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Transpiration, leaf temperature and water potential of rice and barnyard grass in flooded fields
Agricultural Meteorology, 26 (1982) 285--296 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
285
TRANSPIRATION, LEAF TEMPERATURE AND WATER POTENTIAL OF RICE AND BARNYARD GRASS IN FLOODED FIELDS J. C. O'TOOLE and V. S. TOMAR* The International Rice Research Institute, P.O. Box 933, Manila (Philippines) (Received July 23, 1981; revision accepted February 11, 1982) ABSTRACT O'Toole, J.C. and Tomar, V.S., 1982. Transpiration, leaf temperature and water potential of rice and barnyard grass in flooded fields. Agric. Meteorol., 26: 285--296. Few studies are available on crop--water relationships and attendant climatic characterization of irrigated (flooded) rice fields in tropical conditions. We measured transpiration, leaf water potential, leaf diffusive resistance and leaf temperature of rice (Oryza sativa L.), a C3 species, and barnyard grass (Echinochloa crus-galli L.), a C4 species, during the dry season rice crop in the Philippines. Concurrent measurements were made of solar radiation, water vapor pressure deficit and wind speed. Transpiration rate, leaf water potential, and leaf temperature of both species responded to diurnal trends in the meteorological variables. Transpiration rate, during the course of the day was more responsive to vapor pressure deficit and wind speed than solar radiation. Transpiration rate of four rice cultivars averaged 7.2 mm d -1 while barnyard grass was 2.8 m m d -1. During much of the diurnal period, leaf water potential, leaf diffusive resistance and leaf temperature of rice were lower than barnyard grass. Although the leaf area index of barnyard grass was less than that of rice, other characteristics of the two plant groups (C3 and C4) provide partial explanation for the differences observed.
INTRODUCTION E v a p o t r a n s p i r a t i o n f r o m t h e rice c r o p has b e e n s t u d i e d e x t e n s i v e l y in t r o p i c a l Asia. T o m a r a n d O ' T o o l e ( 1 9 8 0 a ) r e v i e w e d t h e l i t e r a t u r e on, a n d basic p r i n c i p l e s a f f e c t i n g , t r a n s p i r a t i o n a n d e v a p o t r a n s p i r a t i o n f r o m w e t l a n d rice in s o u t h a n d s o u t h e a s t Asia. In c o n t r a s t w i t h r e s e a r c h on o t h e r m a j o r f o o d crops they f o u n d a lack of f u n d a m e n t a l i n f o r m a t i o n to describe and u n d e r s t a n d r e l a t i o n s h i p s b e t w e e n t r a n s p i r a t i o n r a t e a n d such v a r i a b l e s as l e a f w a t e r p o t e n t i a l , l e a f d i f f u s i v e r e s i s t a n c e a n d l e a f t e m p e r a t u r e in i r r i g a t e d ( f l o o d e d ) t r o p i c a l rice c r o p s . G e n e r a l l y , air t e m p e r a t u r e , w a t e r v a p o r p r e s s u r e d e f i c i t ( V P D ) a n d s o l a r r a d i a t i o n have b e e n r e l a t e d t o t r a n s p i r a t i o n rate. T h e d i r e c t e f f e c t s o f air t e m p e r a t u r e on t h e s t o m a t a l a p p a r a t u s a n d h e n c e on t r a n s p i r a t i o n are, h o w ever, i n c o n c l u s i v e ( M e i d n e r a n d M a n s f i e l d , 1 9 6 8 ) . T r a n s p i r a t i o n c h a n g e s during t h e c o u r s e o f a d a y in r e s p o n s e t o t h e s e m e t e o r o l o g i c a l v a r i a b l e s r e s u l t in changing leaf water potential and leaf t e m p e r a t u r e . Relationships between * Present address: Department of Soil Science, G. B. Pant University of Agriculture and Technology, Pantnagar District Nainital, U.P., India.
0002-1571/82/0000--0000/$02.75
© 1982 Elsevier Scientific Publishing Company
286 transpiration rate and leaf water potential have been reviewed (Kaufmann, 1976) for several plant species. Changes in leaf temperature and leaf-to-air temperature differential have been associated with evaluation of crop water status (Tanner, 1963; Idso et al., 1980). Views on the direct effects of VPD on diffusive resistance and plant water status are conflicting. Using greatly variable plant material, several workers (Ishihara et al., 1971; Lange et al., 1971; Schulze et al., 1972; Aston, 1976) illustrated the dependence of stomatal resistance on atmospheric humidity. In contrast, other workers have found no such trend (Barrs, 1973; Rawson et al., 1977). The direct relationship between solar radiation and transpiration is more universally accepted. However, rates of crop transpiration have been more frequently related to daily or weekly totals of solar radiation than to short-term or diurnal measurements. Wetland rice grown in tropical conditions has been little studied with regard to the above-mentioned variables and their effect on transpiration and plant water status. The present study was initiated to characterize the diurnal variations in transpiration rate, leaf water potential, leaf-to-air temperature differential (AT) and attendant meteorological variables for a dry season irrigated rice crop. Additionally, a c o m m o n weed species, barnyard grass (Echinochloa crus-galli L.) was also included as preliminary observations showed very different responses when compared to rice. MATERIALS AND METHODS The study was conducted at the experimental farm of the International Rice Research Institute (IRRI), Los Bafios, Laguna, Philippines, on Maahas clay soil (an amorphous soil classified as Typic Tropoquept). Fields were fully irrigated (flooded to a depth of 3--8 cm of water) during the entire crop season. Fields were prepared as puddled wetland and 22-day-old seedlings of four rice cultivars, IR20, IR36, Dular and M1-48, were transplanted at 20 x 20 cm spacing on February 28, 1978. Barnyard grass was seeded one week after the rice had been transplanted because of its more rapid growth and phasic development. The four cultivars of rice and barnyard grass were replicated four times in a completely randomized block design. Each replicate plot of rice or barnyard grass was 8 x 10 m. Fertilizer was applied at 1 2 0 k g N , 60kgP2Os and 6 0 k g K 2 0 per hectare. Agronomic practices included high levels of insect and disease protection. Observations on transpiration rate, leaf temperature, stomatal resistance, leaf water potential and weather conditions were made 42 days after transplanting on two consecutive clear days, April 11 and 12, 1978. Solar radiation was monitored with a pyranograph calibrated against an Eppley pyroheliometer, and relative humidity was measured with a hair hygrothermograph at one of the Institute's meteorological stations less than 0.5 km from the study site. The station was located in an irrigated rice area which included the study site. Wind speed was measured at the research site by a vane anemometer located 2 m above the soil surface.
