EVIDENCE ON THE PATHWAYS OF PHOSPHORUS TRANSFER BETWEEN VESICULAR-ARBUSCULAR MYCORRHIZAL PLANTS

Transcription

1 Neio Phytol. (1986) 104, EVIDENCE ON THE PATHWAYS OF PHOSPHORUS TRANSFER BETWEEN VESICULAR-ARBUSCULAR MYCORRHIZAL PLANTS BY E. I. NEWMAN AND K. RITZ* Department of Botany, University of Bristol, Bristol BS8 1UG, UK {Accepted 6 May 1986) SUMMARY Mycorrhizal hyphae can link one root to another, and it is possible that phosphorus passes from plant to plant by these links. We present evidence on whether this 'direct transfer pathway' is the major route between mycorrhizal plants, or whether most phosphorus passes from the roots of one plant to the soil before heing taken up hy the other plant's roots or its associated mycorrhizal hyphae (the 'soil pool pathway'). The time-course of loss of ''^P from Lolium perentie L. roots to solution was measured after '^P had heen fed to leaves. Another experiment indicated that the amount of ''^P lost to soil was not influenced by mycorrhizal infection. "'-P was applied to soil in which Plantago latueolata L., mycorrhizal or non-mycorrhizal, was growing and the time-course of uptake determined. Llsing these results, two alternative models, the 'direct transfer model' and the 'soil pool model', were used to predict the time-course and amount of '''^P transfer from L. perenne to P. laticeolata. The predictions were then compared with measured transfers between these two species presented here and in a previous paper. The soil pool model's predictions of both amount and time-course of transfer gave the better fit to observation. The evidence thus suggests that direct hyphal links hetween roots are not important in phosphorus transfer between these plants. Key words: Hyphae, mycorrhiza, nutrient transfer, phosphorus, roots. INTRODUCTION If '^P is fed to the shoot of one plant, the isotope can later be detected in neighbouring plants. This has been demonstrated for woody and herbaceous plants, both in pot experiments and under field conditions (Rakhteyenko, 1958; Whittingham & Read, 1982; Chiariello, Hickman & Mooney, 1982; Ritz & Newman, 1984). Such transfer of mineral nutrients between intact plants could be of great ecological significance, if it reduces differences in nutrient concentrations between competing plants. Whittingham & Read (1982) showed that transfer of ^^P can occur between herbaceous plants when they are not infected with vesicular-arbuscular mycorrhiza (YAM), but can be substantially increased by mycorrhizal infection. They measured ' ^P transfer during the first two days after it was fed. Results of Ritz & Newman (1985) suggest that mycorrhizas continue to cause increased transfer over several weeks; however, our mycorrhizal plants were grown at a different time from the non-mycorrhizal plants. In this paper we describe an experiment designed to confirm whether ^-P transfer is enhanced by mycorrhizas over a period of several weeks. Erancis, Einlay & Read (1986) fed * Present address: Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen AB9 2QJ, UK X/86/ $03.00/ The New Phytologist

2 78 E. I. NEWMAN AND K. RITZ nutrients (non-radioactive) to part of the root system of a Festuca ovina or Plantago lanceolata plant (the 'donor'); the other part of the donor's root system mingled with the roots of other plants (the'receivers'). The receivers' shoots increased in phosphorus content if the plants were mycorrhizal but not if they were non-mycorrhizal. In this paper we consider the possible pathways by which ^' ^P, and presumably all phosphorus, moves from plant to plant, and in particular the mechanism by which mycorrhizal fungi increase this movement. Since mycorrhizal fungi can form direct links between roots, including roots of different species (Heap & Newman, 1980; Duddridge, Malibari & Read, 1980; Chiariello et al., 1982), phosphorus might move directly from one root system to another through these hyphal connections ('direct transfer pathway'). Alternatively, it