Stomatal behavior and leaf water status of strawberry in different growth environments

Scientia Horticulturae, 18 (1982/83) 101--110

101

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

STOMATAL BEHAVIOR AND LEAF WATER STATUS OF STRAWBERRY IN DIFFERENT GROWTH ENVIRONMENTS

A. R I C H A R D

RENQUIST,

PATRICK

J. B R E E N

and L L O Y D

W. M A R T I N '

Department of Horticulture, Oregon State University, Corvallis, OR 97331 (U.S.A.) 'North Willamette Experiment Station, 15210 NE Miley, Aurora, OR 97002 (U.S.A.) Oregon Agricultural Experiment Station Technical Paper No. 5956 (Accepted for publication 24 November 1981)

ABSTRACT Renquist, A.R., Breen, P.J. and Martin, L.W., 1982. Stomatal behavior and leaf water status of strawberry in different growth environments. Scientia Hortic., 18: 101--110. Leaves of irrigated (IR) and non-irrigated (NIR) strawberries (Fragaria × ananassa Duch. cultivar 'Olympus') were compared in terms of water potential (~), turgor potential (~p), and leaf conductance (Kl) during diurnal cycles in a growth chamber, glasshouse, and 2 field studies (1977 and 1979). Irrigation was withheld for 3--5 days before measurements were m a d e in the greenhouse and growth chamber, and for 21 days (1977) and 36 days (1979) in the field. In the field, m i n i m u m mid-day leaf ~ was usually near -15 bars in both IR and N I R plants. O n clear days, such as the 1977 date, ~ and ~ p were not greatly affected by irrigation, except that ~ in IR plants dropped more slowly in early morning and recovered faster near sunset. O n the 1979 date, mid-day ~ was higher in IR than N I R plants due to lower solar radiation, but ~p was very similar in the 2 treatments during most of the day. K l rates in N I R plants were half those in IR plants throughout the day, and diurnal patterns of K l were similar in 1977 and 1979 despite the differences in water status of IR plants. While stomata clearly responded to soil moisture deficit, the relationship between K 1 and leaf ~ or ~p depended on the growth environment and irrigation history, as well as other influences on stomata. In contrast to the behavior of field-grown plants, ~ and ~p of N I R glasshouse plants dropped well below those of IR plants as soil dried, probably due to small pot volume. Also, K 1 at a given ~ was lower in the growth chamber than in the field, perhaps due to a direct effect of low light on stomata. A threshold ~ at which strawberry K 1 drops sharply (indicating stomatal closure) was not observed. The existence of such thresholds in other crops is questionable if their occurrence was inferred from leaf resistance (R1) rather than K1 data.

INTRODUCTION

The rate of leaf expansion may be a sensitive indicator of water deficit

in strawberry (Renquist et al., 1982b), yet leaf area can be reduced somewhat without significantly hindering fruit production (Renquist et al., 1982a)