287 T h e t r a n s p i r a t i o n rate was m e a s u r e d at h o u r l y intervals b e t w e e n 0 5 0 0 and 1 8 0 0 h with m i c r o l y s i m e t e r s installed in all plots. Design and testing o f the m i c r o l y s i m e t e r s are discussed elsewhere ( T o m a r and O ' T o o l e , 1980b). T w o m i c r o l y s i m e t e r s were placed in each replication, i.e., eight units f o r each cultivar o f rice or the weed species. A f t e r the e x p e r i m e n t was completed, plants in m i c r o l y s i m e t e r s and in f o u r a d j a c e n t hills in each p l o t were cut to measure leaf area. A Hayashi D e n k o m e t e r Model AAM-7 was used for this purpose. Values o f t r a n s p i r a t i o n rate are p r e s e n t e d on a land area basis, each hill covering 400 cm 2 and also on a per unit leaf area basis, calculated f r o m the t o t a l leaf area o f the plants in each m i c r o l y s i m e t e r . L e a f t e m p e r a t u r e , s t o m a t a l resistance and leaf w a t e r p o t e n t i a l were measured o n t w o fully d e v e l o p e d leaves r e p r e s e n t a t i v e o f the u p p e r c a n o p y and c o m p a r a b l e with plants in the microlysimeters. These m e a s u r e m e n t s were m a d e at each h o u r l y sampling p e r i o d b e t w e e n 0 5 0 0 and 1 8 0 0 h o u r s in each replicate plot. L e a f t e m p e r a t u r e was m e a s u r e d with a biological-type stainless steel micro-syringe with t h e r m i s t o r sensor at the tip. T h e micro-syringe sensor was inserted into the mid-vein on the abaxial leaf surface. Air t e m p e r a t u r e at the c r o p level was m e a s u r e d s i m u l t a n e o u s l y with a shielded e p o x y - c o a t e d thermistor. S t o m a t a l resistance was m e a s u r e d with a L a m b d a diffusive resistance m e t e r m o d e l LI-60, with an LI-15S sensor having an a p e r t u r e o f 3.5 × 20 ram. T h e calibration plate p r o v i d e d a range o f resistances f r o m 0.59 to 33.3 s cm -1 at 25°C. M e a s u r e m e n t s were m a d e on the adaxial leaf surface. L e a f w a t e r p o t e n t i a l was e s t i m a t e d with a pressure c h a m b e r . A fully develo p e d leaf, s e c o n d f r o m the t o p o f a tiller, was w r a p p e d in a m o i s t cheesecloth and a l u m i n u m foil p r o t e c t i v e cover b e f o r e excision and k e p t in t h a t c o n d i t i o n until the m e a s u r e m e n t was c o m p l e t e d . Pressure was applied f r o m a c o m p r e s s e d n i t r o g e n gas s o u r c e at a rate o f 21 bar min -1 . RESULTS
Changes in transpiration with weather conditions Transpiration rate Diurnal p a t t e r n s o f t r a n s p i r a t i o n rate in rice cultivars I R 2 0 and M1-48 and b a r n y a r d grass are p r e s e n t e d in Fig. 1. Because w e a t h e r c o n d i t i o n s on April 11 and 12 were similar, data f o r the first d a y o n l y are presented. T h e response o f I R 2 0 was similar to t h a t o f I R 3 6 , and M1-48 was similar to t h a t o f Dular: thus o n l y e x a m p l e s o f I R 2 0 and M1-48 are p r e s e n t e d in Fig. 1. Peak t r a n s p i r a t i o n rates were r e a c h e d at a b o u t 1500 h. Daily totals and the corres p o n d i n g leaf area i n d e x are given in Table I. Values o f t r a n s p i r a t i o n rate per u n i t leaf area were similar in diurnal t r e n d t o the m e a s u r e d rates per u n i t g r o u n d surface area a l t h o u g h differences in the leaf area b e t w e e n rices and
288
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15
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Fig. 1. Transpiration rate of (a) rice cultivar IR20, (b) rice cultivar M1-48, and (c) barnyard grass, during the light period April 11, 1978. ~ ~, per unit ground area; c~---o, per unit leaf area. TABLE I Daily total transpiration per unit ground area (ram d -1) of rice and barnyard grass and leaf area index (m2/m 2) of plants in microlysimeters Measured transpiration (mm d -1 )
Leaf area index (m2/m 2 )
April 11, 1978 (0600--1800 h)
April 12, 1978 (0600--1800 h)
Rice IR36 IR20 Dular M1-48
8.0 7.1 6.8 5.9
8.8 8.1 7.1 6.4
6.8 5.7 4.3 2.3
Weed species Barnyard grass
2.8
2.8
1.7
weed species were apparent. In later discussions, only the measured trans p i r a t i o n r a t e p e r u n i t g r o u n d s u r f a c e a r e a is u s e d . Transpiration rate began increasing at sunrise and attained the maximum
289
value of 1 . 1 - - 1 . 2 m m h -1 for IR20 and 0 . 9 5 m m h -~ for M1-48 during the period 1 2 0 0 - - 1 5 0 0 h . The maximum transpiration rate for barnyard grass { 0 . 3 5 - - 0 . 4 5 m m h -1) was much lower than the rice cultivars and peaked between (1100 and 1 3 0 0 h . The daily total transpiration of barnyard grass was about one-third that of the rice cultivars (Table I). Weather conditions for April 11 are depicted in Fig. 2. Solar radiation increased steadily up to 1 1 0 0 h and then declined. Total radiation received was 26.76 and 2 7 . 8 5 M J m -2 d -~ on April 11 and 12, 1978, respectively. Relative humidity remained low (~70%) during most of the day hours. Vapor pressure deficit (VPD) increased from sunrise to 1 2 0 0 h , attaining maximum values of a b o u t 20 mbar. Vapor pressure deficit remained around this value until 1700 h and then declined. The wind speed was greater than 2 m s -~ for much of the day. Similar climatic conditions prevailed on April 12, 1978. Thus, both days were relatively clear with high evaporative demand conditions characteristic of the dry season in much of tropical Asia. The relationships of transpiration to solar radiation, vapor pressure deficit and wind speed are illustrated in Fig. 3. Solar radiation and wind speed both show strong interaction with VPD in their effects on diurnal changes in transpiration rate. A characteristic diurnal loop, more circular for rice and elliptical for barnyard grass, was observed in the transpiration--solar radiation relationship of the two species (Fig. 3a). A similar trend is obvious for wind speed with a very strong interaction illustrated in Fig. 3b. The difference in species response is striking in both sets of graphs in Fig. 3. The effect of VPD on the transpiration rate is illustrated in Fig. 4. The transpiration rate of rice (average of four cultivars) increased exponentially while that of barnyard grass increased linearly with VPD. At high values of VPD, a large increase in the transpiration rate was observed for rice cultivars in comparison with that of barnyard grass. ,ii~ r
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Fig. 2. Solar radiation (-w $), water vapor pressure deficit ( v - -v), and wind speed (~>--o) during the light period April 11, 1978.
