Energy exchange and leaf temperature behavior of three plant species

Environmental and Experimental Botany, Vol. 31, No. 3, pp. 295-302, 1991 Primed ill Greal Britain.

0098- 8472/91 $3.00 + 0.00 Pergamon Press pie

E N E R G Y E X C H A N G E A N D LEAF T E M P E R A T U R E B E H A V I O R OF T H R E E P L A N T SPECIES j. L. HATFIELD* and J. J. BURKEt *National Soil Tilth Laboratory, USDA-ARS, Ames, IA 50011-3250, U.S.A. and ~'Plant Stress and Water Conservation Unit, USDA-ARS, Box 215, Lubbock, TX 79401, U.S.A.

(Received 27 November 1989; accepted in final revisedform 19 December 1990) HATFIELDJ. L. and BURKEJ. J. Energy exchange and leaf temperature behavior of three plant species. ENVIRONMENTALAND EXeERIMENTALBOTANY31, 295--302, 1991. Leaf temperature differences have been observed among species; however, no direct comparison has been made relative to species-specific thermal kinetic windows. To evaluate the leaf temperature response within the same environment, a field study was conducted on cotton (Gossypium hirsutum L.), cucumber (Cucumis sativa L.) and bell pepper (Capsfumfrutescens L.). Individual leaves were monitored for leaf temperature, transpiration, and leaf and aerodynamic resistances. An energy balance model was used to estimate the sensible and latent heat fluxes. Cotton exhibited a lower leaf temperature, lower leaf resistance, and higher transpiration than cucumber and pepper under the same environmental conditions. Mid-day leaf resistances for cotton, cucumber, and pepper were 17.5, 64.7, and 25.0 sec/m, respectively, while aerodynamic resistances tbr all species were between 10 and 17.5 sec/m. Leaf" temperatures for this same interval were 26.2°C for cotton, 29.5°C for cucumber and 32. I°C for pepper. Transpiration rates maintained leaf temperatures within the predefined thermal kinetic window. We propose that a unique link exists between the biological and physical energy exchange mechanisms in which the leaf temperature is maintained within optimum limits for physiological function as indicated by enzyme kinetics.

UPCHURCH a n d MAHAN/12) showed that cotton (Gossypium hirsutum L.) when a d e q u a t e l y w a t e r e d OBSERVATIONS of leaf or foliage t e m p e r a t u r e s have and grown in greenhouse conditions m a i n t a i n e d been utilized to characterize crop stress a n d to a foliage t e m p e r a t u r e of 28 _+ 2°C when exposed to define p l a n t response to the environment./6) In high solar r a d i a t i o n and air t e m p e r a t u r e s above the d e v e l o p m e n t of crop stress indices it has been 28°C. T h e y hypothesized that the cotton plants assumed that the observed leaf t e m p e r a t u r e s expressed a limited h o m e o t h e r m i c b e h a v i o r in occurred as a passive physical response to energy that foliage t e m p e r a t u r e was m a i n t a i n e d to peri n p u t as a result of the energy exchange between mit an o p t i m a l e n v i r o n m e n t for p l a n t cell the leaf a n d the atmosphere. WIEBELT and HEN- f u n c t i o n . HAYFIELD et al. !5i suggested that w h e a t DE,SON ~t:~; described the prediction of single leaf (Triticum aestivum L.) and cotton would m a i n t a i n t e m p e r a t u r e s based solely on energy exchange different foliage t e m p e r a t u r e s u n d e r the same characteristics. T h e y found leaf t e m p e r a t u r e s for e n v i r o n m e n t a l conditions. HATFIELD et al. ~5~ showed that w h e a t foliage b u r r oak (Quercus macrocarpa L.) and silver m a p l e (Acer saccharum L.) to be within 0.2°C of the t e m p e r a t u r e s separated fi'om air t e m p e r a t u r e at predicted values with a s t a n d a r d d e v i a t i o n of a b o u t 20°C while cotton s e p a r a t e d at a b o u t 28°C. 4.0°C. However, MAHAN a n d UPCHURCH '7) and These t e m p e r a t u r e s were found by BURKE eta[. c4) 295

INTRODUCTION

296

J.L.

