Irrig Sci (1982) 3:111-121 Irrigation

clence © Springer-Verlag 1982

Effect of Soil Water Osmotic Potential on Growth and Water Relationships in Barley during Soil Water Depletion

C. R. Jensen

Hydrotechnical Laboratory, The Royal Veterinary and Agricultural University, 23 Btiilowsvej, DK- 1870 Copenhagen, Denmark

Received July 26, 1981

Summary. Barley plants (Hordeum distichum, L., cv. 'Zita') grown in a sandy soil in pots were adjusted during a pretreatment period of 5 days to three levels of soil water osmotic potential by percolating 6 1 of a nutrient solution with additional 0, 22.3 and 44.6 mM KC1. A drying cycle was then started and the plants were harvested when the soil water matric potential had decreased to -1.4 MPa, respectively 6, 7 and 8 days later.

No significant differences in dry matter yields, transpiration coefficients and wilting percentages were found between treatments.

During the drying cycle leaf water potential (~0l) decreased concomitantly with decrease in soil water potential (~0s) with almost constant and similar differences (~t -~s) for all treatments despite differences in levels of potentials. The concomitant decrease in leaf osmotic potential (:r) was due partly to dehydration (58%) and partly to increase in leaf solute content (42%) indepen- dent of treatment. The part of total osmotic solutes due to K decreased relatively during the drying cycle.

Close relationships were found between :~ and ~t as functions of relative water content (RWC). Identical curves for the two levels of salt treatment agree with similar concentrations of K, C1, and ash found for salt treated plants indicating that maximum uptake of macro nutrients may have been reached.

During the main part of the drying cycle the turgor potential as function of RWC was higher and decreased less steeply with decreasing RWC in the salt treated than in the non-salt treated plants.

In the beginning of the drying cycle additions of KC1 lowered the transpiration rates of the salt treated plants resulting in a slower desiccation of the soil and hence an increased growth period. A delay in uptake from a limited soil water supply may be advantageous during intermittent periods of drought.

Introduction

The depression of plant growth and of transpiration rate under saline conditions have been widely investigated (e.g. Ehlig etal. 1968; Gale 1975; Mass and

0342-7188/82/0003/0111/$ 02.20

112 C.R.Jensen

Hoffman 1977). Salinity which is associated with high uptake of ions, induces osmotic adjustment in the plant (e.g. Bernstein 1961; Storey and Wyn Jones 1978). Decrease in transpiration rate together with osmotic adjustment may enable salinized plants to maintain turgor during periods of drought. Improvement of plant water balance during drought has been found both in NaC1 salinized halophytes (Gates 1972; Kaplan and Gale 1972) and glycophytes, maize (Stark and Jarrell 1980), and sugar beets (Durrant et al. 1978).

The major osmotic components in leaf cells of glycophytes under non-saline conditions are, however, potassium salts of organic acids and sugars (Hellebust 1976). K is thus important for turgor regulation and hence for cell extension and growth in most crop plants. Furthermore, rate of transpiration may be decreased by supply of K (e.g. Blanchet et al. 1962; Linser and Herwig 1968; Brag 1972; Jensen 1975).

The positive effect of K on turgor and plant water relationships was demon- strated under water stress regimes in an earlier study (Jensen 1981 a). Dehydration and wilting in bean plants well supplied with potassium were considerably reduced when the plants were exposed to water stress due to polyethyleneglycol (PEG MW = 1500).

In a recent investigation it was shown that high K levels in the leaves lowered leaf osmotic potential, supported maintenance of turgor, and reduced wilting in wheat plants at low soil water matric potential 0Pm) (Jensen 1981 b). The surplus of K salts was leached from the soil before the plants were subjected to periods of soil water stress.

Such conditions are, however, different from those prevailing in the field where no leaching of salts can be expected to occur prior to drought. The aim of the present work is an attempt to simulate the field conditions and to include determinations of the soil water potential (~p~), the sum of ~Pm and soil water osmotic potential Qp~). Under conditions of non-salinity and adequate water supply ~p~ amounts to about -0 .1 MPa (Epstein 1972), but under conditions of drought the values of ~p~ decrease steeply with the decrease in soil water content, the more the higher the initial salt concentration (Jensen 1979).