290
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24
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Fig. 3. ( a ) D i u r n a l changes in transpiration rate of rice (average of four eultivars) and barnyard grass as affected by the interaction of solar radiation and vapor pressure deficit. (b) Diurnal changes in transpiration rate of rice (average of four cultivars) and barnyard grass as affected by the interaction of wind speed and vapor pressure deficit.
As shown in Fig. 3, transpiration rate was high at high VPD ( ~ 1 5 mbar), irrespective of solar radiation for both the species. At lower values of VPD, transpiration appears to increase with solar radiation. But the observations at low VPD were very few. Transpiration response to VPD and wind speed is illustrated in Fig. 3b. For both species, transpiration increased linearly with VPD and wind speed. The multiple linear regression relationship for rice was Y = - 0 . 1 1 + 0.01x1 + 0.23X2,
R: = 0.92
291
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Fig. 4. R e l a t i o n s h i p b e t w e e n t r a n s p i r a t i o n rate a n d w a t e r v a p o r p r e s s u r e d e f i c i t for an a v e r a g e of four cultivars of rice (o; :~ = 0 . 0 6 e °'13X, R 2 = 0 . 9 0 ) a n d for b a r n y a r d grass (e; ~ = --0.02 + 0.01X, r 2 = 0.57).
and for barnyard grass ~" = 0 . 0 3 - - 0 . 0 0 4 6 X 1 + 0.1X2,
R 2 = 0.70
where Y is the transpiration rate, X 1 the VPD and X 2 the wind speed. However, our observations are limited to certain specific field conditions where patterns in all the variables may be attributed primarily to diurnal changes.
Leaf temperature, AT and leaf water potential Changes in leaf temperature with the air temperature are plotted in Fig. 5. In general, leaf temperatures were higher than air temperature in early morning and late afternoon hours. Air temperature was higher than leaf temperature from 0900 to 1 6 0 0 h for rice cultivars. Leaf temperature of barnyard grass followed the same diurnal trend but during midday periods it was higher than rice and was approaching air temperature. Leaf temperature of rice cultivar M1-48 was always higher than that of IR20. Temperature difference between leaf and air (AT) decreased with air temperature and followed a curvilinear relationship (Fig. 6). The intercept of the curve was different for both the rice cultivars as well as for barnyard grass. Zero value of AT for IR20 was observed at 30°C air temperature and for M1-48 and barnyard grass at 31 and 32.6°C, respectively. AT for barnyard grass was continuously more positive, indicating a higher leaf temperature. AT values of IR20 were more negative at higher air temperatures. No definite correlation was established between AT and solar radiation though a decreasing trend in AT with solar radiation appeared but the data points were much diffused.
292 3!r
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14
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20
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Fig. 5. Diurnal changes in leaf temperature of rice cultivars IR20 (c>--o), M1-48 (o--o), and of barnyard grass ( H ) in relation to air temperature (D--~) at crop height.
Transpiration, stornatal resistance, AT and leaf water potential relations A linear relationship was established between AT and the decrease in leaf water potential (Fig. 7). Leaf water potential was about --9 bar at zero AT. In early morning and late evening hours, when leaf water potential was about - 0 . 5 bar, the AT was 2--3°C. It will be noted that leaf water potential and AT were generally less negative for barnyard grass than for the rice cultivars. T h e relationship of leaf diffusive resistance and transpiration rate for the rice cultivars IR20 and M1-48 and for barnyard grass is illustrated in Fig. 8. During active transpiration periods on April 11 and 12, leaf diffusive resistances of both the rice cultivars remained low compared to barnyard grass. Variation in diffusive resistance for rice was in the range of 0.5 to 2.0 s cm -] , whereas for barnyard grass it was in the range of 1.0--9.0 s cm -1 . DISCUSSION The diurnal trend of transpiration rate of bot h rice and barnyard grass appears to be more directly influenced by VPD and wind speed than solar radiation. Figure 3 illustrates an interesting l o op relationship between transpiration, solar radiation, and time of observation. Our results suggest that a relatively low transpiration rate of bot h species in the morning hours, when solar radiation is low, is associated with low VPD and wind speed. High transpiration rates in the a f t e r n o o n occur at the same solar radiation level but are strongly influenced by VPD and wind speed at that time. Under controlled
293 55,
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Temperature difference between leaf and air (°C)
Fig. 6. R e l a t i o n s h i p b e t w e e n air t e m p e r a t u r e a n d t e m p e r a t u r e d i f f e r e n c e b e t w e e n leaf a n d air for rice cultivars I R 2 0 (o--o; ~ = 2 9 . 9 0 -- 1 . 6 6 X -- 0 . 1 2 X 2, R 2 -- 0 . 9 5 * * ) a n d M1-48 ('7--v; :~ = 3 0 . 9 9 - - 1 . 7 1 X - - 0 . 2 1 X 2, R 2 = 0 . 9 0 * * ) a n d also for b a r n y a r d grass (o--o; ~ = 32.61 -- 1 . 7 9 X - - 0 . 3 9 X 2, R 2 = 0.95**).
o~ 6
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I -12
I 16
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Fig. 7. R e l a t i o a s h i p o f t e m p e r a t u r e d i f f e r e n c e b e t w e e n leaf a n d air a n d leaf w a t e r p o t e n tial for rice cultivars I R 2 0 (o), M1-48 (v), a n d b a r n y a r d grass (o).
