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Stomatal conductance and photosynthesis in water hyacinth: Effects of removing water from roots as quantified by a foliage-temperature-based plant water stress index
Agricultural and Forest Meteorology, 32 (1984) 249--256 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
249
STOMATAL CONDUCTANCE AND PHOTOSYNTHESIS IN WATER HYACINTH: EFFECTS OF REMOVING WATER FROM ROOTS AS QUANTIFIED BY A FOLIAGE-TEMPERATURE-BASED PLANT WATER STRESS INDEX* S.B. IDSO, P.J. PINTER, Jr., R.J. REGINATO and K.L. CLAWSON U.S. Water Conservation Laboratory, 4331 E. Broadway, Phoenix, AZ 85040 (U.S.A.) (Received January 4, 1984; revision accepted April 4, 1984)
ABSTRACT Idso, S.B., Pinter, P.J., Jr,, Reginato, R.J., and Clawson, K.L., 1984. Stomatal conductance and photosynthesis in water hyacinth: Effects of removing water from roots as quantified by a foliage-temperature-based plant water stress index. Agric. For. Meteorol., 32: 249--256. Stomatal conductance and net photosynthesis measurements were made over a period of a week in mid-October on two stands of water hyacinths floating in sunken metal stock tanks at Phoenix, AZ. On the second day of the experiment, all free water in one of the tanks was removed. Foliage temperature measurements were subsequently used to quantify the water stress experienced by the water-robbed plants; and a plant water stress index derived from the foliage temperature and air vapor pressure deficit data was used to study the effects of developing water stress on the plant physiological parameters being measured. The results obtained were practically identical to those derived from two independent season-long studies of water stress effects in cotton: net photosynthesis decreased linearly to become negative at a plant water stress index of 0.9 (where 1.0 represents the maximum possible stress), while a parameter related to plant water use efficiency first increased with increasing stress to reach a maximum at a plant water stress index of 0.6, after which it dropped off rapidly to zero with additional stress.
INTRODUCTION T h e p l a n t w a t e r s t r e s s i n d e x d e v e l o p e d b y I d s o e t al. ( 1 9 8 1 a ) a n d J a c k s o n e t al. ( 1 9 8 1 ) h a s p r o v e n t o b e o f u s e in a s s e s s i n g m a n y p l a n t p h y s i o l o g i c a l responses to water stress via remote measurements of foliage temperature ( I d s o e t al., 1 9 8 1 b , c , 1 9 8 2 a ; P i n t e r a n d R e g i n a t o , 1 9 8 2 ) . O f p a r t i c u l a r i n t e r e s t in t h i s r e g a r d a r e a s s e s s m e n t s o f l e a f s t o m a t a l c o n d u c t a n c e a n d n e t photosynthesis rates, since these parameters are related to water loss by transpiration and the production of dry matter -- basic plant functions w h i c h f a r m m a n a g e m e n t p r a c t i c e s a r e d e s i g n e d t o i n f l u e n c e in s u c h a w a y as to produce the most yield for the least financial outlay. To date, only one study has been reported which relates the Idso--Jackson p l a n t w a t e r s t r e s s i n d e x t o t h e s e t w o i m p o r t a n t p l a n t p r o p e r t i e s , i.e., t h a t o f I d s o e t al. ( 1 9 8 2 b ) w h e r e c o t t o n ( G o s s y p i u m h i r s u t u m L . ) w a s t h e * Contribution from the Agricultural Research Service, U.S. Department of Agriculture. 0168-1923/84/$03.00
© 1984 Elsevier Science Publishers B.V.
250
experimental crop. Thus, we felt it i m p o r t a n t to c o n d u c t a n o t h e r such study on a second plant species, to look for either similarities or differences in plant behavior relative to the plant water stress index. This paper represents the results o f th a t effort. M A T E R I A L S AND M E T H O D S
To allow for the occurrence of as great a potential difference as possible in response characteristics, we decided to work with an aquatic plant that normally floats with its roots suspended in water and is therefore normally never short o f water and stressed for that reason, i.e., the water hyacinth (Eichhornia crassipes (Mart). Solms). Following upon the work of Idso (1984) with this species, we thus established two good stands of water hyacinths in sunken metal stock tanks of 110- and 230-cm diameter at Phoenix, AZ. Both tanks o f vegetation were nourished on a bi-weekly schedule with a full-strength Hoagland solution 2, as described by J o h n s o n et al. (1957); and bot h were allowed to grow in our experimental setting for a t w o - m o n t h period (mid-August to mid-October) before we began our m eas u r emen t program. The e x p e r i m e n t was initiated by withdrawing sufficient water from both tanks so that, when all remaining free water was p u m p e d from the 110-cm diameter tank, the vegetation would not drop any furt her or spread out laterally. In this setting, measurements (to be described hereafter) were t h e n made on both tanks for one daylight period (Day --1), which began about 1030 and lasted until a b o u t 1500 MST. Subsequently, on Day 0, all of the remaining free water was p u m p e d from the 110-cm diameter tank at a b o u t 1120 MST. Since m uch water was still retained by the massive m at of roots which the hyacinths had pr oduc e d, however, effects of water stress were slow in revealing themselves; and measurements of developing water stress effects were thus able to be made over three of the five following days: Day 1, Day 2, and Day 5. Three basic types of measurements were made on both tanks of water hyacinths on three different schedules. First, foliage and air wet- and drybulb t e m p e r a t u r e measurements were made every 15 minutes of each daylight period in exactly the same m a n n e r as described by Idso (1984), i.e., with a 4-degree field-of-view Everest Interscience* Model 110 infrared t h e r m o m e t e r and an aspirated Bendix p s y c h r o m e t e r . Data acquired over the 230-cm d iamete r tank were used to c o nst ruct the large-leaf non-waterstressed baseline described by Idso et al. (1984); while data acquired over the l l 0 - c m d iamete r tank were used to cons t r uct the plant water stress index for the water-robbed plants. Leaf diffusion resistance measurements were obtained at 20-minute * Trade n a m e s a n d c o m p a n y n a m e s are i n c l u d e d for t h e b e n e f i t o f t h e r e a d e r and i m p l y no e n d o r s e m e n t o r p r e f e r e n t i a l t r e a t m e n t o f t h e p r o d u c t listed b y t h e U.S. D e p a r t m e n t o f Agriculture.
