Xylem development in relation to water uptake ... - Wiley Online Library

New Pliytot. (1993), 125, 93-99

Xylem development in relation to water uptake by roots of grapevine ( Vitis

vinifera L.) BY E. M A P F U M O , D. A S P I N A L L , T.

HANCOCK^

AND M. S E D G L E Y

Department of Horticulture, Viticulture and Oetiology, ^ Department of Plant Science, University of Adelaide, Waite Agricultural Research Institute, Glen Osmond 5064, South Australia {Received 14 October 1992; ciccepted 31 March 1993)

SUMMA RY

Axial resistittice (/?„) was e.stirtiated from xylem diameter tnieasuretnents obtaitied from periodic acid atid toluiditie blue O (PAS-TBO) staitied sectiotis of Vitis vinifera L. cv. Shiraz. Multiple litiear regressioti showed a strotig negative felationship between axial resistance atid either root diameter or distatice frotn the root tip. Water stress treatmetits did not affect the relationships, but plant age significatitly influenced the ititercept of the legrcssion. T h e use of both lignin atid cytoplasm stain showed that some vessels retaitied degenerating protoplastn which would impede water flow. Iticlusioti of these vessels iti calculatiotis of axial resistatTce could account for the underestimatioti reported by some authors in cotiiparisoti with experitnental detertninatiotis. Calculations based on the assumptioti that all xyletn vessels, large atid stnall, ate itivoKed iti axial water cotiductioti showed that use of tneati xylem diatneter tnight result in overestitnatitig \'alues by a tnagtiitude of between 1-7 atid 4-4. The use of individual .xylem x'essel diameters gave tnore accurate estitnatioti of axtal resistatice. Sotne adjacetit secotidary xylem and metaxyletn vessels were observed to merge itito sitigle vessels as a result of breakdown of the wall between thetn. Implicatiotis of sueh a developmetital phetiometion are discussed. Key words: Axial resistatice, xylem vessels, Vitis vinifera (grapevine), roots, water uptake.

I N T H O D II C r I O N

Mathematical tnodellitig of \yater uptake by plant roots has highlighted the need for further experimetital itiformatioti (Latidsberg & Fowkes, 1978; Taylor & Klepper, I 978). Water flow from the soil to and through the phmt eticoutiters resistarices iti both the soil and the plant (Reicosky & Ritchie, 1976; Oosterhuis, 1983). Infortnatioti about the location and magtiitude of platit resistatice to water flow is crucial to the tnathematical atialysis of the hydraulics of water movemetit. A major component of jilant resistance resides iti the roots. Root resistatice is partitiotied itito radial {R,.) and axial {R.J cotnponents and the relati\e importance of these has been stressed (Newtnan, 1976; Landsberg & Fowkes, 1978; Passioura, 1981; St Aubin, Canny & McCully, 1986). It is generally assumed that the path for water flow through the root xylem passes through the ltttnen of adjacetit empty vessel elements and tlius does tiot cross li\ itig membranes. Calculating axial flow resistatice frotn

root xylem dimensiotis usitig the Poiseuille-Hagen (PH) eciuatioti tnay be adequate, but estitnated resistatice \alues do not alvyays agree with those foutid b\^ experitnetit. Iti some cases root axial resistatices haye been reported to be at least double those estimated frotn the PH equatioti (Witid, 1955; Passioura, 1972; Ponsatia, 1975; Frensch & Steudle, 1989), while others haye shown for pea {Pisum sativutn) and wheat {Triticutn aestivum) that experimetitalh- determined R^ values were larger by a factor of 1-3 to 2-3 (Greaceti, Potisatia & Barley, 1976). To be fully functional in water transport the xylem \essel eletnetits tnust lose cross-walls atid protoplastn. Ho\ve\er, recent studies in corn {Zea mays) ha\e shown that despite eotnplete ligtiificatioti, late tnetaxylem eletnents (ofteti very large) ha\ e dela> ed tnatitratioti with persistetit cross-walls atid protoplasm present up to 20-30 cm from the root tip (St Aubiti et al., 1986). Sitnilar finditigs were reported for teosinte {Zea mexicana), sorghum {Sorghntti bicolor) and sudati grass {Sorghu»i Sudanese) where all

