Response of cassava to water shortage III. Stomatal control of plant water status

Field Crops Research, 4 (1981) 297--311 Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands

297

RESPONSE OF CASSAVA TO WATER SHORTAGE III. STOMATAL CONTROL OF PLANT WATER STATUS

D.J. CONNOR* and J. P A L T A

Centro Internacional de Agricultura Tropical (CIAT), A.A. 6713, Cali (Colombia) *Permanent address: School of Agriculture, La Trobe University, Bundoora, Vic. 3083 (Australia) (Accepted 25 May 1981)

ABSTRACT Connor, D.J. and Palta, J., 1981. Response of cassava to water shortage. III. Stomatal control of plant water status. Field Crops Res., 4: 297--311. Diurnal measurements of leaf water potential and the diffusive conductance of the abaxial surface of two cassava cultivars, M Col 22 and M Mex 59, were made on three occasions on field grown plants during a 10-week period of rainfall exclusion. Conductances of about 10 m m s - ' were observed in the rainfed plots but generally the mean conductance was in the range 3--5 m m s -1. The minimum water potential of --1.8 MPa was observed in the rainfed plots. Water shortage caused reduction in mean conductance to < 1 m m s - ' at which level the control of water loss maintained leaf water potential > --1.5 MPa at all times. Stress plots recovered more slowly during the late afternoon but during the day had higher leaf water potentials than the controls. A t the same levels of leaf water potential the conductance of M Mex 59 was less than that of M Col 22 in both control and stress plots. Measurements are also reported of the stomatal distribution, density and pore size for both fully expanded leaves and those whose expansion was seriously restricted by the water shortage.

INTRODUCTION

P l a n t s m a y r e s t r i c t w a t e r l o s s b y e i t h e r a r e d u c t i o n in t h e e x t e n t o f t h e i r evaporating surfaces or by a reduction in the rate of water loss per unit evaporating area. The latter can be achieved by an increase in the resistance to w a t e r f l o w i n t h e c a t e n a b e t w e e n s o i l a n d a t m o s p h e r e a n d in t h i s p a t h w a y t h e m o s t a c t i v e l y v a r i a b l e r e s i s t a n c e is t h a t o f t h e s t o m a t a l p a t h w a y . U n l i k e l e a f loss, stomatal closure does not involve the sacrifice of growth reserves previously assimilated and currently elaborated as potentially active leaf area. Stomatal closure does reduce the current carbon assimilation capacity of the plant but u n d e r c o n d i t i o n s o f w a t e r s h o r t a g e t h e i m p o r t a n t c o n s i d e r a t i o n is n o t h i g h growth rate but rather the most efficient use of the available water, in the maintenance of existing yield and in its increase. For active plants this requires

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© 1981 Elsevier Scientific Publishing Company

298 that the most favourable internal water status should be maintained under existing environmental conditions. Stomatal control of water loss can thus play an important long term as well as short term role in the regulation of water reserves. Furthermore, it is closely tuned to the water status of the important carbon assimilation organs because it operates as the leaf--atmosphere interface. As an "end of p a t h w a y " resistance, stomatal control sets a lower limit to the water status of the entire plant system and is able to respond to the capacitance of the total system which is not always negligible. The stomatal system and its response in cassava is not well known. Williams (1971) accounted for his difficulties in using a pressure drop porometer on three Malaysian cultivars by suggesting the absence of adaxial stomata, but Viegas (1976) reporting wide ranging studies on Manihot spp. comments, w i t h o u t elaboration, t h a t stomata do occur on both leaf surfaces. Apart from isolated measurements (CIAT, 1972) there are no field data available for leaf diffusive conductance of cassava although there are a few recent laboratory studies (Aslam et al., 1977; Mahon et al., 1977) in which an analysis of the physical p a t h w a y of diffusion is made. These measurements support the conclusion o f Williams (1971) that the leaf conductance of cassava is low relative to other crop species but his equipment and the problems he encountered in using it on cassava do not allow either an absolute definition of the conductance p a t h w a y and its response, nor an easy comparison with other species. In this paper we report observations on the diurnal patterns of leaf water potential and leaf diffusive conductance of two cultivars, M Col 22 and M Mex 59, both before and during a period when water was withheld from the crops. METHODS The experimental design and field site are described in detail in the first paper of the series (Connor et al., 1981). Briefly, two cultivars, M Col 22 and M Mex 59, were planted on 25 April 1979 and during the 72-day period from 12 August to 23 October 1979 were subjected to a period of water stress, achieved by the exclusion of rainfall with covers of black plastic sheeting over the soil surface. On one occasion before the plastic was placed, 25 July, and on three occasions during the period of rainfall exclusion, 29 August, 12 September and 26 September, continuous measurements were maintained around three replicate treated plots and their controls of leaf water potential and of leaf diffusive conductance. Measurements were made on recently fully expanded leaves in the upper canopy and generally duplicate measurements of water potential and the diffusive conductance of the abaxial surface, to which stomata are restricted, of four central lobules were measured before passing onto the next plot. To estimate leaf water potential, the xylem pressure potential of the central lobule was measured using pressure chambers. By using the lobule the difficulty arising from latex vessels c o m m o n in this species was minimized. A comparison between xylem pressure potential using lobules