251 intervals, also in the same manner as described by Idso (1984), by means of a Li-Cor Model LI-1600 steady-state porometer. Three abaxial and three adaxial leaf surfaces were sampled on each tank and the results appropriately combined in parallel and then inverted to give a mean stomatal conductance value. These results, as well as the previously described foliage and air wet- and dry-bulb temperature measurements, were further smoothed with time through the day by means of a simple three-term running-averaging procedure. Net photosynthesis rates for each tank were estimated using a technique based on CO2 depletion by leaves in a portable hand-held chamber, much like the m e t h o d employed by Idso et al. (1982b). Prior to the experiment, five vigorous green leaves from the top of each canopy were selected and labeled for repeated measurement t h r o u g h o u t the entire study. The areas of these leaves were obtained by tracing their outlines on paper and sending these cut-outs through an optically-integrating leaf area meter. The chamber was constructed from a pair of hemispherically-shaped domes of transparent plexiglass, which were hinged to facilitate the rapid inclusion of a leaf blade within the resulting 1.73-liter sphere. When closed, the chamber halves were sealed with a soft neoprene gasket; and a high volume blower mixed air continuously within the chamber. Immediately upon clamping onto a leaf, electronically activated mechanisms synchronized the extraction of a reference air sample into a 10-ml plastic syringe. Then, after 15 seconds, another air sample was automatically withdrawn. Separate leaves were sampled at 1.5-min intervals; syringes containing air samples were temporarily stored at ambient temperatures in a styrofoam chest. Within 30 to 35 minutes of the first sample acquisition, CO 2 concentrations of the air samples were determined in the laboratory on an infrared gas analyzer (Analytical Development Co., Type 225 Mk 3) interfaced with a printing integrator. This system was calibrated at the beginning and end of each set of measurements using a 376 pm CO2 primary standard gas. Since complete sets of net photosynthesis measurements could be obtained no faster than about one per hour, no time-averaging of results was performed. RESULTS AND DISCUSSION The solid diagonal line of Fig. 1 represents the large-leaf non-waterstressed baseline developed by Idso et al. (1984) from data acquired over the 230-cm diameter hyacinth tank; while the points represent similarly acquired data over the l l 0 - c m diameter tank. As can be seen, the data appear to segregate into four distinct groups. The first two days' results fall on or near the baseline; while the succeeding three days' results sequentially rise above it in discrete steps to approach the upper foliage--air temperature differential limit. The derivation of this limit has been described previously in considerable detail (Idso, 1981; Idso et al., 1981a, c) and thus will not be repeated here. Suffice it to say t h a t the limit is weakly dependent upon air
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Fig. 1. Foliage--air temperature differential vs. air vapor pressure deficit for large-leaf (mean individual leaf diameter of 18cm) water hyacinths well supplied with water (diagonal solid line) and suddenly deprived of water (solid points). Horizontal lines are different theoretical upper limits. Fig. 2. Plant water stress index of water-robbed water hyacinths as a function of time on several days specified relative to the day of water extraction (Day 0). The vertical arrow marks the time of free water removal from the plants on that day.