HATFIELD and J. ,J. BURKE

to represent the m i d p o i n t of the thermal kinetic window ( T K W ) fi)r each of these species based on their enzyme kinetics. T h e y showed that the enzyme kinetics of each species exhibited a distinct m i n i m u m a p p a r e n t K,, wdue and a relatively n a r r o w range in which the enzymes would function optimally. BURKE and H A T F I E I . D :3 incorp o r a t e d tbliage t e m p e r a t u r e into the reaction process o f g l u t a t h i o n e reductase to suggest that cellular protection from peroxides `*'as aflbrded to `*heat which could m a i n t a i n foliage temperatures near the o p t i m a l value of 20°C. T h e i n c o r p o r a t i o n of the biochemical process with the e n v i r o n m e n t a l physics of leaf temp e r a t u r e opens new possibilities lot interpreting and u n d e r s t a n d i n g plant a t m o s p h e r e interactions. T h e objectives of this study were to determine if d i u r n a l fCiage t e m p e r a t u r e s of different species are related to the species-specific optimal K,, t e m p e r a t u r e s and that leaf t e m p e r a t u r e patterns are related to net radiation, leaf resistance, and transpiration. MATERIALS

AND METHODS

Experimental procedure D u r i n g the s u m m e r of 1988 experimenls were conducted on field-grown plants of cotton, c u c u m b e r (Cucumis sativa I,.) and bell p e p p e r (Capsicum fiulescens I,.) Each species was planted in 76 cm rows and gro`*'n u n d e r well-irrigated conditions in tbur-ro`*' plots 30 m in length. These areas were used to provide p l a n t material fi)r the observations on individual leaves. T h e planted areas were located within an area s u r r o u n d e d by extensive cotton fields in order to minimize any horizontal advcction. In each species r a n d o m l y selected leaves were chosen tbr m e a s u r e m e n t of the leaf t e m p e r a t u r e and energy exchange. Due to the complexities of the e q u i p m e n t and a limited n u m b e r of inti'ared thermometers only one leaf" in each species was measured with the infrared thermometer. Detailed measurements were m a d e on each species in the tollowing manner. S m a l l - b e a d Cu Cn thermocouples (0.1 m m diameter) were placed on the underside of two upper, tully e x p a n d e d leaves, which were fully exposed to the sun. O n l y horizontally oriented, u p p e r leaves of each plant were measured and ibr c u c u m b e r the leaves `*ere

raised to the height of the cotton leaves. A nonaspirated, shielded Cu Cn thermocouple (0.1 m m diameter) was placed a d j a c e n t to the leat, and held in position by a t t a c h i n g the shield to a small metal rod. A 4" field-of-view infrared therm o m e t e r (Everest model 4000 Fullerton, CA) ,*as positioned at a 0 azimuth angle at an angle ot"60 ~ fi'om the perpendicular, to measure the radiative t e m p e r a t u r e of the teaE T h e r m o c o u p l e and intiared t h e r m o m e t e r readings were c o m p a r e d to determine if the infrared t h e r m o m e t e r r e m a i n e d tbcussed on the leaf t h r o u g h o u t the day. In these analyses only the inti'ared t e m p e r a t u r e readings were utilized because of the larger area of the leaf being sampled c o m p a r e d to the small sensing bead of the thcrmocouple. T h e difl'ereiwes between the two a t t a c h e d thermocouptes were less than 0.5'~'C, which is the accuracy limit of the in fi'ared thermometer. T o measure the a m b i e n t environmental conditions, air temperature, water v a p o r pressure, windspeed, and solar radiation were measured at 20 cm above the leaves. Air t e m p e r a t u r e was measured with a 0.1 m m d i a m e t e r Cu Cn thermocouple placed under a radiation shield. W a t e r vapor pressure was measured with a shielded, Phvschem model 10l sensor, windspeed with a pulse a n e m o m e t e r , and solar radiation with a p y r a n o m e t e r . All d a t a were sampled every 10 sec with a C R 2 1 X d a t a logger ( C a m p b e l l Scientific, Logan, UT) with a 5 min average and s t a n d a r d deviations c o m p u t e d fi)r each measurement. Measurements were m a d e tbr several consecutive days tbr each species. For cotton and c u c u m b e r 19 days of simultaneous d a t a were collected and tbr bell peppers 20 days ol" d a t a were obtained. Length and width of each individual leaf with an attached thermocouple `*ere measured at the beginning of the m e a s u r e m e n t cycle to o b t a i n the characteristic dimension. l,eaf photosynthesis and transpiration measurements were m a d e with a spherical, Plexiglas" leaf c h a m b e r a t t a c h e d to an A n a r a d A P P A 3 analyzer. These measurements were m a d e hourly on 10 r a n d o m l y selected leaves, fully exposed to the sun, fiom each species from 0700 to 1600 C D T . Measurements ,*'ere m a d e tbr a 20 sec period to o b t a i n photosynthetic and transpiration rates. L e a f area was d e t e r m i n e d by measuring length and width of the leaf after the

