The diffusive conductivity of sugar beet and potato leaves

Agricultural Meteorology- Elsevier Publishing Company, Amsterdam- Printed in The Netherlands

THE DIFFUSIVE CONDUCTIVITY OF SUGAR BEET AND POTATO LEAVES F. J. BURROWS 1

Nottingham University School of Agriculture, Sutton Bonington, Loughborough (Great Britain) (Received April 10, 1968)

SUMMARY

Curves relating the viscous conductivity, Sv sec/cm, to the diffusive conductivity, So cm-1, have been plotted for leaves of sugar beet and potato with defined stomatal concentrations and dimensions. The relationships and the theory of porometers have been used to obtain estimates of diffusive conductivity from readings taken with a portable porometer, and determinations made with an infiltration technique on leaves of both species. Both techniques have been used to follow the hourly variation in diffusive conductivity of leaves of each species growing under field conditions. Estimates of Su varied between 0.1 and 7.9 cm -1, and between 0.3 and 14.8 cm-1 for leaves of potato and sugar beet, respectively, corresponding to pore widths of approximately 0.1 p and 5 # for both species. Changes in SD could not be related satisfactorily to changes in the level of radiation, nor could they be explained by fluctuations in the relative turgidity of the leaves. Mean daily conductivity was not related to mean daily radiation nor potential transpiration. SD varied greatly with the soil water deficit. The effect of changes in the level of soil water deficit on the actual transpiration rate relative to the potential rate is described. When deficits are of the order 20 cm, actual transpiration rates may be reduced by 60 ~o and 25 ~ of the potential rate for potato and sugar beet, respectively. INTRODUCTION

The rate of evaporation from leaves is influenced by the conductance of their stomata to vapour diffusion (PENMAN and SCHOFIELD, 1951; BANGE, 1953; MILTHORPE and SPENCER, 1957). However, few quantitative estimates of conductance have been made under field conditions due to lack of suitable experimental techniques, since those widely used in the laboratory are unsatisfactory (MIL1 Now at: Macquarie University, North Ryde, N.S.W. (Australia).

Agr. Meteorol., 6 (1969) 211-226

212

E.J. BURROWS

THORPE, 1960). The most useful estimate is that of the integrated conductance of stomata in a defined area of leaf surface, i.e., of the diffusive conductivity. The effects of inherent variation in the size of stomata (ASHBY, 1931) should be minimized by making measurements on large areas of leaf. Measurements should be carried out rapidly and with little disturbance to the leaf and its environment; prolonged or intense interference may affect stomatal aperture directly (HEATH, 1950). These requirements are satisfied by the use of an infiltration technique (SCHORN, 1929) or by portable porometer (ALv~M, 1965). The infiltration technique is empirical and must be calibrated. Measurements taken with the Alvim porometer may be used, with the aid of theory due to PENMAN (1942), and MILTHORPE and PENMAN (1967), to estimate the diffusive conductivity more directly. In this paper the application of these techniques in sugar beet and potato is described and the results of measurements made under varying physiological and environmental conditions are discussed. THEORY

The rate of transpiration per unit area of lear surface may be formally represented by: E

= ns o D e (Ap/p)

cma/sec cm 2

where D e is the effective coefficient of diffusion of water vapour (cm2/sec)--it differs from the Fickian coefficient of diffusion by the effect of streaming in narrow pores (MILTHORPEand PENMAN, 1967)--Ap the partial pressure difference of water vapour across the ends of the stomatal pores, and p the pressure of the atmosphere (dynes/cm~). So is the average diffusive conductance of a stoma (cm) and nSD the diffusive conductivity of the leaf surface (cm-1), where n is the number of stomata per unit area of leaf surface. So is a function of the geometric attributes of the stomatal pore, an approximate value being given by: s o = a/(l +