Abbreviations: ~Ps, soil water potential; ~Pm, soil water matric potential; ~p~, soil water osmotic potential; ~0t, leaf water potential; ~pp, turgor (pressure) potential; ~, leaf osmotic potential; Jrd, part of leaf osmotic potential due to dehydration; arKX RWC, part of leaf osmotic potential at full turgor due to potassium salt content; RWC, relative water content; SD, standard deviation.

Materials and Methods

Growth Conditions and Treatments

Barley plants (Hordeum distichum, L., cv. 'Zita') were grown in soil in plastic pots, 23 cm in diameter, 23 cm in height and with a hole in the bottom for drainage. Each pot with 22 plants contained 13.0 kg dry soil with a bulk density of 1.4 g cm -3. The texture of the soil after passing through a 4 mm sieve was 82% of sand (< 2.0 > 0.02 ram), 11% silt (< 0.02 > 0.002 mm), 7% clay (< 0.002 ram), and 2.3% organic matter. In order to avoid evaporation from the soil during the growth period a 3 cm thick layer of perlite was placed on the surface of the soil.

Soil Water Osmotic Potential Effects in Barley during Drought 113

The pots were placed in a growth chamber with the following climate conditions: relative humidity 60 to 65%; air temperature 14.0+ 1.0 °C. The fight source was Philips HPI 400 W and incandescent lamps giving an irradiance (400-700 nm) of 120 Wm -2 at plant height. Light and dark periods were 13 and 11 h.

Before sowing each pot was watered with a nutrient solution containing: 2.3 mM NO3-N, 4.8 mM NH4-N, 1.1 mM water soluble P, 4.2 mM K, and 1.0 mM MgSO4 7 H20. During a pretreatment period of 25 days the plants were watered whenever the soil water content had decreased to 14% by weight to be rewetted to 18% alternatively with nutrient solution and tap water containing 2-3 mM Ca.

When the plants were 25 days old 3 x 12 pots were percolated with 6 1 of salt solutions per pot. 12 pots were percolated with the nutrient solution only (treatment S0), 12 pots with the nutrient solution containing additional 22.3 mM KC1 (treatment $1), and 12 pots with the nutrient solution containing additional 44.6 mM KC1 (treatment $2). The osmotic potentials of the So, S~, and $2 solutions were-0.04,-0.14, and-0 .24 MPa, respectively.

No further water was applied until the plants were 30 days old. Four pots per treatment were then harvested. The remaining 24 pots were adjusted to a soil water content of 21.1% by weight with tap water. A drying cycle was then started and continued until the soil water matric potential had decreased to -1.4 MPa, corresponding to a soil water content of 4.2% lasting 6 to 8 days. The plants were then without turgor and partly wilted. The wilting began at the tip of the leaves and proceeded downwards.

The pots were rewetted to saturation and harvested 24 hours later. The $1 and $2 plants were then 1 to 2 days older than the So plants.

During the drying cycle each plant had 3 to 4 fully expanded leaves.

Measurements

Soil. The soil water matric potential (~m) was estimated by use of the soil water characteristic curve (Jensen 1979). Soil water potential QPs) was determined as the mean of readings from the two dewpoint hygrometers ('Wescor, Inc., PT-51', 1979) placed in each of five pots per treatment. The hygrometers were imbedded in the soil near the centre of the pot and at two thirds of the total depth. Hygrometer outputs were read with a 'Wescor HR-33 T' microvoltmeter using dew point read outs. The hygrometer gives rectilinear responses in the actual range of measurements (-0.2 to -3.0 MPa) (Nnyamah and Black 1977; Jensen 1979). For correcting both the hygrometer cooling coefficients and the slope of the rectilinear hygrometer response curves for the soil temperature, 14 to 16 °C, the hygrometers were prior to installation calibrated in a 0.50 molal NaC1 solution (-2.20 MPa) at a temperature of 15 °C. The calibration procedure used was in accordance with instructions given in manuals by Wescor, Inc., Logan, Utah, USA.