may pass from the donor roots into the soil and then be taken up by the receiver's roots or associated mycorrhizas ('soil pool pathway'), enhanced transfer being caused by more efficient uptake by mycorrhizal than non-mycorrhizal plants. Determining which of these pathways operates is important, since the direct transfer pathway would allow nutrients to bypass chemical and physical fixation and also immobilization by saprophytic microorganisms in the soil. Francis et al. (1986) have argued that their results demonstrate that the direct transfer pathway was operating in their experiment, but we do not consider the evidence conclusive. In their experiment the donor plant {Festuca ovina or Plantago lanceolata) bad a split root system, each half in a small pot of dune sand. In one of the pots were four receiver plants, two Arabis hirsuta and two F. ovina or P. lanceolata. The receivers were younger and presumably smaller than the donor. Replication in the experiment was low and there were many non-significant differences, but the general trend was that the receiver F. ovina and P. lanceolata increased most in weight and N and P content if they were mycorrbizal and tbe donor was fed nutrients via the other half of its root system (treatment M-Nut), increased less if they were mycorrhizal but the donor was not fed (M-Dist), and least if the donor was fed but they were non-mycorrhizal (NM-Nut). Arabis hirsuta was never mycorrhizal and showed no significant difference in weight between treatments; its nutrient content was not determined. These results could be explained by nutrient transfer between mycorrhizal plants via hypal links. However, there is no definite proof that nutrient transfer between plants occurred at all. In tbe M-Dist treatment the larger 'donor' plant was presumably competing strongly with the smaller 'receiver' plants for the limited amounts of available nutrients in the dune sand. Supplying nutrients to one half of the donor's root system might reduce its uptake through the other half, thus leaving more for the 'receivers'. Their poor growth in the NM-Nut treatment nnay merely reflect a poor ability to grow in tbe dune sand when non-mycorrhizal. In our opinion there is as yet no definite evidence that phosphorus can pass from plant to plant via direct hypal links. The main aim of this paper is to present new evidence on the pathway of phosphorus transfer between intact plants. We have determined the time-course of "'^P loss from a plant's roots after the shoot was fed a pulse of '''^P; we have also added ^' 'P to soil in which an established plant was growing and measured the uptake of '"^P by tbe plant. We describe here the results of these experiments and use them to predict the time-course and total amount of ^^P transfer between plants under the two hypotheses, (i) that all phosphorus has to pass into the soil before it can be taken up by the receiver, and (ii) that all phosphorus passes directly from donor to receiver. These predictions are then

3 Phosphorus transfer between mycorrhizal plants 79 compared with actual ''^P transfer. Time courses of ^'-^P transfer from Lolium perenne to Plantago lanceolata previously measured in three experiments (Ritz & Newman, 1985) are shown in Figure 3. All three show a lag, i.e. the rate of arrival in the receiver shoot increased over some days after ''^P was fed to the donor; this lag has not yet been explained. A similar lag was found by Cooper & Tinker (1978) in uptake of ^^P via mycorrhizal hyphae into clover or onion. They concluded from their results that the lag could not be due to the time taken for build-up of ^^P in a pool within the fungus or host plant, and they suggested that it might indicate an induction period caused by the previous absence of phosphorus in the external medium. Such an explanation could not apply to our results, since the concentrations of phosphorus in various parts of the system never changed suddenly. An aim of this paper is to see