Hydraulic conductance and anatomy of roots of Vicia faba plants infected by Uromyces viciae-fabae

Ph).siologicaland Molecular Plant Patholog~ (1988) 32, 199-207

Hydraulic conductance and anatomy of roots of Vicia faba plants infected by Uromyces viciae-fabae PREENI TISSERA'~a n d P. G. AYRES De,barlment of Biological Sciences, Universityof Lancaster, Lancaster LAI 4 YQ, U.K. (Acceptedfor publication oTune1987)

Infection of faba bean (Vicia faba L.) by rust [Urom.yces riciae-fabae (Pers.) Shroet.] inhibited increasesin the length of tap and primary lateral roots both in soilcolumnsand in solutionculture. In whole root systemsexcised from plants grown in solution culture, rust infectionreduced volume fluxofwater slightlybut increasedhydrauficconductance (Lp) per unit areaofroot or ofendodermal surface; these changes occurred over a range of imposed hydrostatic pressures and temperatures. Rust-induced increasesin Lp occurred in tissueproduced after infection,but not in tissueproduced before infection,the sub-terminal segment. Examination oflateral roots from the upper 18 cm of the soil profileshowed infectionreduced root diameter and increasedspecificroot length (cm root rag- 1dry weight). In tissuesformed after inoculation, infectionhad no effect on the diameter of the cortex or the numberofcortical cellsalong a radius. However, the diameter ofthe endodermal cylinder was reduced in both tap and lateral roots. Since radial resistance to water flow is the major component of total root resistancein faba bean, it is proposed that these changeswere responsiblefor the increased Lp caused by infection.

INTRODUCTION W a t e r a n d turgor potentials of healthy regions of infected leaves, a n d of healthy leaves close to the shoot apex, were lowered when faba beans (Viciafaba L.) growing in drying soil were infected by rust [Uromyces viciae-fabae (Pets.) Shroet] [13]. These changes occurred largely because stomatal control over transpiration was lost when the host's epidermis r u p t u r e d d u r i n g fungal sporulation. However, rust-induced effects on root growth could also have affected shoot water relations. Rust inhibited root dry weight growth, particularly in the mid-depth (moderately low soil water potential) region o f u n w a t e r e d soil columns with a total depth of 54 cm. Such growth differences could, with time, put rusted plants at a n increasing d i s a d v a n t a g e in relation to healthy plants; the faba b e a n is a crop p l a n t whose growth a n d yield are exceptionally sensitive to water stress, even when plants are healthy [3]. This p a p e r reports a study that (1) c o m p a r e d the ability of whole root systems of healthy a n d rusted beans to transport water to the shoot, a n d (2) e x a m i n e d individual roots to determine whether changes observed in whole root systems were localized to those tissues newly formed after inoculation. I n order to control precisely the forces driving water flux, pressure gradients (positive or negative) were applied across excised Abbreviation used in text: Lp, hydraulic conductance. J'Present address: Department of Botany, UniversityofSrijayewardenepura, Nugegoda, Sri Lanka. 0885-5765/88/020199+ 09 503.00]0 © 1988Academic Press Limited

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roots. Changes in volume flux were related to root surface area because (1) length and area, rather than dry weight (the more commonly studied characteristic) are likely to regulate water uptake and transport; and (2) rust infections of groundsel by Puccinia lagenophorae [7], and of wheat by Puccinia striiformis [5], increase specific root lengtb and decrease root diameter. It is probable that changes in root anatomy, which in rusted wheat included a reduction in the diameter of the stele, could affect water transport. Therefore, the anatomy ofroots was studied in the present investigation. MATERIALS AND METHODS