~Ps was determined just before fight was switched on in order to reduce the effect of temperature gradients on response of the hygrometers.

At the end of the drying cycle the soil water osmotic potentials were determined in soil samples taken vertically in the pots. Three 100 g samples per pot were mixed and air dried and the osmotic potential was determined by a vapour pressure osmometer ('Knauer', Berlin) in a filtered extract at a soil-to-water ratio of 1 : 1 (Bower and Wilcox 1965). By calculating to 21.1 weight per cent soil water the soil water osmotic potentials Qp,~) were -0.05_+0.023, -0.13_+ 0.009, and -0.32+0.018 MPa for treatments So, $1, and $2, respectively.

Exchangeable K was determined in 0.5 M ammonium acetate extracts from 100 g dry soil samples per pot. K was analyzed using a flame photometer.

Plants. During the drying cycle transpired water was determined by weighing the pots daily just after fight was switched on. Change in weight of the growing plants was ignored.

Stomatal conductance of water vapour (on the abaxial leaf surface) was determined by using a diffusion porometer ( 'Crump Sci. Instr., Automatic Diffusion Porometer No 502', 1977, England) and given as the mean of determinations on four fully expanded leaves per pot.

Leaf water potential QPl), leaf osmotic potential (~), and relative water content (RWC = fresh W t - d r y Wt/turgid W t - dry Wt) were measured in the same one fully expanded leaf

114 C.R.Jensen

(the third leaf). The pressure chamber technique of Scholander, as modified by Millar and Hansen (1975), was used to determine ~01 after which the leaf was transversely divided into segments for estimating the two other parameters.

Immediately after measuring ~0t, RWC was determined according to Barrs and Weather- ley (1962). Three segments (1.5 em) from near the centre of the leaf, were weighed, placed on the surface of distilled water at 18.0___0.1 °C and exposed to weak light for 4 h in order to obtain the weight at full turgor. Finally the segments were oven dried at 80 °C for 24 hours, and RWC was calculated.

The remaining segments of the leaf were frozen a t - 15 ° C, After thawing and squeezing ar was determined in the leaf sap by using a vapour pressure osmometer ('Knauer', Berlin). Values ofar refer to a leaf temperature of 17 °C.

The determinations of stomatal conductance, ~0 l and RWC were made 1 to 3 h after light was switched on.

The part of osmotic potential due to dehydration (arc/) was calculated from the difference:

ard= ~r - (ar X RWC)

where ar x RWC is the solute component of the osmotic potential at full turgor when RWC equals 1 (Jones and Turner 1978).

At the start and end of the drying cycle the component of ar × RWC due to K salt content (arKx RWC) was derived from osmotic potentials due to K concentrations in the leaf water at RWC = 1. For RWC = 1 the leaf water content was found to be 7.07 (+-_0.20) g per g leaf dry matter. The leaf water was regarded as pure water and the K salts as KC1. ark was estimated at leaf temperature (17 °C) from the osmotic potential of a NaC1 solution with the same freezing point as the KC1 solution (Lang 1967).

The turgor potential 0/2/,) was calculated according to the equation:

~0t= ~0p+ar

The criterion used for evaluation of wilting was the appearance of the leaves or leaf parts. Wilted leaves had turned yellow, were without turgor and appeared dead. At harvest, one day after rewatering, the leaves were divided into fresh green and wilted leaves or leaf parts.

Plant dry weights were obtained by oven drying at 80 °C for 24 h. At final harvest the root length was determined in three pots, one from each treatment

according to Marsh (1971). The root density was high and at final harvest 1.3___0.18 cm root length per cm 3 soil was found. The resistance against water uptake was therefore attributed to resistance of transport of water in the plant only (Hansen 1974).

The ash content of the leaves was determined after dry ashing at 550 °C. In the ash K was determined by flame photometry. C1 was determined in leaf material dried at 80 °C by using a potentiometric electrode.

Results and Discussion

Table 1 shows that dry matter yields of non-wilted leaves of $1 and $2 plants, and for the latter when corrected for ash content, were significantly larger than for the So plants. No significant differences in total top dry matter yields (excluding ash content), transpiration coefficients and wilting percentages were found between treatments at the end of the drying cycle. The growth response is a confirmation of earlier results (Mass and Hoffman 1977) indicating that barley is a rather salt tolerant species.