0304-4238/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

102 Stomatal behavior is an aspect of strawberry leaf physiology which may be more critical than leaf expansion in determining the yield response to drought. Fruit yield losses may be avoided if periods of severe leaf water deficit are minimized by stomatal closure in response to soil moisture or high evaporative demand. Leaf water status and light are 2 influences on stomatal aperture which have received considerable attention. Direct effects of humidity have also been observed in some species (Kaufmann, 1976; Maier-Maercker, 1979). The greatest changes in guard cell turgot and stomatal aperture during daylight are generally attributed to changes in leaf water status. However, it is probable that there are simultaneous responses to, or interactions with, other factors. Interpretation of environmental effects would be much easier if the mechanism(s) of stomatal m o v e m e n t were known. Darrow and Dewey (1934) observed microscopically that in drought conditions strawberry stomata are open for only a brief period in the morning. However, transpiration may decrease appreciably with reductions in stomatal aperture below the minimum opening which can be resolved by light microscopy (Shimshi, 1970). The visual study of strawberry, therefore, may n o t provide an accurate picture of transpiration control throughout the day, as is possible from leaf conductance (Kl) data. We have measured diurnal K 1 and addressed the question of h o w it is related to leaf water status and light flux density. An additional question, which has been studied b y Davies (1977), is whether or n o t these relationships are consistent in different environments (i.e. indoors and field). MATERIALS AND METHODS 'Olympus' strawberry plants were established in 3-1 pots in a growth chamber and a glasshouse, as well as direct-planted in the field. The container medium was a 3:1 mix of silty clay loam soil and peat, fertilized as needed with soluble 30--4--8 NPK. The temperature in the growth chamber was 18°C. A mix of fluorescent and incandescent lights provided a 15-h photoperiod with a photosynthetic p h o t o n flux density (PPFD) at plant height of 210--280 pE m-~s-1. In the glasshouse, the mid-day PPFD was 120--400 gE m-2s -1 in the winter, and 250--1400 gE m-2s-1 in spring and autumn. The temperature was 18°C at night and reached 22--32°C at mid-day. Sodium vapor lights extended the p h o t o p e r i o d to 15 h. Old leaves were removed, so that plants had a uniform c a n o p y of 7--10 fully expanded leaves and several younger ones at the start of each experimental period. Measurements were made on leaves which had been fully expanded for approximately 1 week. Depending on the evaporative demand, irrigation was withheld from half of the greenhouse and growth chamber plants for 3--5 days prior to the start of measurements. F o r all experiments, data were taken on at least 4 well-irrigated (IR) and 4 non-irrigated (NIR) plants at 4--6 sampling-times per 24 h. In most studies, leaf ¢, K 1 and solute potential (¢s) were measured concurrently.

103 The field studies were on first-year plantings, using drip irrigation and raised beds (Renquist et al., 1982a). The dates of the experimental periods, along with light, temperature, and irrigation date, are listed in Table I. TABLE

I

Environmental data during the experimental periods for 2 field plantings of 'Olympus' strawberry. Planting dates were in May of 1977 and 1979 Environmental data

Sampling dates

Maximum temperature (° C) Minimum temperature (° C) Mid-day average PPFD

10 August 1977

25 August 1979

39.5 17.8 2000

29.5 12.2 1100

(~E m-2s -I )

Wind run (km/day) Class A pan evaporation (cm/day) Cloud condition Duration of irrigation treatments (days)

88 1.42 Clear 21

48 0.41 Clouds 1200--1500

36

The concurrent measurement of ~ and Ss allows turgor potential ($p) to be determined from the equation $ = ~ p + ~s + ~ m" The matric potential ($ m) was assumed to be zero for the degree of tissue hydration involved (Boyer, 1967). Values were expressed as bars (1 bar = 10 s Pascals). The leaf conductance was measured with a ventilated diffusion porometer (Turner and Parlange, 1970), which was calibrated using a drilled plate of known theoretical conductance (Kanemasu and Tanner, 1969). Drift of the LiC1 sensor was minimized by frequent replacement of the silica gel desiccant. Strawberry leaves are reported to have stomata on the lower surface only (Darrow, 1966), which we verified for 'Olympus' using silicon rubber imprints. K 1 (which is primarily a measure of stomatal conductance) was therefore calculated from measurements on the abaxial leaf surfaces. From 1 to 3 fully illuminated leaves on each plant were horizontally positioned and used for K 1 measurements t h r o u g h o u t the diurnal cycle to reduce variability. Leaf ~ was estimated with a pressure chamber (Waring and Cleary, 1967) and Ss was estimated with a dewpoints hygrometer (Campbell et al., 1973), as previously described (Renquist et al., 1982b). RESULTS

AND

DISCUSSION

L e a f ~ a n d s t o m a t a . - - On clear mid-summer days, such as on the 1977

date, leaf $ in NIR plants was lower than that in IR plants only in the early morning and late afternoon (Fig. 1C vs. 1D). There was no treatment difference in ~p (data n o t shown). On the 1979 date, evaporative demand was