t e m p e r a t u r e and t h a t t h e h o r i z o n t a l d a s h e d line r e p r e s e n t s t h e u p p e r limit f o r t h e m e a n air t e m p e r a t u r e on t h e a f t e r n o o n o f D a y 5, while the t w o h o r i z o n t a l solid lines r e p r e s e n t t h e e x t r e m e s o f t h e u p p e r limit f o r t h e highest a n d l o w e s t air t e m p e r a t u r e s e n c o u n t e r e d d u r i n g t h e study. F o l l o w i n g t h e p r o c e d u r e s o f I d s o et al. ( 1 9 8 1 a , b , c ) , t h e l l 0 - c m d i a m e t e r t a n k d a t a o f Fig. 1 w e r e t r a n s f o r m e d into p l a n t w a t e r stress i n d e x values a n d p l o t t e d as dally t r e n d s in Fig. 2. H e r e again it is e v i d e n t t h a t d e t e c t a b l e levels o f w a t e r stress did n o t o c c u r until t h e d a y f o l l o w i n g w a t e r r e m o v a l , a n d t h a t w a t e r stress did n o t b e c o m e m a x i m a l until five d a y s later. With this c h a r a c t e r i z a t i o n o f p l a n t w a t e r stress d e v e l o p m e n t , t h e s t o m a t a l c o n d u c t a n c e values o b t a i n e d o n t h e w a t e r - r o b b e d t a n k were n e x t divided b y t h o s e o b t a i n e d o n t h e n o n - w a t e r - s t r e s s e d t a n k to f o r m a r a t i o w h i c h s h o u l d t h e o r e t i c a l l y v a r y f r o m 1.0 w h e n t h e r e is no stress to 0.0 w h e n stress is m a x i m a l . T h u s , b y m a t c h i n g these s t o m a t a l c o n d u c t a n c e ratios w i t h t h e c o r r e s p o n d i n g p l a n t w a t e r stress i n d e x values o f Fig. 2, t h e p l o t o f Fig. 3 was d e v e l o p e d , w h e r e t h e h a n d - d r a w n t r e n d line t h r o u g h the d a t a p o i n t s has b e e n c o n s t r a i n e d to begin and e n d at t h e t w o t h e o r e t i c a l limits. We n e x t d e v e l o p e d a similar ratio o f s t r e s s e d - t o - n o n - s t r e s s e d n e t p h o t o s y n t h e s i s values w h i c h w e p l o t t e d against t h e p l a n t w a t e r stress i n d e x as in Fig. 4. In this case t h e r e is again a readily c o m p r e h e n s i b l e t h e o r e t i c a l u p p e r limit, b u t t h e l o w e r limit c a n n o t b e specified o n p u r e l y t h e o r e t i c a l g r o u n d s ,
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Fig. 3. The ratio o f s t o m a t a l c o n d u c t a n c e s of water-robbed water hyacinths to those o f normal free-floating water hyacinths vs. the plant w a t e r stress index of the water-robbed plants. Fig. 4. The ratio of net p h o t o s y n t h e s i s rates o f water-robbed water hyacinths to those of normal free-floating water h y a c i n t h s vs. the plant w a t e r stress index o f the w a t e r - r o b b e d plants. The linear relationship d e p i c t e d by the solid diagonal line c o m e s directly from a study of c o t t o n c o n d u c t e d b y Idso et al. (1982b).
since under severe stress conditions respiration may become greater than gross photosynthesis. Thus, net photosynthesis may become negative, and the lower bound can only be determined by experiment. Consequently, we have only a theoretical starting point for determining the functional relationship appropriate to the data. Rather than utilize any fitting procedure to derive such a function directly, however, what we have done in Fig. 4 is superimpose upon our water hyacinth data the functional relationship statistically derived by Idso et al. (1982b) for cotton. In viewing the result, it must be admitted that any other procedure would probably not significantly alter the slope of the line and that the net photosynthetic responses of these two vastly different plants to developing water stress are essentially identical. Now the information contained in Figs. 3 and 4 can be combined to tell us something of considerable significance for farm management practices, in this case about irrigation schemes. If, for instance, we form the ratio of these two ratios, i.e., if we divide the net photosynthesis ratio of Fig. 4 by the stomatal conductance ratio of Fig. 3 and normalize the result to yield a m a x i m u m value of unity, we create a function analogous to plant water use efficiency, which should be related to the ultimate total dry matter or economic yield produced by the plant per total a m o u n t of water used in the transpiration process. This "normalized plant water use efficiency" parameter for water hyacinth is plotted as a function of the plant water
254
stress index in Fig. 5, from whence it can be seen that, although maximum net photosynthesis occurs under conditions of no water stress, the most efficient net fixation of CO2 in terms of the potential for water loss via the transpiration process occurs just past the midway stress point, i.e., at a plant water stress index of about 0.6. How does this inference for water hyacinth compare with what is known about agronomic crops? At first glance, it may appear somewhat different; for most studies of yield--evapotranspiration relationships generally show plant water use efficiency to continue to increase with increasing evapotranspiration right up to the maximum values attained by both parameters (Downey, 1972). In a few studies where extraordinarily high evapotranspiration rates have been maintained, however, decreases in plant water use efficiency of the type portrayed in Fig. 5 have been encountered (Grimes et al., 1969; Howell et al., 1984). The most extensive of these data sets which we have been able to locate is that of Grimes et al. (1969), which fortuitously pertains to cotton, the agronomic crop we are comparing with water hyacinth. Transforming their plot of relative lint yield vs. evapotranspiration to one of normalized lint water use efficiency vs. total seasonal evapotranspiration and superimposing it upon our water hyacinth relationship of Fig. 5 produces the results of Fig. 6, where it can be seen that the two curves have a remarkable correspondence to each other. In view of the fact that cotton and water hyacinths differ so greatly in
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Fig. 5. N e t p h o t o s y n t h e s i s ratios derived from Fig. 4 divided by stomatal c o n d u c t a n c e ratios derived from Fig. 3 - - w i t h this result divided b y the m a x i m u m value o f the subseq u e n t set o f numbers - - p l o t t e d against the plant water stress i n d e x o f the w a t e r - r o b b e d plants. Fig. 6. N o r m a l i z e d lint water use e f f i c i e n c y vs. total seasonal evapotranspiration for c o t t o n (dashed curve) and n o r m a l i z e d plant water use e f f i c i e n c y vs.plant water stress i n d e x for w a t e r h y a c i n t h (solid curve).
