Scientia Horticulturae, 7 (1977) 9--17 9 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands WATER RELATIONS OF LETTUCE. I. INTERNAL PHYSICAL ASPECTS FOR TWO CULTIVARS M.H. BEHBOUDIAN and H.M.C. van HOLSTEIJN Department of Horticulture, Agricultural University, P.O.B. 30, Wageningen (The Netherlands) Publication 437 (Received 26 January 1977) ABSTRACT Behboudian, M.H. and Holsteijn, H.M.C. van, 1977. Water relations of lettuce. I. Internal physical aspects for two cultivars. Scientia Hort., 7 : 9--17. Experiments were performed to assess physical aspects of internal water relations (reported in this paper), and the gas exchange properties as affected by drought (reported in a second paper). A Dutch cultivar ('Amanda Plus') and a French cultivar ('Sucrine') were used. Water was withheld from plants for 5 or 9 days, and the latter group was rewatered to observe the pattern of recovery 1 day after rewatering. Measurements were made of plant water potential (hygrometry), sap osmotic potential (hygrometry and cryoscopy), sap electrical conductivity, and plant relative water content (RWC). Hygro- metry was found suitable for measurement of plant water and sap osmotic potentials, although equilibration time for the former parameter was long. With suspension of irriga- tion, all the parameters decreased except for the electrical conductivity which increased. Leaf water and osmotic potentials were lower in 'Amanda Plus' than in 'Sucrine', but pressure potential in the former was better maintained. Significant parabolic regressions existed for RWC and osmotic potential on plant water potential. The significant linear regressions of plant water and sap osmotic potentials on sap electrical conductivity were thought to provide easy methods for measurements of the 2 former parameters. INTRODUCTION In water relations studies, less attention has been paid to vegetables {especially leafy vegetables) as compared to field crops. Vegetable production has perhaps been considered as being closely related to abundance of water. Since water as a critical resource is becoming increasingly important, further studies of vegetable responses to drought seem appropriate. Available reports on water relations of lettuce have dealt with the monthly course of transpira- tion in relation to radiation (Neale, 1956), and the effects of different water regimes on the growth under glass (Majmudar and Hudson, 1957). Data on the physical aspects of internal water relations and important plant responses to drought seem to be lacking for this species. In water relations research it is necessary to measure the plant water status.
Transcript
Scientia Horticulturae, 7 (1977) 9--17 9 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
WATER RELATIONS OF LETTUCE. I. INTERNAL PHYSICAL ASPECTS FOR TWO CULTIVARS
M.H. BEHBOUDIAN and H.M.C. van HOLSTEIJN
Department of Horticulture, Agricultural University, P.O.B. 30, Wageningen (The Netherlands)
Publication 437
(Received 26 January 1977)
ABSTRACT
Behboudian, M.H. and Holsteijn, H.M.C. van, 1977. Water relations of lettuce. I. Internal physical aspects for two cultivars. Scientia Hort., 7 : 9--17.
Experiments were performed to assess physical aspects of internal water relations (reported in this paper), and the gas exchange properties as affected by drought (reported in a second paper). A Dutch cultivar ( 'Amanda Plus') and a French cultivar ( 'Sucrine') were used. Water was withheld from plants for 5 or 9 days, and the latter group was rewatered to observe the pattern of recovery 1 day after rewatering. Measurements were made of plant water potential (hygrometry), sap osmotic potential (hygrometry and cryoscopy), sap electrical conductivity, and plant relative water content (RWC). Hygro- metry was found suitable for measurement of plant water and sap osmotic potentials, although equilibration time for the former parameter was long. With suspension of irriga- tion, all the parameters decreased except for the electrical conductivity which increased. Leaf water and osmotic potentials were lower in 'Amanda Plus' than in 'Sucrine', but pressure potential in the former was better maintained. Significant parabolic regressions existed for RWC and osmotic potential on plant water potential. The significant linear regressions of plant water and sap osmotic potentials on sap electrical conductivity were thought to provide easy methods for measurements of the 2 former parameters.
INTRODUCTION
In water relations studies, less at tention has been paid to vegetables {especially leafy vegetables) as compared to field crops. Vegetable product ion has perhaps been considered as being closely related to abundance of water. Since water as a critical resource is becoming increasingly important , further studies of vegetable responses to drought seem appropriate. Available reports on water relations of let tuce have dealt with the month ly course of transpira- tion in relation to radiation (Neale, 1956), and the effects of different water regimes on the growth under glass (Majmudar and Hudson, 1957). Data on the physical aspects of internal water relations and important plant responses to drought seem to be lacking for this species.
In water relations research it is necessary to measure the plant water status.