Table 2 shows that high exchangeable K content in the soil in treatments $1 and $2 was reflected in high K concentrations in leaf dry matter. Almost the same concentrations of K, C1, and ash were found in the salt treated plants which indicates that max imum values have been reached. It confirms findings for barley by other workers that increasing concentrat ion of already high concentration of K

Soil Water Osmotic Potential Effects in Barley during Drought 115

Table 1. Yield characteristics at start and end of the drying cycle, SD is of the mean of 8 ob- servations. Any two values within column are significantly different at the 5% level, if they have no letter in common. Letters in the brackets refer to test based on yields when ash content is subtracted from dry matter yield

Treat- Start of End of drying cycle ment drying cycle

Top dry Dry matter Dry matter Wilting of "Top dry bTranspira_ matter of non-wilted of wilted leaf dry matter tion

leaves leaves matter coefficient g pot -1 g pot -1 g pot -~ per cent g pot -a g /g± SD

So 26.8 a 11.3 a (a) 8.5 a (a) 42.9 a (a) 36.4 a (a) 254 ± 25 a (a) Sl 28.0 a 13.4 b (ab) 8.0 a (a) 37.4 a (a) 38.6 ab (a) 230± 16 a (a) $2 28.9a 14.1 b (b) 8.7 a (a) 38.2 a (a) 40.4 b (a) 212±11 a(a)

a Includes wilted and non-wilted leaves and stems b (Ag transpired water)/(Ag top dry matter during the drying cycle)

and Na in the root medium solution has little effect on the corresponding concentration in the plant (Pitman 1965; Storey and Wyn Jones 1978), and is in accordance with the suggestion (Pitman 1972) that plant growth controls uptake of nutrients by the shoot.

Figure 1 shows the decreases in transpiration rate, leaf water potential (~Pl), soil water potential 0Ps) and soil matric potential (g'm) during the drying cycle. The steep decrease in ~Ps reflects the influence o f the decrease in soil water osmotic potential (%0, most pronounced for treatments $1 and $2. Until ~Pm was -0.8 MPa the ~0t-curves run almost parallel to the ~Ps-curves resulting in small variations in the potential difference OPt - ~Ps) shown in Fig. 2 as a function o f ~0 m .

For all treatments the leaf osmotic potential (z 0 was lower than ~ps (Fig. 2), but the curves tended to merge at low ~Pm-values. Thus, the influence o f n in maintaining a potential gradient seemed to cease at a b o u t - 1 . 4 MPa.

The component of z~ due to dehydration (rid) decreased with decreasing ~Pm, firstly progressively and then diminishing towards constancy.

F rom the results in Fig. 2 it was calculated that, during the drying cycle and independent o f treatments, about 58% of the decrease in leaf osmotic potential was due to dehydration and hence 42% to net increase in solute content (A(n × RWC)).

T a b l e 2. Exchangeable K in soil and concentrations of K, C1, and ash in leaf dry matter at the end of the drying cycle. SD of the mean of 8 determinations

Treat- Soil Leaf dry matter ment

Exchangeable K K % C1% Ash % rag/100 g soi l iSD ___SD _+SD ±SD

9.8±0.42 6.5±0.10 1.9±0.10 15.8±0.52 $1 26.0±1.20 8.3±0.17 4.7±0.02 18.5±0.18 $2 35.0±3.50 8.7±0.15 5.3±0.17 19.0±0.26

116 C. R. Jensen

3 " - A u : l

O

£ --

E m

30 32 3~ 3B 38 30 32 3l., 36 38 30 32 3/., 36 38 /.,0 DAYS DAYS DAYS

Fig. l. Soil water matric potential (~0r,), soil water potential (~0~), leaf water potential (~01), and rate of transpiration (T) during the drying cycle. Bars denotes _+SD of the mean of 3 determinations for ~t, of 5 determinations for ~s, and of 8 determinations for ~0m and T