0.5 rh)

where a is the area of the pore, 1 its length and r h the hydraulic radius (calculated as perimeter divided by area). Similarly the conductance for viscous flow, Sv, is defined by the expression: s v = ax/l

where x, the specific conductivity of the pore, accounts for the normal Poiseuille flow through capilliaries and includes a correction of slip along the sides of the pore (MILTHORPE and PENMAN, 1967). The relationship between viscous and Agr. M e t e o r o l . ,

6 (1969) 211-226

213

DIFFUSIVE CONDUCTIVITY OF SUGAR BEET AND POTATO LEAVES

diffusive conductivity (Sv = nsv and So = nSD) derived from measured values of a and 1, assumed values of r h, and a knowledge of stomatal densities, may be used to obtain estimates of diffusive conductivity from measurements with a mass flow porometer. METHODS

Stomatal densities and dimensions The number of stomata per unit area was assessed in discs of 1 cm radius, cut from leaves of potato and sugar beet, fixed in absolute alcohol and stained with phenol iodide in potassium iodide. Allowance was made for the shrinkage in leaf area that occurred during the fixing process ( 1 . 7 ~ and 2 . 2 ~ in sugar beet and potato, respectively). Mean densities of stomata in 103 per cm 2 were:

Potato Sugar beet

Upper surface

Lower surface

Total

4 . 2 + 0.4 12.9 _ 3.5

ll.3 + 3.7 19.9 _ 12.9

15.5 32.8

The dimensions of stomata were determined by microscopic examination of sections of leaf tissue cut by microtome and fixed in absolute alcohol. Average values used in subsequent calculations were:

Potato Sugar beet

Major axis (It)

Minor axis (It)

Length (It)

2l 26

0-11 0-17

14

14

The Alvim pororneter ]'he porometer cup was made from two circular soft rubber annuli, and mounted in a modified medical clamp for easy attachment to a leaf. The radii to the inner and outer walls of the cup were 0.45 and 0.74 cm, respectively. Compressed air was supplied to the cup from a reservoir (volume, V = 245 cm 3) and escaped across a segment of a leaf (area, A = 0.64 cm 2) bounded by the cup; the initial pressure was established by use of a bulb pump and the subsequent decrease in pressure followed with a differential pressure gauge. The permeability to flow through the leaf was determined using the relation: k ....

V t

In

(Ao~ \

~

cm3/sec

Ap J

where t was the time taken for the air in the reservoir to fall from the initial value Aop to the final value As. In practice Aop never exceeded 5.36 • 104 dynes/cm2; the final pressure varied between 2.36 • 104 and 5.36 • 104 dynes/cm 2. Estimates of Sv may be obtained from the leaf permeability measured with a porometer, provided the ratio of the conductivities of the upper and lower leaf Agr. Meteorol., 6 (1969) 211-226

214

F.J. BURROWS

surface is known and the resistance to air flow presented by the mesophyll, m, is determined (PENMAN, 1942). Penman assumed the ratio of the conductivities of the leaf surfaces to be the same as the corresponding ratio of the concentrations of stomata; for sugar beet and potato 0.65 and 0.36, respectively. For these ratios the relationship between viscous conductivity and leaf resistance is close to linearity and is displaced only slightly, suggesting that only small inaccuracies would be caused if the ratio was assumed to be unity. Moreover, differences in the stomatal size partly compensate the differences in density (BURROWS, 1961, 1965). For the Alvim parameter of defined dimensions, assuming the conductivities of the leaf surfaces are identical: log (mSv 1) = 1.078 (log mk) + 0.3 where Sv 1 is the viscous conductivity of the lower leaf surface and the total viscous conductivity of the leaf Sv = 2SvL The estimates of Sv permit the determination of So from the curves relating Sv and S o for leaves of sugar beet and potato of given stomatal dimensions and densities (Fig.l). For the purpose of computation the pores of sugar beet and