LEAF T E M P E R A T U R E BEHAVIOR OF THREE PLANT SPECIES c h a m b e r was removed and m u l t i p l y i n g by the leaf correction factor. L e a f area was measured on the last sample of the d a y with a leaf a r e a meter to d e t e r m i n e the correction factor. Mathematical equalions T h e energy b a l a n c e of a leaf can be described as tbllows: Rn = H + L E + J

(1)

where Rn is the net r a d i a t i o n (Jm '-' sec a), H t h e sensible heat flux (Jm 2 sec l), L E the latent heat flux (Jm ~ sec- i), and J t h e storage of heat within the leaf volume (Jm 2 sec i). In this study, J w a s assumed to be 0.0. Net r a d i a t i o n was calculated as tbllows: Rn = ( 1 - o : ) S l + g ~ t , aT4-~.j,.~aaTi~,.~,

t t = pCp( T~,.,,,- T..,)/r~,h

with 7 the psychrometric constant (kPa/C), <,(Tj~,,l) the s a t u r a t i o n v a p o r pressure at the leaf t e m p e r a t u r e (kPa), e~, the v a p o r pressure of the air (kPa), r,, the a e r o d y n a m i c resistance fbr water v a p o r (sec/m), and r, the leaf resistance to water v a p o r exchange (sec/m). In this study latent heat flux was calculated fi'om E q u a t i o n (1) and leaf resistance fiom Equations (1) and (5). T h e measurements were i n c o r p o r a t e d into the a p p r o p r i a t e equations to calculate the net radiation, transpiration rate, leaf resistance and aerod y n a m i c resistance fbr each 5-min period t h r o u g h o u t the day. These d a t a were c o m p a r e d to photosynthesis and transpiration measurements on individual leaves. RESULTS

(2)

where ~ is the a l b e d o of" the leaf, St the incident solar r a d i a t i o n (Jm ~ sec 1), g~k> and gl~a the e m i t t a n c e of the sky and leak a the Stefan Boltzm a n n constant (5.67 x 10 UJm 2sec J K 4),and T, and TI¢.,j the air and leaf" t e m p e r a t u r e (K), respectively. Emissivity of the sky was calculated t?om the equation given by BRUNT ~ and leaf emissivity was assumed constant at 0.97. Sensible heat flux fi-om a leaf was calculated as

DOY 202 IRRIGATED COTTON - AIR TEMP

16

.

0

,v" " ~ , . .

(5)

.

2

.

.

4

.

.

6

.

.

8

.

.

.

.

.

.

.

.

.

.

.

.

.

.