255 m a n y respects, plus the fact t h a t the w a t e r h y a c i n t h e f f i c i e n c y curve was d e t e r m i n e d f r o m s h o r t - t e r m d a t a involving daily and even h o u r l y changes in t h e p e r t i n e n t p a r a m e t e r s , while the c o t t o n d a t a were o b t a i n e d f r o m a seasonlong s t u d y , t h e similar end results observed in Fig. 6 are t r u l y striking. O n e c o n s e q u e n t l y w o n d e r s if o t h e r plants exhibit this same t y p e o f response, and w h y t h e y either do or d o not. Research o f a similar n a t u r e on y e t o t h e r species m a y thus be a p r o f i t a b l e c o u r s e o f a c t i o n to p u r s u e in o u r c o n t i n u i n g q u e s t to b e t t e r u n d e r s t a n d the scientific basis f o r increasing the e f f i c i e n c y with w h i c h w a t e r is utilized in agriculture.
ACKNOWLEDGEMENT We are i n d e b t e d to M.G. A n d e r s o n for t a n k s used to m a i n t a i n the w a t e r h y a c i n t h s plants f r o m the alligator p o n d at t h e P h o e n i x u p k e e p and n u t r i t i o n p r i o r to and t h r o u g h o u t
c o n s t r u c t i n g the e x p e r i m e n t a l in the field, f o r collecting the Zoo, and f o r a t t e n d i n g to their the e x p e r i m e n t .
REFERENCES Downey, L.A., 1972. Water-yield relations for nonforage crops. J. Irrig. Drain. Div., Proc. Am. Soc. Cir. Engin., 98 (IR1): 107--115. Grimes, D.W., Yamada, H. and Dickens, W.L., 1969. Functions for cotton (Gossypium hirsutum L.) production from irrigation and nitrogen fertilizer variables: I. Yield and evap otranspiratio n. Agron. J., 61 : 769--773. Howell, T.A., Davis, K.R., Yamada, H. and Walhood, V.T., 1984. Limited irrigation of narrow row cotton: III. Evapotranspiration--yield relationships and water use efficiency. Agron. J., in press. Idso, S.B., 1981. Surface energy balance and the genesis of deserts. Arch. Meteorol. Geophys. Bioklim., Ser. A, 30: 253--260. Idso, S.B., 1984. Transpiration by water hyacinths: Environmental control of stomatal conductance. Agric. For. Meteorol., submitted. Idso, S.B., Jackson, R.D., Pinter, P.J., Jr., Reginato, R.J. and Hatfield, J,L., 1981a. Normalizing the stress-degree-day parameter for environmental variability. Agric. Meteorol., 24: 45--55. Idso, S.B., Reginato, R.J., Jackson, R.D. and Pinter, P.J., Jr., 1981b. Measuring yieldreducing plant water potential depressions in wheat by infrared thermometry. Irrigation Sci., 2: 205--212. Idso, S.B., Reginato, R.J., Reicosky, D.C. and Hatfield, J.L., 1981c. Determining soilinduced plant water potential depressions in alfalfa by means of infrared thermometry. Agron. J., 73: 826--830. Idso, S.B., Reginato, R.J. and Farah, S.M., 1982a. Soil- and atmosphere-induced plant water stress in cotton as inferred from foliage temperatures. Water Resour. Res., 18: 1143--1148. Idso, S.B., Reginato, R.J. and Radin, J.W., 1982b. Leaf diffusion resistance and net photosynthesis in cotton as related to a foliage temperature based plant water stress index. Agric. Meteorol., 27: 27--34. Idso, S.B., Reginato, R.J., Clawson, K.L. and Anderson, M.G., 1984. On the stability of non-water-stressed baselines. Agric. For. Meteorol., 32: 177--182.