94

E. Mapfumo and others

roots reached lengths of about 30 cm before the late metaxylem elements matured (Wenzel, McCully & Canny, 1989). These findings with monocotyledons raise the question of whether dicotyledons develop similarly. In 10- to 15-d-old roots of soyabean {Glycine max) Kevekordes, McCully & Canny (1988) reported that the first large metaxylem (LMX) matured to open vessels at a mean distance of 17 cm proximal to the tip. Calculations using the PH equation showed that the potential flow in a single central LMX vessel was 50 times that in the total of all early metaxylem vessels of a typical primary root of soyabean. Morphological and anatomical development of roots varies from one plant species to another, so information on xylem maturation and axial resistance must be obtained for each, to derive water uptake models which are biologically correct. The aims of this research were to monitor the pattern of xylem differentiation and maturation in roots of grapevine (Vitis vinifera L. cv. Shiraz). The effects of water stress and plant age on the maturation of xylem were also studied. Grapevines were chosen for these studies because of the apparent lack of information concerning xylem development for dicotyledons in general and also because modelling water uptake is an integral feature of irrigation scheduling in grapevines (Kasimatis, 1967). M ATl'.Kl ALS AND M E T H O D S

dehydrated via methoxy-ethanol, ethanol, propanol, butanol, butanol and glycol methacrylate (GMA) mixture (1:1) and GMA, and embedded in G M A (O'Brien & McCully, 1981), Transverse and longitudinal sections of 3 //m thickness were cut midway through each root segment. Root diameters and stele diameters of the sections were measured using a light microscope. Xylem maturation was assessed by staining sections with periodic acid-SchifF's reagent and toluidine blue O (PAS-TBO) (O'Brien & McCully, 1981). Diameters of all completely lignified xylem vessels were measured. TBO stained the primary walls of the young stelar cells bright pink. Wall staining shifted to dark blue as lignification began and became bright turquoise in the completely lignified xylem elements. PAS stained starch and some polysaccharides of the cell wall red or magenta. The lignified xylem vessels were subdivided into two groups based on whether or not the xylem contained any protoplasmic contents, starch, or cross walls, 'l'he xylem vessels without contents were classified as mature, whereas those with contents were classified as occluded. Two data sets were generated, one using mature vessels only, and the other using both mature and occluded vessels. The first group was designated 'mature', and the latter 'lignified'. The effective diameter of the xylem vessels in any root section (/J,,) was calculated from the ecjuation of Rendig & Taylor (1989).

Experimental design The experimental design was a 2 x 4 factorial randomized complete block design. Two watering treatments imposed on the plants: well-watered (daily watered), and stressed (watered once in 2-3 d). The plant roots were harvested 54, 85, 140 and 21 2 d after planting. The treatments were replicated twice to give a total of 16 experimental units. Plant material Cuttings of Vitis vinifera L. cv. Shiraz were grown in warm sandy soil for 29 d until the roots had grown to between 5 and 10 cm. The plants were then transplanted to pots of 100 cm height and 24-5 cm diameter in a shadehouse. The pots were filled with peat/sand (1:1). Watering was done daily for 2 wk after which watering treatments were imposed. Hoagland nutrient solution (Hoagland & Arnon, 1938) was applied once every 2 wk.

/tm.

(1)

where af, = diameter of an individual xylem element (jim), n = number of vessels. Axial resistance (/?,,) was then calculated Irom the modified PH equation (Newman, 1976) using the effective diameter (/),,) above. R,^ = \ 2-H/(nDl) MPa s mm '^.

(2)

For comparison purposes, axial resistances (Z?,,,,,,,) were also calculated using the mean xylem diameter and the number of xylem elements. Jl^^^^^^ = 1 2-H/(niiDl,..,J MPa s mm-', where!),,,,,.,,, = mean diameter of xylem vessels (//m), n = number of xylem vessels. Multiple linear regression analysis, with water stress and plant age as factors, and calculations were carried out with the GiCNSTAT 5 package (Genstat 5 Committee, 1987).