299 and entire leaves showed t h a t no difference existed. A recent study (Ike et al., 1978) used p s y c h r o m e t r y to demonstrate t h a t the pressure chamber accurately determines leaf water potential in cassava. Diffusive conductance was measured with L a m b d a autoporometers. The sensors were calibrated before and after each occasion of measurement. Observations were also made on the stomatal characteristics of the cultivars. Leaf surface impressions were collected by applying a thin coating of plastic spray (artists fixative) to the leaf surface and then transferring it on transparent cellulose tape to a glass microscope slide (Clemens and Jones, 1978). Impressions were taken of both leaf surfaces from fully expanded leaves both before and during the stress treatment. Measurements of stomatal density and pore length were made at 400× magnification. The areas of the leaves from which t h e y were taken were also measured to describe the extent of stress on leaf development (Connor and Cock, 1981). In order to establish the perspective of the stomatal responses reported here, calculations were made of crop water-use using the conductance patterns, leaf temperatures, crop leaf area and free water evaporation appropriate to 26 September, the d a y of measurement in the second half of the stress period. For this a combination equation (Slatyer, 1967) was used. It was assumed that the measured daffy total free water evaporation occurred sinusoidally during daylight hours, t h a t the aerodynamic resistance of the crops was 0.3 s cm -1 and t h a t for h y p o s t o m a t o u s leaves the canopy resistance is estimated by leaf resistance/leaf area index (Szeicz et al., 1973). RESULTS

General climatic data are available from a site 500 m from the experimental plots. Data for the four days of measurement are presented in Table I. The diurnal observations of diffusive conductance and of leaf water potential are presented in Figs. 1 and 2--4. Fig. 1 portrays the response before the rainfall exclusion period, 25 July, and for this reason contains only a cultivar comparison. Figs. 2--4 are identical in construction and compare, during the stress period, the gradual response of the crops to water shortage. Air temperature and q u a n t u m flux density included in these figures were measured at the experimental site. The leaf temperature measurements are those from the porometers as used in the calculation of leaf conductance. Since the porometers were used routinely on the abaxial (under) leaf surface there was little difficulty in avoiding overheating due to sunlight striking the sensor. In Table II a summary has been made of the mean leaf diffusive conductance obtained during the morning and during the afternoon in each cultivar--treatment comparison. The relationship between diffusive conductance and leaf-air vapour pressure difference was investigated for observations taken when q u a n t u m flux density exceeded 900 pE m -2 s -1. No evidence of a threshold response was seen so that the linear trends t h a t are evident in some of the data sets can be satisfactorily

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Fig. 2. Diurnal q u a n t u m f l u x d e n s i t y and air temperature and the e f f e c t o f water stress o n the leaf water potential, leaf diffusive c o n d u c t a n c e and leaf t e m p e r a t u r e o f t w o cassava cultivars. M e a s u r e m e n t s t a k e n 2 9 A u g u s t 1 9 7 9 , 16 w e e k s after planting and 2 w e e k s after the start o f the period o f rainfall e x c l u s i o n . (The vertical bars represent 1 standard error.)