256 Jackson, R.D., Idso, S.B., Reginato, R.J. and Pinter, P.J., Jr., 1981. Canopy temperature as a crop water stress indicator. Water Resour. Res., 17: 1133--1138. Johnson, C.M., Stout, P.R., Broyer, T.C. and Carlton, A.B., 1957. Comparative chlorine requirements of different plant species. Plant Soil, 8 : 337--353. Pinter, P.J., Jr. and Reginato, R.J., 1982. A thermal infrared technique for monitoring cotton water stress and scheduling irrigations. Trans. ASAE, 25: 1651--1655.
249
STOMATAL CONDUCTANCE AND PHOTOSYNTHESIS IN WATER HYACINTH: EFFECTS OF REMOVING WATER FROM ROOTS AS QUANTIFIED BY A FOLIAGE-TEMPERATURE-BASED PLANT WATER STRESS INDEX* S.B. IDSO, P.J. PINTER, Jr., R.J. REGINATO and K.L. CLAWSON U.S. Water Conservation Laboratory, 4331 E. Broadway, Phoenix, AZ 85040 (U.S.A.) (Received January 4, 1984; revision accepted April 4, 1984)
ABSTRACT Idso, S.B., Pinter, P.J., Jr,, Reginato, R.J., and Clawson, K.L., 1984. Stomatal conductance and photosynthesis in water hyacinth: Effects of removing water from roots as quantified by a foliage-temperature-based plant water stress index. Agric. For. Meteorol., 32: 249--256. Stomatal conductance and net photosynthesis measurements were made over a period of a week in mid-October on two stands of water hyacinths floating in sunken metal stock tanks at Phoenix, AZ. On the second day of the experiment, all free water in one of the tanks was removed. Foliage temperature measurements were subsequently used to quantify the water stress experienced by the water-robbed plants; and a plant water stress index derived from the foliage temperature and air vapor pressure deficit data was used to study the effects of developing water stress on the plant physiological parameters being measured. The results obtained were practically identical to those derived from two independent season-long studies of water stress effects in cotton: net photosynthesis decreased linearly to become negative at a plant water stress index of 0.9 (where 1.0 represents the maximum possible stress), while a parameter related to plant water use efficiency first increased with increasing stress to reach a maximum at a plant water stress index of 0.6, after which it dropped off rapidly to zero with additional stress.
INTRODUCTION T h e p l a n t w a t e r s t r e s s i n d e x d e v e l o p e d b y I d s o e t al. ( 1 9 8 1 a ) a n d J a c k s o n e t al. ( 1 9 8 1 ) h a s p r o v e n t o b e o f u s e in a s s e s s i n g m a n y p l a n t p h y s i o l o g i c a l responses to water stress via remote measurements of foliage temperature ( I d s o e t al., 1 9 8 1 b , c , 1 9 8 2 a ; P i n t e r a n d R e g i n a t o , 1 9 8 2 ) . O f p a r t i c u l a r i n t e r e s t in t h i s r e g a r d a r e a s s e s s m e n t s o f l e a f s t o m a t a l c o n d u c t a n c e a n d n e t photosynthesis rates, since these parameters are related to water loss by transpiration and the production of dry matter -- basic plant functions w h i c h f a r m m a n a g e m e n t p r a c t i c e s a r e d e s i g n e d t o i n f l u e n c e in s u c h a w a y as to produce the most yield for the least financial outlay. To date, only one study has been reported which relates the Idso--Jackson p l a n t w a t e r s t r e s s i n d e x t o t h e s e t w o i m p o r t a n t p l a n t p r o p e r t i e s , i.e., t h a t o f I d s o e t al. ( 1 9 8 2 b ) w h e r e c o t t o n ( G o s s y p i u m h i r s u t u m L . ) w a s t h e * Contribution from the Agricultural Research Service, U.S. Department of Agriculture. 0168-1923/84/$03.00
© 1984 Elsevier Science Publishers B.V.