Structural studies

K li S111, T S

Roots were carefully washed in water, and the longest root per plant with an intact tip was cut into successive segments, each of 0-5 cm length, and fixed for at least 1 wk in 3 % glutaraldehyde in 0-025 M phosphate buffer (pH 6-8). The segments were

Three regions of the young grapevine root can be clearly distinguished; the vascular cylinder, the cortex and the epidermis. The vascular cylinder at the centre of the root is composed of xylem and phloem, together with associated parenchyma cells.

(3

Xylem dezH-lopment in Vitis \ inifera

" • , '

95

1'.

\ ., T

... - ; •'•tar' - ' -• '*'"^

• '

Vr

Figures 1—7. 'rnmsNcr.sc sections oi rods ol Vitis z'inifcra L. cw Sliiraz Figure 1. Mature proto.vylcMii (p) and meta.\yleni (in) \cssels at .^-25 cm (Voni the tip of a 212-d-old jjrape\ine root ji;i'('wn under wcll-watcrcd conditions (bar 30/mi). Figure 2. Secondary .\ylem tievelopment (arrow) has started between the primary xylem poles in the region between 4-{) and 4-.S cm trom the tip of 212-d-old wellwatered grapevine root (bar .30/mi). Figure 3. Secondary .\>'lem (s) formation in the interpole regions with parenchyma rays (r) opposite the protoxylem poles at 24'75 cm trom the tip of 212-d-old stressed grapevine root. Note the large xylem vessels (arrowhead) and the parenchyma eore (arrow) (bar 150//m). Figure 4. Degenerated protoplasm (arrowhead) in occluded xylem vessels, and mature merged vessels at 24 7.S cm from the root tip of 212 d-old stressed grapevine root (bar 10//m). Figures 5-7. Consecutive sections of tlie same well-watered root at 140 d between 29-5 cm and 30 cm from the tip showing the merging of two x>lem vessels (bar 10/mi).