Sugar b e e t ~

'E

o

/

toto

0

J

I

I

__I

4

3

2 I 0 I Log Sv (cm/sec)

I

I

I

I

I

I

I

2

3

4

5

Fig.1. Theoretical curves relating log SD and log Sv for sugar beet and potato.

potato were assumed to be straight-sided elliptical tubes--the length of the minor axis varying over the stated range and that of the major axis remaining constant. The area of the pores was taken to be that at their mouths and is thus an overestimate of their effective aperture. The error is reproduced in estimates of both S v and So and the relationship between the two is only slightly altered. Moreover, in the absence of information on the changes in overall pore dimensions relative to those at the mouth no realistic correction can be applied.

Agr. Meteorol., 6 (1969) 211-226

215

DIFFUSIVE CONDUCTIVITY OF SUGAR BEET AND POTATO LEAVES

The resistance to air flow presented by the mesophyll, m, was determined using techniques described by HEATH (1941). Average values were 3.5 _ 1.2 and 3.4 _+ 1.1 sec/cm 3 for sugar beet and potato, respectively. Individual values showed appreciable variation. However, high accuracy in the determination of m is unnecessary in leaves with similar conductivities for the upper and lower surface (MILTHORPE and PENMAN, 1967).

Calibration of the infiltration method The infiltration series developed by SCHORN (1929) employs mixtures of isobutyl alcohol and ethylene glycol in differing proportions, and xylol (Table I), TABLE I R E L A T I O N B E T W E E N T H E S O L U T I O N N U M B E R O F T H E S C H O R N SERIES,

N,

AND THE DIFFUSIVE CONDUC-

T I V I T Y O F LEAVES O F S U G A R BEET A N D P O T A T O

N

0 1 2 3 4 5 6 7 8 9

Composition of solution

xylol 100 ~ 90~ 80~ 70~ 60 ~ 50 % 40 ~ 30 ~

isobutyl IBA IBA IBA IBA IBA IBA IBA

alcohol (IBA) 1 0 ~ ethylene 200 / ethylene 3 0 ~ ethylene 40 ~ ethylene 50 ~ ethylene 60 ~ ethylene 70 ~ ethylene

log (SD cm 1) sugar beet

glycol glycol glycol glycol glycol glycol glycol

2.96 i.46 i.84 0.31 0.77 0.86 0.85

± ± ± ±

0.210 0.358 0.294 0.180

potato

i.30 i.66 i.91 0.24 0.54

± ± ± ± L

0.734 0.238 0.132 0.109 0.104

-

providing a series of solutions of different surface tensions. When applied to a leaf, the degree of opening of a large number of stomata may be classified into one of nine categories. The method was calibrated for leaves of sugar beet and potato from measurements made with the porometer (HEATH, 1941) using a detachable cup (SPANNER and HEATH, 1951). A portion of leaf was enclosed within the cup, the resistance measured and the diffusive conductivity subsequently estimated. The leaf was removed from the porometer cup, and the solution of highest surface tension just penetrating the abaxial surface of the portion of leaf previously within the cup was found. The penetrating solution was defined as that which produced the largest number of discrete patches on the leaf surface within five seconds of application. Calibration curves are presented in Fig.2. There was considerable variation in the values of diffusive conductivity represented by a particular solution; the standard deviation was high (Table I). However, solutions found to penetrate most frequently during calibration were later observed to enter leaves most often when determinations were conducted in the field. Solutions with surface tensions Agr. Meteorol., 6 (1969) 211-226

216

o.J. BURROWS

Sugar b e e t . /

•

~'0 E

~ o

d

0 Solution

number

Fig.2. The relationship between mean conductivity, log SD, and number of solution of the Schorn series for sugar beet and potato. greater than those corresponding to solution numbers 6 (for sugar beet) and 5 (for potato) never penetrated. The relationship between log S D and solution number was represented by linear equations over the limited range examined: for sugar beet: for potato:

log So = 0.437N + L004 log S D = 0.300N + 7.036

where N is the solution number. There are few calibrations in the literature. F. L. Milthorpe (unpublished data) has obtained similar results for wheat; DALE (1961b) produced a linear relationship between solution number and leaf resistance for cotton. Variations in determinations of diffusive conductivity using the Schorn series can be attributed to several sources of error including the effects of surface differences between leaves of the same species. Field measurements