Growth in soil Plants offaba bean ( Viciafaba L., cv. Giant Exhibition Longpod) were grown from seed in a soil mixture of Shamrock compost (Irish Peat Development Authority) and coarse sand ( 1 : 1) in columns 54 cm long x 5 cm diameter. These were constructed from Layflat nylon tubing (Labap Ltd, Huddersfield, U.K.) and were pierced at the bottom to allow drainage. The soil was packed to uniform density, and columns were maintained at room temperature or cooled to 12___2 °C by being held in an aluminium frame immersed in a cooled, constant-temperature water bath. Plants were maintained in a controlled environment room with a temperature of 20 4- 2 °C during the light period and 16 4- 2 °C during the dark period; vapour pressure deficits were 0.58 and 0.45 kPa, respectively. A photon flux density of 200 lamol m - z s- 1 PAR at the height of the stem apex was provided by a mixture ofmercury vapour and sodium lamps (Lowpak, Thorn Industries) for 16 h per day. Plants were well watered each day. Growth in solution culture Seeds were germinated in moist vermiculite in the controlled environment room. Ten days after sowing, when the primary root was approximately 12 cm long, seedlings were transferred to a frame which immersed the roots in a tank, of capacity 2201, ofcooled (12-1-2 °C) and aerated nutrient solution. The solution was made by adding Sangral Soluble Fertilizer (Sinclair Horticulture and Leisure plc, Wigford House, Brayford Pool, Lincoln, U.K.) to distilled water. The fertilizer provided macronutrients as follows: total nitrogen, 62 mg 1-1; phosphorus 27 mg 1-l; and potassium, 52 mg 1-1. Micronutrients supplied were: magnesium oxide, 58lag l-X; iron 23pg l - l ; manganese, 14lag 1-1; copper, 5 lag 1-1; molybdenum, 5 lag 1-x; boron, 7 lag 1-1; zinc, 5 lag I-1. The solution was changed at 3-day intervals and maintained at pH 6.0-1-0-1. The whole system was kept in the controlled environment room described above.

Inoculation Seedlings were inoculated with uredospores of rust [Uromyces viciae-fabae (Pers.) Shroet.] when the third leafwas fully expanded. Spores were freshly collected from stock plants and a suspension made in distilled water was applied to both leaf surfaces using a camelhair brush. Control and inoculated plants were covered with polythene bags to provide a saturated atmosphere for 24 h. Yellow flecks appeared on inoculated leaves after approximately 5 days. Sporulation from approximately 3.4 lesions cm -2 commenced after approximately 8 days, at which time the fifth leaf was unfolding.

Roots of Uromyces-infected Vicia faba plants

201

Effecls of rust and soil temperature on root growth One healthy and one rusted plant shared each soil column. Half the columns were maintained at 12 °C while the other half were kept at 20 °C. Plants were harvested 8 and 15 days after inoculation. The root system was separated into three portions, 0-18, 18-36 and 36-54 cm deep, and then washed to remove soiI particles. The total length ofthe tap root and of primary lateral roots in each portion were determined by a light intercept method as described by Rowse & Phillips [11]; diameters of primary lateral roots were calculated from measurements of length and volume. The roots were then blotted dry and the total volume from each soil region was measured by determining the amount of water displaced from a graduated cylinder. Dry weights were determined after drying to constant weight in a forced draught oven at 80 °C.