96

E. Mapfumo and others

The xylem occupies the central part of the root. The 22-5 cm from the tip (Table 1). ln the region between number of protoxylem poles in grapevine is com- the tip and 0-5 cm from the tip the largest proportion monly five (pentarch arrangement. Fig. 1), but in of the axial water fiow was contributed by some of some cases tetrarch and hexarch xylem arrangements the EMX elements which were mature. Subwere found. sequently, the bulk of the flow was carried by LMX The longest roots sampled were 30 cm long at and secondary xylem vessels. 212 d after planting, and protoxylem, metaxylem Table 2 shows data obtained at increasing disand secondary xylem development was monitored tances from the tip of 212-d-old grapevine main from the tip to the base. Protoxylem elements roots subjected to two water treatments. The mean appeared at the beginning of vascular differentiation diameter of mature xylem increased steadily from and were already mature at a distance of 0-25 cm the tip toward the base. A similar pattern was from the root tip. Between the tip and 4 0 cm from observed for the mean diameter of both mature and the tip the early metaxylem (EMX) vessels developed lignified xylem. The standard errors and coefficients towards the centre of the stele, and were larger and of variation indicate a very wide range in the sizes of more lignified than the protoxylem vessels (Fig. 1). protoxylem and metaxylem elements which makes Further metaxylem differentiation and maturation the use of mean diameter in our resistance calculaincreased the effective diameter of the xylem. tions inappropriate. For comparison purposes howSecondary xylem development was observed to start ever, calculated ratios of axial resistances using mean at about 4-5 cm from the root tip (Fig. 2). The diameter to axial resistance using effective diameters secondary xylem developed from cambia between (-R,,,,,,,/i?,,,,,), and ratios of axial resistances of mature the poles to form a continuous cylinder of xylem xylem to axial resistances of lignified xylem (/?,j,,,/ outside a parenchymatous core. More layers of R^i), are also presented in the table. secondary xylem developed between the protoxylem Multiple linear regression analysis of the effective poles, with parenchyma rays developing at the poles diameter of mature xylem (i)^,,,,) showed that the best (Fig. 3). Up to 25 cm from the tip some large and regression model to describe the data must include lignified secondary xylem elements still retained distance from the root tip, and root diameter. The degenerated protoplasm, which is likely to impede regression analysis indicated no significant difaxial water How (Fig. 4). In most of the roots ferences between stressed and well-watered plants. protoxylem stretching was apparent at about 7 cm However, while there were sigtiificant differences from the root tip, and stretching and subsequent between the intercepts of the equations, there was no destruction of some of the protoxylem elements was indication of a significant interaction between disseen at further distances from the tip. Large xylem tance and plant age or root diameter and plant age. vessels were present throughout the secondary xylem Thus in the following estimated equations the and late metaxylem (LMX) (Fig. 3), some of which regression coefiFicients are identical. appeared to arise by merging of adjacent xylem vessels via breakdown of tbe common walls (Figs Day 54: 5-7). Most of the merging of xylem was observed in (D,,,,,),..,^ = 8-38 4-1.6846 (distance),.,.,. large secondary xylem vessels and LMX devoid of + 4-82 (diameter),.^,., (4) protoplasm, although in some instances merged Day 85: xylem vessels with degenerating cytoplasm were (A.n,),:,/,- = 15-44+ 1.6846 (distance),,,,, seen (Fig. 4). In 140-d-old roots of stressed plants + 4-82 (diameter),j,., (5) the number of merged xylem vessels increased from one at 15 cm to seven at 30 cm from the tip. Day 140: However, in roots of well-watered plants no merged (£>en,),v.- = 14-38 + 1.6846 (distance),^, vessels were seen until 20 cm from the tip. The + 4-82 (diameter),,,., (6) number of merged xylem vessels increased from two Day 212: at 20 cm to four at 30 cm from the tip. In older roots (^>.m)o*- = 6-02 + 1.6846 (distance),j,. xylem merging was seen closer to the root tip. For + 4-82(diameter),j,., (7) example in 212-d-old roots of stressed grapevines two merged xylem vessels were seen at 10 cm and (percent variance accounted for = 93 %), this increased to eleven at 22-5 cm from the tip. where / = harvest dates 1, 2, 3, 4 corresponding to Similarly for 212-d-old roots of well-watered plants plant ages of 54, 85, 140 and 212 d after planting; the merged xylem vessels were seen closer to the root i = l , 2 for well-watered and stressed plants retip, increasing from one at 1 5 cm to three at 22-5 cm spectively, and k = number of segments examined from the tip. for each root. These ecjuatiotis show a positive Calculations of relative contribution to axial flow relationship between the effective diameter and root of protoxylem elements, which were mostly less than diameter and distance from the root tip. 5 /j,m in diameter, showed a decrease in protoxylem Multiple linear regression of the logarithmically contribution from 25-1% at 0 25 cm to 0-3% at transtormed data of axial resistances showed that the

97

Xyletn developtnent in Vitis \'inifera Table 1. Relative axial flozv contribution of protoxylem elements at successive distatices from the root tip of a 212-d-old grapevine (Vitis vinifera L. cv. Shiraz) root growti under well-watered cotiditions Distance from the root tip (cm)

Relative axial contribution ("„)*

0-25 1-25 2-25 4-25 7-25 12-25 17-25 22-25

25-1 17-5

ficients associated with distance from the root tip and root diameter. Hovve\'er there were significant differences between the intercepts of the regression equations. The predicted regression equations tor the different plant ages are shown below. Day 54: = 3-897-0-1813 (distance),,, — 1-5 15 (diameter),,.,.,

(8)

Day 85:

9-1

= 3-043-0-1813 (distance),,, -1-515 (diameter),.,.,,

4-2 3-1 1-8 0-6 0-3

(9)

Day 140:

* Relative axial contribution was calculated from the effeetive diameter of protoxylem elements raised to the fourth power as a percentage of the total effective diamctor of all mature xylem elements raised to the fourth power.

best regression model to describe the data was the regression model including both the distance trom the root tip and the root diameter. There was no significant difference between well-watered and stressed plants. When separate multiple linear regressions were fitted to each plant age there were no significant differences between the regression coef-

= 3-244-0-1813 (distance),,., -1-515 (diameter),,.,,

(10)

Day 212: ^og,{R.,JuK- = 4-068-0-1813 (distance),,, -1-515 (diameter),,,,

(11)