(a) In 1962, estimates of So were obtained using the Schorn infiltration series. A single determination of the solution which penetrated three leaves-young, mature and o l d - - f r o m each of three plants of both species was made at hourly intervals from sunrise to sunset on discrete days. In 1963 two readings with the Alvim porometer were taken on each leaf; measurements were made hourly on four consecutive days separated by intervals of two weeks. Damaged or visibly senescing leaves were avoided and wet leaves blotted before a determination was made. (b) Additional information on the water status of leaf material was gained from determinations of relative turgidity (WEATHERLEY, 1950) in 1963. MeasureAgr. Meteorol., 6 (1969) 211-226

DIFFUSIVE CONDUCTIVITY OF SUGAR BEET AND POTATO LEAVES

217

ments were made every two hours to correspond with alternate estimates of SD. Turgid weights were determined after leaf material had been floating on water for periods of between 12 and 24 h. (c) Incoming solar radiation during the experimental periods was measured with a Kipp solarimeter, recording in 1962, on a Siemens electrolytic meter from which hourly integrated values were read, and in 1963 on a chart recorder. (d) Direct measurement of evaporation at the experimental site was not available. Potential transpiration and soil water deficit were calculated from daily meteorological records using the Penman formula (PENMAN, 1963). RESULTS

Diurnal variation Estimates of conductivity on several days provide examples of the variation during daylight hours (Fig.3). Time/course relationships of S D differed each day 1963 E x p e r i m e n t

15

Sugor beet ~

A

u oe 247 August June Potato o 7 June gus+

10

T

~5

3~00

121.00

GMT

21,~30

Fig.3. Changes in the conductivity of sugar beet and potato.

(Table II; BURROWS, 1965), and throughout the season (Table lI, III). The conductivity of sugar beet leaves was consistently higher than that of potato leaves; estimates of S D varied between 0.3 and 14.8 cm -1 and 0.1 and 7.9 cm -~, respectively. There was no evidence of a fixed diurnal rhythm in conductivity. If this were present, minimum values might be expected to increase as daylength decreased (Table II, III). Maximum values of conductivity were rarely observed at 12h00 G.M.T. (BURROWS, 1965). It was regularly observed with both species that an initial rapid increase in SD ceased at a time coinciding with the disappearance, by evaporation, of dew or rain from the leaves. This observation was an indication that plants may have been suffering from the effects of water shortage. Agr. Meteorol., 6 (1969) 211-226

June June July July August August

4.32 1.72 1.86 0.57 2.77 2.38

0.86 0.22 0.11 0.22 0.22 0.36

2.93 0.98 0.57 0.35 1.17 1.32

7.06 7.06 7.06 5.39 5.79 6.90

0.28 0.28 0.76 0.60 1.80 1.37

min (cm-a)

max (cm-1)

mean (cm t)

max (em -1)

min (era -1)

sugar beet - -

2.16 3.44 3.06 2.16 3.38 4.88

mean (cm_l)

POTENTIAL TRANSPIRATION

Conductivities So potato

E X P E R I M E N T . C O N D U C T 1 V I T I E S O F S U G A R BEET A N D P O T A T O ; R A D I A T I O N ,

(N.B. Irrigation reduced the deficit under sugar beet by 2.5 cm.)

