Plant Physiol. (1974) 53, 360-365
Stomatal Behavior and Water Status of Maize, Sorghum, and Tobacco under Field Conditions II.
AT LOW SOIL WATER POTENTIAL Received for publication June 18, 1973 and in revised form October 16, 1973
NEIL C. TURNER Department of Ecology anid Climatology, The Connecticut Agricultural Experiment Station, New Haven, Connecticut 06504 ABSTRACT Diurnal changes in the vertical profiles of irradiance incident upon the adaxial leaf surface (I), leaf resistance (r,), leaf water potential (4'), osmotic potential (7r), and turgor potential (P) were followed concurrently in crops of maize (Zea mays L. cv. Pa6O2A), sorghum (Sorghum bicolor [L.] Moench cv. RS 610), and tobacco (Nicotiana tabacum L. cv. Havanna Seed 211) on several days in 1968 to 1970 when soil water potentials were low. The r1, measured with a ventilated diffusion porometer, of the leaves in the upper canopy decreased temporarily after sunrise [H0530 hours Eastern Standard Time] as I increased, but then r1 increased again between 0700 and 0830 hr Eastern Standard Time as the A', measured with a pressure chamber, decreased rapidly from the values of -7, -4 and -6 bars at sunrise to minimal values of -18, -22 and -15 bars near midday in the maize, sorghum, and tobacco, respectively. The 7r, measured with a vapor pressure osmometer, also decreased after sunrise, but not to the same degree as the decrease in A', so that a P of zero was reached in some leaves between 0730 and 0800 hours. The lower (more negative) -r of leaves in the upper canopy than those in the lower canopy gave the upper leaves a higher P at a given 4, than the lower leaves in all three species; leaves at intermediate heights had an intermediate P. This difference between leaves at the three heights in the canopy was maintained at all values of 4'. The ri remained unchanged over a wide range of P and then increased markedly at a P of 2 bars in maize, -1 bar in sorghum, and near zero P in tobacco: r1 also remained constant until 4' decreased to -17, -20, and -13 bars in leaves at intermediate heights in maize, sorghum, and tobacco, respectively. In all three species r1 of leaves in the upper canopy increased at more negative values of 4 than those at the base of the canopy, and in tobacco, leaves in the upper canopy wilted at more negative values of 4' than those in the lower canopy.
Laboratory studies on environmental factors that influence stomatal behavior of field crops have indicated that light and water are the two factors most likely to have the greatest effect on stomatal resistance under field conditions. In part I of this paper, Turner and Begg (24) observed that in maize, sorghum, and tobacco in the field at high soil water potentials, the diurnal range in stomatal resistance was the result primarily of the change in incident radiation. They observed that the rela-
tionship between stomatal resistance and irradiance was hyperbolic in all three species but varied with species, leaf surface, and leaf senescence. Although leaf water potentials varied over a considerable range throughout the day, low potentials had no observable effect on stomatal resistance in any of the three crops. However, the observed decrease in turgor potential with decreasing leaf water potential suggested that tobacco is likely to wilt at a higher leaf water potential than maize, and maize at a higher leaf water potential than sorghum. The variation in the vertical profiles of irradiance, stomatal resistance, leaf water potential and osmotic potential of maize, sorghum, and tobacco measured in the field at low soil water potentials is the subject of the present paper. At these low soil water potentials the leaf water potentials decreased to lower (more negative) values than previously and provided additional data that allowed us to study the effects of low leaf water potentials and low turgor potentials on stomatal behavior. The three species were chosen because of their differences in water use efficiency and tolerance of drought (2, 15), and the role of stomata in these differences is discussed briefly.