whether the measured time-courses of ^^P transfer can be better explained by the direct transfer or soil pool pathways. The time-course of ^^P loss from roots was measured on plants growing in solution culture; these plants were not mycorrhizal. We wish also to know whether loss from roots to soil is influenced by the mycorrhizal status of the roots. Since ''^P cannot be quantitatively extracted from soil without grave danger of extracting some of the root -'^P as well, we used a non-mycorrhizal plant growing in the same pot as a 'detector'. Plantago lanceolata, either mycorrhizal or not, was grown with Brassica oleracea, which does not normally form mycorrhizas; ^^P was fed to the Plantago and the amount taken up by the Brassica was later determined. MATERIALS AND METHODS General Plants were grown in 10 cm pots of soil in a glasshouse, with supplementary light and heat in the colder months. About a week before ' ^P apphcation they were transferred to a growth room where the experiments took place; the temperature was constant at 20 C, the relative humidity 50 to 75 %, with 16 h photoperiod at 150 //mol m ' ^ s"i. The soil was a clay-loam, low in available phosphorus, collected from permanent pasture, with ph in water 4-8, phosphorus extractable by Truog's reagent 2-4//g g"^ When mycorrhiza-free soil was required it was subjected to 1 Mrad y-irradiation. Subsequent reinoculation, if required, was with roots taken from the pasture where the soil was collected. At least two VAM fungi were present, one of them a flne endophyte {Glomus tenue sensu lato). To measure the time-course of arrival of ^' 'P in Plantago lattceolata L. shoots, marked leaves were placed every 1 or 2 d in a Perspex holder attached to a scintillation probe, and radiation counted for 30 or 60 s. A background count, with the leaf removed, was taken immediately afterwards. Total ^'^P in shoots and roots was determined by digesting in H2SO4- -Na2SO4 +Se followed by liquid scintillation counting. All counts quoted have been corrected for background, colour quenching and isotope decay. Amount of mycorrhizal infection was assessed on spare plants, grown at the same time as those for each experiment. Roots were stained with trypan blue (Phillips & Hayman, 1970), and percentage infection measured by the gridline intersect method (Giovannetti & Mosse, 1980). Experiment 1 : '^' ^P transfer between plants One Lolium perenne L. cv S23 and one Plantago lanceolata L. were grown per 10 cm pot. The soil was either non-mycorrhizal (irradiated) or mycorrhizal

4 8o E. I. NEWMAN AND K. RITZ (irradiated and reinoculated). After 44 weeks each Lolium plant was fed '''^P by cutting the tips off two or three leaves and immersing the cut ends in 1-3 MBq of carrier-free ''^P as HgPO^ in 0-5 ml water in a glass vial. After 24 h the vial and the immersed portions of the leaves were removed. There were five replicate pots per treatment. The shoots were harvested 29 d after ^^P feeding, and the ' ^P content of the shoots was determined by digestion and liquid scintillation counting. Experiment 2: loss of '^^P from roots Lolium perenne was grown in the growth room in aerated nutrient solution. The containers were glass beakers painted black on the outside; there were four plants per beaker and four replicate beakers. The solution contained NH^NOg 7'2mM, NaH^PO^ 3-3 mm, KCl 3 0 mm, CaCl^ 2-5 mm, MgSO^ 1-0 mm, FeNaEDTA 0018 mm, plus trace quantities of B, Mn, Cu, Zn and Mo. After 5 weeks each plant was fed 2'5 MBq carrier-free '"'P via leaf tips. The amount of ^' ^P in the solution bathing the roots was determined on days 1, 2, 3, 4 and thereafter every 2 d by removing 5 ml (days 1 to 3) or 15 ml from each beaker, and using a liquid scintillation counter to measure ^^P by Cerenkov radiation. The solution level in the beakers was made up daily with water, and from day 4 onwards the solution was replaced by fresh nutrient solution every 2 d just after the sample had been taken. The plants were