10 12 14 16 18 20 22 24 TIME OF DAY

Fro. 1. Leaf (Tit.~f) and air temperature (7~,) patterns throughout a 24-hr day measured on irrigated cotton on DOY 202. 32

gav -[- r c

n

20

DO~'202 . . . . . . .: IRRIGATED CUCUMBERS • - AIR TEMP •'"

(4)

with u the windspeed above the leaf, and d the characteristic dimension. A d j u s t m e n t for water v a p o r relative to m o m e n t u m resistance was r:,, = r, x 0.93 a n d for sensible heat was r:., = r, x 0.83. L a t e n t heat flux can be solved fbr as a residual of the energy balance [ E q u a t i o n (1)] or independently as

'~

DISCUSSION

32,

a n d W A G G O N E R ~1° as

L E = pCp (e~(T,~ar)-e,)

AND

Leaf-air lemperature patterns Cotton, c u c u m b e r and bell p e p p e r leaf" temperatures exhibited difl'erent patterns t h r o u g h o u t one d a v (Figs 1-3/. O n d a y - o f year (DOY) 202

(3)

where pC]0 is the volumetric heat c a p a c i t y of the air (Jm ~ sec i) and r,h the a e r o d y n a m i c resistance for sensible heat exchange (see/m). T h e value fbr m o m e n t u m resistance, r~,, was calculated fi'om the characteristic dimension and windspeed tbllowing the p r o c e d u r e of MONTE~TH, ~ PARKHURST el al., ~ PARLANGE el al. ll~ and PARLANGE r~, = 6 . 9 8 ( , / d ) ,

297

V-

" ~..~.~

. . . . . . . .

" . ".'~, °.v" "t,,,..%

24 20 16 0

. . . . . . . . . . . . . . . . . . . . "~. 2 4 6 8 10 12 14 16 18 20 22 2 4 TIME OF" DAY

Fig. 2. Leaf (Tl~.,r) and air temperature (7~,) patn-rns throughout a 24-hr day measured on irrigated cucumbers on DOY 202.

298

,J. L. HATFIELD and J. J. BURKE 40 35 ~, 0 "~

30 25 20

p.

15

DOY 237 IRRIGATED BELL PEPPERS - AIR TEMPERATURE • LEAF TEMPERATURE

5 0

0

4

8

12

16

20

24

TIME O F D A Y

FIO. 3. Leaf {T, ) and air temperature (T~,) patterns throughout a 24-hr day measured on irrigated bell peppers on DOY 937. cotton and cucumbers were measured simuhaneously; with leaves of basically similar dimensions (Table 1) there were distinctly different leaf-air temperature patterns (Figs 1 and 2). Cotton leaf temperatures were slightly below air temperatures until about 1000 CST at which time the leaves became 2-3°C cooler than the air temperature. The l e a f air temperatures did not merge together until the next morning (Fig. 1). Cucumber leaves did not cool below air until 1800 CST at which time radiative cooling became the dominant [hctor in the energy tmdget (Fig. 2). 1)uring mid-day, the cucumber leaves were 2 3"C warmer than the air while cotton leaves were 2 4"C cooler (Figs 1 and 2). O n all days, tbr the three species the leaves during the early morlfing (0 hr to sunrise) were slightly cooler than the air temperature. These data for cotton and cucumber show that although the environment is the same, each individual species may exhibit a different leaf temperature pattern relative to air temperature. The data shown in Figs l and 2 are from a day in which the soil water availability

Table 1. Characlerislic dimension,s ojindividual eollon, cucumber and bell pepper leaves as measured with an inj?ared Ihermometer during the 1988 study

Species Cotton Cucumber Pepper

Length (m)

Width (m)