7 25 11 24 13 28

Date

1962

TABLE II SOIL WATER DEFICIT

771 560 438 252 340 229

4.44 4.30 2.60 2.06 2.82 1.60

7.6 13.3 17.7 20.3 20.5 18.7

7.6 13.3 17.7 20.3 18.0 16.1

Daily Accumulated potential soil_ water deficit transpiration potato sugar beet (cm) ( m W h / e m 2) ( m m ) (cm)

Total daily radiation

AND ACCUMULATED

B

t'~

t~

7

ba

III

4- 7 24-27 15-18 5- 8 26-29 16-19

Period

June June July August August September

6.93 5.16 3.88 3.55 4.76 -

3.31 2.05 1.72 1.39 1.90 -

5.17 3.43 2.79 2.58 3.06 11.57 12.88 13.13 13.40 12.83

mean m a x (cm-1)

mean ( c m 1)

mean m a x ( c m 1)

mean min (cm-1)

sugar beet

Conductivities St~ potato

3.87 4.31 3.93 4.63 3.49

mean min (era 1)

7.39 8.69 9.35 9.00 8.38

mean (era 1)

573 415 353 250 300 248

3.47 2.73 2.27 1.65 1.44 1.12

Mean Mean daily daily radiation potential ( m W h / c m 2) transpiration (mm)

9.75 11.59 10.22 11.92 12.27 10.66

Mean accumulated soil water deficit (cm)

1963 EXPERIMENT. CONDUCTIVITIES OF SUGAR BEET AND POTATO; RADIATION, POTENTIAL TRANSPIRATION AND ACCUMULATED SOIL WATER DEFICIT

TABLE

t,~

.<

© ...] ,-d ©

7'

7~

©

C3

C3 © 7"

220

F.J. BURROWS

Estimates on two days were examined statistically to ascertain the amount of variation contributed by each sampling component (Table IV). Significant TABLE IV THE ANALYSIS OF VARIANCE FOR POTATO,

6 JUNE, 1963 AND SUGAR BEET, 2 7 JUNE, 1963

D.F.

Times (T) Plants (P) Times × plants Leaves (L) Leaves × times Leaves × plants L×P×T

Error

17 2 34 2 34 4 68 162

Mean squares potato

sugar beet

29.5062 0.0525 1.7888 77.6128 1.2750 3.0393 0.9752 0.6730

96.2355 15.8131 16.2719 184.2409 12.6172 5.1556 4.1365 1.3868

differences were obtained between "times" and "leaves" and a significant "times × leaves" interaction. An important contribution to the variation of a single reading was from the error or "within leaves" component of the analysis. This variation between two areas on the same leaf did not appear to be associated with a shock reaction of the stomata in one area caused by the application of the porometer cup in another; the second reading on the leaf (7.66 _+ 0.28 and 4.83 _+ 0.30 era- 1 for sugar beet and potato, respectively) did not differ significantly from the first (7.68 _ 0.26 and 5.07 _+ 0.13 cm-1). Despite the large standard error (_+ 2.63 and _ 1.69 for a single reading from sugar beet and potato, respectively) significantly lower conductivities were recorded in younger leaves. These differences, occurring at a time when soil water deficits and potential transpiration rates were not particularly high, suggests that younger leaves possess morphologically smaller pores which do not open to the same extent as the pores on the older leaves. An interesting feature exhibited by the young leaves of sugar beet was the appreciable reduction in SD at midday (Fig.4). This response, if general, may be associated with the frequent observations that old leaves wilt, while young leaves remain upright and turgid in the middle of the day. Hourly variation in conductivity might be expected to be related to the intensity of light, a function of incoming radiation. In 1963 there was a positive relationship between SD and incoming radiation measured during the interval of time when hourly conductivities were determined, but there was considerable variation within and between days (BURROWS, 1965) and between the periods (Table V). This variation could not be attributed to soil water deficit which remained relatively constant nor to differences in potential transpiration which varied widely (BURROWS, 1965). Agr. Meteorol., 6 (1969) 211-226

221

DIFFUSIVE CONDUCTIVITY OF SUGAR BEET AND POTATO LEAVES 15 .Sugar beet 27June 1963 e

o old leaves e m a t u r e leaves • young leaves

~

10

.