harvested on day 22, and the '^P contents of the roots determined by digestion and liquid scintillation counting. Experiment 3 : effect of mycorrhizal status of donor This experiment measured ^^P transfer from Plantago lanceolata to Brassica oleracea L. cv January King. Ten pots were filled with irradiated soil; five of these were reinoculated. In each pot one P. lanceolata and one B. oleracea were grown. Each P. lanceolata seed was planted into a column of soil 20 mm diameter x 25 mm high, contained within a plastic tube. The roots passed through this column into the remaining soil of the pot. After 12 weeks the P. lanceolata was fed '^P by removing the plastic tube and washing away the soil column, revealing the top of the root system. Three first-order lateral roots were then cut off and the ends dipped into a micro test tube containing 0 5 MBq of carrier-free H^-^^PO^ in water. After 24 h the tube and dipped roots were removed. (This system of labelling was used instead of foliar feeding because P. lanceolata often took up only small amounts of liquid through its leaf ends.) After labelling the pots were watered via their saucers only. The donor and receiver shoots were separated by a Perspex screen. The plants were harvested 19 d after labelling. Shoots and roots were dried, weighed, digested and their ''^P content determined as described above. Experiments 4 and 5 : uptake from soil Plantago lanceolata was grown in 10 cm pots of soil which had been irradiated; half the pots had been reinoculated with mycorrhizal roots. There were five plants per treatment in each experiment. After 14 weeks (Expt 4) or 13 weeks (Expt 5) carrier-free H-j^^PO^ dissolved in 25 ml water was poured on to the surface of the soil in each pot. Independent tests (A. P. Jupp, unpublished) showed that 90% of the '^P distributed itself fairly evenly through the top 4 cm of soil, only 10% reaching the remaining 6 cm. The ''^P content of leaves was measured every 1 to 2 d with the scintillation probe, in Expt 4 on four leaves per plant, in Expt 5 on all leaves which were large enough to fit into the leaf holder. The plants were harvested 20 or 22 d (F>xpts 4 and 5 respectively) after labelling; they were then digested and their ^^P contents were determined as described above.

5 Phosphorus transfer between mycorrhizal plants 8i RESULTS Transfer of ^'^P betwen pla?its In Expt 1 the shoots of the donor {Lolium) and receiver {Plantago) were similar in size when mycorrhizal, but in the non-mycorrhizal treatment Plantago grew poorly and was greatly exceeded in size by Lolium (Table 1). The roots were too densely intermingled to be separated for weighing or ^'^P determination. In the mycorrhizal treatment the combined root systems had 70 % of their root length infected (SE = 3). There was no mycorrhizal infection in roots from the control treatment. Transfer of ^'^P from Lolium to Plantago was increased by mycorrhizal infection (Table 1). Even after allowing for differences in size of the receiver plants, transfer was three times as much in mycorrhizal pots. Loss of ^'^P from roots In the solution culture experiment (Expt 2) there was already significant loss of ^T to the solution 24 h after the start of feeding to the leaves (Fig. 1). Loss from the plants continued over the 22 d of measurement, though the rate declined Table 1. Shoot dry weight and amount of ^^P transfer in Experiment 1 (a) Shoot dry weight (g) at final harvest. +VAM -VAM Donor (Lolium perenne) Receiver (Plantago lanceolata) 'T in receiver shoot -xloo P in donor shoot in receiver shoot/receiver shoot wt (g) '^^P in donor shoot (b) Amount of ''^P transferred from donor to receiver. O ±0-009 P < f xloo ±0-06 P < 0-05t t Statistical significance of difference. Figures are means+ SE. Fig. 1. Cumulative loss of ''^P from roots of Lolium peremie to nutrient solution in Expt 2; ''-^P was fed to shoots on day 0. Vertical hars show standard errors. All values have heen corrected for isotope decay.