250
experimental crop. Thus, we felt it i m p o r t a n t to c o n d u c t a n o t h e r such study on a second plant species, to look for either similarities or differences in plant behavior relative to the plant water stress index. This paper represents the results o f th a t effort. M A T E R I A L S AND M E T H O D S
To allow for the occurrence of as great a potential difference as possible in response characteristics, we decided to work with an aquatic plant that normally floats with its roots suspended in water and is therefore normally never short o f water and stressed for that reason, i.e., the water hyacinth (Eichhornia crassipes (Mart). Solms). Following upon the work of Idso (1984) with this species, we thus established two good stands of water hyacinths in sunken metal stock tanks of 110- and 230-cm diameter at Phoenix, AZ. Both tanks o f vegetation were nourished on a bi-weekly schedule with a full-strength Hoagland solution 2, as described by J o h n s o n et al. (1957); and bot h were allowed to grow in our experimental setting for a t w o - m o n t h period (mid-August to mid-October) before we began our m eas u r emen t program. The e x p e r i m e n t was initiated by withdrawing sufficient water from both tanks so that, when all remaining free water was p u m p e d from the 110-cm diameter tank, the vegetation would not drop any furt her or spread out laterally. In this setting, measurements (to be described hereafter) were t h e n made on both tanks for one daylight period (Day --1), which began about 1030 and lasted until a b o u t 1500 MST. Subsequently, on Day 0, all of the remaining free water was p u m p e d from the 110-cm diameter tank at a b o u t 1120 MST. Since m uch water was still retained by the massive m at of roots which the hyacinths had pr oduc e d, however, effects of water stress were slow in revealing themselves; and measurements of developing water stress effects were thus able to be made over three of the five following days: Day 1, Day 2, and Day 5. Three basic types of measurements were made on both tanks of water hyacinths on three different schedules. First, foliage and air wet- and drybulb t e m p e r a t u r e measurements were made every 15 minutes of each daylight period in exactly the same m a n n e r as described by Idso (1984), i.e., with a 4-degree field-of-view Everest Interscience* Model 110 infrared t h e r m o m e t e r and an aspirated Bendix p s y c h r o m e t e r . Data acquired over the 230-cm d iamete r tank were used to c o nst ruct the large-leaf non-waterstressed baseline described by Idso et al. (1984); while data acquired over the l l 0 - c m d iamete r tank were used to cons t r uct the plant water stress index for the water-robbed plants. Leaf diffusion resistance measurements were obtained at 20-minute * Trade n a m e s a n d c o m p a n y n a m e s are i n c l u d e d for t h e b e n e f i t o f t h e r e a d e r and i m p l y no e n d o r s e m e n t o r p r e f e r e n t i a l t r e a t m e n t o f t h e p r o d u c t listed b y t h e U.S. D e p a r t m e n t o f Agriculture.
251 intervals, also in the same manner as described by Idso (1984), by means of a Li-Cor Model LI-1600 steady-state porometer. Three abaxial and three adaxial leaf surfaces were sampled on each tank and the results appropriately combined in parallel and then inverted to give a mean stomatal conductance value. These results, as well as the previously described foliage and air wet- and dry-bulb temperature measurements, were further smoothed with time through the day by means of a simple three-term running-averaging procedure. Net photosynthesis rates for each tank were estimated using a technique based on CO2 depletion by leaves in a portable hand-held chamber, much like the m e t h o d employed by Idso et al. (1982b). Prior to the experiment, five vigorous green leaves from the top of each canopy were selected and labeled for repeated measurement t h r o u g h o u t the entire study. The areas of these leaves were obtained by tracing their outlines on paper and sending these cut-outs through an optically-integrating leaf area meter. The chamber was constructed from a pair of hemispherically-shaped domes of transparent plexiglass, which were hinged to facilitate the rapid inclusion of a leaf blade within the resulting 1.73-liter sphere. When closed, the chamber halves were sealed with a soft neoprene gasket; and a high volume blower mixed air continuously within the chamber. Immediately upon clamping onto a leaf, electronically activated mechanisms synchronized the extraction of a reference air sample into a 10-ml plastic syringe. Then, after 15 seconds, another air sample was automatically withdrawn. Separate leaves were sampled at 1.5-min intervals; syringes containing air samples were temporarily stored at ambient temperatures in a styrofoam chest. Within 30 to 35 minutes of the first sample acquisition, CO 2 concentrations of the air samples were determined in the laboratory on an infrared gas analyzer (Analytical Development Co., Type 225 Mk 3) interfaced with a printing integrator. This system was calibrated at the beginning and end of each set of measurements using a 376 pm CO2 primary standard gas. Since complete sets of net photosynthesis measurements could be obtained no faster than about one per hour, no time-averaging of results was performed. RESULTS AND DISCUSSION The solid diagonal line of Fig. 1 represents the large-leaf non-waterstressed baseline developed by Idso et al. (1984) from data acquired over the 230-cm diameter hyacinth tank; while the points represent similarly acquired data over the l l 0 - c m diameter tank. As can be seen, the data appear to segregate into four distinct groups. The first two days' results fall on or near the baseline; while the succeeding three days' results sequentially rise above it in discrete steps to approach the upper foliage--air temperature differential limit. The derivation of this limit has been described previously in considerable detail (Idso, 1981; Idso et al., 1981a, c) and thus will not be repeated here. Suffice it to say t h a t the limit is weakly dependent upon air