294
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04 06 08 Transpirah0n rate (ram I•-I)
o
o °o
I0
12
Fig. 8. Leaf diffusive resistance of the adaxial surface in relation to transpiration rate of rice cultivars IR20 and M1-48, and b a r n y a r d grass. S y m b o l s as in Fig. 7.
environmental conditions, Horie (1979) and Rawson et al. (1977) also observed a positive linear relationship between transpiration and VPD. At a given value of VPD, the transpiration rate of rice was higher than that of barnyard grass (Fig. 4). Two consequences of the diurnal trend in transpiration are obvious in both species. Increased transpiration decreased leaf temperature and leaf water potential in both rice and barnyard grass. Thus, it is interesting that the two rices and the weed species show different values for leaf temperature during the day (Fig. 5) and different air temperature at which AT = 0 (Fig. 6). The rice cultivar IR20, which was bred and selected for irrigated conditions, T A B L E II C o m p a r i s o n of several plant characteristics during periods of peak transpiration ( ~ 1500 b) for two rice cultivars IR20 and M1-48 (selected for irrigated and dryland culture, respectively) and barnyard grass
Transpiration rate ( m m h -! ) Leaf t e m p e r a t u r e (°C) Air t e m p e r a t u r e at A T ---- 0 (°C) Leaf water potential ( b a r ) S t o m a t a l resistance (adaxial) (s c m -! )
IR20
M1--48
1.15 30.2 29.9 --16.5 0.9
0.95 31.5 31.0 --13.0 1.4
"~
B a r n y a r d grass ~ ~ ~ ~ ~
0.37 32.0 32.6 --11.0 2.1
295
maintained the lowest leaf temperature and air temperature at AT = 0, while M1-48, a cultivar selected for dryland rice culture, had higher temperatures, and barnyard grass was the highest. These overall observations are summarized in Table II which shows consistent trends among the rice and weed species for transpiration rate, leaf temperature, air temperature at which AT = 0, leaf water potential and adaxial stomatal resistance. Much has been written about C3 (rice) and C4 (barnyard grass) species and their adaptation to tropical conditions (Ludlow, 1976). It is clear that in the conditions of this experiment, barnyard grass transpired about half as much water as the rice cultivar M1-48, which "had a leaf area index similar to barnyard grass (Table I). Rawson et al. (1977) and Hasegawa (1977) demonstrated that in general the transpiration rate of C3 plants was higher than C4 plants and concluded that water use efficiency was highest in C4 xerophytes and lowest in C3 mesophytes. As illustrated in Fig. 5, the leaf temperature of both the species, C3 and C4, appears to be closely associated with diurnal changes in air temperature (Hasegawa, 1978). Moreover, leaf temperature of barnyard grass remains higher than rice for much of the day. The leaf temperature of the drylandadapted rice cultivar, M1-48, was higher than that of wetland-adapted, highyielding rice cultivar, IR20. Under controlled conditions, Hasegawa (1977) observed that in both wet and dry atmospheric conditions, leaf temperatures of C3 plants were lower than those of C4 plants. Under irrigated conditions, the lower leaf temperature of rice compared to barnyard grass, is probably related with the lower optimum leaf temperature for net photosynthesis of C3 plants as compared to that of C4 plants (Ludlow, 1976; Vong and Murata, 1977). The difference observed in leaf-to-air temperature and diurnal leaf temperature trend between dryland-adapted and wetland-adapted rice cultivars leads to an interesting question about the possibility of similar intraspecies adaptation for optimal leaf temperature within the rice species. Leaf water potential of --15 to - 1 7 bar appears to be a significant degree of water deficit and is perhaps unexpected for flooded rice. It is obvious, however, that this water potential was not causing stomatal closure (Fig. 8) or leaf temperature increase (Fig. 6) and was supporting the high flux rates of IR20 in these observations. Our results concur with those mentioned above which state that the transpiration rate of C3 plants is higher than that of C4 plants because leaf diffusive resistance of C4 plants is higher even during peak transpiration periods.
REFERENCES Aston, M.J., 1976. Variation of stomatal diffusive resistance with ambient humidity in sunflower (Helianthus annus). Aust. J. Plant Physiol., 3: 489--501. Barrs, H. D., 1973. Controlled environment studies of the effects of variable atmospheric water stress on photosynthesis, transpiration and water status of Zea mays L. and other species. In: R. O. Slatyer (Editor), Plant Response to Climatic Factors. Proc. Uppsala Syrup., UNESCO, Paris, pp. 249--259.