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Among the parameters indicating this status, plant water potential (q~ plant) and relative water conten t are frequently used. Water potential can best be measured with pressure chamber, thermocouple psychrometer , and dew point hygrometer (Baughn and Tanner, 1976). The pressure chamber method, which is the easiest, cannot be used for lettuce due to its unsuitable leaf anatomy. The other two methods are not available universally. Therefore, indirect determination of water potential by calibration against some readily measurable at tr ibute deserves consideration.
To gain information on the reactions of lettuce to drought, water relation parameters and gas exchange rates were measured in drying-cycle experiments. The results are presented in two papers. This paper deals with physical aspects of internal water relations such as variations in plant water potential, relative water content , and sap osmotic potential and electrical conductivi ty with drought. The second paper will deal with gas exchange properties as affected by drought. Since ecological diversity of cultivars could be reflected in their water relation properties, as exemplified by the results of Pochard (1973) with eggplant, 2 lettuce cultivars of different origin were used.
MATERIALS AND METHODS
Seeds of Lactuca sativa L. cultivar 'Amanda Plus' (a Dutch winter cultivar of but terhead type used in glasshouses) and 'Sucrine' (a French summer cultivar of cos-lettuce type used in the Mediterranean area) were sown in 5 X 5 X 5 cm peat blocks on 19 September 1975. On 10 October 1975, each block was placed in one plastic po t containing a mixture of garden peat and loamy soil. The plants were kept in a Venlo glasshouse with average day/night temperatures of 19.8°/14.5°C. Ten days before measurements, the plants were transported to a climate room for imposition of water stress. The dominating environmental condit ions in the climate room were: day/night temperatures of 21.5°/20°C, corresponding vapour pressure deficits of 10.2 and 7.5 millibars, an irradiance (400--700 nm) of 58 W m -2 (obtained from 400-watt HPLR lamps during a 16 h photoper iod) at the po t level, and a windspeed range of 40--50 cm sec -1. Water was withheld from the plants according to a programme based on preliminary observations. The mild and severe stress treatments were not watered for 5 and 9 days, respectively. The recovery group was deprived of water for 9 days, followed by irrigation I day before appropriate measure- ments. Determinations of plant relation parameters and gas exchange proper- ties began on 10 November 1975.
Plant water potential was measured with the dew point hygrometer described by Campbell et al. (1973). A Wescor HR-33 Dew point microvoltmeter was employed in conjunction with the Wescor C-52 sample chamber. These measure- ments were carried ou t in the above-mentioned climate room. The equilibration time was observed to be in the range of 1.5 to 2 hours. For each cultivar, 3 replicates were used per treatment.
The relative water content was determined by the method of Barfs and Weatherley (1962) using 6 replicates per treatment. Further recommendations
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of Barrs (1968) were followed. For determinat ion of both parameters, the samples were taken from fully grown leaves.
For measurement of sap osmotic potential , plant samples were placed in a deep freeze (--20°C), thawed, and pressed for sap extraction. Determinations were done by cryoscopy using a Knauer osmometer . Dew point hygromet ry was used for comparison. Electric conduct ivi ty of the same sap was measured with a direct-reading conduct ivi ty meter. The last 2 parameters were measured at 22°C.
RESULTS
Table 1 shows the variations of plant water and sap osmotic potentials for cultivars 'Amanda Plus' and 'Sucrine' for different watering-treatments. Both parameters were measured by the dew point hygrometer . Values of plant water potential decreased as watering was suspended. For the control treat- ment , water potential was significantly higher in 'Sucrine' than in 'Amanda Plus' (* P < 0.05). For the other treatments, water potentials were higher in 'Sucrine' but no t significantly so. One day after the severely stressed plants were rewatered (recovery t reatment) , water potential increased by 6 bars but remained below the control values. This was presumably due to a higher resistance to water t ransport through plants which occurs after a severe water stress, as shown by Boyer (1971) for sunflower. Osmotic potential also decreased when water stress increased and recovery values were lower than those of controls. For the same treatments, osmotic potentials in Table 1 (measured by hygromet ry) generally agree with those shown in Fig. 1 (mea- sured by cryoscopy) . Values of osmotic potentials ment ioned hereaf ter belong to cryoscopic measurements.
Figure 1 shows the regressions of sap osmotic potential and RWC on plant water potent ial for the 2 cultivars. Significant linear regressions existed for both osmotic potentials and RWC on plant water potential. Addition of
TABLE 1
Variations of plant water potential a n d s a p osmotic potential (both measured by a dew point hygrometer) in relation to different watering-treatments for cultivars 'Amanda Plus' and 'Sucrine'
Treatment Days Plant water potential Sap osmotic potential without (bars ± SE) (bars)
Fig. 1. Regressions of sap osmotic potential (~I, os) and plant relative water content (RWC) on plant water potential (~I, plant) for cultivars 'Amanda Plus' (triangles) and 'Sucrine' (circles). Closed symbols represent recovery values.
quadratic terms to the equations resulted in significantly bet ter (* P < 0.05) regressions. The latter tests of significance were carried out by the "extra sum of square principle" as described by Draper and Smith (1966, pp. 67--69). The same reference was consulted for testing the significance of correlation coefficients for the parabolic regressions of Fig. 1.