MATERIALS AND METHODS
Adjacent plots of maize (Zea mays L. cv. Pa6O2A), sorghum (Sorghum bicolor [L.] Moench cv. RS610), and tobacco (Nicotiana tabacum L. cv. Havanna Seed 211) were established in a well fertilized fine sandy loam at the Lockwood Farm, Mt. Carmel, Connecticut, in 1968, 1969, and 1970. All plots were sown in north-south rows 0.76 m apart; the 0.5 ha of maize and 0.2 ha of sorghum were sown in mid-May and had final plant populations of 47,000 to 71,000 plants/ha, whereas the 0.2 ha of tobacco was transplanted as seedlings in early June to give final plant populations of 22,000 to 28,000 plants/ha. Concurrent measurements of stomatal resistance, irradiance, leaf water potential, and osmotic potential were obtained at five or six heights in the three crops after the canopies were fully developed. Some observations were taken over 24 hr from midday to midday, but generally measurements were begun before sunrise (~ 0530 hr EST) and concluded at 1400 hr EST. Meteorological conditions were measured at the Mt. Carmel Meteorological Station, 400 m from the experimental site. The stomatal resistance (r.) of the horizontal portion of a leaf was measured with a ventilated diffusion porometer (26). The adaxial and abaxial stomatal resistances were measured 1 Abbreviations: EST: index. 360
Standard Time; LAI: leaf area
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separately on adjacent portions of the leaf, and the leaf resistance (r1) was calculated assuming that the two leaf surfaces acted as parallel resistors. Immediately after the completion of the measurements of r,, the irradiance incident upon the adaxial epidermis (I), in the region that resistance measurements were obtained, was measured with a model 756 Weston Sunshine Illumination Meter (Weston Instrument Co., Newark, N.J.). The meter was calibrated in energy units with an Eppley pyranometer (Eppley Laboratory, Inc., Newport, R.I.); the calibration was obtained on several days with the illumination meter and pyranometer situated above the crop. The leaf water potentials (0) were measured with a pressure chamber (25), a modification of that described by Scholander et al. (18). The pressure chamber provides reasonable estimates of + in all three crops (6, 7). The osmotic potentials (7r) were measured with a model 302B Hewlett Packard (Avondale, Pa.) vapor pressure osmometer. Leaf samples from the opposite half of the leaf used for the measurement of & were sealed into a test tube and quickly frozen in Dry Ice. The frozen samples were returned to the laboratory and then allowed to thaw before the cell constituents were expressed from the leaf and quickly transferred to the osmometer. The t and 7r, concurrently sampled and measured on the same leaf, were used to determine the turgor potential (P) from the equation: += r+p The matric potential was assumed to be zero. The leaf area was obtained in 20-cm strata from four randomly selected plants in each crop. For maize and sorghum, the formula (12, 20): Leaf area = length X maximum width X 0.75 was used to determine the area of each leaf. In tobacco the total leaf weight and the ratio of leaf area to dry weight of three to five discs per leaf in each stratum were used to calculate the total leaf area per stratum. The discs were 2.85 cme in area and included midribs and major veins. The LAI, i.e., the total leaf area per unit ground area, could then be calculated. A simple index of leaf flaccidity was obtained in tobacco by measuring the projected width (W,) at the point of maximal leaf width (Win) and calculating the wilting ratio (W :W.). The ratio was measured just before sampling on all leaves sampled for l and r.
leaves at sunrise (-6.5 hr in Fig. lc, i.e., 0530 hr EST), decreased in the upper leaves as I increased (-6.0 hr), and then by 0730 hr EST (-4.5 hr) were similar to those at sunrise and even higher by late morning (-1 hr). A similar sequence was observed in maize and sorghum, but on the particular days presented, dew prevented the measurement of stomatal resistances at sunrise and the initial decrease in resistance was missed (Fig. 1, a and b). Data obtained on days when there was no dew confirmed that ri was high at sunrise and fell to the values observed in Figure 1 at 0800 hr EST (-4 hr). The mean ,6 at sunrise (-6.5 hr in Fig. 1) was -7 bars, -4 bars, and -6 bars in the maize, sorghum, and tobacco, respectively, suggesting that the soil water potential was not more decreased rapidly after negative than these values. The sunrise in all leaves, to reach minimal values in the upper leaves of -18, -22, and -15 bars in the maize, sorghum, and tobacco, respectively. In the first 2 to 2.5 hr, 0fi decreased the most in sorghum and the least in tobacco; thereafter the rate of decrease in all three species was similar. Unlike the situation when water was adequately available (24), the gradient in between the upper and lower leaves was small. The mean 7r at sunrise (-6.5 hr in Fig. 1) was approximately -12 bars in all three species and was lower (more negative) than v& in all leaves. The decrease in 7r during the morning was less than the decrease in i, particularly in