Hydraulic properties of roots and their anatomy The hydraulic properties of whole root systems were determined by methods similar to those used previously [14]. Briefly, root systems were excised and sealed in a stainless steel pressure vessel filled with the same nutrient solution as that in which plants had grown. The vessel was connected to a cylinder ofcompressed air. Aeration was maintained via an air stone at the bottom of the vessel, and temperature regulated by a cooling coil inside the vessel which was connected to an external cooled water bath. When the effect of hydrostatic pressure on exudation was examined, root systems were maintained at a constant temperature (16-1- 1 °C), while the pressure was altered in steps of 0.1 MPa. When the effect of temperature was examined, root systems were held at a constant pressure (0-3 MPa) and exudation rates were measured at three different temperatures. After allowing 15 rain for equilibration at each new pressure, exuded xylem sap was removed from the cut stump with a piece offilter paper and the subsequent exudate was collected on pre-weighed, dried filter paper enclosed in a short length of nylon tubing attached to the cut stump of the root system. At regular intervals, usually of 15 rain, the volume of exudate was determined from measurements of the weight of the wet filter paper. The osmotic potential of the exudate was determined after enclosing a disc ofsaturated filter paper in the sample chamber of a vapour pressure osmometer (Wescor, 5100C, Logan, Utah, U.S.A.). After completion of the pressure cycle, the root system was removed from the vessel and root length was measured by the light intercept method. Root volume was measured as above. Surface areas were calculated from measurements of root volume and length. It was assumed that all the roots were perfect cylinders. This is approximately correct for lateral roots, but involves some error for the tap root which tapers slightly from base to tip. The relationship between the exudation rate, osmotic potential and hydrostatic pressure was then used to determine hydraulic conductance (Lp), calculated according to Milburn [6]. To compare the hydraulic properties of root tissues formed before and after inoculation, 7-day-old lateral roots were tagged with cotton thread 1 cm behind the root apex on the day of inoculation. Extension of these lateral roots, and of similarly tagged tap roots, was then measured directly at 3-day intervals. When the tag was approximately 6 c m behind the tip (8-12 days after inoculation), the terminal i cm was excised and discarded, and two further 5-cm long segments (apical, formed after inoculation, and sub-apical, formed before inoculation) were excised. For each segment, the proximal cut

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end was sealed with silicon rubber (Lastic, Kettenbach Dental, Eschenburg, F.R.G.) in a 100-1al capillary tube (Microcap, D r u m m o n d Sci. Co.). The capillary tube was carefully passed through a rubber "suba-seal" which fitted into a perspex plate holding 12 such units. T h e roots were immersed in 500 c m 3 aerated nutrient solution while the capillaries were enclosed in an air-tight perspex chamber. Roots were held at 20 °C and a partial vacuum equivalent to 40 kPa was applied to the chamber. The volumes ofxylem sap that accumulated in the capillary tubes over 30 rain were measured. The cut surface of the distal end of root segments was sealed with silicon rubber which prevented entry of water, or water soluble dyes, into cut xylem vessels. When studies of water flux were completed, the central 1 cm length of each root segment was excised for anatomical studies. Segments were fixed for 24 h in formalin : acetic acid:alcohol ( 5 : 5 : 9 0 , v/v), dehydrated in alcohol, embedded in wax, and 14-pm thick sections were cut on a rotary microtome. Following staining in safranin, mounted sections were magnified using a projecting microscope so that 1 m m of the image represented 1 lain ofthe tissue. Measurements were made of the diameter of the whole root, of the endodermal cylinder, and ofindividual xylem vessels. Since the Hagen-Poiseuille Law, Q = (H r4/ql) A P (where Q is flow per conduit, m 3 s - x, r is radius, l is length, r I is viscosity, m 3 s - I , and AP is the difference in hydrostatic pressure across the system), holds that most axial flow in a root or stem will occur through a few large vessels, for comparisons between tissues from healthy and rusted plants, the vessels were grouped into three arbitrary size classes, diameter > 70 Ilm, 40-70 lain and < 40 I~m. RESULTS

Growth in soil columns The tap root had reached the lowest third ofthe soil colums by 8 days after inoculation at both 12 °C and room temperature. Increase of length between 8 and 15 days after inoculation was slightly, but not significantly, less in rusted plants than in controls at both 12°C (4-8 and 5-1 cm, respectively) and 20°C (7-7 and 8-0cm respectively). Increase oftotal length ofprimary laterals in the lower regions ofthe columns, 18-36 cm and 36-54 cm deep, was inhibited by the lower temperature (Table 1). Rust infection reduced total length of laterals, except below 18 cm depth 15 days after inoculation. T h e diameter of later/xl roots sampled in different regions of the profile was greater in tile upper soil region than in the lowest region and there was a tendency for radius to be reduced by rust infection (Table 1). Mean specific root length (cm m g - t dry weight) of primary laterals in the region 0-18 cm deep increased with time in all plants, except rusted at 12 °C where there was no significant extension, and at both temperatures (Table 2). Mean specific root length was increased by infection. Growth in solution culture Rust infection inhibited the extension of both the tap root and of tagged lateral roots (Fig. 1). In tissues produced after inoculation, the diameter oflateral roots, and of the