(percent variance accounted for = 88'\,). These equations suggest a negative t-elationship between the root diameter and axial resistance such that as the roots become thicker the axial resistance becomes less and less important as a component of the term for whole root resistance. Similarly root elongation results in the reduction of axial resistance

T a b l e 2. Mecjn diameter of mature xylem, mean diameter of ligtrified xylem, standard errors, coefficients of variation {% CV), effective diameters of tnature and ligttified .xylem, ratio of axial resistance ttstng mean diatneter to axial resistance tising effective diattieter (R,,,,,,//?,,,,,), and ratio of axial resistance of tnature .xylem to axial resistatice of lignifted xylem {R.,,,,JR.J of 212-d-old roots of grapevine (Vitis vinifera L. cv. Shiran) grozvn under ivell-watered attd stressed conditiotis

Treatment Well watered

Stressed

EfTective diameter (/mi)

Mean xylem diameter (/fm)*

("-„ CV)

from root tip (cm)

Mature

Lignified

Mature

Lignified

Mature

Lignified

ratio

ratio

0-25 1-25 2-25 3-25 4-25 7-25 12-25 17-25 22-25

3-5 (0-5) 4-4 (0-4) 5-8 (0-6) 5-0 (0-3) 5-9 (0-3) 8-3 (0-4) 9-1 (0-4) 10-7 (0-6) 13-8(0-8)

3-6 (0-4) 4-7 (0-5) 6-0 (0-5) 5-7 (0-3) 6-3 (0-2) 9-1 (0-4) 9-9 (0-3) 11-4(0-5) 14-0(0-7)

42-4 48-0 52-1 43-2 45-6 36-5 39-7 51-7 51 -6

37-3 55-3 48-8 41-3 41-2 36-8 36-8 50-5 50-4

7-5 13-1 17-3 16-1 22-8 25-8 33-1 42-9 54-5

8-2 15-2 17-9 18-9 25-0 30-3 38-6 49-4 58-9

1-4

0-25

4-7 (0-5) 6-8 (0-5) 7-2 (0-6) 7-4 (0-5) 6-8 (0-4) 6-9 (0-4) 8-7 (0-5) 9-8 (0-5) 10-9(0-3) 10-6(0-5)

5-5 (0-5) 6-7 (0-4) 7-6 (0-4) 7-4 (0-3) 7-3 (0-3) 7-2 (0-3) 9-1 (0-4) 9-8 (0-4) 11-1 (0-3) 11-1 (0-4)

45-1 34-2 47-8 45-5 43-8 45-5 45-8 50-7 46-0 59-0

41-8 33-6 42-2 38-6 37-2 37-7 40-4 46-5 46-6 55-2

12-3 17-4 22-3 23-6 24-1 25-4 31-9 40-8 57-6 56-4

14-9 17-5 24-7 25-9 28-0 27-9 36-0 44-0 60-5 60-5

2-2 3-1 2-8 2-3 2-7 1-8 2-4 3-3 2-8 2-6 1-7 2-5 2-6 2-7 2-7 2-6 3-0 3-1

Distance

1-25 2-25 3-25 4-25 7-25 12-25 17-25 22-25 27-25

4-2

1-8

.") -9

1-4 -9

1-8 -8 -4 ;).2 -0 -5 -4 -8 -5 -6

-4 -2 1 -3

Standard errors in parentheses. * Number of nvatu.-e vessels varied between 10 and 260, and that of lignified \-esscls varied between 4 and 288.

98

E. Mapfumo and others

of the older part of the root compared to the younger part of the root. The total axial resistance will however increase with root length. DISCUSSION