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303 TABLE I Climatic data for the four days of measurement Temperature (°C)

Day

25 29 12 26

July August September September

Maximum Minimum

Shortwave radiation (ca] cm -2)

Pan evaporation (mm)

Vapour pressure (kPa)

31.0 28.0 29.0 31.0

438 456 426 433

7.8 4.8 3.9 3.5

1.76 1.92 2.01 1.96

17.0 17.0 18.0 18.0

TABLE II Mean conductance (mm s -1 ) of the abaxial surface of leaves of two cultivars of cassava during the morning (08.30--12.00 h) and the afternoon ( 1 2 . 3 0 - 1 6 . 3 0 h) in response to water stress (each value is the mean of at least 100 measurements) Cultivar

Treatment Date 25 July 79

29 August

12 September

26 September

am

pm

am

pm

am

pm

am

pm

Control Stress

7.0 --

4.5 --

4.3 3.2

3.4 2.0

5.4 2.7

5.4 1.7

3.4 1.2

2.8 1.4

M Mex 59 Control Stress

6.6 --

2.2 --

3.7 3.0

4.4 1.3

4.3 2.0

5.8 1.6

3.1 1.2

2.8 1.0

M Col 22

s u m m a r i z e d as t h e s i m p l e c o r r e l a t i o n c o e f f i c i e n t s o f T a b l e I I I . T h e r e l a t i o n s h i p h o l d s s t r o n g l y i n t h e stress p l o t s a n d e s p e c i a l l y i n t h e c u l t i v a r M M e x 59. S t o m a t a l d e n s i t y a n d size a r e p r e s e n t e d i n T a b l e I V t o g e t h e r w i t h m e a s u r e m e n t s o n f o u r o t h e r cultivars f r o m a parallel e x p e r i m e n t m a i n t a i n e d at the s a m e t i m e . T h e c a l c u l a t i o n s o f d i u r n a l c r o p t r a n s p i r a t i o n a r e p r e s e n t e d i n Fig. 5. T h e s e c o m b i n e t h e e f f e c t s o f r e d u c e d l e a f a r e a ( s p e c i f i e d i n Fig. 5) a n d r e d u c e d l e a f c o n d u c t a n c e d u e t o t h e stress t r e a t m e n t .

Fig. 3. Diurnal q u a n t u m flux density and air temperature and the effect of water stress on the leaf water potential, leaf diffusive conductance and leaf temperature of two cassava cultivars. Measurements taken 12 September 1979, 18 weeks after planting and 4 weeks after the start of the period of rainfall exclusion. (The vertical bars represent 1 standard error. )

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305 T A B L E III C o r r e l a t i o n b e t w e e n leaf diffusive c o n d u c t a n c e a n d leaf-air v a p o u r p r e s s u r e d i f f e r e n c e for t w o cassava cultivaxs at a m b i e n t q u a n t u m flux d e n s i t y above 900 # E m -2 s-1 Cultivar

Treatment

Date 25 J u l y

29 A u g u s t

12 S e p t e m b e r

26 S e p t e m b e r

M Col 22

Control Stress

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--0.22 0.57*

0.26 --0.65*

--0.35 --0.58

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Control Stress

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--0.28 -0.63**

--0.20 --0.79***

--0.50 0.94***

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Fig. 4. Diurnal q u a n t u m flux d e n s i t y a n d air t e m p e r a t u r e and t h e e f f e c t o f w a t e r stress o n t h e leaf w a t e r p o t e n t i a l , leaf diffusive c o n d u c t a n c e a n d leaf t e m p e r a t u r e o f t w o cassava cultivaxs. M e a s u r e m e n t s t a k e n 26 S e p t e m b e r 1979, 20 w e e k s a f t e r p l a n t i n g and 6 w e e k s a f t e r t h e s t a r t o f t h e p e r i o d o f rainfall exclusion. (The vertical bars r e p r e s e n t 1 s t a n d a r d error. )