0.132 0.165 0.101

0.115 0.157 0.042

was optimum to avoid any possible interactive effects of soil water and species response. The data shown here are typical of the days in which the soil water was adequate. O n D O Y 202 the cotton leaf temperature averaged 26.2°C between 1000 and 1600 CST with a standard deviation of 1.1 while the cucumber averaged 29.5°C with a standard deviation of 1.4 during the same interval. The leaf temperatures would have averaged higher during this interval if the mid-morning period of cloudiness had not occurred which reduced the net radiation load on the leaf To evaluate other leaves, during the mid-day ( 13001400 CST) leaf temperature was measured with a handheld infrared thermometer on surrounding leaves of similar exposure and size. It was tbund that the means of" these data were within the accuracy limit of the stationary int?ared thermometer and that the leaf" being measured was typical of other leaves in the canopy (Figs 1 and

2). Pepper leaf temperatures behaved similarly to those observed tbr cotton although the characteristic dimensions were smaller and leaf width was considerably smaller (Table 1). Leaf" temperature [bllowed air temperature until after 1200 CST at which time the leaves became cooler than the air until after 1900 CST when the leaf temperatures became warmer than the air temperature (Fig. 3). Leaf temperatures during the period fi'om 1200 to 1900 C S T fluctuated between 30 and 35°C and were 2-3°C cooler than the air temperature. These patterns of pepper leaf temperatures were typical of the days in which measurements were made. These results show that the leaf temperatures attained each day were influenced by r~tctors other than only the physical exchange processes. All three species were adequately watered throughout the study period to insure that soil water was not a limitation to the transpiration process.

Net radialion and transpiration

For each 5-rain interval net radiation and transpiration fluxes were calculated using Equations (2) and (5). Net radiation exhibited the typical response throughout the 24-hr day with values tit night averaging - 1 5 0 J m ~ sec ~ and then tbllowed the pattern of solar radiation throughout

LEAF T E M P E R A T U R E BEHAVIOR OF THREE PLANT SPECIES " ~ 1500

T

DOY 202 IRRIGATED COTTON NET RADIATION ~ ,.~ TRANSPIRATION .':~ .""~.~. • .i ."

1200

900

E

"

600 c E

:~t.

3oo

0

-300

J 0

4

8

12

16

20

24

TIME OF DAY

FIG. 4. Calculated net radiation (Rn) and transpiration (Transp) tbr a 24-hr day on irrigated cotton on DOY 2O2. 1200

T

800

DOY 202 TRANSPIRATIONi 1 ~ ~ _

E

400

/

c

"-

0

-400 2

4

6

8

10 12 14 16 18 20 22 24 TIME OF DAY

FIG. 5. Calculated net radiation (Rn) and transpiration (Transp) tbr a 24-hr day on irrigated cucumber on DOY 202. " " 1000

• DOY 257 800 - - NET RADIATION

I

I.~

600 400 "

200 0

£ -200 m -400

IRRIGATED BELL PEPPERS 4

8

12 16 TIME OF" DAY

20

24

Fro. 6. Calculated net radiation (Rn) and transpiration (Transp) tbr a 24-hr day on irrigated belt peppers on DOY 237. the d a y t i m e hours (Figs 4-6). I n all three species the peak net r a d i a t i o n exceeded 700 J m 2 sec and varied t h r o u g h o u t the d a y in response to clouds. T r a n s p i r a t i o n rates were d e p e n d e n t u p o n the