296 Hasegawa, S., 1977. Agroclimatological studies on Ca-plants and C4-plants. (3) Transpiration and leaf temperatures. J. Agric. Meteorol., 33: 129--136. Hasegawa, S., 1978. Agroclimatological studies on C3-plants and C4-plants. (4)Diurnal variations of leaf temperature and transpiration rates of rice and Japanese barnyard millet. J. Agric. Meteorol., 34: 119--124. Horie, T., 1979. Studies on photosynthesis and primary production of rice plants in relation to meteorological environments. II. Gaseous diffusive resistances, photosynthesis and transpiration in the leaves as influenced by atmospheric humidity and air and soil temperatures. J. Agric. Meteorol., 35:1--12. Idso, S.B., Reginato, R.J., Hatfield, J.L., Walker, G.K., Jackson, R.D. and Pinter, P.J., Jr., 1980. A generalization of the stress-degree-day concept of yield prediction to accommodate a diversity of crops. Agric. Meteorol., 21 : 205--211. Ishihara, K., Ishida, Y. and Ogura, T., 1971. The relationship between environmental factors and behavior of stomata in the rice plant. 2. On the diurnal movement of tbe stomata. Proc. Crop Sci. Soc. Jpn., 40: 497--504. Kaufmann, M.R., 1976. Water transport through plants: Current perspectives. In: I.F. Wardlaw and J.B. Passioura (Editors), Academic Press, New York, NY, pp. 313 -327. Lange, O.L., Losche, R., Schulze, E.-D. and Kappen, L., 1971. Responses of stomata to changes in humidity. Planta, 100: 76--86. Ludlow, M.M., 1976. Ecophysiology of C4 grasses. In: O.L. Lange, L. Kappen and E.-D. Schulze (Editors), Ecological Studies. Analysis and Synthesis, Vol. 19. Water and Plant Life. Springer-Verlag, Berlin and Heidelberg, pp. 364--386. Meidner, It. and Mansfield, T.A,, 1968. Physiology of Stomata. McGraw-Hill, London, 178 pp. Rawson, H.M., Begg, J.E. and Woodward, R.G., 1977. The effect of atmospheric humidity on photosynthesis, transpiration and water use efficiency of leaves of several plant species. Planta, 123: 5--10. Schulze, E.-D., Lange, O.L., Buschbom, U., Kappen, L. and Evenari, E., 1972. Stomatal responses to changes in humidity in plants growing in the desert. Planta {Berlin), 108: 259--270. Tanner, C.B., 1963. Plant temperatures. Agron. J., 55: 210--211. Tomar, V.S. and O'Toole, J.C.~ 1980a. Water use in lowland rice cultivation in Asia: A review of evapotranspiration. Agric. Water Manage., 3 : 8 3 - - 1 0 6 . Tomar, V.S. and O'Toole, J.C., 1980b. Design and testing of a microlysimeter for wetland rice. Agron. J., 72: 689--692. Vong, N.Q. and Murata, J., 1977. Studies on the physiological characteristics of C3 and C4 crop species. (1) The effects of air temperature on the apparent photosynthesis, dark respiration and nutrient absorption of some crops. Jpn. J. Crop Sci., 46: 45--52.
285
TRANSPIRATION, LEAF TEMPERATURE AND WATER POTENTIAL OF RICE AND BARNYARD GRASS IN FLOODED FIELDS J. C. O'TOOLE and V. S. TOMAR* The International Rice Research Institute, P.O. Box 933, Manila (Philippines) (Received July 23, 1981; revision accepted February 11, 1982) ABSTRACT O'Toole, J.C. and Tomar, V.S., 1982. Transpiration, leaf temperature and water potential of rice and barnyard grass in flooded fields. Agric. Meteorol., 26: 285--296. Few studies are available on crop--water relationships and attendant climatic characterization of irrigated (flooded) rice fields in tropical conditions. We measured transpiration, leaf water potential, leaf diffusive resistance and leaf temperature of rice (Oryza sativa L.), a C3 species, and barnyard grass (Echinochloa crus-galli L.), a C4 species, during the dry season rice crop in the Philippines. Concurrent measurements were made of solar radiation, water vapor pressure deficit and wind speed. Transpiration rate, leaf water potential, and leaf temperature of both species responded to diurnal trends in the meteorological variables. Transpiration rate, during the course of the day was more responsive to vapor pressure deficit and wind speed than solar radiation. Transpiration rate of four rice cultivars averaged 7.2 mm d -1 while barnyard grass was 2.8 m m d -1. During much of the diurnal period, leaf water potential, leaf diffusive resistance and leaf temperature of rice were lower than barnyard grass. Although the leaf area index of barnyard grass was less than that of rice, other characteristics of the two plant groups (C3 and C4) provide partial explanation for the differences observed.
INTRODUCTION E v a p o t r a n s p i r a t i o n f r o m t h e rice c r o p has b e e n s t u d i e d e x t e n s i v e l y in t r o p i c a l Asia. T o m a r a n d O ' T o o l e ( 1 9 8 0 a ) r e v i e w e d t h e l i t e r a t u r e on, a n d basic p r i n c i p l e s a f f e c t i n g , t r a n s p i r a t i o n a n d e v a p o t r a n s p i r a t i o n f r o m w e t l a n d rice in s o u t h a n d s o u t h e a s t Asia. In c o n t r a s t w i t h r e s e a r c h on o t h e r m a j o r f o o d crops they f o u n d a lack of f u n d a m e n t a l i n f o r m a t i o n to describe and u n d e r s t a n d r e l a t i o n s h i p s b e t w e e n t r a n s p i r a t i o n r a t e a n d such v a r i a b l e s as l e a f w a t e r p o t e n t i a l , l e a f d i f f u s i v e r e s i s t a n c e a n d l e a f t e m p e r a t u r e in i r r i g a t e d ( f l o o d e d ) t r o p i c a l rice c r o p s . G e n e r a l l y , air t e m p e r a t u r e , w a t e r v a p o r p r e s s u r e d e f i c i t ( V P D ) a n d s o l a r r a d i a t i o n have b e e n r e l a t e d t o t r a n s p i r a t i o n rate. T h e d i r e c t e f f e c t s o f air t e m p e r a t u r e on t h e s t o m a t a l a p p a r a t u s a n d h e n c e on t r a n s p i r a t i o n are, h o w ever, i n c o n c l u s i v e ( M e i d n e r a n d M a n s f i e l d , 1 9 6 8 ) . T r a n s p i r a t i o n c h a n g e s during t h e c o u r s e o f a d a y in r e s p o n s e t o t h e s e m e t e o r o l o g i c a l v a r i a b l e s r e s u l t in changing leaf water potential and leaf t e m p e r a t u r e . Relationships between * Present address: Department of Soil Science, G. B. Pant University of Agriculture and Technology, Pantnagar District Nainital, U.P., India.