306 DISCUSSION

In previous papers reporting aspects of this e x p e r i m e n t it was shown t hat t h e exclusion o f rainfall had significant effects on the p r o d u c t i o n and distrib u t i o n o f dry m a t t e r (Connor et al., 1981) and u p o n leaf p r o d u c t i o n and senescence (Connor and Cock, 1981). Without except i on the data support the view t h a t th e response was to water shortage. For this reason it is i m p o r t a n t to c o m m e n c e the present discussion with a consideration of the leaf water potential data since leaf water potential is generally accepted as the definitive p a ra m eter o f plant water status. T h e data presented in Figs. 1--4 dem ons t r a t e , at least during the day-time hours when measurements were made, t ha t t he plants of b o t h cultivars from b o t h t r e a t m e n t s showed remarkably consistent diurnal patterns of leaf water potential. There is evidence of slower and incomplete (Figs. 2--4) evening recovery u n d er stress b u t overwhelmingly the impression is one of the maintenance o f internal water status in the face of a t r e a t m e n t which had severe effects o n plant f o r m and productivity. Thus leaf water potential cannot be used here as an index of the differential stress experienced by the cultivar-t r e a t m e n t combinations but rather as a s t a t e m ent of the success of the stomatal response in maintaining stable water status. This was perhaps generally overd o n e , since d a y t i m e leaf water potential is if anything higher in the stressed plants. In this discussion we t h e r e f o r e deal principally with stomata and their c o n t r i b u t i o n towards the maintenance of internal plant water status but we conclude with an analysis of the c om bi ned and interacting effects of stomatal and leaf area modifications on t he pat t er n of crop water use. T h e p o r o m e t e r s used in this study were calibrated over the range 0.3--20 m m s -1 and on no occasion was the c o n d u c t a n c e of t he upper (adaxial) leaf surface seen to rise into this range. S t o m a t a were n o t observed on the upper surface in these, or other, cultivars so t h a t in r out i ne m easurem ent of leaf diffusive c o n d u c t a n c e emphasis was c o n c e n t r a t e d on the behaviour of the lower leaf surface. Thus Figs. 1--4 present t he diffusive c o n d u c t a n c e of one leaf surface acting in parallel with a c o m p l e m e n t a r y surface o f effectively zero c o n d u ctan ce. Deter m i na t i on of t he cuticular conduct i vi t y of the adaxial surfaces was n o t possible with t he available instruments. However, it is certainly 0.3 m m s -1. Thus abaxial c o n d u c t a n c e and leaf c o n d u c t a n c e are equivalent fo r cassava b u t th e distinction must be maintained in comparisons between conductances o f h y p o s t o m a t o u s and a m p h i s t o m a t o u s leaves. T h e mean leaf diffusive c o n d u c t a n c e of cassava may, under favourable conditions, ap p r o ach 10 m m s -1 (Figs. 1--3). Thus on t he basis of the stomatal p a t h w a y at least, field grown cassava should have t he potential for high rates o f photosynthesis and hence also of transpiration. However it is also clear f r o m t h e data t h a t even th e visually unstressed cassava of t he control plots, generally o p e ra t ed at considerably lower c o n d u c t a n c e (3--5 m m s -~) and this is probabl y t h e range o f m a x i m u m c o n d u c t a n c e e n c o u n t e r e d in a rainfed environment. At this level, th e c o n d u c t a n c e of the physical pa t hway is likely t o limit photosynthetic rates as well as provide a control on transpiration.

307 Under stress the conductance patterns were considerably altered. Mean conductances show t h a t the plants operate at extremely low exchange rates with average conductances for large samples frequently less than 1 m m s -1 (Figs. 2--4). Under stress conditions the high variability of leaf conductance, a characteristic of the stomatal behaviour under well watered conditions, is considerably reduced. Low conductances are consistent with an effective limitation to water loss. However, the comparison of these values with conductances of lower leaf surfaces during the night and upper leaf surfaces at any time {~ 0.3 m m s -1) show that the stomata are not closed and that they remain responsive. The continuing growth of the storage roots and the limited but continuing development of the leaf system that was observed during this stress cycle (Connor et al., 1981; Connor and Cock, 1981) suggest that a positive carbon balance was maintained under these conditions but measurements of photosynthesis under field conditions are needed to define carbon assimilation under stress. A positive carbon balance was maintained down to a leaf water potential of - 1 2 bar with p o t t e d plants in the laboratory (J. Palta, unpublished). As in the field experiment reported here, the plants responded to induced stress by a restriction of stomatal aperture which was not associated with notable changes in the diurnal pattern of leaf water potential. Cassava is n o t alone with this form of response. Similar behaviour has been f o u n d in siratro (Macropunctilium atropurpureum) (Ludlow and Ibaraki, 1979; Wilson et al., 1980), cowpea (Vigna unguiculata) (Hall and Shultze, 1980), kenaf (Hibiscus cannabinus) (Muchow et al., 1980) and even with soybean undergoing a slow drying cycle under field conditions (Reicosky and Deaton, 1979). The summary of mean leaf conductance during morning and afternoon periods presented in Table II highlights important features of the data presented in Fig. 1--4. Firstly the conductance of M Mex 59 is generally lower t h a n t h a t of M Col 22, indicating a potentially important genotypic difference. M Mex 59 is a more vigorous variety than M Col 22 and, although it maintained a higher leaf area in b o t h the control and stress treatments, it was n o t possible to detect a more extensive root system (Connor et al., 1981). Low conductance compensates for high leaf area in the control of transpiration. The second feature is that the afternoon conductance in both cultivars, and especially under stress, is less than t h a t of the morning. Two anatomical features of cassava could contribute to this. Firstly it has been shown (Connor et al., 1981) t h a t cassava has a remarkably sparse root system in which root densities are c o m m o n l y < 10 -3 m m -2, a characteristic likely to lead to supply difficulties later in any day as the soil in proximity to the roots dries and its hydraulic conductance falls, but which would become a more serious problem as the profile dries and its general hydraulic conductance falls. Secondly the plant itself has at least the potential for a significant internal capacitance involving both storage roots and stems. The significance of the internal capacitance in the water relations of cassava has yet to be evaluated but certainly a plant which can contain up to 8 1 of water (D.J. Connor and J. Palta, unpublished) and be faced, under the conditions reported here, with a m a x i m u m