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Fig. 1. Diurnal leaf conductance (K1) and leaf water potential (~) in irrigated (IR) and non-irrigated (NIR) strawberries in 2 first-year plantings. A, K1 in IR plants; B, K1 in NIR plants; C, ~ in IR plants; D, ~ in NIR plants. Dates and weather data are listed in Table I. K1 data are plotted together for the 2 years. Morning dew prevented K 1 determination at 06.00 h in 1979. The bars represent ±SE. lower as a result of morning dew, mid-day clouds, and lower light levels (see Table I). In these conditions, ~ was higher in IR than in NIR plants (Table II), however ~p was again similar in the 2 treatments, except during a brief mid-day p e r i o d (Table II). The minimum ~ values for NIR plants were near -15 bars in 1977, 1979 (Fig. 1D), and on other dates n o t reported. Minimum ~ of IR plants was also near -15 bars on sunny days. In both years, ~ of IR plants recovered more quickly at dusk than that of NIR plants (Fig. 1C vs. 1D). Such transitory differences in leaf water status are n o t known to be significant, however. Stomata of NIR plants were less open than those in IR plants at all times (Fig. 1A and 1B). All plants had m a x i m u m K 1 rates in mid-morning, although the stomata of NIR plants apparently opened more slowly. Darrow and Dewey (1934) f o u n d t h a t 80% of the s t o m a t a of irrigated strawberries were visibly open at 09.00 h, only 25% at 12.00 h, 50% at 15.00 h and all were closed

105 T A B L E II D i u r n a l w a t e r p o t e n t i a l (4), s o l u t e p o t e n t i a l ( ~ . ) , a n d t u r g o t p o t e n t i a l ( ~ n ) (all -+SE) o f leaves f r o m irrigated ( I R ) a n d n o n - i r r i g a t e d ( N I R ) s t r a w b e r r i e s in t h e field o n 25 A u g u s t 1 9 7 9 . E n v i r o n m e n t a l d a t a are in T a b l e I Time

Treatment

~ (bars)

~s (bars)

~ p (bars)