tobacco, and thus the P of 5, 8, and 6 bars in maize, sorghum, and tobacco, respectively, at sunrise quickly decreased. Zero turgor was reached in some leaves by 0730 to 0800 hr EST; in sorghum all leaves were at zero turgor at this time whereas in maize and tobacco only the lowest leaf had a P of zero. However, it should be noted that in maize this was only one of three observations at all times of day when P was zero. The leaves in the lower part of the canopy tended to have a lower P than leaves higher in the canopy in maize and tobacco. In a number of cases, particularly in leaves low in the canopy, P was less than zero; i.e., turgor potentials were negative. Leaf Water Potential, Osmotic Potential, and Turgor Potential. The concurrent measurements of i&, 7r, and P revealed that all three parameters decreased throughout the morning (Fig. 1), reached minimal values in midafternoon, and then increased again in the late afternoon and overnight. In Figure 2, the data from Figure 1 plus other data at intermediate times and from days when the soil water potentials were high (24) are replotted to show the change in P with change in for all three species. In all cases, the leaves in the upper part of the canopy had a higher P at a given qi than leaves low in the canopy at all values of qi: midcanopy leaves were intermediate between the upper and lower leaves, but the data are omitted for clarity. Within the scatter of the points, P decreased linearly with decreasing qi in the three species, irrespective of the leaf position in the canopy. In the sorghum and tobacco AP/if,6was approximately 0.8, whereas in maize
Diurnal Changes in the Vertical Proffles of Irradiance, Leaf Resistance, Leaf Water Potential, Osmotic Potential, and Turgor Potential. Concurrent measurements of r1, ,1, 7r, and it was only 0.5; no significant difference in AP/Aif was obP are presented in Figure 1; additional data were obtained at served between leaves situated in the upper, intermediate, or intermediate times but are omitted for clarity. Clear sunshine lower canopy. Although the decrease in P for each 1 bar decharacterized the 3 days and other meteorological conditions crease in il was similar in sorghum and tobacco, sorghum varied within a narrow range. Since the vertical profiles of leaves reached zero P at the lowest (most negative) if, viz: -21 irradiance were similar to those presented previously (24), bars in leaves in the upper canopy and -17 bars in leaves in they were omitted from Figure 1. The irradiance within the the lower canopy, whereas tobacco leaves reached zero P at three canopies rose from zero on all leaves at sunrise to values the highest (least negative) of the three species, viz: -13 and of 1.0 to 1.25 cal cm-2 min-' at the top of the canopy at mid- -8 bars for leaves in the upper and lower canopy, respectively. day. At this time the irradiance within the canopy was 0.25 In maize, zero P occurred at a of -19 and -12 bars in cal cm2 min' on the lower leaves of all three crops. upper and lower leaves, respectively. This suggests that leaves The values of ri were high in all leaves at sunrise (~ 0530 at the base of the canopy should wilt at a higher Vi than leaves hr EST), decreased as I increased, and then increased again to in the upper part of the canopy. high values between 0700 hr and 0830 hr EST. This sequence Figure 3 shows that in the tobacco the lower leaves did, can be seen in tobacco (Fig. ic): the resistances were high in all indeed, wilt at a higher 0 than the upper leaves. At a 0 of, if
(b) SORGHUM 3 AUG 1970 LAI- 2.5
10 AUG 1970 LAI * 3.8
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0 S s
13 AUG 1970 LAI a 3.0
II 0 50
4. 75 (Cr)
0 OSMOTIC POTENTIAL (T)
TUR6OR POTENTIAL (P)
boer bors em' FIG. 1. Vertical profiles of leaf resistance (r1), leaf water potential (#6), osmotic potential (7r), and turgor potential (P) within crops of maize (a), sorghum (b), and tobacco (c) at three or four times of day. Times are calculated from noon EST; e.g., -6.5 is 0530 hr EST. sec
changed with decreasing P in all three species until a marked increase in r1 occurred at about 2 bars in maize, -1 bar in sorghum, and about 0 bars in tobacco; at values of P lower than these r1 was high in the majority of the leaves. Also, r1 remained unchanged as 7,r decreased in all three species until a "critical" V was reached at which r1 increased markedly. This critical varied with species and with the position of the canopy. Leaf Resistance, Leaf Water Potential, and Turgor Potential. leaf in the canopy. In the leaves at intermediate heights The effect of and P on ri of leaves at intermediate height in presented in Figure 4, the critical q& was -17, -20, and -13 the canopy is given for the three species in Figure 4; only bars in maize, sorghum, and tobacco, respectively; the critical leaves that were irradiated at greater than 0.6 cal cm72 min' + was higher (less negative) for leaves low in the canopy and were selected since r1 increased markedly at low irradiances in was lower (more negative) for leaves at the top of the canopy all three crops (23, 24). Figure 4 shows that r1 remained un- in all three species.
