial (approximately 38%) decrease in stubble protein was also observed. Although the relative abundance of several soluble proteins in roots decreased during the initial recovery from defoliation, no evidence was found for vegetative storage protein (VSP). Furthermore, rhizomes did not act as a N storage compartment. CONCLUSIONS: Production of new leaf area was entirely reliant, during the first week after defoliation, on N stores present in the plant. Mobilized N originated mainly from free amino acids and soluble proteins located in roots, and less so from proteins in stubble. Presence of VSP in the roots was not confirmed. The data suggest that rhizomes played an important role in N transport but not in N storage.     

Extension rate and respiratory activity in the growth zone of wheat roots: Time-course for adjustments after defoliation

The time-course for adjustments in the rate of extension of wheat (Triticum aestivum L. cv. Alexandria) roots, and the activity and capacity of respiratory pathways in the root apex, were determined after pruning the shoot to the ligule of the first leaf. Leaf pruning reduced the extension rate of both seminal and lateral roots. The onset of the response occurred within 1 h of pruning for laterals and between 2 and 3 h for seminals. The reduction in rate appears to be the result of a decrease in carbohydrate availability because (1) in seminal roots it was preceded by a decrease in soluble sugar content of the apical part of the growth zone (0–5 mm behind the root apex) and (2) supplying glucose (50 mM) to the roots of plants defoliated 24 h earlier led to a steady increase in extension rate of both seminal and lateral roots compared to non-fed controls. Supplying 3-O-methyl glucose had no effect. The reduction in extension rate of seminal roots was accompanied (or slightly preceded) by a reduction in respiratory O₂ uptake in the apical part of the growth zone (0–5 mm). Changes in respiratory activity in the basal part of the growth zone (5–10 mm) only occurred several hours later. At the time root extension rate was reduced, the rate of O₂ uptake could be stimulated with FCCP, which indicates that respiration was under the fine control of adenylates. From these results we suggest the following sequence of events occurs after defoliation. Firstly, defoliation reduces the supply of sugars to the root apex, this leads to a reduction in rate of extension through some form of coarse control by carbohydrates on cell division and expansion, which in turn reduces the rate of respiratory O₂ uptake because of a smaller demand for ATP. The results also indicate that there is a rapid (<1.5 h) reduction in respiratory capacity in the root apex after defoliation which occurs before any change in the overall rate of respiration.     

Regrowth by Swards of Subterranean Clover after Defoliation. 1. Growth, Non-structural Carbohydrate and Nitrogen Content

Swards of subterranean clover (Trifolium subterraneum L.) atLAl 6 grown in N-free nutrient solution were subjected to threedefoliation treatments which removed 30, 70 and 80% of shootdry weight. Subsequent regrowth and changes in the concentrationsof carbohydrate and nitrogen in plant components were measuredat 0, 1, 5, 9 and 13 d after defoliation and compared with thosein uncut swards. The rate of shoot regrowth declined with increasing severilyof defoliation. In all defoliation treatments, growth was confinedto leaves for up to 5 d. Root growth ceased in all treatmentsfor a longer period. Reestablishment of the leaf area in severely-defoliatedswards was facilitated by the rapid opening of developing leavesand by changes in the allocation of carbon which favoured leafover branch and root, and lamina over petiole growth. Loss of carbohydrate and nitrogen from roots and branches lasting5–9 d was observed in the more severe defoliation treatments.Loss of protein (N x 6.25) exceeded that of total non-structuralcarbohydrate, and could have accounted for the nitrogen contentof new leaf during this period. Branches lost 62% of their initialcarbohydrate content compared with 25% from roots in the 80%cut swards. In contrast, roots, by virtue of their greater mass,were the principle source of mobilized nitrogen. Nitrogen accumulationceased in 80% cut swards for 9 d. However, carbohydrate levelsin the crown nodules were not severely depleted. It was concluded that partitioning of growth to lamina and themobilization of carbohydrates and nitrogen were important forrecovery from defoliation. Carbohydrates, carbon partitioning, defoliation, nitrogen, mobilization, regrowth, subterranean clover, Trifolium subterraneum L     

Increased Defoliation Frequency Depletes Remobilization of Nitrogen for Leaf Growth in Grasses