05.00

IR NIR

1.1+- 0.3 - 4.3 -+ 0.9

- 1 5 . 3 + 1.3 - 1 8 . 8 + 1.5

14.2 .+ 1.1 14.5 -+ 1.2

08.00

IR NIR

1.3.+ 0.3 5.0-+ 1.3

-16.4.+ -19.7.+

0.7 2.0

15.1 +-0.6 14.7 -+ 2.2

11.00

IR NIR

- 1 1 . 1 -+ 0.9 -14.8.+ 1.0

-18.7-+ 1.1 -22.4-+ 1.2

7.6 -+ 2.0 7.6 .+ 1.1

14.00

IR NIR

-10.0-+ 1.1 -15.3-+ 0.9

-20.6.+ -21.6.+

1.4 1.2

10.6 +- 2.1 6.3 .+ 1.8

17.00

IR NIR

-10.6-+ 0.9 - 1 5 . 6 - + 0.9

-19.5-+ 0.4 -22.4.+ 0.7

8.9 + 0.7 6.8 .+ 1.2

20.00

IR NIR

- 2 . 3 + 0.3 7.5-+ 1.2

-17.4-+ 1.0 -22.4+- 1.6

15.1 .+ 0.7 14.9 + 2.7

at 17.00 h (2.5 h before sunset). Under drought conditions, they observed that s t o m a t a opened more slowly and the most drought-resistant cultivar had a m a x i m u m of 20% of its stomata open at 09.00 h, compared to 70--80% for other cultivars. Drought caused the stomata of all cultivars to appear closed by 11.00 h. The K l results in Fig. 1B reinforce this earlier work, since a conductance of 0.15 cm s-1, which was reached by 13.30 h, corresponded to s t o m a t a which appeared closed on silicon rubber imprints. Further reductions in K 1 below 0.15 cm s-1 did decrease the rate of transpiration in a separate pot-weighing experiment (data not shown), so visual examination of strawberry stomata provides an incomplete picture of transpiration control. o f l e a f w a t e r s t a t u s . - - There has been much research on a wide range of crop species to determine if a relationship exists between leaf or ~ p and K I. Since K 1 can be a major determinant of the rate of drop in $, it is possible that the ~ level can be self-regulated to a degree by exerting feedback control on K 1. To show the K l vs. ~ relationship for strawberry w i t h o u t the effects of light, the measurements between 08.00 h and 14.00 h for 1 growth chamber and 2 field studies were plotted together (Fig. 2). The field data are for IR plants only. In both environments, K 1 rates dropped gradually as ~ decreased, and in the growth chamber stomata were essentially closed when ~ was -15 bars (K 1 < 10% of maximum). Field ob-

K 1 as a f u n c t i o n

106 servations of K! at high ~ are lacking because of the presence of dew and also because $ dropped considerably b y the time stomata were fully open in the morning. Stomatal closure in the growth chamber occurred over a wide range of leaf $ with no sharp decline in K l at a single value of 4. In N I R growth-chamber plants, K l was reduced from 0.31 to 0.06 cm s-1 as ~ decreased from - 5 to - 1 5 bars. However, the difference between the maximum K l of N I R plants (0.31 cm s-1) and the K l of IR plants (about 0.60 cm s-~) was even greater, despite the fact that ~ differed by only 2 bars (-3.5 vs. -5.5) (Fig. 2, broken line). The assumption that continuity in the K 1 vs. ~ relationship exists between IR and NIR chamber-grown plants is open to question, since K l at a given ~ in the field was consistently lower in NIR than in IR plants (see Fig. 1). This difference in the K l vs. $ relationship due to irrigation could n o t be precisely defined, since K1 data from NIR field plants appeared to result from a mixture of responses to both water status and light level, and are therefore omitted from Fig. 2. Yet it can be concluded that no unique relationship exists between leaf ~ and K 1. K l is n o t a function of ~ p either (see Fig. 1 and Table II). Apparently, stomata in NIR plants were responding to a guard cell water deficit which was n o t detected in terms of bulk leaf ~ or ~p. For example, the stress may involve the supply of water through the epidermis, which is considered by some workers to regulate stomatal aperture independently of bulk $ p (Maier-Maercker, 1979). GROWTHCHAMBER: III o HIR •

1.(:

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J I -6

* I -8

I -10

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{ BARS ) Fig. 2. Leaf c o n d u c t a n c e (K1) vs. leaf water potential ( 4 ) for strawberries in a growth chamber or in the field. All observations were m a d e b e t w e e n 0 8 . 0 0 and 1 4 . 0 0 h. Growth. chamber plants were either irrigated (IR) or non-irrigated (NIR). Field data are from IR plants on days in 1977 and 1979. The stars represent treatment means in the field in 1 9 7 9 w h e n mid-day light was l o w (IR ffi o p e n star, NIR ffi solid star; see text for discussion). The dashed line approximates the K 1 vs. ~ relationship in N I R growth-chamber plants during the first 4 days of treatments. The bars represent -+SE.