Roots of Uromyces-infected Vicia faba plants

203

TABLE 1 Effects of rust infection and soil temperature on total length, and diameter, ofprlmary lateral roots ofVicia faba in different regions of the soil profile. Each value is a mean of six replicates with standard error Root length (cm) Days after inoculation

Soil depth (cm)

Healthy

Rusted

Root diameter (mm) Healthy

Rusted

Soil temp. 12 °C 0-18 36-54 0-18 18-36 36-54

5844-51 . . 10664-86 804-78 .

0-18 18-36 36-54 0-18 18-36 36-54

7434-57 2094-38 894-53 1272-+98 419_+72 89-+53

8

18-36

15

6874-67* 1-104-0.02 . . . . . . 7294-48*** 1-064-0-02 --'l" 0.74-t-0.02 . . .

0.98-+0.04*** 1.04-+0-02ns --t

Soil temp. 20 °C 8

15

5894-51"** 108___13"** --t 11294-68 3674-95 ns 444-35 ns

1"084-0"02 1"064-0"02 0-78--+0-14 0-98+0"02 !.00-+0.04 0-76___0-04

1-044-0"02"* 1"02_0-08 ns 0.64-+0.02 ns 0"844-0"02*** 0.92-+0.04** 0-72-+0-02 ns

***, **, *Difference between healthy and rusted significant in t-test at P=0.001, 0-01 and 0-05, respectively; ns, not significant. "['Nocomparative value for rusted plants.

TABLE 2 Effect of rust infection and soil temperature on specific root length (cm rag- l dry weight) of lateral roots of Vicia faba growing at 0-18 cm depth in soil columns. Each value is a mean of six replicates with stan'dard error

Temperature 12°C 20°C

Days after inoculation 8 15 8

15

Healthy

Rusted

1-79-t-0-09 1'86_+0"12 1"55_+0"13 2"874-0"15

2-514-0-13"** 2"!7_+0"11"* 1"754-0-10" 3'874-0"42***

***, **, *Difference between healthy and rusted significant in t-test at P=0-001,0-01 and 0.05, respectively.

endodermal cylinder of both tap and lateral roots, was reduced by infection (Table 3), but the diameter of the cortex and the number of cortical cells along a radius were not affected in either type of root by infection. The m a x i m u m diameter of xylem vessels in tap roots was not affected by infection but fewer small vessels were present in rusted than in healthy plants. M a x i m u m vessel diameter in lateral roots was reduced by infection.

Hydraulic properties of roots Exudation ofsap from whole excised root systems at 16 °C increased as the hydrostatic pressure gradient increased, and decreased as hydrostatic pressure decreased (Fig. 2).

204

P. Tissera and P. G. Ayres

5

S~ f •~, 3

I

~ ' F ' i

0

3

i

i

I

I

6 9 12 Time after inoculation (days}

15

FiG. 1. Effect ofrust injection on extension of tap ( - - - ) and tagged lateral (

) roots of Hcia

faba grown in solution culture. 0 , Health},; e , rusted. Each point is a mean orB replicates with standard error.