308 daily w a t e r loss o f a r o u n d 4 1 d o e s h a v e a significant c a p a c i t y t o b u f f e r its w a t e r loss p e r h a p s especially in t h e e a r l y p a r t o f t h e d a y . C o n d u c t a n c e p a t t e r n s c h a n g e d u n d e r stress b u t so also did t h e a n a t o m y o f t h e s t o m a t a l a p p a r a t u s . E v e n u n d e r relatively well w a t e r e d c o n d i t i o n s w h e n leaf e x p a n s i o n is n o t seriously r e s t r i c t e d , cassava s t o m a t a are small a n d d e n s e relative t o o t h e r c r o p species ( T a b l e I V ) . U n d e r stress, h o w e v e r , w h e n leaf e x p a n s i o n is seriously r e d u c e d , t h e s t o m a t a are b o t h smaller a n d m o r e d e n s e p e r u n i t area. T h e d a t a in T a b l e I V d e m o n s t r a t e t h a t t h e e f f e c t is n o t s i m p l y a r e d u c t i o n in l e a f e x p a n s i o n , b e c a u s e u n d e r stress t o t a l s t o m a t a per leaf as well as leaf size are r e d u c e d . T h e s t o m a t a l census d a t a allow i m p o r t a n t c o n c l u s i o n s . T A B L E IV

The effect of water stress on the stomatal characteristics of six cultivars of cassava Plant age

3 months

7 months

Cultivar

Control Plots

Control Plots

Stress Plots

Density (mm -2)

Density (mm -2)

Density (ram -2)

Pore length

(.m) M Col 22 M Mex 59 M Ven 218 M Col 72 M Col 638 M Col 1684 LSD (P < 0.05)

509 474 516 602 538 497 26

Sample number

20

15.8 15.0 12.9 15.6 13.6 14.7 0.3 160

620 643 648 614 626 593 36 24

Pore Leaf length area

(~m)

(cm 2)

16.7 17.1 19.6 15.5 17.8 17.5 0.3

150 180 295 199 110 168 43

856 812 919 880 812 863 57

12

24

48

Pore Leaf length area

(~m)

(cm 2)

8.5 11.7 13.8 9.7 10.9 7.0 0.4

42 71 40 54 39 69 16

48

12

Firstly, c a l c u l a t i o n s o f leaf c o n d u c t a n c e f r o m s t o m a t a l d i m e n s i o n s ( M o n t e i t h , 1 9 7 3 ) d e m o n s t r a t e t h a t f o r well e x p a n d e d cassava leaves t h e r e is s u f f i c i e n t c o n d u c t i v e c a p a c i t y t o e x p l a i n t h e m e a s u r e d high c o n d u c t a n c e s if o n l y a r o u n d 10% o f t h e s t o m a t a are o p e n t o 0.3 o f t h e i r p o t e n t i a l m a x i m u m size. M a x i m u m size was c a l c u l a t e d as t h e circle w i t h c i r c u m f e r e n c e o f t w i c e t h e l e n g t h o f t h e closed p o r e . T h e value o f 0.3 is b a s e d u p o n t h e g r e a t e s t a p e r t u r e s o b s e r v e d in t h e s t o m a t a l i m p r e s s i o n s . L a r g e n u m b e r s o f small s t o m a t a are a f e a t u r e o f t h o s e p l a n t s t h a t exercise e f f e c t i v e s t o m a t a l c o n t r o l b u t t h e significance o f this a n a t o m i c a l c h a r a c t e r i s t i c is n o t u n d e r s t o o d . I n d i v i d u a l cassava leaves are k n o w n t o r e m a i n p h o t o s y n t h e t i c a l l y active f o r p e r i o d s in excess o f 1 0 0 d a y s (J.H. C o c k , u n p u b l i s h e d d a t a ) so p e r h a p s during this t i m e t h e c o n t r o l o f gas e x c h a n g e is n o t a l w a y s t h e t a s k o f all or t h e s a m e s u b s e t o f s t o m a t a . T h e census d a t a f u r t h e r suggest t h a t even t h o u g h t h e s t o m a t a l d e n s i t y increases u n d e r stress, t h e r e d u c t i o n in p o r e size m o r e t h a n c o u n t e r a c t s it. T h u s f o r leaves f o r m e d u n d e r stress, t h e c o n d u c t a n c e f o r a n y p r o p o r t i o n o f t h e t o t a l s t o m a t a l c o u n t