Plants ofLolium perenneandFestuca rubrawere grown in sand culturereceiving all nutrients as a complete nutrient solution containing1.5 mMNH₄NO₃, and subjected to one of three defoliation treatments:undefoliated, defoliated on one occasion, or defoliated weekly.¹⁵Nlabelling was used to determine the rate of N uptake, allowingthe amount of N remobilized from storage for the growth of thetwo youngest leaves (subsequently referred to as ‘newleaves’) growing over a 14 d period after defoliationto be calculated. The total plant N uptake by both species wasreduced, compared with undefoliated plants, by both a singleand repeated defoliation, although neither caused complete inhibitionof uptake. Regularly defoliatedL. perennehad a greater reductionin root mass, concomitant with a greater increase in N uptakeper g root than did regularly defoliatedF. rubra. In both species,the amount of N derived from uptake recovered in the new leaveswas unaffected by the frequency of defoliation. BothL. perenneandF.rubramobilized nitrogen to the new leaves after a single defoliation,mobilization being sufficient to supply 50 and 41%, respectively,of the total nitrogen requirement. In regularly defoliated plants,no significant nitrogen was mobilized to the new leaves inL.perenne, and only a small amount was mobilized inF. rubra. Plantsachieved greater leaf regrowth when only defoliated once. Weconclude that increasing the frequency of defoliation of bothL.perenneandF. rubrahad little effect on the uptake of nitrogenby roots which was subsequently supplied to new leaves, butdepleted their capacity for nitrogen remobilization, resultingin a reduction in the rate of growth of new leaves. Lolium perenne; Festuca rubra; defoliation frequency; mobilization; root uptake; nitrogen     

Regrowth by Swards of Subterranean Clover after Defoliation. 2. Carbon Exchange in Shoot, Root and Nodule

The carbon economy of subterranean clover swards subjected tothree defoliation treatments (removal of 30, 70 and 80% of shootdry weight) was compared with that of uncut swards. Carbon dioxideexchange in shoots and roots was measured independently 0, 4,8 and 12 d after defoliation. The respiration linked to nitrogenaseactivity was estimated by comparing root respiration measuredin an atmosphere containing 3% oxygen with the respiration in21% oxygen. Net photosynthesis fell by up to 100% immediately after defoliation.There was a decline of over 60percnt; in root respiration bythe end of the first light period, composed of a rapid declineof 70% in nitrogenase-linked respiration in all treatments anda slower decline of nearly 40% in root plus nodule growth andmaintenance respiration in the more severe treatments. Recoveryof net photosynthesis to rates achieved by uncut swards occurredover 4 d in the 30% cut treatment and at least 12 d in moresevere treatments. Whilst recovery of photosynthesis was theprinciple determinant of recovery of net positive carbon balance,the early reduction in respiration facilitated this outcome.After the immediate decline in nitrogenase-linked respiration,recovery in this component of respiration appeared to be linkedwith the recovery in net photosynthesis (approximately 10% ofnet photosynthesis). Carbon budgets revealed priorities in allocation towards leafin the first 5 d and later also towards root growth in severelydefoliated swards. Root respiration comprised a large respiratorycost (up to 75% of net photosynthesis) during early regrowth. Carbon budget, defoliation, N₂ fixation, photosynthesis, regrowth, respiration, subterranean clover, Trifolium subterraneum L     

Effect of Nutrition on the Short-term Response of Molinia caerulea to Defoliation

Overwintering, rooted basal internodes of Molinia caerulea (L.)Moench were taken from the field and subjected to four nutritiontreatments [an adequate (high) and suboptimal (low) level ofnitrogen (N) x an adequate (high) and suboptimal (low) levelof phosphorus (P)] and three degrees of defoliation (12 treatmentsin total). Growth parameters were studied using both non-destructivemeasurements and destructive harvesting. High N supply and highP supply increased both the number of tillers and mass of eachtiller. Interactions between the effects of N and P did occurfor several growth variables; in general this was due to anabsence of a response to P supply at low N. Defoliation reducedthe dry weight of basal internodes and roots produced and temporarilyreduced leaf dry weight per tiller. There were no effects ofdefoliation on tillering. Interactions of defoliation with bothN supply and P supply were observed. Leaf extension rate wasincreased by defoliation at low, but not high N, and the adverseeffects of defoliation on root dry weight and root/shoot (R/S)ratios were proportionally greater at low N. The results arediscussed in relation to other investigations which have reportedcontrasting aspects of Molinia growth in response to both nutrientsand defoliation. Molinia caerulea, purple moor grass, nitrogen, phosphorus, defoliation     