TABLE 3

Effect of rust infection on the anatomy of roots 0f Vicia faba. Measurements were made on sections taken approximatelyfrom 2 cm behind the tip. Each ralue is the mean offour replicates. The experiment was repeated with similar results Healthy Tap Diameter ofroot (pro) Diameter ofendodermal cylinder (pro) No. ofcortical cellsalong the radlus No. of large xylem vessels, > 70 laindiam. No. of medium xylemvessels,40-70 pm diam. No. of small xylem vessels, <40 pm diam. Diameter of largest xylem vessel (pro)

Lateral

1345_+29 5694-14 465_+ 19 1954-6 14.5 4-1-1 7 4-0-2 34- 0-2 --i" 2 ! 184-4.0 10+ !.4 78.7 4-0.8 42-7_ 2-9

Rusted Tap

Lateral

1295±68ns 5344-11"* 3934-10"** 1594-9"** l14-0-4ns 74-0.4ns 24-0.5 --t 34-0-2 1 124-2.0 104-0.6 77-0±l-0ns 38-7±0-8*

***, **, *Differencebetween healthy and rusted significantin t-test at P=0-001, 0.01 and 0.05 respectively,ns, not significant. tNa comparative value for rusted.

Osmotic potential of the sap decreased (from --0-37-t-0.01 M P a for healthy a n d --0.38 M P a for rusted) as pressure increased a n d increased as pressure decreased. At each pressure, the potential of sap from rusted plants was lower t h a n that from healthy plants; for example at 0.4 M P a hydrostatic pressure, the potential of sap from healthy plants was - - 0 . 2 8 + 0 . 0 1 M P a , sap from rusted plants was --0-304-0.01 M P a . At all

Roots of Uromyces-infected Vicia faba plants

/

(a) 0-8 m _o

~

0-6

205 (b) 6O

,r T

g.

15 ~.0

-

Y

E

I0

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20

.-~ E

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o'.,

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o~,

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Fro. 2. Effect ofrust infectionon (a) exudation ofsap from, and (b) hydraulic conductance, Lp, of, excised root systems of Viciafaba held at different hydrostatic pressures at 16 °C. Ascending pressure series ( ), descending pressure series ( - - - ) . Open symbols healthy, closed symbols rusted plants. O, • , Lp of root surface area; [], • , Lp ofendodermal surfacearea. Resultsare from one experiment, which was repeated four timeswith similar results.

TABLE; 4

Effect of rust infection on exudation rate and h.ydraulic conductance of 5-cm segments of individual lateral roots of healthy and rust-infected Vicia faba. A partial vacuum equivalent to 40 k Pa was applied to segments. Each value is a mean offive replicates with standard error

Healthy

Rusted

Exudation rate (m3s- ! x 10-9) Sub-apical segment 0.58 4-0.08 0-58-I-0.08 ns Apical segment 0-70 + 0.14 1.02 -t-0.20" Hydraulic conductance (m3 m-2 s- z MPa- t × 10-6) Sub-ap~ealsegment 8-34- 1-2 8"34-1-3 ns Apical segment 9.64- 1-4 14.44-2-7"* **, *Difference between rusted and healthy significant in t-test at P=0.01 and 0.05, respectively;ns, not significant.

pressures, e x u d a t i o n was more rapid from h e a h h y plants t h a n from rusted plants. However, Lp was increased b y infection whether calculated for u n i t area of root surface or for unit area of e n d o d e r m a l cylinder (Fig. 2). Lp at a constant hydrostatic pressure of 0"3 M P a increased with increasing t e m p e r a t u r e in the range 12, 16, 20 °C, a n d at each t e m p e r a t u r e was greater in root systems from rusted plants t h a n in controls (data not presented). Rust promoted sap e x u d a t i o n from apical segments, formed after inoculation, b u t not from sub-apical segments, formed before inoculation, when a partial v a c u u m equivalent to 40 kPa was applied to excised lateral roots ( T a b l e 4). E x u d a t i o n rate a n d Lp were greater in apical t h a n in sub-apical segments in rusted b u t not in healthy plants.