0002-1571/82/0000--0000/$02.75
© 1982 Elsevier Scientific Publishing Company
286 transpiration rate and leaf water potential have been reviewed (Kaufmann, 1976) for several plant species. Changes in leaf temperature and leaf-to-air temperature differential have been associated with evaluation of crop water status (Tanner, 1963; Idso et al., 1980). Views on the direct effects of VPD on diffusive resistance and plant water status are conflicting. Using greatly variable plant material, several workers (Ishihara et al., 1971; Lange et al., 1971; Schulze et al., 1972; Aston, 1976) illustrated the dependence of stomatal resistance on atmospheric humidity. In contrast, other workers have found no such trend (Barrs, 1973; Rawson et al., 1977). The direct relationship between solar radiation and transpiration is more universally accepted. However, rates of crop transpiration have been more frequently related to daily or weekly totals of solar radiation than to short-term or diurnal measurements. Wetland rice grown in tropical conditions has been little studied with regard to the above-mentioned variables and their effect on transpiration and plant water status. The present study was initiated to characterize the diurnal variations in transpiration rate, leaf water potential, leaf-to-air temperature differential (AT) and attendant meteorological variables for a dry season irrigated rice crop. Additionally, a c o m m o n weed species, barnyard grass (Echinochloa crus-galli L.) was also included as preliminary observations showed very different responses when compared to rice. MATERIALS AND METHODS The study was conducted at the experimental farm of the International Rice Research Institute (IRRI), Los Bafios, Laguna, Philippines, on Maahas clay soil (an amorphous soil classified as Typic Tropoquept). Fields were fully irrigated (flooded to a depth of 3--8 cm of water) during the entire crop season. Fields were prepared as puddled wetland and 22-day-old seedlings of four rice cultivars, IR20, IR36, Dular and M1-48, were transplanted at 20 x 20 cm spacing on February 28, 1978. Barnyard grass was seeded one week after the rice had been transplanted because of its more rapid growth and phasic development. The four cultivars of rice and barnyard grass were replicated four times in a completely randomized block design. Each replicate plot of rice or barnyard grass was 8 x 10 m. Fertilizer was applied at 1 2 0 k g N , 60kgP2Os and 6 0 k g K 2 0 per hectare. Agronomic practices included high levels of insect and disease protection. Observations on transpiration rate, leaf temperature, stomatal resistance, leaf water potential and weather conditions were made 42 days after transplanting on two consecutive clear days, April 11 and 12, 1978. Solar radiation was monitored with a pyranograph calibrated against an Eppley pyroheliometer, and relative humidity was measured with a hair hygrothermograph at one of the Institute's meteorological stations less than 0.5 km from the study site. The station was located in an irrigated rice area which included the study site. Wind speed was measured at the research site by a vane anemometer located 2 m above the soil surface.
287 T h e t r a n s p i r a t i o n rate was m e a s u r e d at h o u r l y intervals b e t w e e n 0 5 0 0 and 1 8 0 0 h with m i c r o l y s i m e t e r s installed in all plots. Design and testing o f the m i c r o l y s i m e t e r s are discussed elsewhere ( T o m a r and O ' T o o l e , 1980b). T w o m i c r o l y s i m e t e r s were placed in each replication, i.e., eight units f o r each cultivar o f rice or the weed species. A f t e r the e x p e r i m e n t was completed, plants in m i c r o l y s i m e t e r s and in f o u r a d j a c e n t hills in each p l o t were cut to measure leaf area. A Hayashi D e n k o m e t e r Model AAM-7 was used for this purpose. Values o f t r a n s p i r a t i o n rate are p r e s e n t e d on a land area basis, each hill covering 400 cm 2 and also on a per unit leaf area basis, calculated f r o m the t o t a l leaf area o f the plants in each m i c r o l y s i m e t e r . L e a f t e m p e r a t u r e , s t o m a t a l resistance and leaf w a t e r p o t e n t i a l were measured o n t w o fully d e v e l o p e d leaves r e p r e s e n t a t i v e o f the u p p e r c a n o p y and c o m p a r a b l e with plants in the microlysimeters. These m e a s u r e m e n t s were m a d e at each h o u r l y sampling p e r i o d b e t w e e n 0 5 0 0 and 1 8 0 0 h o u r s in each replicate plot. L e a f t e m p e r a t u r e was m e a s u r e d with a biological-type stainless steel micro-syringe with t h e r m i s t o r sensor at the tip. T h e micro-syringe sensor was inserted into the mid-vein on the abaxial leaf surface. Air t e m p e r a t u r e at the c r o p level was m e a s u r e d s i m u l t a n e o u s l y with a shielded e p o x y - c o a t e d thermistor. S t o m a t a l resistance was m e a s u r e d with a L a m b d a diffusive resistance m e t e r m o d e l LI-60, with an LI-15S sensor having an a p e r t u r e o f 3.5 × 20 ram. T h e calibration plate p r o v i d e d a range o f resistances f r o m 0.59 to 33.3 s cm -1 at 25°C. M e a s u r e m e n t s were m a d e on the adaxial leaf surface. L e a f w a t e r p o t e n t i a l was e s t i m a t e d with a pressure c h a m b e r . A fully develo p e d leaf, s e c o n d f r o m the t o p o f a tiller, was w r a p p e d in a m o i s t cheesecloth and a l u m i n u m foil p r o t e c t i v e cover b e f o r e excision and k e p t in t h a t c o n d i t i o n until the m e a s u r e m e n t was c o m p l e t e d . Pressure was applied f r o m a c o m p r e s s e d n i t r o g e n gas s o u r c e at a rate o f 21 bar min -1 . RESULTS
Changes in transpiration with weather conditions Transpiration rate Diurnal p a t t e r n s o f t r a n s p i r a t i o n rate in rice cultivars I R 2 0 and M1-48 and b a r n y a r d grass are p r e s e n t e d in Fig. 1. Because w e a t h e r c o n d i t i o n s on April 11 and 12 were similar, data f o r the first d a y o n l y are presented. T h e response o f I R 2 0 was similar to t h a t o f I R 3 6 , and M1-48 was similar to t h a t o f Dular: thus o n l y e x a m p l e s o f I R 2 0 and M1-48 are p r e s e n t e d in Fig. 1. Peak t r a n s p i r a t i o n rates were r e a c h e d at a b o u t 1500 h. Daily totals and the corres p o n d i n g leaf area i n d e x are given in Table I. Values o f t r a n s p i r a t i o n rate per u n i t leaf area were similar in diurnal t r e n d t o the m e a s u r e d rates per u n i t g r o u n d surface area a l t h o u g h differences in the leaf area b e t w e e n rices and