6 82 E. I. NEWMAN AND K. RITZ somewhat. The total loss during this period was 26 % of the ''^P present in the roots at the end. In the Plantago-Brassica experiment (Expt 3) the inoculated Plantago had 20% of the root length infected with VA mycorrhiza (se = 7). There was no VA infection in the Brassica or the uninoculated Plantago. There was no consistent difference in root or shoot weight between mycorrhizal and non-mycorrhizal Plantago. Transfer of ''^P from donor to receiver, expressed as (''^P in receiver shoot/^^p in donor root) x 100, was 14 in +VAM pots and 1-0 in VAM pots. This difference was not statistically significant by ; test; the pooled standard error was 0-6. Uptake of ^^P from soil Mycorrhizal infection did not influence shoot weight in Expt 4; roots were not harvested in this experiment. In Expt 5 mycorrhizal infection caused a small reduction in shoot weight and a two-fold reduction in root weight (Table 2). There was no significant effect of mycorrhizas on ' *P uptake in either experiment. Eigure 2 shows the time-courses of uptake of ''^P into the leaves. Using the MLP computer package (Ross, 1980) curves of the form /. max n.. e (1) Table 2. Mycorrhizal infection, plant weight and ^^P uptake from soil by Plantago lanceolata in Experiments 4 and 5 Mycorrhizal status + VAM -VAM Pooled SE Statistical significance of diflerencef Experiment 4 Mycorrhiza (%) Shoot dry weight (g) '' 'P in shoots (cpm) ] 1-7 NS NS Experiment 5 Mycorrhiza (%) Shoot dry weight (g) Root dry weight (g) '^P in shoots! "'^P in roots! NS NS t By t test. * P < 0-05; NS, not significant.! Expressed as % of that applied to soil. have been fitted, where P^ = ^^P counts at time t, P^^^ is the asymptotic ^'^P count, and /s is a constant. The curves were forced through the origin. The fit was close in all cases, as shown by the high r'^ values (Table 3). Individual leaves gave similar uptake curves, irrespective of age, so leaf ageing was not a major influence on the shape ofthe curves. The k values (Table 3) are a measure of uptake rate; e.g. when k = 0169 d"s at any time the plant is taking up 16-9% of the ^^p remaining in the soil per day. To obtain an indication of variability, a curve was fitted to the data from each replicate pot, and the replicate k values thus obtained were used to calculate standard errors. There were no significant differences between any ofthe four k values in Table 3, by analysis of variance.

7 Phosphorus transfer between mycorrhizal plants (b) i Days Fig. 2. Cumulative uptake of -'"P from soil into leaves of Ptantago tanceotata; "^P was added to the soil on day 0. All values have been corrected for isotope decay, (a) Expt 4, (b) Expt 5. 9 Plants mycorrhizal; O non-mycorrhizal. Continuous lines are best fit to Eqn {]) (see text). Table 3. Values of r^ {goodness of fit to Eqn I), atid k {instantaneous uptake rate) for uptake of '^^P from soil by Plantago lanceolata in Expts 4 and 5 Experiment 4 -I-VAM -VAM Pooled SE Experiment 5 + VAM -VAM Pooled SE r-' k (d-') DISCUSSION Experiment 1 confirmed that mycorrhizal infection increases ^^P transfer between intact plants. The plants were much older than those we used previously to study transfer (Ritz & Newman, 1985), and the period for ^' ^P transfer was much longer than that allowed by Whittingham & Read (1982). The time-course of ^^P uptake by Plantago from soil fitted well to negative exponential curves (Fig. 2, Table 3), which agrees with the hypothesis that each day the plant took up a certain set proportion of the labile pool of phosphorus. This proportion is estimated by the mean k value to be 0'160 per day, indicating that the average half-life or residence time of a phosphorus atom in the labile pool is 4-3 d. Uptake of ^^P had virtually ceased by the end of the experiments, yet in Expt 4 more than half of the ^^P added still remained in the soil (Table 2). Evidently the ^^P can rapidly move into forms or sites not exploitable by roots or mycorrhizal fungi. In neither Expt 4 nor 5 was there any significant difference in ^'^P uptake between mycorrhizal and non-mycorrhizal plants (Table 2). The root weight was substantially reduced by mycorrhizal infection, and this may have offset any promotion of phosphorus uptake by the mycorrbizal fungus. Using diameter measurements made on P. lanceolata growing in this soil (Christie, 1976) we