The Effects of Nitrogen Supply and Defoliation on the Seasonal Internal Cycling of Nitrogen in Molinia caerulea

Plants of Molinia caerulea were grown in pots for two seasonsat two levels of nitrogen (N) supply and two levels of defoliation.All N supplied was enriched with ¹⁵N in the first season andwas at natural abundance in the second season. This allowedthe contribution of remobilization from overwintering storesto be discriminated from current root uptake in supplying Nfor new shoot growth in the second season. The effects of Nsupply and defoliation upon the internal cycling of N in M.caerulea were quantified. N was remobilized from both roots and basal internodes to supportnew shoot, especially leaf, growth in spring. Roots suppliedmore N than basal internodes. Since the remobilization mainlyoccurred before the onset of root N uptake, internal cyclingwas important for the earliest period of shoot growth. An increasedN supply increased the amount of N remobilized to new shootgrowth, however, the proportion of N remobilized from overwinteringstores was independent of N supply. Defoliation increased theamount of N remobilized from the roots, and had no effect onthe ¹⁵N content of basal internodes of plants receiving a lowsupply of N. Remobilization of N from leaves of undefoliatedplants occurred later in the season. Remobilization from leavessupplied flowers in plants receiving a low N supply and bothflowers and new basal internodes in plants receiving a higherN supply. Key words: Molinia caerulea, internal cycling, nitrogen, defoliation     

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.     

Effects of phosphorus availability and vesicular–arbuscular mycorrhizas on the carbon budget of common bean (Phaseolus vulgaris)

Low phosphorus availability is often a primary constraint to plant productivity in native soils. Here we test the hypothesis that root carbon costs are a primary limitation to plant growth in low P soils by assessing the effect of P availability and mycorrhizal infection on whole plant C budgets in common bean ( Phaseolus vulgaris L.). Plants were grown in solid-phase-buffered silica sand providing a constant supply of low (1 μ m ) or moderate (10 μ m ) P. Carbon budgets were determined weekly during the vegetative growth phase. Mycorrhizal infection in low-P plants increased the root specific P absorption rate, but a concurrent increase in root respiration consumed the increased net C gain resulting from greater P uptake. The energy content of mycorrhizal and non-mycorrhizal roots was similar. We propose that the increase in root respiration in mycorrhizal roots was mainly due to increased maintenance and growth respiration of the fungal tissue. Plants grown with low P availability expended a significantly larger fraction of their total daily C budget on below-ground respiration at days 21, 28 and 35 after planting (29–40%) compared with plants grown with moderate P supply (18–25%). Relatively greater below-ground respiration in low P plants was mainly a result of their increased root:shoot ratio, although specific assimilation rate was reduced significantly at days 21 and 28 after planting. Specific root respiration was reduced over time by low P availability, by up to 40%. This reduction in specific root respiration was due to a reduction in ion uptake respiration and growth respiration, whereas maintenance respiration was increased in low-P plants. Our results support the hypothesis that root C costs are a primary limitation to plant growth in low-P soils.     

Effect of defoliation upon root growth,phosphate absorption and respiration in nutrient-limited tundra graminoids

Summary Moderate experimental defoliation stimulated root respiration and phosphate absorption in two tundra graminoids, Eriophorum vaginatum and Carex aquatilis, growing under nutrient-limited field conditions in northern Alaska. The increase in phosphate absorption rate following defoliation of Eriophorum was associated with a decrease in root phosphate and available carbohydrate contents per unit root length but a constant root nitrogen content. Only after four repeated defoliations did phosphate absorption rate decrease below control levels. We suggest that the stimulation of root respiration and phosphate absorption immediately following defoliation resulted from lowered root phosphorus status as nutrient reserves were reallocated to support shoot regrowth. Root growth was affected more severely by defoliation than was root activity. Two or more defoliations reduced root elongation, initiation and weight per unit length, but root mortality increased only after four defoliations. Carex aquatilis, a species with large belowground biomass, was less sensitive to defoliation than Eriophorum. Phosphate absorption rate increased only after four defoliations in this species, and root elongation, initiation and mortality were affected only by the most severe clipping regimes. Responses of plants to repeated defoliation over two growing seasons were consistent with results of short-term studies.     