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t e m p e r a t u r e and t h a t t h e h o r i z o n t a l d a s h e d line r e p r e s e n t s t h e u p p e r limit f o r t h e m e a n air t e m p e r a t u r e on t h e a f t e r n o o n o f D a y 5, while the t w o h o r i z o n t a l solid lines r e p r e s e n t t h e e x t r e m e s o f t h e u p p e r limit f o r t h e highest a n d l o w e s t air t e m p e r a t u r e s e n c o u n t e r e d d u r i n g t h e study. F o l l o w i n g t h e p r o c e d u r e s o f I d s o et al. ( 1 9 8 1 a , b , c ) , t h e l l 0 - c m d i a m e t e r t a n k d a t a o f Fig. 1 w e r e t r a n s f o r m e d into p l a n t w a t e r stress i n d e x values a n d p l o t t e d as dally t r e n d s in Fig. 2. H e r e again it is e v i d e n t t h a t d e t e c t a b l e levels o f w a t e r stress did n o t o c c u r until t h e d a y f o l l o w i n g w a t e r r e m o v a l , a n d t h a t w a t e r stress did n o t b e c o m e m a x i m a l until five d a y s later. With this c h a r a c t e r i z a t i o n o f p l a n t w a t e r stress d e v e l o p m e n t , t h e s t o m a t a l c o n d u c t a n c e values o b t a i n e d o n t h e w a t e r - r o b b e d t a n k were n e x t divided b y t h o s e o b t a i n e d o n t h e n o n - w a t e r - s t r e s s e d t a n k to f o r m a r a t i o w h i c h s h o u l d t h e o r e t i c a l l y v a r y f r o m 1.0 w h e n t h e r e is no stress to 0.0 w h e n stress is m a x i m a l . T h u s , b y m a t c h i n g these s t o m a t a l c o n d u c t a n c e ratios w i t h t h e c o r r e s p o n d i n g p l a n t w a t e r stress i n d e x values o f Fig. 2, t h e p l o t o f Fig. 3 was d e v e l o p e d , w h e r e t h e h a n d - d r a w n t r e n d line t h r o u g h the d a t a p o i n t s has b e e n c o n s t r a i n e d to begin and e n d at t h e t w o t h e o r e t i c a l limits. We n e x t d e v e l o p e d a similar ratio o f s t r e s s e d - t o - n o n - s t r e s s e d n e t p h o t o s y n t h e s i s values w h i c h w e p l o t t e d against t h e p l a n t w a t e r stress i n d e x as in Fig. 4. In this case t h e r e is again a readily c o m p r e h e n s i b l e t h e o r e t i c a l u p p e r limit, b u t t h e l o w e r limit c a n n o t b e specified o n p u r e l y t h e o r e t i c a l g r o u n d s ,
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Fig. 3. The ratio o f s t o m a t a l c o n d u c t a n c e s of water-robbed water hyacinths to those o f normal free-floating water hyacinths vs. the plant w a t e r stress index of the water-robbed plants. Fig. 4. The ratio of net p h o t o s y n t h e s i s rates o f water-robbed water hyacinths to those of normal free-floating water h y a c i n t h s vs. the plant w a t e r stress index o f the w a t e r - r o b b e d plants. The linear relationship d e p i c t e d by the solid diagonal line c o m e s directly from a study of c o t t o n c o n d u c t e d b y Idso et al. (1982b).
since under severe stress conditions respiration may become greater than gross photosynthesis. Thus, net photosynthesis may become negative, and the lower bound can only be determined by experiment. Consequently, we have only a theoretical starting point for determining the functional relationship appropriate to the data. Rather than utilize any fitting procedure to derive such a function directly, however, what we have done in Fig. 4 is superimpose upon our water hyacinth data the functional relationship statistically derived by Idso et al. (1982b) for cotton. In viewing the result, it must be admitted that any other procedure would probably not significantly alter the slope of the line and that the net photosynthetic responses of these two vastly different plants to developing water stress are essentially identical. Now the information contained in Figs. 3 and 4 can be combined to tell us something of considerable significance for farm management practices, in this case about irrigation schemes. If, for instance, we form the ratio of these two ratios, i.e., if we divide the net photosynthesis ratio of Fig. 4 by the stomatal conductance ratio of Fig. 3 and normalize the result to yield a m a x i m u m value of unity, we create a function analogous to plant water use efficiency, which should be related to the ultimate total dry matter or economic yield produced by the plant per total a m o u n t of water used in the transpiration process. This "normalized plant water use efficiency" parameter for water hyacinth is plotted as a function of the plant water
254
stress index in Fig. 5, from whence it can be seen that, although maximum net photosynthesis occurs under conditions of no water stress, the most efficient net fixation of CO2 in terms of the potential for water loss via the transpiration process occurs just past the midway stress point, i.e., at a plant water stress index of about 0.6. How does this inference for water hyacinth compare with what is known about agronomic crops? At first glance, it may appear somewhat different; for most studies of yield--evapotranspiration relationships generally show plant water use efficiency to continue to increase with increasing evapotranspiration right up to the maximum values attained by both parameters (Downey, 1972). In a few studies where extraordinarily high evapotranspiration rates have been maintained, however, decreases in plant water use efficiency of the type portrayed in Fig. 5 have been encountered (Grimes et al., 1969; Howell et al., 1984). The most extensive of these data sets which we have been able to locate is that of Grimes et al. (1969), which fortuitously pertains to cotton, the agronomic crop we are comparing with water hyacinth. Transforming their plot of relative lint yield vs. evapotranspiration to one of normalized lint water use efficiency vs. total seasonal evapotranspiration and superimposing it upon our water hyacinth relationship of Fig. 5 produces the results of Fig. 6, where it can be seen that the two curves have a remarkable correspondence to each other. In view of the fact that cotton and water hyacinths differ so greatly in
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Fig. 5. N e t p h o t o s y n t h e s i s ratios derived from Fig. 4 divided by stomatal c o n d u c t a n c e ratios derived from Fig. 3 - - w i t h this result divided b y the m a x i m u m value o f the subseq u e n t set o f numbers - - p l o t t e d against the plant water stress i n d e x o f the w a t e r - r o b b e d plants. Fig. 6. N o r m a l i z e d lint water use e f f i c i e n c y vs. total seasonal evapotranspiration for c o t t o n (dashed curve) and n o r m a l i z e d plant water use e f f i c i e n c y vs.plant water stress i n d e x for w a t e r h y a c i n t h (solid curve).