Table 3. The solute component ( ~ x RWC), the part of the solute component due to K (z~K x RWC) of the leaf osmotic potential, and the ratio of the components at the start and end of the drying cycle. SD are of the mean of 8 observations

Start of drying cycle (30 days old)

x RWC__ SD, z~K x RWC +_ SD, (~/~ x R W C ) / ( ~ X RWC) MPa MPa

So - 1.07+_0.044 - 1.04+_0.066 0.97 $1 - 1.17+_0.026 - 1.06+_0.017 0.91 $2 - 1.32 _+ 0.031 - 1.15 _ 0.029 0.87

End of drying cycle (37-39 days old)

z~ x RWC__SD, Z~K X RWC +_ SD, (~K x RWC) / (~ x RWC) MPa MPa

So - 1.49 +_ 0.043 -0.98 +_ 0.011 0.66 $1 - 1.86 +_ 0.039 - 1.24+ 0.026 0.67 $2 - 1.97 +_ 0.052 - 1.29 +_ 0.056 0.66

Soil Water Osmotic Potential Effects in Barley during Drought 117

At the start of the drying cycle the main part of ~ x RWC was associated with K salt concentration (97 to 87%). This result is in agreement with the well known observation that K salts, charge balanced by inorganic anions and by organic acid accumulation, are the main osmoticum of leaf cell sap in most glycophytes (Cram 1976; Wyn Jones et al. 1979). During the drying cycle, however, concentrations of other solutes increased relatively more than K salt concentrations as the ratio of (~K× RWC)/(s~× RWC) decreased from about 0.9 to 0.66 at the end of the drying cycle independent of treatment (Table 3). These other solutes are presumably organic compounds and the amino acid, proline, may have significantly contributed as large increases in proline content have been found at moderate to severe levels of water stress in barley (Singh et al. 1973; Hanson et al. 1977) and other species (Jones et al. 1980). In a preceeding study with spring wheat (Jensen 1981b) and during a similar drying cycle only about 20% of the decrease in total zc was attributable to uptake of ions and/or increase in organic solutes compared with the above 42% in barley in the present study. In barley the decrease in zcXRWC expressed as absolute values were about -0.4 MPa for So plants and about -0.7 MPa for $1 and $2 plants and in wheat -0.15 to -0.2 MPa independent of treatment and in both cases associated with a fall in RWC to about 65-70%. The results indicate a difference between the two species in their ability to osmotic adjustment. It should be noted, however, that initial s~XRWC values were about 0.2 MPa lower in the wheat plants resulting in potentials of zc x RWC approaching the same values in wheat and barley at the end of the drying cycle.

-05

-1.0

I

-15

~zo

-2.5

-3.0

, I i i , i

\ TREATMENT S 0

\T h ' ' ~ ~'+-~

\. "+,,,,

I I I

-0.4 -0.8

.... v,,, \ \

I I I L L I I I I

-1.2 -0./-, -0.8 -1.2 ~m(MPa) ~rn(MF~)

i i i , i i i

+~ REATMENT S 2

L I I I L I

-0.4 -0.8 - 1.2 ~m (MPa)

Fig. 2. Leaf osmotic potential (~), the component of zc due to dehydration (zcd), the potential difference (~Pl-~Ps), and soil water potential (~Ps) in relation to soil water matric potential 0Pm)- Bars denotes _+SD of the mean of 3 determinations for s~, ~d and (~Pl-~Ps), and of 5 determinations for ~Ps

118 C.R.Jensen

-1.o

-[5 t

l=

-25

-3.0

0 Q_

-05

-1.0

-1.5

~ '-2p

-3.0

-~.s f 100

' I I

\

I I +

V~+~_.~ S1--A . - - I I - - $2= []

\

X \ \

\

' i

9'0 8'0 70 60 RWC (°/o)

Fig. 3. Leaf osmotic potential (~), turgor potential (~pa) and leaf water potential (VI) as functions of relative water content (RWC). - - denotes treatment So, and --- treatments S~ and $2. Bars denotes +__ SD of the mean of 3 determinations

Figure 3 shows ~, turgor potential (Vp) and Vl as functions of RWC. The identical curves referring to $1 and $2 plants indicate that the concentrations of solutes in the cell sap must have been similar in the plants of the two treatments. This is supported by the rather similar concentrations of K, C1, and ash (Table 2) and of solutes (Table 3) at the end of the cycle.