Allocation of carbon to shoots,roots, soil and rhizosphere respiration by barrel medic (Medicago truncatula) before and after defoliation

The allocation of carbon to shoots, roots, soil and rhizosphere respiration in barrel medic (Medicago truncatulaGaertn.) before and after defoliation was determined by growing plants in pots in a labelled atmosphere in a growth cabinet. Plants were grown in a ¹⁴CO₂-labelled atmosphere for 30 days, defoliated and then grown in a ¹³CO₂-labelled atmosphere for 19 days. Allocation of ¹⁴C-labelled C to shoots, roots, soil and rhizosphere respiration was determined before defoliation and the allocation of ¹⁴C and ¹³C was determined for the period after defoliation. Before defoliation, 38.4% of assimilated C was allocated below ground, whereas after defoliation it was 19.9%. Over the entire length of the experiment, the proportion of net assimilated carbon allocated below ground was 30.3%. Of this, 46% was found in the roots, 22% in the soil and 32% was recovered as rhizosphere respiration. There was no net translocation of assimilate from roots to new shoot tissue after defoliation, indicating that all new shoot growth arose from above-ground stores and newly assimilated carbon. The rate of rhizosphere respiration decreased immediately after defoliation, but after 8 days, was at comparable levels to those before defoliation. It was not until 14 days after defoliation that the amount of respiration from newly assimilated C (¹³C) exceeded that of C assimilated before defoliation (¹⁴C). This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

Root Flooding of Muskmelon (Cucumis melo L.) Affects Fruit Sugar Concentration But Not Leaf Carbon Exchange Rate

Flooding the roots of greenhouse-grown muskmelon (Cucumis meloL. cv. Noy Yizreel) plants for 4 days reduced sucrose accumulation36% in the inner mesocarp and 88% in the outer mesocarp of developingfruit. Concentration of the translocated sugars raffinose andstachyose were also lower in fruit on flooded plants than inthose from nonflooded plants. In contrast, fruit hexose concentrationwas similar in both flooded and nonflooded plants. There wasno alteration in activities of enzymes associated with sucrosemetabolism in the fruit which could explain the decreased sucroseconcentration. Four days of root flooding caused no reductionin leaf carbon exchange rate or assimilate export rate, indicatingthat the reduction in fruit sucrose accumulation was not dueto source limitation. Root respiration, measured as CO₂ evolution,was approximately 30% lower in anaerobic roots than in aerobicroots. When viewed as carbohydrate consumed, a doubling of glycolyticactivity occurred in the anaerobic root mass. Increased demandfor carbohydrates by anaerobic roots may lead to a reductionin translocated carbohydrates available for sucrose biosynthesisin the developing fruit. (Received August 29, 1990; Accepted February 21, 1991)     

Root Respiration and Carbohydrate Status of Two Wheat Genotypes in Response to Hypoxia

To investigate root respiration and carbohydrate status in relationto waterlogging or hypoxia tolerance, root respiration rateand concentrations of soluble sugars in leaves and roots weredetermined for two wheat (Triticum aestivum L.) genotypes differingin waterlogging-tolerance under hypoxia (5% O₂) and subsequentresumption of full aeration. Root and shoot growth were reducedby hypoxia to a larger extent for waterlogging-sensitive Coker9835. Root respiration or oxygen consumption rate declined withhypoxia, but recovered after 7 d of resumption of aeration.Respiration rate was greater for sensitive Coker 9835 than fortolerant Jackson within 8 d after hypoxia. The concentrationsof sucrose, glucose and fructose decreased in leaves for bothgenotypes under hypoxia. The concentration of these sugars inroots, however, increased under hypoxia, to a greater degreefor Jackson. An increase in the ratio of root sugar concentrationto shoot sugar concentration was found for Jackson under hypoxicconditions, suggesting that a large amount of carbohydrate waspartitioned to roots under hypoxia. The results indicated thatroot carbohydrate supply was not a limiting factor for rootgrowth and respiration under hypoxia. Plant tolerance to waterloggingof hypoxia appeared to be associated with low root respirationor oxygen consumption rate and high sugar accumulation underhypoxic conditions.Copyright 1995, 1999 Academic Press Oxygen consumption rate, sugar accumulation, Triticum aestivum L., waterlogging tolerance     