,

~

~,

~

I

3.00

i

12.O0

GMT

21.OO

Fig.4. Changes in the conductivity of leaves of sugar beet with time and with age of leaf. TABLE V THE COEFFICIENTS OF LINEAR CORRELATIONS BETWEEN THE MEAN DIFFUSIVE CONDUCTIVITY OF LEAVES OF SUGAR BEET AND POTATO AND INCOMING RADIATION

Period

4, 6 and 7 June 24-27 June 15-18 July 5- 8 August 26-29 August 16-19 September

Potato n

a

b

r2

n

Sugar beet

53 69 64 61 57 -

4.15 2.85 2.43 2.07 2.46 -

0.066 0.043 0.034 0.034 0.055

0.533 0.208 0.260 0.260 0.358 -

. 68 33 61 56 55

a

.

. 6.13 6.33 6.42 7.02 5.94

b

r ~-

. 0.097 0.197 0.318 0.182 0.254

0.155 0.535 0.636 0.411 0.639

n is the n u m b e r of observations, a is the intercept on the ordinate, b is the slope of the line, and r 2 the correlation coefficient. 1963 Experiment

Sugar beet • 7 August e 24 June Potato o 7 June 5 August

130

~

120 110

°

~

~

"~ 1 0 0

9o 8o 7o

o ~. 6o

50

3.'o0

1210o

21.oo G i',4T Fig.5. Changes in the re|ative turgidity o f ]eaves o f sugar beet and potato.

Changes in the conductivity could not be related to changes in the relative turgidity of the leaves (cf. Fig.3, 5) which might be expected to decrease with Agr. Meteorol., 6 (1969) 211-226

222

F.J. BURROWS

increasing soil water deficit and increase of net radiation. Relative turgidities were consistently low. This may be a valid reflection of the water status of the larger, and therefore older, leaves used to provide sufficient material for the determination. However, dark injection patches were occasionally observed on turgid leaf discs. This would cause the turgid weight determinations to be erroneously high, reducing still further the estimate of relative turgidity. Seasonal variation

There was a decline in the mean daily conductivity of potato as the season progressed. Conversely, the conductivity of sugar beet tended to increase. These trends could not be attributed to changes in daily radiation or potential transpiration rate (Table II and III). There was, however, a relationship between SD and soil water deficit when the data for two seasons were compounded (Fig.6).

\Sugar

I0

beet

9 8

~-

6

u

4

8

Potato

o

r~ 2 I

°"---3_0_ o, O

0i

~ ~ ~ ib ~'21:4I~ 18 2o 2'2 S,W.D. c m

Fig.6. The relationship between soil water deficit and conductivity for leaves of sugar beet and potato. Data from Table II, III.

The low conductivities of sugar beet associated with low soil water deficit and high transpiration rate (7 June, 1962) suggest that seedlings of this species are particularly susceptible to water stress. Sugar beet produces a tap root which elongates as the season progresses and might be expected to exhibit effects of water stress in early growth. Potato is normally planted about a month earlier than sugar beet at a time when the soil water content is much closer to field capacity. The species produces a superficial root system under relatively favourable conditions and is able to withstand water stress during early growth. The roots, however, remain close to the surface throughout the season particularly under conditions of water stress (WEAVER, 1926). A decline in conductivity, even when soil water deficits remain relatively constant (in 1963), may reflect the failure of the root system to extract sufficient moisture from the soil to fulfil the requirements of the continually increasing growth. Agr. Meteorol., 6 (1969) 211-226