299

species, incident radiation, a n d the l e a ~ a i r temp e r a t u r e differences. In cotton, transpiration rates exceeded net r a d i a t i o n d u r i n g m i d - d a y as i n d i c a t e d by the leaves being cooler than the air (Fig. 4). For cucumbers on the same d a y the t r a n s p i r a t i o n rates did not exceed net r a d i a t i o n (Fig. 5) and leaf t e m p e r a t u r e s were higher than air temperature. Thus, the two species u n d e r identical e n v i r o n m e n t a l conditions exhibited different transpiration rates when net r a d i a t i o n was near its peak. Peppers exhibited a p a t t e r n of net r a d i a t i o n and t r a n s p i r a t i o n similar to those observed in cotton with the transpiration rates either equal to or slightly above net r a d i a t i o n when leaf t e m p e r a t u r e s were below air temp e r a t u r e (Fig. 6). T r a n s p i r a t i o n rates for cotton on D O Y 202 when plotted relative to leaf t e m p e r a t u r e exhibited an interesting pattern. T r a n s p i r a t i o n rates and leaf t e m p e r a t u r e s increased in the m o r n i n g with increasing net radiation; however, transpiration exhibited a wide range of'values within a relatively n a r r o w leaf t e m p e r a t u r e range of 23 28°C (Fig. 7a and b). HATFIELD et al. 5 suggested that t r a n s p i r a t i o n m a y be used to m a i n t a i n foliage or leaf t e m p e r a t u r e s within the thermal kinetic w i n d o w as defined by BURKE el al. ~4~ O n D O Y 202 calculated rates of transpiration (Fig. 7a) showed transpiration m a i n t a i n e d leaf temperatures below 28°C, except for a fi:w isolated leaf t e m p e r a t u r e s between 28 ° and 31°C. T h r o u g h o u t the d a y cotton exhibited larger t r a n s p i r a t i o n rates than c u c u m b e r by 20°/,. W e hypothesized that this m a y occur because the m i d p o i n t of the thermal kinetic window for cotton is 28°C c o m p a r e d to 35°C for cucumber. T h e larger transpiration rate m a y be related to an increased metabolic activity in cotton; however, these relationships are yet to be defined. This direct species comparison suggests a mechanism within the leaf which links stomatal opening to enzyme t e m p e r a t u r e response. Photosynthesis was significantly correlated to leaf t e m p e r a t u r e in c u c u m b e r but not in cotton (Table 2). A linear relationship was found between leaf t e m p e r a t u r e and t r a n s p i r a t i o n when leaf t e m p e r a t u r e s were outside the T K W (Fig. 7). W i t h i n the T K W a range of transpiration or photosynthetic rates was measured while leaf t e m p e r a t u r e r e m a i n e d faMy constant. Thus, leaf t e m p e r a t u r e remains faMy

300

.]. L. HATFIELD and J. J. BURKE

Io

1500,

.........

DOY 2 0 2 IRRIGATED COTTON

1250

1. . . . . I

i . . . . . I

1000

j

750

I

< n,.

500

I

u~

250

i

<

~-

10

~-

15

20

25

30

35

40

(oc)

LEAF TEMPERATURE 35

04

i

E o

'

30 25

I I

20

I

%t

'

or; ~

I dl~l

I

Z o I--

< n,-

'15

IIoI~Qe

10

I

~

5

< r~

o

ca]

k-

•

0

J i

. . . . . . . . . . . . . . . . . . I0 15 20 25 30

35

lated plants were measured, the aerodynamic resistances were small, e.g. l0 15 sec/m. Calculations o[" the leaf resistances were restricted from 0800 to 1600 C S T when the net radiation values were positive. Aerodynamic resistance exhibited less variation throughout the day than did leaf resistance tot all species (Figs 8-10). Cucumbers had an aerodynamic resistance of approximately 12 sec/m with a leaf resistance ot"64.7 sec/m with a standard deviation of 13.5 sec/m (Fig. 9); the leaf" resistance of cotton was 17.5 sec/m with a standard deviation of 6.2 sec/m. This difference between cotton and c u c u m b e r would be expected based on the similar net radiation values but different transpiration rates. Peppers had aerod y n a m i c resistance values near 10 sec/m and average leaf resistances at 25 sec/m with a standard deviation of 7.5. The dift~rent leaf temperatures (26.2°C for cotton and 29.5°C for

40

LEAF TEMPERATURE ( o c )

Fro. 7. Transpiration rates relative to measured leaf temperatures calculated (a) and measured with a porometcr (b) tbr irrigated cotton on DOY 202.

4O DOY 2 0 2 IRRIGATED c o T r o N - r Leaf • r Air

35

E

30 25

20 L

15

-

Table 2. Simple linear correlations among leaf temperature, transpiration and photoayntheaia jbr cotton and cucumber (n = 46)

..J ~-

• ,,*

~..