Variation in phosphorus uptake efficiency by genotypes of cowpea (Vigna unguiculata) due to differences in root and root hair length and induced rhizosphere processes

The lengths of roots and root hairs and the extent of root-induced processes affect phosphorus (P) uptake efficiency by plants. To assess the influence of variation in the lengths of roots and root hairs and rhizosphere processes on the efficiency of soil phosphorus (P) uptake, a pot experiment with a low-P soil and eight selected genotypes of cowpea (Vigna unguiculata (L) WALP) was conducted. Root length, root diameter and root hair length were measured to estimate the soil volume exploited by roots and root hairs. The total soil P was considered as a pool of Olsen-P, extractable with 0.5 M NaHCO₃ at pH 8.5, and a pool of non-Olsen-P. Model calculations were made to estimate P uptake originated from Olsen-P in the root hair zone and the Olsen-P moving by diffusion into the root hair cylinder and non-Olsen-P uptake. The mean uptake rate of P and the mean rate of non-Olsen-P depletion were also estimated. The genotypes differed significantly in lengths of roots and root hairs, and in P uptake, P uptake rates and growth. From 6 to 85% of total P uptake in the soil volume exploited by roots and root hairs was absorbed from the pool of non-Olsen-P. This indicates a considerable activity of root-induced rhizosphere processes. Hence the large differences show that traits for more P uptake-efficient plants exist in the tested cowpea genotypes. This opens the possibility to breed for more P uptake-efficient varieties as a way to bring more sparingly soluble soil P into cycling in crop production and obtain capitalisation of soil P reserves.     

Respiration Rate of Barley Roots: its Relation to Growth, Substrate Supply and the Illumination of the Shoot

The respiration rate of roots on intact barley plants grownin 16 h light 8 h dark cycles shows an exponential decay inthe dark, rises on re-illumination and there is a transientfall 12–14 h into the photoperiod Roots of plants placedin the dark for up to 48 h show a continued exponential decay,and a rather small fall in soluble carbohydrate levels The respirationof roots excised from predarkened plants does not rise on additionof sucrose to the medium bathing them Respiration rate, measured10 h into the photoperiod, shows a constant relation to rootweight in plants 8–24 days old, during which time rootcarbohydrate content first falls and later rises It is concludedthat root respiration rate is not a simple function of carbohydratesupply from the shoot The importance of root respiration inthe carbon budget of barley plants is evaluated and the levelsof control operating on root respiration rate are briefly discussed Hordeum distichum (L ) Lam, barley, respiration rate, light, carbohydrate     

Ion Absorption and Allocation of Carbon Resources in Excised Pea Roots Grown in Liquid Medium in Absence or Presence of NaCl

Freshly excised pea roots (Pisum sativum cv. Alaska) when transferredto growth medium (130 mOsm) or growth medium containing salt( 370 mOsm) suffer an initial osmotic shock and lose water.Contol roots tended to accumulate potassium, particularly inthe apical zone, while those exposed to NaCl accumulated mainlysodium, potassium accumulation being depressed. Exposure tosalinity for 6 d caused increases in root protein, cellulose,uronic acid and lignin content per cell. In roots supplied with¹⁴C-glucose for 24 h immediately after excision there was littledifference in uptake of glucose and in its use in respirationbetween control and salt treated roots. However, there werenoticeable differences in incorporation of labelled carbon intoseveral cell fractions, and particularly into the cellulosefraction in the upper parts of the root. When roots were grownfor six days in culture before being supplied with [¹⁴C]glucose,uptake per root was greater in the 120 mM NaCl treatment, andthe fraction diverted to respiration was decreased by salinity.On a per cell basis incorporation into soluble starch, uronicacid and cellulose fractions was increased in the salt treatedroots. The data obtained are in accord with the previous findingsand are suggestive of increased synthesis of cell wall materials.No conclusion could be drawn as to whether the changes describedare of adaptive value. Pisum sativum L. cv. Alaska, root culture, salinity, osmoregulation, cell wall     

Sectorial root growth in cuttings of Coleus rehneltianus in response to localized aerial defoliation