DIFFUSIVE C O N D U C T I V I T Y OF S U G A R BEET A N D P O T A T O LEAVES

223

DISCUSSION The application of the theory due to PENMAN (1942), and M1LTHORPEand PENMAN (1967) to sugar beet and potato provides estimates of maximum diffusive conductivity of 14.8 and 7.9 cm- 1 or 1.80 and 0.95 cm/sec, respectively. Published estimates of maximum conductivity range from 10.0 to 0.2 cm/sec; e.g., 0.2 for bean (KuIPER, 1961), 0.6 for turnip (GAASTRA, 1959), 0.6 for cotton (SLATYERand BIERHUIZEN,1964), 1.4 for sunflower (PENMANand SCHOFIELD,1951), 2.0 for sugar beet (BURROWS, 1961), 2.5 for grass (MONTEITH, 1963), and 10 for lucerne (VAN BAVEL,1967). Differences in the maximum conductivity of sugar beet and potato recorded under field conditions largely reflect differences in stomatal concentrations. They represent similar stomatal conductances, viz., 4.5 • l0 -4 and 5.1 • l0 -4 cm for sugar beet and potato, respectively, corresponding to similar pore apertures, i.e., approximately 5/~. Minimum values were, in general, much higher than would normally be expected; the stomata were never recorded as having closed completely. It is possible that opening occurred at some time prior to the commencement of measurements at levels of light intensity much lower than could be accurately measured with the instrumentation available. However, values of 0.3 and 0. l cm-1 were recorded for sugar beet and potato, respectively. These estimates are better than could be achieved with techniques employing the light microscope and correspond to pore widths of approximately 0.1/~ for both species. The Alvim porometer (used in 1963), which should effectively measure air flow through a leaf, consistently produced higher estimates of conductivity than the Schorn series (used in 1962). This might be attributed in part to the unsatisfactory calibration of the solutions at wide stomatal apertures and in part to leakage between the porometer cup walls and the leaf lamina leading to overestimates of SD, particularly at the narrow stomatal apertures. However, soil water deficits in 1962 were considerably higher than in 1963 and the lower estimates may be a valid reflection of the effects of water shortage. Both techniques are satisfactory for use under field conditions, but the calibration of the Schorn series is markedly subjective and comparisons between different operators are unreliable. This is a disadvantage which limits the widespread application of the technique. Linear correlations have been presented to describe the variation of the relationship between conductivity and radiation. Additional data (BURROWS, 1965) suggest this relationship may be curvilinear and similar to that observed by GAASTRA (1959) and KUIPER (1961). Under field conditions the true relationship represents an ideal which can only be achieved when environmental conditions other than radiation do not significantly influence the response of the stomata. Evidence presented here suggests that water shortage was a limiting factor in 1963. The relationship between conductivity and radiation may also be confused by Agr. Meteorol., 6 (1969) 211-226

224

E.J. BURROWS

a relationship between conductivity and time of day (DALE, 1961a). No evidence of an inherent diurnal rhythm was fbund for either species but, for reasons similar to those given above, it would be difficult to confirm the existence of such a rhythm under field conditions. The failure to explain the hourly variation in conductivity by fluctuations in the relative turgidity of leaf material might be attributed to shortcomings in the application of the technique; time did not permit the maintainance of a rigid experimental schedule as suggested by BARRSand WEATHERLEY(1962) or WEATHERLEY (1950). However, transpiration, a function of conductivity, and relative turgidity, though influenced by soil water deficit, are not related to each other (SLATVER, 1955; RUTTER and SANDS, 1958). The determination of the potential transpiration rate from weather parameters (PENMAN, 1963) relies on the assumption that the stomata remain fully open, i.e., the conductivity of the leaves is at a maximum, during daylight hours (PENMAN and SCHOFIELD, 1951). MILTHORPE (1960) described the influence of decreasing conductivity on the actual transpiration rate, E, relative to the potential rate, E , His data and the extrapolated curves (drawn by hand) relating mean daily conductivity and soil water deficit (Fig.6) can be used to examine the effect of increasing soil water deficit on the ratio E/E, for both sugar beet and potato (Fig.7). Clearly the estimates of soil water deficit based on the calculation of a 1.0 i 0.9 0.8 0.7 0.6 E -~t 0.5

0.4 0.3 0.2 0.1 0c

S.W.D.