say -11 bars, the upper leaves had a wilting ratio of 0.9, whereas leaves in intermediate and lower canopy positions had wilting ratios of 0.7 and 0.3, respectively. Alternatively, a wilting ratio of 0.6 occurred at -4 bars and -12 bars in leaves at lower and intermediate heights in the canopy and did not occur even at a i& of -16 bars in leaves in the upper
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FIG. 2. Relation between turgor potential (P) and leaf water potential (4/) for maize (a), sorghum (b), and tobacco (c) leaves in the upper and lower canopies when soil water potential (4,/) was high (greater than -2 bars) or low (-4 to -7 bars). The lines are the fitted linear regressions for leaves from the upper and lower canopy. The upper canopy leaves were numbers 1 to 2 in maize and sorghum, and 2 to 4 in tobacco, whereas the lower canopy leaves were 8 to 10, 6 to 8, and 15 to 18 in maize, sorghum, and tobacco, respectively, when numbered consecutively from the top of the plant. Data at high soil water potential from Turner and Begg (24). 0
POTENTIAL ( /), bars FIG. 3. Change in wilting ratio with decreasing leaf water potential (4) for leaves in the upper (0), intermediate (A), and lower (-) canopy of tobacco on 15 and 16 August 1968 (LAI = 2.9). Leaves 4 to 11, 12 to 18, and 19 to 22, numbered from the top of the plants, were categorized upper, intermediate, and lower canopy
leaves, respectively. DISCUSSION
The observed diurnal changes in I, r1, xr, and P (Fig. 1) have demonstrated that at low soil water potentials, estimated i,
to be between -4 and -7 bars in this study, the development of low i and P, and not irradiance, has the predominant effect on the diurnal pattern of r1. This contrasts with the observations at high soil water potential in which no effect of Vi or r1 was observed and the daily march of r1 was attributable to the diurnal changes in I (24). With the development of lower values of i, a marked increase in ri at a critical f was observed in all three species (Fig. 4). Such an abrupt increase in r1 at a particular qi has been observed previously (1, 3, 9-11, 14), with the critical if varying with species from -6 bars in onion (14) to -14 bars in vine (11) and sunflower (1). Moreover, in snap beans the critical if varied with the leaf surface, the stomata on the adaxial epidermis closing at a higher if than those on the abaxial epidermis (1). In the present study the critical if varied from -13 bars in tobacco to -20 bars in sorghum; maize was intermediate at -17 bars. In previous studies with maize and sorghum, stomatal closure occurred at a lower l in sorghum than in maize (17), but the V/ at which stomatal closure was observed was higher (less negative) than in the present study (3, 17). However, both the former studies were conducted in a growth room, and other studies have indicated that field-grown plants have a lower critical if than those grown in a growth room (1, 4, 9, 10, 14) possibly because of a lower 7r in field-grown material (14). Although the flux of water through the plant is dependent on the difference in potential between the leaf and soil, the aperture of the stomata, and hence r1, is dependent on the difference in turgor between guard cells and subsidiary cells. Surprisingly, the general leaf turgor, P, decreased over a considerable range without affecting r1 in all three species (Fig. 4). This was most marked for sorghum in which P decreased from 11 bars to less than 1 bar before there was an increase in r1. The fact that the guard cells remain open over a wide range of P and then close sharply over a narrow range of P suggests
Plant Physiol. Vol. 53, 1974
WATER POTENTIAL (*), bars
TURGOR POTENTIAL (P), bars
FIG. 4. Relation between leaf resistance (r,) and leaf water potential (4&) or turgor potential (P) for maize (a), sorghum (b), and tobacco (c) at high (@), i.e., greater than -2 bars, and low (U), i.e., -4 to -7 bars, soil water potentials for leaves irradiated at greater than 0.6?,cal cm-' min-'. Data at high soil water potential from Turner and Begg (24).