8 4 E. L NEWMAN AND K. RITZ estimate the density of roots in Expt 5 to be about 100 to 200 cm cm"'; at sucb a high density the increase in phosphorus capture due to roots having external mycorrhizal hyphae may be limited. In Expt 3 the Brassica plants, which never became mycorrhizal, were intended as 'sensors' of the ''^P released by the donor roots into the soil labile pool. Assuming they did act in this way, the results indicate that mycorrhizal infection of the donor has no effect on the rate of loss of *'^P from its roots. Thus mycorrhizas influenced neither the loss of ' ^P from the donor to soil nor its uptake from soil by the receiver, yet they did increase transfer from donor to receiver. This is in itself an interesting finding. It is known that some plants, including some grass species, do not show increased growth or phosphorus uptake as a result of mycorrhizal infection, at least on some soils (Powell, 1977; Bethlenfalvay, Bayne & Pacovsky, 1983; Allen & Allen, 1984). However, the sharing of phosphorus among such species, and hence their relative success when growing together, could still be influenced by mycorrhizas. When a phosphorus atom in soluble form passes from a living root to the surrounding soil its possible fates are to be (1) reabsorbed by the root that it came from, (2) taken up by another root of the same plant, (3) taken up by a free-living microorganism, (4) fixed chemically or physically, or (5) taken up by another plant; (1), (2) and (5) could involve mycorrhizas. Of these five sinks for phosphorus, (1) and (3) are likely to be greater close to the surface of the donor root than in bulk soil; therefore when mycorrhizas promote plant-to-plant phosphorus transfer more than they promote uptake by plants from soil, it is likely to be by reducing the relative amounts of phosphorus going to (1) and (3). This they could do by direct transfer, i.e. hyphae capturing phosphorus while it is still within the donor root, or by mycorrhizal hyphae growing abundantly close to the surface of the donor root. Mycorrhizal hyphae, by virtue of their ability to exploit 'microbe sized' microsites, are likely to be better placed than the receiver root itself to compete against free-living microorganisms for phosphorus released by the donor. We therefore need to consider our results more carefully to decide whether the direct transfer pathway or the soil pool pathway predominated in the movement of phosphorus from donor to receiver. PREDICTING PHOSPHORUS TRANSFER We now attempt to predict, using the results of Expts 2 to 5, the amount of ^^P transferred between plants. We use two alternative hypotheses. (1) The 'soil pool model' assumes that all '^P which passes from donor to receiver does so by passing out of the donor's roots and mixing with the pool of available phosphorus in the soil before being taken up by roots or mycorrhizal fungi of the receiver plant. (2) The 'direct transfer model' assumes that all '' ^P which passes from donor to receiver does so by a direct pathway which involves no mixing with any soil pool. The predictions are compared with actual transfers measured between mycorrhizal and between non-mycorrhizal plants. If most transfer between mycorrhizal plants is by hyphal links we should expect the observed transfer between mycorrhizal plants to conform to predictions by the direct transfer model but transfer between non-mycorrhizal plants to fit the soil pool model. The assumptions underlying the soil pool model are as follows. (1) The loss of '^P from roots to soil is the same, both in time-course and in amount (expressed as a proportion of ''^P in the root), as in the solution culture