309 open to the same degree is less than for leaves formed under relatively well watered conditions. This anatomical modification under stress could be interpreted as a significant adaptive mechanism since the maintenance of lower conductance of the same physiological status has direct significance to growth a n d survival under stress. Crop water use is controlled not only b y stomatal closure which can modify energy and gaseous exchange per unit leaf area, b u t also by modification to the extent of the evaporating surface itself. It has been shown (Connor and Cook, 1981) that leaf production and expansion in cassava are particularily susceptible to water shortage, so that crop water-use under stress is controlled b y significant modifications to b o t h parts of this water-use equation. The calculations in Fig. 5 demonstrate this important interaction using the physiological and environmental data collected for 26 September, late in the stress cycle. It was mentioned earlier that M Mex 59 matches its higher transpiring surface with a lower leaf conductance. This is clearly seen in the calculated transpiration behaviour of the stressed plants in Fig. 5. The lower conductance of M Mex 59 was able to offset its higher leaf area to provide a comparable daily transpiration pattern, which it is tempting to suggest, more closely corresponds to the capacity of a similar root system to supply water. These data make it possible to define important characteristics of stomatal form and function in cassava. The stomata have the physical capacity to present relatively high conductances (10 mm s -1) to the pathway between the atmosphere and mesophyll cells b u t even under relatively well watered conditions the c o m m o n l y encountered conductances were generally lower than this (3--5 mm s -1). Under stress, the pattern of conductance is substantially modified. With significant intercultivar differences, the conductances are greatly reduced, less variable, and capable of causing significant reductions in crop water use. The expansion of leaves formed under stress was greatly reduced and the anatomical characteristics of the stomata were considerably altered also. The resulting pattern of more dense b u t smaller stomata itself contributes to the maintenance of lowered conductances under stress. The diurnal behaviour of the plant suggests that it is regularly source limited in its water relations b y its sparse r o o t system b u t the quantitative significance of r o o t density and of the hydraulic capacitance of the storage root--stem system have yet to be evaluated. The data, however, throw limited light on the basis of the stomatal response. Even with some allowance for possible osmotic adaptation (Acevedo et al., 1979), it seems unlikely that feed back control b y leaf water potential (Raschke, 1975) could explain the observed patterns of leaf conductance and leaf water potential. In view of the correlations that exist within these data b e t w e e n conductance and leaf-air vapour pressure difference (Table III) a feed forward control (Farquhar, 1978; Cowan, 1977) mediated b y peristomatal transpiration is a more likely candidate. A number of species are considered to behave in this w a y (Hall et al., 1976; Muchow et al., 1980; Ludlow and Ibaraki, 1979; Sheriff and Kaye, 1977). There are, however, other alternatives including the possible involvement of stomatally active c o m p o u n d s (Loveys

310 a n d K r i e d e m a n n , 1 9 7 3 ; B e a r d s e l l a n d C o h e n , 1 9 7 5 ) . A d d i t i o n a l l y it s h o u l d b e stressed that any stomatal mechanism that maintains stable internal water s t a t u s in t h e f a c e o f v a r y i n g e v a p o r a t i v e d e m a n d w o u l d s h o w a s i m i l a r c o r r e l a tion between conductance and the driving force for water loss, the leaf-air vapour pressure difference. ACKNOWLEDGEMENTS D.J.C. wishes to thank La Trobe University for the opportunity through its Outside Studies Programme, to undertake these studies and wishes to thank CIAT for its encouragement, assistance and hospitality. We both wish to express our appreciation to the members of the Cassava Physiology Team for t h e i r c a r e f u l a s s i s t a n c e in t h i s w o r k . D r s . F i s c h e r , H s i a o , L u d l o w a n d W h i t f i e l d offered valuable comments on the manuscript.

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