The results indicate that higher turgor potentials were maintained over longer periods of the drying cycle in the salt treated compared with the non-salt treated plants. The negative values of turgor potentials, found in the present work are

Soil Water Osmotic Potential Effects in Barley during Drought 119

Fig. 4. Soil water content as a function of time. Each point as a mean of 8 determinat ions

22

20

,y,18

16

m 14

O

~.10 i,I ° 8 rY

I I I I I , I I I i

~A o= TREATMENT S O" [ ] • = // Sl.

ko ~ ~ n = II S2

30 31 32 33 34 35 36 37 38 39 DAYS

probably a consequence of underestimating ~ by using the applied method (Nasser et al. 1980). In the process of freezing and thawing the cell sap is diluted by almost pure apoplastic water (Jones and Turner 1978). As the volume of apoplastic water is considered constant as RWC declines (Tyree 1976) the dilution effect increases with water stress and at a greater rate the higher the solute concentration in the cell sap.

As already mentioned similar potential gradients for water uptake were maintained during the drying cycle in both non-salt and salt treated plants due to osmotic adjustment. Nevertheless transpiration rates of non-salt and salt treated plants differed considerably as evident in Fig. 1. In the first days of the drying cycle and before soil water became deficient a lower rate of transpiration was found in the salt treated compared with the non-salt treated plants. Stomatal conductances in the same period were 0.88+0.08, 0.79_+0.04, and 0.73_+0.09 cm s -1 in the So, $1 and $2 plants, respectively, in fair agreement with findings by several workers (e.g. Thomas 1970; Brag 1972; Jensen 1975) that reduction of rate of transpiration may be induced by K uptake affecting stomatal closure.

The marked differences in transpiration rate resulted in a slower desiccation of the soil with the $1 and $2 plants than with the So plants. Thus, after the first three days of the drying cycle the So plants had transpired 77% whereas the $1 and $2 plants had transpired only 66 and 55% of the available soil water (Fig. 4). This should be considered in connection with the fact that the difference in rate of transpiration was reversed from the 33rd day after which the rate became highest in the salt treated plants until all available soil water was taken up (Fig. 1). At this time the $1 plants were 1 day and the $2 plants 2 days older than the So plants. It should be noted that these maximum differences in age were almost reached at 10 weight per cent water in the soil when 33% of available soil water remained.

In spite of differences in transpiration rate during the drying cycle, dry matter yields (excluding ash content) for all treatments, as already mentioned, were very similar when the total amount of soil available water had been absorbed resulting in similar transpiration coefficients (Table 1).

120 C.R. Jensen

With limited water supply near proportionality between dry matter production and transpiration and hence constant transpiration coefficients have been reported by several workers (De Wit 1958; Friis-Nielsen 1963; Jensen 1980). Thus, by using transpiration as a measure of dry matter production, the results o f the salt treated plants indicate a salt induced yield depression in the beginning of the drying cycle compensated by a continued dry matter production towards the end of the drying cycle.

Conclusively, throughout the drying cycle the salt treatments effected osmotic adjustments in the salt treated plants resulting in similar water potential gradients as those in the nontreated plants. The main effect of the salt treatments was a slower water uptake from a given water supply and hence a longer growth period of the salt treated compared with non-salt treated plants. The results demonstrate that a rationing of soil water during periods of drought might be of importance by increasing the chance of survival or reducing loss of yield.

Acknowledgement. The author wishes to thank dr. agro. Bodil Friis-Nielsen, M. phil. S. G. Heintze, and lic. agro. V. O. Mogensen for their help and suggestions in preparing the manuscript, Marie Louise Langkj~er and Eva Svenstrup, and Tove Vadskj~er for typescript and drawings, respectively. The project was financially supported by the Danish Agricultural and Veterinary Research Council who is gratefully acknowledged (project No. 513-10166).

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