9 Phosphorus transfer between mycorrhizal plants 85 experiment (Fig. 1). Since no difference in loss of ^'^P between mycorrhizal and non-mycorrhizal plants was found in Expt 3, the data of Fig. 1 are assumed to apply to both. Some extrapolation of these data was necessary to estirnate ^-P loss after the last measurement. Since the rate of loss showed no consistent increase or decrease during the last 8 d the mean rate for that period was assumed to continue after day 22. (2) The ^^P lost mixes fully with a pool in the soil. From this a constant proportion is removed each hour; since there was no significant difference between the four k values in Table 3, their mean value of 0-16 d~' (= h"') is used for this uptake rate. (3) Of this loss from the soil pool, 24% goes into the receiver shoots. This is the mean of the + VAM and VAM values from Expt 5 (Table 2), which did not differ significantly from each other. A simple iterative computer program calculated ^ ^P movement into the receiver shoot each hour by the following equations: R^ = j, (2) where 5j = amount of ^^P in soil pool at time t, i?^ = amount of ' ^P removed from the soil pool during that hour, R^ = amount of ^^P accumulated in the receiver shoot up to time t, and Lj = loss of ' ^P from the donor to the soil pool during that hour. The direct transfer model assumes that '^'^P passes from the donor immediately to the receiver at the same rate as the loss to the solution in Expt 2. This views the mycorrhizal fungus as absorbing any ^^P released across plasmalemmas of root cells. Of this ^^P passing to the receiver plant, 62 % goes into its shoot, this being the mean value for mycorrhizal (60-9%) and non-mycorrhizal (62-8%) in Expt 5. Table 4 gives the total amount of ^' 'P transfer to the receiver shoot predicted by the two models and the amounts actually measured in whole digested shoots. All the measured values are for Lolium perenne (left intact throughout the experiment) as donor and Plantago lanceolata as receiver, all growing in the same soil as was used in this paper. Figure 3 shows the time-courses predicted by the models compared with those measured by Ritz & Newman (1985). To aid comparison of the time-courses, each curve is expressed as a percentage of its value on day 22, except for the experiment (symbol ) which only extended to day 19, which has been set on that day to a similar value to the other experiment with mycorrhizal plants. The predictions should be viewed with caution, since the models are simple. It is unlikely that plants grown in phosphorus-deficient soil will lose ^^P at exactly the same rate as plants in nutrient solution with ample phosphorus. It is also unlikely that transfer from plant to plant by mycorrhizal hyphae, if it occurs, will be exactly the amount lost by roots to solution culture: the actual amount might be either more or less, more because the fungus obtains ^^P that would never reach the outer surface of the root, or less because part of the root length has no fungal infection. However, the ttnie-course of direct transfer is likely to follow the direct transfer model even if the total amount of transfer does not. The models also contain no information about amounts of root or distances between donor and receiver roots, which may influence the time-course (3) (4)

10 86 E. I. NEWMAN AND K. RITZ Table 4. Amounts of ^^P transferred from Lolium perenne to Plantago lanceolata, predicted by two alternative models, and as measured in experiments Time* (d) Predicted transfer Soil pool model Direct transfer model Measured transfer -VAM (0-4) VAM (3-3) Source of datat Here 2 * Days after ''^P fed to donor shoot. t 1, 2, 3, Expts 1, 2 and 3 (respectively) of Ritz & Newman (1985); Here, this paper, Expt I. Transfer expressed as (-''^P in receiver shoot/-"p in donor root) X 100. ( ) indicates -'^P in donor root was not measured, so calculation assumes ratio of (''^P in donor shoot/'^p in donor root) was the same as in the corresponding treatment of another experiment. Fig. 3. Time courses of transfer of ^^P from Lo/ium perenne to P/antago /aneeolata predicted by two models and measured in three experiments hy Ritz & Newman (1985). -'"'P fed on day 0. Transfer up to day 22 set to 100%, except for Expt 3 where final value on day 19 set to 80%. Measured values: Expt 1 (mycorrhizal), Expt 3 (mycorrhizal), O Expt 2 (non-mycorrhizal). Predictions:, direct transfer model;, soil pool model. and total amount of transfer. In spite of these limitations we believe comparison of the predicted and observed