107 For several species, a threshold value or narrow range of ~ or ~p has been reported to correspond to stomatal closure, as shown by a rapid increase in diffusion resistance (Rl) (Kanemasu and Tanner, 1969;Turner, 1974). The choice of R 1 rather than its reciprocal, K l, may have unforseen consequences. The mathematical nature of a reciprocal transformation includes the possibility of converting a linear function into a quadratic one. If the inflection point in the quadratic is to be given physiological significance, then there must also be a physiological basis for choosing the quadratic function. However, transpiration is usually found to be a linear function of K l (Turner, 1975) and consequently cannot be as simply related to its reciprocal, R 1. Since a rapid increase in R 1 at the inflection point does n o t correspond to an equally rapid decrease in transpiration, the ~ value at the inflection should not be considered a threshold value for stomatal closure. The choice of R 1 rather than K 1 as the measure of stomatal behavior in many earlier studies caused the data to be transformed before it was examined. Several literature reports of the K 1 vs. ¢ relationship (Cutler and Rains, 1977; Davies and Lakso, 1978; Ackerson, 1980) and the results for strawberries in Fig. 2 all fail to show a threshold ¢ at which K l drops abruptly. The concept of a threshold ¢ has probably remained attractive because it fits the notion that a trigger mechanism exists for the synthesis and release of ABA from chloroplasts (Mansfield et al., 1978). While ABA modulation of stomata no d o u b t occurs, it may exert its effects over a wider range of ~ than was initially thought. ABA-induced closure may also be preceded by a decrease in K 1 in response to other factors (Mansfield et al., 1978; Maier-Maercker, 1979). Apparent reductions in threshold ~ due to stress pre-conditioning or growth environment (Cutler and Rains, 1977; Ackerson, 1980) might be more meaningfully described in terms of changes in the slope of the K 1 vs. ~ relation. A lower slope for the K 1 response to decreasing means that stomata are closing more slowly over a wide range of 4, rather than at a lower apparent threshold ~ (but the same rate), as previously suggested. This is in accord with the observation that sorghum stomata closed over a 10-bar range when stress developed at a rate representative of field conditions (Jones and Rawson, 1979). E f f e c t s o f g r o w t h e n v i r o n m e n t . -- The question of what influence the type

of environment has on K 1 and $ (or components of 6 ) is an important one, since most data on crop species have been obtained from growth-chamber and glasshouse-grown plants. One visual difference between controlled environments and the field is the irradiance. The consistently lower K 1 at each level of @ in the growth chamber vs. the field (see Fig. 2) is perhaps an effect of the much lower PPFD in the chamber. This is supported by the observation that the 2 treatment means for K 1 in the field, when mid-day light was low (see Fig. 2, stars), fell very close to the growth-chamber curve. An alternative explanation is that there were morphological differences, such as frequency of stomata, in plants grown in the different environments.

108

Another difference between growth environments, in terms of strawberry behavior, was that while diurnal $ of IR and NIR plants was quite similar on sunny days in the field, ~ was lower in NIR plants in the glasshouse. The separation between treatment means increased during the course of a drying cycle (Fig. 3). This was apparently due to the combined effects of 2 environmental effects. The lower level of solar radiation in the glasshouse reduced the mid-day drop in ~ in the IR plants, much as it did in the field on the cloudy day in 1979 (see Fig. 1C). The second factor was the small soil volume in pots, which resulted in greater depletion of soil water and a very low minimum $ in NIR plants in the glasshouse. $ fell to -23 bars on the third day of the drying cycle, of which only the first 2 days are presented in Fig. 3. In contrast, ~ never dropped below -16 bars in the field even when evaporative demand was very high (see Table I), and overnight recovery of ~ and S p always occurred. 0~

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OF DAY

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1000

1400

1800 2200

(PDT)

Fig. 3. Diurnal leaf water potential ( ~ ) of irrigated (IR) and non-irrigated (NIR) strawberries in a greenhouse on 6 May 1978. The bars represent ±SE. CONCLUSIONS

Stomatal response to soil water deficit was apparent in strawberry, but the relationship of K 1 to leaf ~ or Sp depended on the growth environment and the irrigation history (IR vs. N I R plants), and was also subject to alteration by other influences on stomata. While the different functional responses of plants to water stress are probably integrated by general metabolic factors, it is also possible that distinct facets of water stress exist. Strawberry stomata may respond to some facet which is n o t reflected in bulk leaf ~ or ~p. Until the internal factors that relate water status to stomatal

109 f u n c t i o n are defined, it m a y be m o r e useful to utilize K 1 m e a s u r e m e n t s d i r e c t l y as an i n d e x o f stress in strawberries r a t h e r t h a n m e a s u r e w a t e r potential or t u r g o t and a t t e m p t t o i n t e r p r e t their significance. Used t o g e t h e r with m e a s u r e m e n t s o f leaf e l o n g a t i o n rate ( R e n q u i s t et al., 1 9 8 2 b ) , it m a y be feasible t o establish a physiological basis f o r s t r a w b e r r y irrigation requirem e n t s w i t h o u t having t o wait for t h e m a n y c o m p l e x w a t e r - - g r o w t h relationships t o be elucidated.