Fig.7. The relationship between soil water deficit and relative transpiration rate.

potential transpiration rate on a daily basis will be high. Moreoever, it would seem unwise to apply the same value of deficit to two species when the agronomic practises involved during their cultivation will differentially affect rates of water loss from the soil surface. Nevertheless, in the absence of other suitable measurements, it provides a useful guide to the levels of water stress during the experiments. The actual transpiration rates are reduced to 90 700 of the potential rate when soil water deficit exceeds approximately 8 cm and 14 cm for potato and sugar beet, respectively. During 1962 when the deficit increased from 8 cm to 20 cm the actual transpiration rate was reduced from 0.85E t to 0.4E t and 0.98E t to 0.75E t Agr. Meteorol., 6 (1969) 211-226

DIFFUSIVE CONDUCTIVITY OF SUGAR BEET AND POTATO LEAVES

225

for potato and sugar beet, respectively. In 1963 the comparable reductions were to 0.75E t and 0.95E t for potato and sugar beet, respectively. Under crop conditions the change in the transpiration rate is unlikely to be as great as that obtained from measurements of the conductivity of a single leaf. Nevertheless, the reduction in the transpiration rate indicates that for potato, in both 1962 and 1963, estimates of soil water deficit based on potential transpiration rates alone would be too high. This ability of potato to reduce transpiration rates by lowering the conductivity of the leaves through closure of the stomates might prove advantageous to other growth processess. Frequent observations of wilting in sugar beet suggested that the stomates of this species failed to close sufficiently to alleviate water stress. Under such circumstances the high conductivities associated with high transpiration rates might prove deleterious (STOCKER, 1960). The demonstration of differential crop responses to water shortage through the application of well-established theory and relatively simple experimental techniques emphasizes the need to obtain meaningful estimates of a stomatal factor to be included in calculations of transpiration. ACKNOWLEDGEMENTS

The work described in this paper was in partial fulfilment for the degree of P h . D . at Nottingham University. It is a pleasure to thank Professor F. L. Milthorpe and Dr. I. R. Cowan for their advice and assistance. REFERENCES ALVIM, P. DE T., 1965. A new type of porometer for measuring stomatal opening and its use in irrigation studies. Proc. Syrup. Eco-Physiol., U.N.E.S.C.O., Arid Zone Res., 25: 325-329. Asna¥, E., 1931. Comparison of two methods of measuring stomatal aperture. Plant Physiol. (Lancaster), 6: 715-719. BANGE, G. G. J., 1953. On the quantitative explanation of stomatal transpiration. Acta Botan. Need., 2: 255-297. BARRS, H. O. and WEATHERLEY, P. E., 1962. A reexamination of the relative turgidity technique for estimating water deficit in leaves. Australian J. Biol. Sci., 15: 413-428. BURROWS, F. J., 1961. The Stomatal and Cuticular Conductance of Sugar Beet. Thesis, Nottingham University, 52 pp. BURROWS, F. J., 1965. The Stomatal Conductance of Sugar Beet and Potatoes. Thesis, Nottingham University, 142 pp. DALE, J. E., 1961a. Investigations into the stomatal physiology of upland cotton, 1. The effects of hour of day, solar radiation, temperature and leaf water-content on stomatal behaviour. Ann. Botany, (London), 25: 39-52. DALE, J. E., 1961b. Investigations into the stomatal physiology of upland cotton, 2. Calibration of the infiltration method against leaf and stomatal resistances. Ann. Botany, (London), 25: 94-103. GAASTRA,P., 1959. Photosynthesis of crop plants as influenced by light, carbon dioxide, temperature, and stomatal diffusion resistance. Mededel. Landbouwhogeschool Wageningen, 59: 1-68.

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