Asymmetries in root growth in response to localized aerial defoliation were examined in Coleus rehneltianus (Lamiaceae). We confirmed that assimilate transport was sectorial by examining the distribution of ¹⁴C-labeled carbohydrates following a 24-h chase period. Integrated physiological units (IPUs), or sectors, extended from the leaves into the roots, and this was reflected in the differential growth of roots following artificial defoliation of part of the leaf canopy. When defoliation was localized within leaves or leaf halves within sectors, roots grew asymmetrically, with decreased root growth in defoliated sectors. Three root populations were identified by their location and growth responses: stem side, stem corner, and bottom side roots, and asymmetric growth was observed in all three populations. Only the growth of stem corner roots, which made up 35–90% of dry mass of the total root population, was influenced by the pattern of aerial defoliation. In contrast, asymmetries in the growth of the other two root populations appeared to reflect the distribution of leaf biomass prior to defoliation.     

Root Distribution, Growth, Respiration, and Hydraulic Conductivity for Two Highly Productive Agaves

Cultivated Agave mapisaga and A. salmiana can have an extremelyhigh above-ground dry-weight productivity of 40 Mg ha^–1yr^–1. To help understand the below-ground capabilitiesthat support the high above-ground productivity of these Crassulaceanacid metabolism plants, roots were studied in the laboratoryand in plantations near Mexico City. For approximately 15-year-oldplants, the lateral spread of roots from the plant base averaged1.3 m and the maximal root depth was 0.8 m, both considerablygreater than for desert succulents of the same age. Root andshoot growth occurred all year, although the increase in shootgrowth at the beginning of the wet season preceded the increasein growth of main roots. New lateral roots branching from themain roots were more common at the beginning of the wet season,which favoured water uptake with a minimal biomass investment,whereas growth of new main roots occurred later in the growingseason. The root: shoot dry weight ratio was extremely low,less than 0.07 for 6-year-old plants of both species, and decreasedwith plant age. The elongation rates of main roots and lateralroots were 10 to 17 mm d^–1, higher than for various desertsucculents but similar to elongation rates for roots of highlyproductive C₃ and C₄ agronomic species. The respiration rateof attached main roots was 32 µmol CO₂ evolved kg^–1dry weight s^–1 at 4 weeks of age, that of lateral rootswas about 70% higher, and both rates decreased with root age.Such respiration rates are 4- to 5-fold higher than for Agavedeserti, but similar to rates for C₃ and C₄ agronomic species.The root hydraulic conductivity had a maximal value of 3 x 10^–7ms^–1 MPa^–1 at 4 weeks of age, similar to A. deserti.The radial hydraulic conductivity from the root surface to thexylem decreased and the axial conductivity along the xylem increasedwith root age, again similar to A. deserti. Thus, although rootsof A. mapisaga and A. salmiana had hydraulic properties perunit length similar to those of a desert agave, their highergrowth rates, their higher respiration rates, and the greatersoil volume explored by their roots than for various desertsucculents apparently helped support their high above-groundbiomass productivity Key words: Crassulacean acid metabolism, productivity, root elongation rate, root system, water uptake     

The Development of Tomato Root System in Relation to the Carbohydrate Status of the Whole Plant

The decrease in growth rate of the root system or complete cessationof its growth in developed, fruit-bearing tomato plants areknown phenomena. It has been suggested that a limited supplyof carbohydrates to this organ, due to its relative weaknessin competition with the flowers and developing fruitlets isthe main cause for these disorders. This theory was tested inthe present study with plants grown in an aerohydroponic systemup to the appearance of 12–13 trusses per plant, 172 dafter transplanting. The changes in the contents of carbohydratesin the various organs during this period were monitored. Theconcentrations of soluble sugars and starch in the leaves increasedwith the increase in truss number. The upper stem was foundto contain more carbohydrates than the lower stem, while nosignificant changes in the concentration of these compoundscould be detected in the roots throughout the experiment. Nevertheless,120–130 d after transplanting, the roots of the plants,bearing five to six trusses and two to three inflorescences,ceased growing and remained at the same or a slightly reducedsize for another 40–50 d. Calculations show that at thestage of five to six trusses, 38 g total soluble sugars and35 g starch were stored in the vegetative organs. Therefore,it seems unlikely that carbon deficiency caused by the competitionwith the reproductive organs (mainly developing fruits), affectedthe root growth. Instead, it is suggested that some other factoris responsible. Lycopersicon esculentumMill; carbohydrates; root growth rate; sink-source relationship; soluble sugars; starch