255 m a n y respects, plus the fact t h a t the w a t e r h y a c i n t h e f f i c i e n c y curve was d e t e r m i n e d f r o m s h o r t - t e r m d a t a involving daily and even h o u r l y changes in t h e p e r t i n e n t p a r a m e t e r s , while the c o t t o n d a t a were o b t a i n e d f r o m a seasonlong s t u d y , t h e similar end results observed in Fig. 6 are t r u l y striking. O n e c o n s e q u e n t l y w o n d e r s if o t h e r plants exhibit this same t y p e o f response, and w h y t h e y either do or d o not. Research o f a similar n a t u r e on y e t o t h e r species m a y thus be a p r o f i t a b l e c o u r s e o f a c t i o n to p u r s u e in o u r c o n t i n u i n g q u e s t to b e t t e r u n d e r s t a n d the scientific basis f o r increasing the e f f i c i e n c y with w h i c h w a t e r is utilized in agriculture.
ACKNOWLEDGEMENT We are i n d e b t e d to M.G. A n d e r s o n for t a n k s used to m a i n t a i n the w a t e r h y a c i n t h s plants f r o m the alligator p o n d at t h e P h o e n i x u p k e e p and n u t r i t i o n p r i o r to and t h r o u g h o u t
c o n s t r u c t i n g the e x p e r i m e n t a l in the field, f o r collecting the Zoo, and f o r a t t e n d i n g to their the e x p e r i m e n t .
REFERENCES Downey, L.A., 1972. Water-yield relations for nonforage crops. J. Irrig. Drain. Div., Proc. Am. Soc. Cir. Engin., 98 (IR1): 107--115. Grimes, D.W., Yamada, H. and Dickens, W.L., 1969. Functions for cotton (Gossypium hirsutum L.) production from irrigation and nitrogen fertilizer variables: I. Yield and evap otranspiratio n. Agron. J., 61 : 769--773. Howell, T.A., Davis, K.R., Yamada, H. and Walhood, V.T., 1984. Limited irrigation of narrow row cotton: III. Evapotranspiration--yield relationships and water use efficiency. Agron. J., in press. Idso, S.B., 1981. Surface energy balance and the genesis of deserts. Arch. Meteorol. Geophys. Bioklim., Ser. A, 30: 253--260. Idso, S.B., 1984. Transpiration by water hyacinths: Environmental control of stomatal conductance. Agric. For. Meteorol., submitted. Idso, S.B., Jackson, R.D., Pinter, P.J., Jr., Reginato, R.J. and Hatfield, J,L., 1981a. Normalizing the stress-degree-day parameter for environmental variability. Agric. Meteorol., 24: 45--55. Idso, S.B., Reginato, R.J., Jackson, R.D. and Pinter, P.J., Jr., 1981b. Measuring yieldreducing plant water potential depressions in wheat by infrared thermometry. Irrigation Sci., 2: 205--212. Idso, S.B., Reginato, R.J., Reicosky, D.C. and Hatfield, J.L., 1981c. Determining soilinduced plant water potential depressions in alfalfa by means of infrared thermometry. Agron. J., 73: 826--830. Idso, S.B., Reginato, R.J. and Farah, S.M., 1982a. Soil- and atmosphere-induced plant water stress in cotton as inferred from foliage temperatures. Water Resour. Res., 18: 1143--1148. Idso, S.B., Reginato, R.J. and Radin, J.W., 1982b. Leaf diffusion resistance and net photosynthesis in cotton as related to a foliage temperature based plant water stress index. Agric. Meteorol., 27: 27--34. Idso, S.B., Reginato, R.J., Clawson, K.L. and Anderson, M.G., 1984. On the stability of non-water-stressed baselines. Agric. For. Meteorol., 32: 177--182.
256 Jackson, R.D., Idso, S.B., Reginato, R.J. and Pinter, P.J., Jr., 1981. Canopy temperature as a crop water stress indicator. Water Resour. Res., 17: 1133--1138. Johnson, C.M., Stout, P.R., Broyer, T.C. and Carlton, A.B., 1957. Comparative chlorine requirements of different plant species. Plant Soil, 8 : 337--353. Pinter, P.J., Jr. and Reginato, R.J., 1982. A thermal infrared technique for monitoring cotton water stress and scheduling irrigations. Trans. ASAE, 25: 1651--1655.