206 DISCUSSION

P. Tisseta and P. G. Ayres

A rust-induced reduction in the rate ofroot extension was a feature common to tap and lateral roots in soil, at two temperatures, and in solution culture. Some recovery in zones below 18 cm was indicated 15 days after inoculation at a soil temperature of 20 °C. This can probably be attributed to the increasing influence on root development of newly expanded healthy leaves. While reductions in root dry weight have been noted in m a n y plants infected by rusts [1], the few investigations where root length has also been measured suggest that length is either less affected than weight, e.g. in wheat infected by P. striiformis [5], or is not affected, e.g. groundsel infected by P. lagenophorae [7]. T h e present results are, however, similar to those with groundsel in that specific root length was increased by infection, i.e. roots became narrower (Tables I and 3). The effect of rust would probably have been greater if it had been possible to exclude from the measurements those laterals formed before inoculation and, presumably, of fixed diameter. Changes in specific root length enable rusted plants to use dry weight with increased efficiency in tile exploration of soil for unexploited reserves of water or nutrients. However, overall, reductions in root length and depth ofrooting probably have harmful effects on water relations in the field, particularly since the faba bean is characteristically a shallow-rooted species [3, 4]. When root systems o f h e a h h y and rusted plants were given access to an unlimited supply of water and subjected to equal driving forces, due to either an increased or a decreased hydrostatic pressure gradient, systems from healthy plants transported more water than systems from rusted plants. This Was in spite of the greater Lp of each unit of root surface area or endodermal area of rusted plants. Similar results were previously found in whole root systems of barley infected by powdery mildew (Erysiphe graminis hordei) [14]. Reductions in water and turgor potentials which occur soon after infection offaba bean by rust [13] derive, therefore, from the reduced ability ofthe roots to supply the shoot with water, as well as the decreased ability of the shoot to retain water. In the long term, water relations of rusted plants will be modified by changing root to shoot ratios, root distribution, the availability and distribution of soil water and, in some circumstances, by root competition for water with neighbours [8]. Values for volume flux and Lp were similar to those reported for other dicotyledonous root systems (see Salim & Pitman [12] for full references). T h e present work shows for the first time that changes in Lp caused by foliar infection are localized in tissues formed after infection. It is possible that changes in conductance observed when root systems are subjected to increased hydrostatic pressure could be artifacts which arise because water is forced into intercellular spaces normally occupied by air and, consequently, radial transport does not follow the normal route. Therefore, it is important that the effects of rust on conductance were confirmed when water was "pulled through" roots as the result of a partial vacuum applied to the upper end of root segments. Effects of rust on tissues near the root apex are critical because, firstly, conductance normally increases towards the root tip, the terminal 5 cm in the faba bean having greatest conductance [2], and, secondly, root tips have greatest access to untapped reserves ofwater in drying soil. It is proposed that increased Lp is the direct result of the altered anatomy of root tissues. In healthy faba beans the radial resistance to water flow through roots is much greater than the axial resistance, thus, the latter contributes little to total root resistance [9,10]. It should be noted that, from the Hagen-Poiseuille equation, and using the tissue

Roots of U r o m y c e s - i n f e c t e d Vicia faba plants

207

d i m e n s i o n s in T a b l e 3, it is c a l c u l a t e d that rust w o u l d h a v e i n c r e a s e d axial resistivity [9] from 0-291 s m - 3 to 0"386 s m - 3 . Nevertheless, a n increase o f o n l y 3 3 % in this m i n o r c o m p o n e n t is u n l i k e l y to h a v e a m a j o r effect o n total root resistance. T h e results show t h a t L p was i n c r e a s e d b y rust. T h e r e f o r e , c o u n t e r c h a n g e s i n the r a d i a l p a t h w a y o f w a t e r m o v e m e n t were m u c h m o r e i m p o r t a n t a n d o u t w e i g h e d c h a n g e s in the axial p a t h w a y . T h e e n d o d e r m i s p r o b a b l y plays a key role in such c h a n g e s since it represents the first p o i n t o n the r a d i a l p a t h w a y at w h i c h w a t e r m u s t m o v e from a p o p l a s t to symplast. T h e d i s t a n c e b e t w e e n the e n d o d e r m i s a n d the x y l e m vessels was s h o r t e n e d b y infection, a n d this c h a n g e a l o n e m a y h a v e i n c r e a s e d c o n d u c t a n c e , o r it is possible t h a t infection affected t r a n s p o r t properties o f the e n d o d e r m a l m e m b r a n e s . T h e reasons for i n c r e a s e d r a d i a l c o n d u c t a n c e c a n o n l y b e s p e c u l a t e d u p o n , p a r t i c u l a r l y since there is n o d e t a i l e d k n o w l e d g e o f factors g o v e r n i n g r a d i a l flow o f w a t e r in roots o f h e a l t h y plants. P. T i s s e r a t h a n k s the C o m m o n w e a l t h S c h o l a r s h i p C o m m i s s i o n a n d the British C o u n c i l for f i n a n c i a l s u p p o r t .