=.~

°.

,.

.

,

5 0 8

10

12

14

16

TIME OF DAY

Species Cotton Cucumber

Transpiration

Photosynthesis

0.43* 0.60"

0.25 n.s. 0.37"

non-significant. *Significant at 0.01 level.

l"m. 8. Calculated leaf Irh.~,) and aerodynamic (r~,i,) resistances [br the 0800 1600 CST period for irrigated cotton on DOY 202. 120

n.s.,

DOY 2 0 2 IRRIGATED CUCUMBERS

~'100

- -

r Leof • r Air

80

constant while the other e n v i r o n m e n t a l variables, e.g. net radiation, air temperature, and windspeed vary in response to a c h a n g i n g euergy balance between the leaf and the air. Leaf and aerodynamic resistances were calculated from the leaf energy balance model to provide another comparison a m o n g species. I n L u b b o c k windspeeds are typically greater than 4 m/see d u r i n g the day a n d as a result of the windspeed and the fact that only upper leaves on i s o -

60 t_ 40 "a

20 0 8

10

12

14

16

T I M E OF D A Y

Fro. 9. Calculated leaf (rL,.~,) and aerodynamic (r,,i,) resistances for the 0800 1600 CST period tor irrigated cucumber on DOY 202.

LEAF TEMPERATURE BEHAVIOR OF THREE PLANT SPECIES 60

m

"

DOY 2 3 7 IRRIGATED B E L L P E P P E R S rLeaf

/ ,/"

4o

20

.5 L-

9

10

11

12 13 TIME OF DAY

14

15

6

FIG. 10. Calculated leaf (r,,.~,) and acrodynalnic (r~,~,) resistances fi~r the 0800 1600 CST period tbr irrigated ])ell peppers on DOY 237.222. cucumber) a n d leaf resistances for two species within the same e n v i r o n m e n t suggest a link between enzyme kinetics and leaf temperature response. These same results were found throughout the study. Leaf resistance variation within each species is a result of variations in net radiation, leaf temperature, and windspeed. In all three species the leaf resistances began to increase in the afternoon atier 1400 C S T as the net radiation and air temperature began to decrease. This p a t t e r n shows that as the air temperature a n d net radiation decreased, lower transpiration rates resulted in a m a i n t e n a n c e of leaf temperatures within the thermal kinetic window. T h e lack of symmetry a b o u t solar noon is a result of an increasing radiation load in the m o r n i n g and a decreasing radiation load in the afternoon with w a r m e r air temperatures. CONCLUSIONS

Ditt~rent leaf t e m p e r a t u r e patterns t h r o u g h o u t the day were observed for three species, cotton, cucumber, a n d pepper and could be predicted based on an enzymatically d e t e r m i n e d speciesspecific thermal kinetic window. T h e m i d p o i n t of the thermal kinetic window was 28°C for cotton, 35~C tbr cucumber, and 32°C for pepper as shown by BURKE. 2' T r a n s p i r a t i o n m a i n t a i n e d leaf temperatures within the thermal kinetic window u n d e r conditions when air temperatures were above the T K W . Cotton had the lowest T K W m i d p o i n t and exhibited the largest transpiration rate while c u c u m b e r had the lowest transpiration rate and the highest T K W . Leaf resistances

301

showed that cotton had a lower leaf resistance than c u c u m b e r while pepper was intermediate. T h e data shown here were typical of all days in which soil water availability was optimal. Although a complete u n d e r s t a n d i n g of the mechanisms invoked to m a i n t a i n leaf ten> perature within the thermal kinetic window is not yet available, the results of the present study suggest a link between the biological and physical systems. F u r t h e r studies will be required to explore fully all of the possible mechanisms.

Acknowledgement Contribution from USDA-ARS. Mention of a specific tradename or product does not imply endorsement or preferential treatment by the United States Department of Agriculture, Agricultural Research Service.

REFERENCES

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