288
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(
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Fig. 1. Transpiration rate of (a) rice cultivar IR20, (b) rice cultivar M1-48, and (c) barnyard grass, during the light period April 11, 1978. ~ ~, per unit ground area; c~---o, per unit leaf area. TABLE I Daily total transpiration per unit ground area (ram d -1) of rice and barnyard grass and leaf area index (m2/m 2) of plants in microlysimeters Measured transpiration (mm d -1 )
Leaf area index (m2/m 2 )
April 11, 1978 (0600--1800 h)
April 12, 1978 (0600--1800 h)
Rice IR36 IR20 Dular M1-48
8.0 7.1 6.8 5.9
8.8 8.1 7.1 6.4
6.8 5.7 4.3 2.3
Weed species Barnyard grass
2.8
2.8
1.7
weed species were apparent. In later discussions, only the measured trans p i r a t i o n r a t e p e r u n i t g r o u n d s u r f a c e a r e a is u s e d . Transpiration rate began increasing at sunrise and attained the maximum
289
value of 1 . 1 - - 1 . 2 m m h -1 for IR20 and 0 . 9 5 m m h -~ for M1-48 during the period 1 2 0 0 - - 1 5 0 0 h . The maximum transpiration rate for barnyard grass { 0 . 3 5 - - 0 . 4 5 m m h -1) was much lower than the rice cultivars and peaked between (1100 and 1 3 0 0 h . The daily total transpiration of barnyard grass was about one-third that of the rice cultivars (Table I). Weather conditions for April 11 are depicted in Fig. 2. Solar radiation increased steadily up to 1 1 0 0 h and then declined. Total radiation received was 26.76 and 2 7 . 8 5 M J m -2 d -~ on April 11 and 12, 1978, respectively. Relative humidity remained low (~70%) during most of the day hours. Vapor pressure deficit (VPD) increased from sunrise to 1 2 0 0 h , attaining maximum values of a b o u t 20 mbar. Vapor pressure deficit remained around this value until 1700 h and then declined. The wind speed was greater than 2 m s -~ for much of the day. Similar climatic conditions prevailed on April 12, 1978. Thus, both days were relatively clear with high evaporative demand conditions characteristic of the dry season in much of tropical Asia. The relationships of transpiration to solar radiation, vapor pressure deficit and wind speed are illustrated in Fig. 3. Solar radiation and wind speed both show strong interaction with VPD in their effects on diurnal changes in transpiration rate. A characteristic diurnal loop, more circular for rice and elliptical for barnyard grass, was observed in the transpiration--solar radiation relationship of the two species (Fig. 3a). A similar trend is obvious for wind speed with a very strong interaction illustrated in Fig. 3b. The difference in species response is striking in both sets of graphs in Fig. 3. The effect of VPD on the transpiration rate is illustrated in Fig. 4. The transpiration rate of rice (average of four cultivars) increased exponentially while that of barnyard grass increased linearly with VPD. At high values of VPD, a large increase in the transpiration rate was observed for rice cultivars in comparison with that of barnyard grass. ,ii~ r
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290
g
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1530
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24
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Fig. 3. ( a ) D i u r n a l changes in transpiration rate of rice (average of four eultivars) and barnyard grass as affected by the interaction of solar radiation and vapor pressure deficit. (b) Diurnal changes in transpiration rate of rice (average of four cultivars) and barnyard grass as affected by the interaction of wind speed and vapor pressure deficit.
As shown in Fig. 3, transpiration rate was high at high VPD ( ~ 1 5 mbar), irrespective of solar radiation for both the species. At lower values of VPD, transpiration appears to increase with solar radiation. But the observations at low VPD were very few. Transpiration response to VPD and wind speed is illustrated in Fig. 3b. For both species, transpiration increased linearly with VPD and wind speed. The multiple linear regression relationship for rice was Y = - 0 . 1 1 + 0.01x1 + 0.23X2,
R: = 0.92
291
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Fig. 4. R e l a t i o n s h i p b e t w e e n t r a n s p i r a t i o n rate a n d w a t e r v a p o r p r e s s u r e d e f i c i t for an a v e r a g e of four cultivars of rice (o; :~ = 0 . 0 6 e °'13X, R 2 = 0 . 9 0 ) a n d for b a r n y a r d grass (e; ~ = --0.02 + 0.01X, r 2 = 0.57).
and for barnyard grass ~" = 0 . 0 3 - - 0 . 0 0 4 6 X 1 + 0.1X2,
R 2 = 0.70
where Y is the transpiration rate, X 1 the VPD and X 2 the wind speed. However, our observations are limited to certain specific field conditions where patterns in all the variables may be attributed primarily to diurnal changes.
Leaf temperature, AT and leaf water potential Changes in leaf temperature with the air temperature are plotted in Fig. 5. In general, leaf temperatures were higher than air temperature in early morning and late afternoon hours. Air temperature was higher than leaf temperature from 0900 to 1 6 0 0 h for rice cultivars. Leaf temperature of barnyard grass followed the same diurnal trend but during midday periods it was higher than rice and was approaching air temperature. Leaf temperature of rice cultivar M1-48 was always higher than that of IR20. Temperature difference between leaf and air (AT) decreased with air temperature and followed a curvilinear relationship (Fig. 6). The intercept of the curve was different for both the rice cultivars as well as for barnyard grass. Zero value of AT for IR20 was observed at 30°C air temperature and for M1-48 and barnyard grass at 31 and 32.6°C, respectively. AT for barnyard grass was continuously more positive, indicating a higher leaf temperature. AT values of IR20 were more negative at higher air temperatures. No definite correlation was established between AT and solar radiation though a decreasing trend in AT with solar radiation appeared but the data points were much diffused.
292 3!r
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i
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( hi I
Fig. 5. Diurnal changes in leaf temperature of rice cultivars IR20 (c>--o), M1-48 (o--o), and of barnyard grass ( H ) in relation to air temperature (D--~) at crop height.