This study has shown that water stress has only minor effects on the structure and axial resistance of grapevine roots. The first elements to differentiate were the protoxylem, but their contribution decreased with increasing distance from the root tip, mainly due to development and maturation of larger metaxylem and secondary xylem vessels. The destruction of some of the protoxylem elements observed in these studies agrees with the report of Esau (1965) that the protoxylem elements are the first to mature but are unable to keep pace with the extension of adjacent cells and are therefore stretched and completely destroyed. In the region between the tip and 10 cm from the tip, the differentiation and gradual maturation of the EMX elements, which were larger than the protoxylem, was responsible for the increase in effective diameter and hence most of the axial flow contribution. From about 15 cm from the tip axial flow became dominated by the large secondary xylem and LMX elements. Unlike soyabean (Kevekordes et al., 1988), the development of secondary xylem and that of LMX elements of grapevine roots occur concurrently. Results from regression analysis of the effective diameter show dependence on the distance from the root tip, root diameter and plant age. Plant age affects the intercept of the regression equation in particular, such that at a given distance from the root tip and at a given root diameter, the effective diameter increases steadily from planting. There is also a negative relationship between axial resistance and root diameter. As the roots become thicker the axial resistance to water flow decreases in a logarithmic fashion such that continued root thickening may cause the axial resistance to be a negligible component of the root resistance term in water uptake models. This result is in partial agreement with the results of Fowkes & Landsberg (1981) who found a significant decrease of axial resistance per unit length of apple roots when root diameter increased. Generally the radial resistance increases with distance from the root tip (Rowse & Goodman, 1981), The negative relationship between axial resistance and distance from the root tip therefore suggests that the axial resistance to radial resistance ratio will decrease with distance from the root tip. However, the total axial resistance of the root tends to increase with age of the root or with the increase of root length so that old long roots have higher total axial resistances than new short roots (Glinski & Lipiec, 1990), An unusual phenomenon observed in older parts of the roots is merging of xylem vessels. In paired

xylem vessels separated by a thin lignified wall it is likely that there is interxylem water flow. The wall finally breaks and the vessels become a single large xylem vessel. However, after the wall has broken two lignified flanges are still visible, but become shorter basipetally. In most instances merged xylem vessels were devoid of contents, although in some cases degenerating protoplasm was seen. More merged xylem vessels were seen closer to the root tip of stressed plants compared with well-watered plants. However, there were no significant differences between calculated axial resistances of stressed plants compared with well-watered plants and this suggests that the stress imposed on the plants was not enough to significantly reduce axial resistance through xylem merging. The older roots had more merged xylem vessels closer to the root tip compared with young roots. Measurements taken and calculations of axial resistances made for some of the merged xylem vessels indicated a substantial decrease in the axial resistance as a result of xylem merging. For example two mature xylem elements of diameters 27 and 17 fim were observed to merge into one large xylem vessel of diameter 36 /im further from the root tip. Calculations of axial resistance showed a decrease by 2-7 in magnitude as a result of merging. This is equivalent to an increase of relative axial flow contribution from 8-1°/, before merging to 22-2% after merging. Similar calculations of water flow in 14 and 115 //m xylem elements which merged into a 21-5 /xm xylem vessel showed a 3-8 times decrease in the axial resistance corresponding to an increase of relative axial flow contribution from 5-4 "^ before merging to 20-5% after merging. It is possible that such a developmental process might be a natural process in the roots of grapevine which creates a low resistance pathway for axial flow of water. However the phenomenon has so far been seen in pot-grown grapevine roots only and it is not known whether it exists in field-grown grapevines. Calculations of the effective diameter and axial resistance of mature xylem, and that of all lignified xylem, showed that using ligniflcation alone as the indication of xylem maturity results in underestimating the axial resistance by a magnitude of between 1-0 and 2-2, This result may partly explain why theoretical axial resistances are generally less than measured resistances (Tyree & Zimmermann, 1971; Petty, 1978). Although it is likely that lignified but occluded xylem vessels are partly functional, it is diflicult to evaluate their contribution. The use of both cytoplasm and lignin stains such as the PASTBO combination is essential to discriminate fully functional from partly functional and non-functional vessels. In maize, Frensch & Steudle (1989) found that measured values of R.^ were smaller than estimates from xylem diameters by a factor of between 2 and 5. One suggested reason for the deviation was the