Both Table ! and Fig. 1 show that, for the same treatments, osmotic poten- tials measured with hygrometry as well as cryoscopy were lower in 'Amanda Plus' than in 'Sucrine'. In the range of plant water potentials cited in Table 1, osmotic potentials decreased from --8.15 t o - - 1 6 . 5 2 bars in 'Amanda Plus' (Fig. 1), recovering to --9.07 bars after rewatering. For 'Sucrine', the corres- ponding decrease was from --6.98 to --10.92 bars with a recovery value of --7.07 bars. In 'Amanda Plus', RWC declined from a value of 90.20 % in con- trols to 59 .61% in severely stressed plants. The corresponding figures for 'Sucrine' were 91.17 and 68.9 %, respectively. For both cultivars this parameter recovered to the level of controls in spite of still lower plant water potentials.
Figure 2 shows the linear regression lines of plant water and sap osmotic potentials on sap electrical conductivity. Pooled values of the 2 cultivars were used for calculation of regressions. Correlation coefficients were significant at the indicated levels. From control to severe stress t reatment in 'Amanda Plus', the electrical conductivi ty increased from 19.95 to 31.50 m mhos/cm. The corresponding values for 'Sucrine' were 16.80 and 22.5. After rewatering, this parameter recovered to 22.50 and 18.30 m mhos/cm in 'Amanda Plus' and 'Sucrine', respectively. Since plant water potential is the algebraic summation of osmotic, matric, and pressure potentials (Slatyer, 1967, p. 145)
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1 8 j ~ bos=508 .067EC ~=B "°~*09"/ ~*IP<O001 /
161_ ~pLant='127 06!EC ~=8 ~*=OS2 **P<O01
12~ Z~Z~ ~
0 ~ t I I I J_ i I i J 16 18 20 22 24 26 28 30 32
EC (rn m h o s / c m )
Fig. 2. Linear regressions of sap osmotic potential (~l, os , circles) and plant water potential (,I, plant, triangles) on sap electrical conductivity (EC) for cultivars 'Amanda Plus' (open symbols) and 'Sucrine' (closed symbols).
and matric potentials are assumed to be negligible in our experiment (see Discussion), the difference between the ordinates of regression lines in Fig. 2 would represent the pressure potential component .
DISCUSSION
The results ment ioned in Table 1 indicate the sensitivity of the dew point hygrometer for measuring plant water and sap osmotic potentials. Successful applications of this method have been reported for some other plant species {Neumann et al., 1974; Dube et al., 1974; Baughn and Tanner, 1976). In our experiment, the equilibration time was relatively long (1.5 to 2 hours). Unless this could be reduced, the method would remain inefficient, in spite of its accuracy, for experiments involving large numbers of replicates. Presence of leaf cuticle is presumably responsibte for the slow equilibration process. To shorten the equilibration time, Neumann et al. {1974) applied xylene to corn, sorghum, sunflower, and soybean. Since for soybean no improvement was observed, they carefully scraped its leaf samples with a razor blade. No such methods were tried in our experiment because it was feared that the condition of leaves would alter and would not be parallel to those in photosynthesis chambers. For measurement of sap osmotic potentials {Table 1), the equilibra- tion times were not more than a few minutes. The lower plant water potentials in 'Amanda Plus' could partly be due to its higher transpiration rates per plant because of its larger leaf area (unpublished own results). At tempts were made to obtain data on the extent of root systems, bu t with the soil type used, reliable results could not be obtained. It is also possible that there was a differ-
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ence in internal resistance of the 2 cultivars to water transport for which data were not obtained.
The components of plant water potential are osmotic, matric, and pressure potentials (Slatyer, 1967, p. 145). These potentials may play different roles in various physiological processes. For example, the total water potential decides water transfer phenomena in plants, osmotic potential is a measure of protein hydrat ion and enzymatic activities, and pressure potential has direct bearing on stomatal opening (Slavik, 1975). The values of each component might vary according to the stage of water stress in plants. Hsiao ( i973) maintained that matric potential could be neglected unless much tissue water (e.g. 50 %) is lost. Therefore, the components of concern in our study are assumed to be osmotic and pressure potentials. It is a common occurrence that osmotic potential decreases when plant water potential declines. Osmotic potential has been shown to decrease gradually at the higher values of plant water potentials, followed by a sharper decline such as in pepper (Barrs, 1968), or it could decrease linearly at all values of water potentials such as in apple (Goode and Higgs, 1973). Our results with the 2 lettuce cultivars (Fig. 1) show a relationship similar to that of pepper (Barrs, 1968). The decline of osmotic potential has simply occurred due to water loss, as shown by the similar trends of RWC and osmotic potential curves in Fig. 1. It is still a mat ter of speculation if, in water stressed shoots of higher plants, solute accumulation occurs leading to osmotic adjustment or osmoregulation (Hsiao, 1973). Erlandsson (1975) showed that when substrate water potential in wheat plant was lowered, ion absorption by plants decreased, a process which will hinder osmoregulation.