REFERENCES 1. AYRES,P. G. (1978). Water relations ofdiseased plants. In Water DeficitsandPlant Growth, Vol. V, IVaterand Plant Disease, Ed. by T. T. Kozlowski, pp. 1-60. Academic Press, New York. 2. BROUWER,R. (1953). Water absorption by the roots of Viciafaba at various transpiration strengths. II. Caffsal relation between suction tension, resistance and uptake. Proceedings A'oninklijke aVederlandse Akademie Van IVetenschappen56, 129-136. 3. HEBBLETHWAITE,P. (1982). The effects ofwater stress on the growth, development and yield of Vieiafaba L. In Faba Bean Improvement, Ed. by G. Hawtin & C. Webb, pp. 165-176. Martinus Nijhoff, Amsterdam. 4. KAX~,.~IANOS,A.J. (1981). Case examples of research progress in drought-stress physiology: Viciafaba. In Water Stress on Plants, Ed. by G. M. Simpson, pp. 199-234. Praeger, New York. 5..~IARTIN, g. E. & HENDRnX,J.W. (1974). Anatomical and physiological responses of Baart wheat roots affected by stripe rust. Washington Agricultural Experimental Station, Technical Bulletin 77, 1-17. 6. *ItLBURN,J.A. (1979). Water Flow in Plants. Longman, London. 7. PAUL,N. D. & AYRES,P. G. (1986). The effects ofnutrient deficiency and rust infection on tile relationship between root dr). weight and length in groundsel (Seneciovulgaris L.). Annals of Botany 57, 353-360. 8. PAUL,N. D. & AYRES,P. G. (1987). Water stress modifies intraspecific interference between rust (Purcinia lagenophorae) infected and healthy groundsel (Seneciovulgaris). New Ph.)'tologist 106, 555-566. 9. REtD,J. B. & HUTCtttSON,B. (1986). Soil and plant resistances to water uptake by l'z~iafaba L. Plant andSoil 92, 431-441. 10. RowsE, H. R. & GOOD.~tAN,D. (1981). Axial resistance to water movement in broad bean (Viciafaba) roots..]ournal of Experimental Botany 32, 591-598. 11. RowsE, H. R. & PmLLtPs, D. A. (1974). An improvement for estimating the total length ofroots in a sample. Journal of Applied Ecolog~ 11,309-314. 12. SALt.xt~`\I. & PxT~tAN~M. G. ( ~984 ). Pressure-induced water and s~ute ~ w thr~ugh p~ant r~ts. ~urnal ~f Experimental Botany 35~ 869-88 !. 13. TXSSERA,P. & AYRES,P. G. ( ! 986). Transpiration and the water relations offaba bean (Vidafaba) infected by rust ( Uromycesvidae-fabae), aVewPhytologist 102, 385-395. 14. WALTERS,D. R. & AYRES,P. G. (1982). Water movement through root systems excised from healthy and mildewed barley: relationships with phosphate transport. PhysiologicalPlant Pathology20, 275-284.