Xylem development in Vitis vinifera

99

uncertainty in identifying mature xylem using a Esau K. 1965. J'laiit anatomy, 2iid odn. New York: Jolin Wiley ;ind .Sons. lignin stain (toluidine blue O). The results of our Fo-wkes ND, Landsberg JJ. 1981. Optimal rool systems in terms experiment suggest that this might be a minor reason of water uptake and movement. In : Rose D.-\, Charles-Kdwards DA, eds. MathciiKxtics ami filaut plivsialocrv. .\c-.idomic Press, for the deviation, but that the most probable reasoti lt)9 I2.S. is the use of mean xylem diameter in calculating axial Frensch J, Steudle E. 1989. .A.\ial and radial hydraulic resistance resistances. Our results show that using mean xylem to roots of maize (Zra may.s L.). Plant Pliysiology 91: 71') 726. vessel diameter causes overestimation of the axial Genstat 5. Committee. 1987. Geitstat n'fereiue moiuiot. Oxford: Clarendon Press. flow resistance. The ratio of axial resistance calcu- Glinski J, Lipiec J. 1990. Soil pliysical condition.^ ai:,/ plant loots. lated from mean diatneter of mature .xylem \-esseIs to Boea-Katon, Florida, L'SA : CRC Press. axial resistance oi elective diameter of mature xylem Greacen EL, Ponsana P, Barley KP. 1976. Resistance to water vessels {R.^m,Jt^^un) was calculated to demonstrate the How- in the roots of eereals. I n : L a n g e O L , K a p p e n L, Schulze I'^-D, eds. Water and plant life, piobtems and modern approaches. error introduced when using mean xylem diameter, Ecolo!;i(al Studies, vol. 1'). Berlin: .Springer-\'erlag. considering that all xylem, large and small, is Hoagland DR, Arnon DI. 1938. Groivinfi ptants witlioiit soit by the water-culture method. Berkeley, California: Ihiiversity of equally functional in axial water flow. The axial California, 1 — 16. resistance was overestimated by a tnagnitude of Kasimatis AN. 1967. Grapes and berries. In: I lagan RM. Haise liR, l-"dminister T W , eds. Irrigation of agricidtiirat lands. between 17 and 4'4 times when the mean diameter Agronomy Scries, N o . 11, 7 ] ' i - 7 6 S . was used in the calculations instead of the effective Kevekordes KG, McCully ME, Canny MJ. 1988. Late diameter obtained frotn individual xylem diameters. maturation of large meta.xylem vessels in soyabean roots: signiHeance for w-ater and nutrient supply to the shoot. Annals Canny (1991) wrote that, '.xylem as a n-ii.\ture of nf Botany 62: lO.S-117. elements of huge and small diameter, presents a Landsberg JJ, Fowkes ND. 1978. Water movement through paradox when viewed as a conductor of sap flow, plant roots. .Annals of Botany 42: 493-.S08. since the quantity carried is determined by the N e w m a n EL 1976. Water movement through plant roots. Philosophical Transactions of the Royal Society of London B 273 : largest diameter elements'. It therefore appears that 463 47S. development and maturation of large xylem elemet-its O'Brien TP, McCully ME. 1981. The study of plant structure prineiples and selected methods. .Melbourne, ."Xustralia: Termaresults in them dominating axial water flow and carphi Pty. Ltd. thereby renders smaller elements useless. However, Oosterhuis DM. 1983. Resistances to water How- through the Canny (1991) suggested that water entering the soil-plant system. South African Journal of Science 79: 4.'i9-465. small elements from the .xylem parenchyma would Passioura JB. 1972. The etfect of t-oot geometry on yield of wheat gi-owing on stored water. Australian Journal of .4gricultur(d flow radially to the larger elements, where it would Research 23: 745-752. flow upwards. The small elements thus would serve Passioura JB. 1981. Water colleetion by roots. In: Paleg LG. .•Xspitiall D, eds. The physiology and biochemistry of drought as an extension of the collecting catchment that feeds resistance in ptants. .Australia: Academic Ptess. the large elements. In such a case where smaller Petty JA. 1978. l-'luid flow- through vessels of bireh wood. Journat xylem elements would not be involved in axial water of Experimental Botany 29: 1463-1469. flow, it is likely that the use of the mean diameter of Ponsana P. 1975. Drainage and water uptake terms in the water balance. PhD thesis, I'nivei-sity of .-Adelaide. larger xylem elements only might give calculated /?,, Reicosky DC, Ritchie JT. 1976. Relative importance of soil values closer to the measured values, compared with resistance and plant resistance in root water absorption. Soil .Science Society of .imerica Journat 40: 243-2