REFERENCES Ackerson, R.C., 1980. Stomatal response of cotton to water stress and abscisic acid as affected by water stress history. Plant Physiol., 65: 455--459. Boyer, J.S., 1967. Matric potentials of leaves. Plant Physiol., 42: 213--217. Campbell, E.C., Campbell, G.S. and Barlow, W.K., 1973. A dewpoint hygrometer for water potential measurements. Agric. Meteorol., 12: 113--121. Cutler, J.M. and Rains, D.W., 1977. Effects of irrigation history on responses of cotton to subsequent water stress. Crop Sci., 17: 329--335. Darrow, G.M., 1966. The Strawberry. Holt, Rinehart and Winston, New York. Darrow, G.M. and Dewey, G.W., 1934. Studies on the stomata of strawberry varieties and species. Proc. Am. Soc. Hortic., 32: 440--447. Davies, F.S. and Lakso, A.N., 1978. Water relations in apple seedlings: changes in water potential components, abscisic acid levels and stomatal conductances under irrigated and non-irrigated conditions. J. Am. Soc. Hortic. Sci., 103: 310--318. Davies, W.J., 1977. Stomatal response to water stress and light in plants grown in controlled environments and in the field. Crop Sci., 17: 735--740. Jones, M.M. and Rawson, H.M., 1979. Influence of rate of development of leaf water deficits upon photosynthesis, leaf conductance, water use efficiency, and osmotic potential in sorghum. Physiol. Plant., 45: 103--111. Kanemasu, E.T. and Tanner, C.B., 1969. Stomatal diffusion resistance of snap beans. I. Effect of leaf water potential Plant Physiol., 44: 103--111. Kaufmann, M.R., 1976. Stomatal response of Engelmann spruce to humidity, light and water stress. Plant Physiol., 57: 898--901. Maier-Maercker, U., 1979. "Peristomatal transpiration" and stomatal movement: a controversial view. I. Additional proof of peristomatal transpiration by hygrophotography. Z. Pflanzenphysiol., 91: 25--43. Mansfield, T.A., Wellburn, A.R. and Moreira, T.J.S., 1978. The role of abscisic acid and farnesol in the alleviation of water stress. Philos. Trans. R. Soc. London, Set. B, 284: 471--482. Renquist, A.R., Breen, P.J. and Martin, L.W., 1982a. Effect of polyethylene mulch and summer irrigation regimes on subsequent flowering and fruiting of 'Olympus' strawberry. J. Am. Soc. Hortic. Sci., 107: 373--376. Renquist, A.R., Breen, P.J. and Martin, L.W., 1982b. Influences of water status and temperature on leaf elongation in strawberry. Scientia Hortic., 18: 77--85. Shimshi, D., 1970. The effect of nitrogen supply on transpiration and stomatal behavior of beans (Phaseolus vulgaris L.). New Phytol., 69: 405--412. Turner, N.C., 1974. Stomatal behavior and water status of maize, sorghum, and tobacco under field conditions. II. At low soil water potential. Plant Physiol., 53: 360--365. Turner, N.C., 1975. Concurrent comparisons of stomatal behavior, water status, and evaporation of maize in soil at high or low water potential. Plant Physiol., 55: 932--936.

110 Turner, N.C. and Parlange, J.Y., 1970. Analysis of operation and calibration of a ventilated diffusion porometer. Plant Physiol., 46: 175--177. Waring, R.H. and Cleary, B.D., 1967. Plant moisture stress: evaluation by pressure bomb Science. 155: 1248--1254.