Transpiration, stornatal resistance, AT and leaf water potential relations A linear relationship was established between AT and the decrease in leaf water potential (Fig. 7). Leaf water potential was about --9 bar at zero AT. In early morning and late evening hours, when leaf water potential was about - 0 . 5 bar, the AT was 2--3°C. It will be noted that leaf water potential and AT were generally less negative for barnyard grass than for the rice cultivars. T h e relationship of leaf diffusive resistance and transpiration rate for the rice cultivars IR20 and M1-48 and for barnyard grass is illustrated in Fig. 8. During active transpiration periods on April 11 and 12, leaf diffusive resistances of both the rice cultivars remained low compared to barnyard grass. Variation in diffusive resistance for rice was in the range of 0.5 to 2.0 s cm -] , whereas for barnyard grass it was in the range of 1.0--9.0 s cm -1 . DISCUSSION The diurnal trend of transpiration rate of bot h rice and barnyard grass appears to be more directly influenced by VPD and wind speed than solar radiation. Figure 3 illustrates an interesting l o op relationship between transpiration, solar radiation, and time of observation. Our results suggest that a relatively low transpiration rate of bot h species in the morning hours, when solar radiation is low, is associated with low VPD and wind speed. High transpiration rates in the a f t e r n o o n occur at the same solar radiation level but are strongly influenced by VPD and wind speed at that time. Under controlled
293 55,
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Fig. 6. R e l a t i o n s h i p b e t w e e n air t e m p e r a t u r e a n d t e m p e r a t u r e d i f f e r e n c e b e t w e e n leaf a n d air for rice cultivars I R 2 0 (o--o; ~ = 2 9 . 9 0 -- 1 . 6 6 X -- 0 . 1 2 X 2, R 2 -- 0 . 9 5 * * ) a n d M1-48 ('7--v; :~ = 3 0 . 9 9 - - 1 . 7 1 X - - 0 . 2 1 X 2, R 2 = 0 . 9 0 * * ) a n d also for b a r n y a r d grass (o--o; ~ = 32.61 -- 1 . 7 9 X - - 0 . 3 9 X 2, R 2 = 0.95**).
o~ 6
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Fig. 7. R e l a t i o a s h i p o f t e m p e r a t u r e d i f f e r e n c e b e t w e e n leaf a n d air a n d leaf w a t e r p o t e n tial for rice cultivars I R 2 0 (o), M1-48 (v), a n d b a r n y a r d grass (o).
294
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o
o °o
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Fig. 8. Leaf diffusive resistance of the adaxial surface in relation to transpiration rate of rice cultivars IR20 and M1-48, and b a r n y a r d grass. S y m b o l s as in Fig. 7.
environmental conditions, Horie (1979) and Rawson et al. (1977) also observed a positive linear relationship between transpiration and VPD. At a given value of VPD, the transpiration rate of rice was higher than that of barnyard grass (Fig. 4). Two consequences of the diurnal trend in transpiration are obvious in both species. Increased transpiration decreased leaf temperature and leaf water potential in both rice and barnyard grass. Thus, it is interesting that the two rices and the weed species show different values for leaf temperature during the day (Fig. 5) and different air temperature at which AT = 0 (Fig. 6). The rice cultivar IR20, which was bred and selected for irrigated conditions, T A B L E II C o m p a r i s o n of several plant characteristics during periods of peak transpiration ( ~ 1500 b) for two rice cultivars IR20 and M1-48 (selected for irrigated and dryland culture, respectively) and barnyard grass
Transpiration rate ( m m h -! ) Leaf t e m p e r a t u r e (°C) Air t e m p e r a t u r e at A T ---- 0 (°C) Leaf water potential ( b a r ) S t o m a t a l resistance (adaxial) (s c m -! )
IR20
M1--48
1.15 30.2 29.9 --16.5 0.9
0.95 31.5 31.0 --13.0 1.4
"~
B a r n y a r d grass ~ ~ ~ ~ ~
0.37 32.0 32.6 --11.0 2.1
295
maintained the lowest leaf temperature and air temperature at AT = 0, while M1-48, a cultivar selected for dryland rice culture, had higher temperatures, and barnyard grass was the highest. These overall observations are summarized in Table II which shows consistent trends among the rice and weed species for transpiration rate, leaf temperature, air temperature at which AT = 0, leaf water potential and adaxial stomatal resistance. Much has been written about C3 (rice) and C4 (barnyard grass) species and their adaptation to tropical conditions (Ludlow, 1976). It is clear that in the conditions of this experiment, barnyard grass transpired about half as much water as the rice cultivar M1-48, which "had a leaf area index similar to barnyard grass (Table I). Rawson et al. (1977) and Hasegawa (1977) demonstrated that in general the transpiration rate of C3 plants was higher than C4 plants and concluded that water use efficiency was highest in C4 xerophytes and lowest in C3 mesophytes. As illustrated in Fig. 5, the leaf temperature of both the species, C3 and C4, appears to be closely associated with diurnal changes in air temperature (Hasegawa, 1978). Moreover, leaf temperature of barnyard grass remains higher than rice for much of the day. The leaf temperature of the drylandadapted rice cultivar, M1-48, was higher than that of wetland-adapted, highyielding rice cultivar, IR20. Under controlled conditions, Hasegawa (1977) observed that in both wet and dry atmospheric conditions, leaf temperatures of C3 plants were lower than those of C4 plants. Under irrigated conditions, the lower leaf temperature of rice compared to barnyard grass, is probably related with the lower optimum leaf temperature for net photosynthesis of C3 plants as compared to that of C4 plants (Ludlow, 1976; Vong and Murata, 1977). The difference observed in leaf-to-air temperature and diurnal leaf temperature trend between dryland-adapted and wetland-adapted rice cultivars leads to an interesting question about the possibility of similar intraspecies adaptation for optimal leaf temperature within the rice species. Leaf water potential of --15 to - 1 7 bar appears to be a significant degree of water deficit and is perhaps unexpected for flooded rice. It is obvious, however, that this water potential was not causing stomatal closure (Fig. 8) or leaf temperature increase (Fig. 6) and was supporting the high flux rates of IR20 in these observations. Our results concur with those mentioned above which state that the transpiration rate of C3 plants is higher than that of C4 plants because leaf diffusive resistance of C4 plants is higher even during peak transpiration periods.
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