that the turgor balance between the guard cells and subsidiary cells is largely independent of changes in bulk leaf turgor, and this presumably is of adaptive advantage in maintaining high rates of CO2 exchange in the field where diurnal ranges of P can be considerable. Unlike the critical 4, the P at which the stomata closed varied little with species (Fig. 4). The three species varied in the 4' at which the critical leaf turgor for stomatal closure was encountered because they differed in both the 7r at the full turgor and the A.P/A4 (Fig. 2). The 7r at full turgor can vary with site and pretreatment, but since the three species were grown in adjacent plots and were measured at approximately the same time, site and pretreatment differences should be slight and the lower r at full turgor in sorghum compared to maize and tobacco represents an adaptation to drought for this species. Maize, on the other hand, had a higher at full turgor than sorghum, but the .AP/ A¾ was less than in the other two species (Fig. 2). Expressed in another way, the 7r of maize decreased to a greater degree with increasing water deficit than in the other two species. The magnitude of a decline in 7r with a decline in 4 depends on the degree of concentration of solutes in the vacuole and the degree of elasticity of the cell walls. Weatherley (27) and Meyer and Boyer (13) have reported -
considerable adjustment in 7v to water stress by mobilization and concentration of solutes in leaf cells, but in the present case, the greater adjustment of 7r in maize is probably due to its greater cell wall elasticity (17). Wilting of lower leaves is often observed before those higher in the canopy, particularly in tobacco. The wilting of leaves has been taken as an indication of low 4, and this led Hoffman and Splinter (8) to expect lower (more negative) values of 4' in wilted tobacco leaves at the base of the canopy than in unwilted leaves at the top of the canopy. However, slightly higher values of 4' were observed in the lower leaves of tobacco (Fig. 1), and the data in Figure 3 demonstrate that wilting as an index of 4 in a canopy of leaves is misleading because the basal leaves wilt at a higher 4 than leaves in the upper canopy. The basal leaves wilt and the stomata close at a higher 4' than in upper leaves because of the high 7r of the basal leaves at full turgor and not because of differences in AP/A4' (Fig. 2). Although the basal leaves were also the older leaves in all three species, the less negative -r of these leaves is not considered to result from their age, but from the low I incident upon leaves at the base of the canopy throughout much of the daily cycle (24). Supportive evidence for this was obtained from a study in which the stomatal response to A
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STOMATAL RESPONSES TO LIGHT AND WATER.
compared in lower leaves of similar height and age but from plants in the center or at the southerly edge of an irrigated tobacco plot spaced at very high density. At high irradiances, the stomata in the leaves at the center of the plot that were normally at low I closed at a of -5 bars, whereas the stomata in the normally well irradiated leaves at the edge of the plot had not closed at a 6 of -10 bars (Turner, un-
published), presumably because of their lower7r. Negative turgors were observed in the basal leaves of all
three species, particularly maize and tobacco, and in the upper leaves of sorghum and tobacco (Fig. 2). Slatyer (19) reported values of P of -5 bars and -10 bars in tomato and privet and concluded from work by Buhmann (5) that the adhesion between the cell wall and protoplast could withstand negative turgors as low as -10 bars. Thus a P of -4.7 bars is not unreasonable. Since, however, P was calculated from the measured and 7r, it is possible that the negative values of P arise from errors in either measurement. This could certainly account for a P of -1 to 2 bars but is less likely to account for the more negative values of P observed in the lower leaves of tobacco and sorghum. However, De Roo (7) showed that at low the pressure chamber gave more negative estimates of V than the thermocouple psychrometer for tobacco. Moreover, the discrepancy between the pressure chamber and psychrometer was greater in the more mature leaves than in the younger leaves. Thus, correction for the effect of aging in the lower older leaves could increase P by 2 to 3 bars without altering AP/ Aip. It is clear that of the three species studied, the two tropical grasses, or C4 species, were able to withstand a lower before stomatal closure and wilting than the C, species, tobacco. If arise from stomatal reductions in photosynthesis at low closure and not from changes in the mesophyll resistance, as suggested by Troughton (22) and Boyer (3), the lower critical +i for stomatal closure in maize and sorghum will confer an advantage on these two species over the tobacco in areas of restricted water supply. Moreover, the greater yielding ability of sorghum over corn in areas of limited water may not arise from differences in rooting depth and density (15), but from the ability of the sorghum to keep its stomata open at a lower than maize. Acknowledgments-I thank J. E. Begg for collaboration in the 1st year of this study, P. F. Tomlinson for technical assistance, and G. S. Taylor for the supply of tobacco seedlings. LITERATURE CITED
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