transfers is illuminating. The predicted transfer is, as would be expected, substantially less by the soil pool model than the direct transfer model (Table 4). Even so, transfer predicted by the soil pool pathway is higher than or about equal to the actual transfer, between either mycorrhizal or non-mycorrhizal plants. Thus it is not necessary to postulate any direct transfer to account for the amount of ^^p transferred. The soil pool model predicts more closely than the direct transfer model the time-course of ^^P transfer between intact plants (Fig. 3). It correctly predicts that there will be a lag in transfer, although the observed lag was in fact greater than predicted, especially in non-mycorrhizal pots. The lag is due to the soil pool taking some days to build up its concentration of ^^p. The predictions of the direct transfer model are less firmly based than those of the soil pool model, but it is reasonable to

11 Phosphorus transfer between mycorrhizal plants 87 assume, at least, that there would be a greater total amount transferred by direct hyphal links than via the soil pool and with less lag, if any. It appears, therefore, that movement of phosphorus between Lolium perenne and Plantago lanceolata, whether mycorrhizal or not, was predominantly by loss of phosphorus from L. perenne roots to soil, followed by its uptake by P. lanceolata roots or mycorrhizal hyphae. ACKNOWLEDGEMENT This research was supported by a grant from the Natural Environment Research Council REFERENCES ALLEN, E. B. & ALLEN, M. F. (1984). Competition between plants of different successional stages: mycorrhizas as regulators. Canadian Journal of Botany, 62, BETHLENFALVAY, G. J., BAYNE, H. G. & PACOVSKY, R. S. (1983). Parasitic and mutualistic associations between a mycorrhizal fungus and soybean: the eflect of phosphorus on host plant-endophyte interactions. Physiologia Plantarutn, 57, CHIARIELLO, N., HICKMAN, J. C. & MOONEY, H. A. (1982). Endomycorrhizal role for interspecific transfer of phosphorus in a community of annual plants. Science, 217, CHRISTIE, P. (1976). Interactions between root micro-organisms and grassland plant species in mixtures and monocultures. Ph.D. thesis. University of Bristol. COOPER, K. M. & TINKER, P. B. (1978). Translocation and transfer of nutrients in vesicular-arbuscular mycorrhizas. II. Uptake and translocation of phosphorus, zinc and sulphur. Neiv Phytologist, 81, DuDDRiDGE, J. A., MALIUARI, A. & READ, D. J. (1980). Structure and function of mycorrhizal rhizomorphs with special reference to their role in water transport. Nature, 287, FRANCIS, R., FINLAY, R. D. & READ, D. J. (1986). Vesicular-arbuscular mycorrhiza in natural vegetation systems. IV. Transfer of nutrients in inter- and intra-specific combinations of host plants. Nexo Phytologist, 102, GIOVANNETTI, M. & MOSSE, B. (1980). An evaluation of techniques for measuring vesicular-arbuscular mycorrhizal infection in roots. New Phytologist, 84, HEAP, A. J. & NEWMAN, E. 1. (1980). Links between roots by hyphae of vesicular-arbuscular mycorrhizas. New Phytologist, 85, PHILLIPS, J. M. & HAYMAN, D. S. (1970). Improved procedures for clearing roots and staining parasitic and vesicular-arbuscular mycorrhizal fungi for rapid assessment of infection. Transactions of the British Mycologieal Society, 55, POWELL, C. L. (1977). Mycorrhizas in hill country soils. V. Growth responses in ryegrass. Neto Zealand Journal of Agricultural Research, 20, RAKHTEYENKO, I. N. (1958). The transfer of mineral nutrient substances from one plant to another through the interaction of their root systems. Botaniclmii Zlnirnal, 43, RITZ, K. & NEWMAN, E. I. (1984). Movement of-'^p between intact grassland plants of the same age. Oikos, 43, RITZ, K. & NEWMAN, E. I. (1985). Evidence for rapid cycling of phosphorus from dying roots to living plants. Oikos, 45, Ross, G. J. S. (1980). Maximum Likelihood Program. Rotbamsted Experimental Station, Harpenden, Herts. WHITTINGHAM, J. & READ, D. J.(1982). Vesicular-arbuscular mycorrhiza in natural vegetation systems. III. Nutrient transfer between plants with mycorrhizal interconnections. New Phytologist, 90,

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