The declines of osmotic potentials shown in Fig. 1 have led to maintenance of pressure potentials (mainly for 'Amanda Plus') which are represented by differences in ordinates of regression lines in Fig. 2. If pressure potential had been so maintained, it is expected that processes depending on it, such as stomatal opening, should have kept at normal levels even at the low plant water potentials observed in the experiment. Stomatal opening depends on the turgot of guard and subsidiary cells while pressure potentials in Fig. 2 represent average potentials of mesophyll and epidermal cells, which might be different from those of guard and subsidiary cells. Moreover, the synthesis of abscisic acid at lower plant water potentials and its closing effects on stomata (Raschke, 1975) has to be considered. Stomata of both cultivars generally showed a closing pattern after water was withheld from the plants (unpublished own results). For wheat plants, Simmelsgaard (1976) reported increasing values of pressure potentials concomitant with stomatal closure. He attr ibuted the discrepancy to the disparity of guard cell pressure potentials and those of the bulk leaf.
Although RWC is a popular me thod for expressing the plant water status, it is generally insensitive at low water deficits (Hsiao, 1973). This s ta tement applies to our results shown in Fig. 1. For water potentials down to --6 bars, diminution of RWC was not as conspicuous as for the 2 measurements taken
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at lower potentials. Halevy (1960) studied various indicative criteria for irrigation of gladiolus and reported that water saturation deficit (= 100 -- RWC) was the most sensitive to soil moisture tension. It seems that the applicability of this me thod depends on the environmental conditions. In situations of low evaporative demand such as in our experiment, low sensitivity of RWC could be expected, while in field conditions of higher evaporative demand such as those employed by Halevy (1960), higher sensitivity of RWC could be observed. It is generally maintained that for a given decrease in plant water potential, drought resistant plants decrease their RWC less than drought sensitive ones (e.g. Sanchez-Diaz and Kramer, 1971). For the same range of plant water potentials, the RWC in our experiment decreased more than those shown for tomato (Slatyer, 1967, p. 147) and sweet pepper (Janes, 1970). Therefore, it could be suggested that lettuce is less drought resistant than those 2 species. Since the RWC--~ - l a n t relationship depends on many factors, such a conclusion might not be entirely justified.
The main components acting osmotically in plants are, in decreasing order of importance, inorganic salts, sugars, and organic acids and their salts (Slavik, 1959). To measure it easily in the field, Shimshi and Livne (1967) assumed that osmotic potential is the result of contribution from electrolytes and metabolites. For 17 plant species, they measured the 2 components by con- duct imetry and refractometry, respectively. Addition of the 2 measurements gave a good approximation of plant osmotic potentials. We correlated their conduct imetry and refractometry results with their cryoscopic measurements of osmotic potentials. Correlation of conduct imetry results with cryoscopic data was bet ter than that of refractometry results with cryoscopic data (***r = 0.79 vs. ***r = 0.74, ***P < 0.001). The highly significant correlation between conductivity data and osmotic potentials in our experiment corrobor- ates the content ion of Slavik (1959) and the results of Shimshi and Livne (1967) that electrolytes are important contributors to osmotic potentials. This should make it possible, at least for lettuce, to measure the osmotic potential easily by conductivity measurements. In the methods of osmotic potential measurements described by Slavik (1974, pp. 75--109), this procedure is not included. In view of satisfactory results with lettuce in the present experiment, and also with tomato, cucumber, and sweet pepper (Behboudian, 1977), this method seems to deserve consideration. The highly significant correlation between plant water potential and sap electrical conductivity (Fig. 2) also provides a satisfactory method for measuring water potential indirectly. Measurements of plant water status and especially plant water potential are very important in water relations studies, as well as in timing of irrigation based on plant factors. For lettuce, which is no t suitable for the pressure chamber, the above indirect method might provide a possibility wherever more sophisticated instruments are no t available. This method, if proved accurate for other species, seems easier than most of the methods reviewed by Slavik (1974, pp. 15--74) for determination of water potential. Since the osmotically active components vary according to environment, plant age, and physiological conditions, appropriate calibrations should be done for each set of conditions.
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ACKNOWLEDGEMENT
We are grateful to Mr. F. Kuiper (M.Sc.) of Plant Physiological Research Department for permitting us to use the Knauer osmometer.
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