Soil Temperature and Moisture Stress Effects on Kernel Water Uptake ...

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Soil Temperature and Moisture Stress Effects on Kernel Water Uptake and Germination of. Winter Wheat. G. P. Lafond and D. B. Fowler*. ABSTRACT.
Published May, 1989

Soil Temperature and Moisture Stress Effects on Kernel Water Uptake and Germination of Winter Wheat G. P. Lafond and D. B. Fowler* ABSTRACT Direct seeding into standing stubble from a previous crop (stnbbling-in) has allowed successful production of winter wheat (Triticum aestivum L.) on the northern edge of the North American Great Plains. Soil at seeding is usually very dry with this production system, and the producer is often left with the dilemma of either seeding at the optimum date into a dry seedbed or delaying seeding until after a rain. The objectives of this study were to determine the importance of soil temperature and moisture potential on kernel water uptake and germination, so that the minimum requirements for successful crop establishment could be identified. Controlled environment studies demonstrated that germination could occur at moisture contents as low as 512 g water kg-' kernel dry weight. Therefore, differences in rate of water uptake observed for kernels placed in a Typic Haploboroll soil at -0.2, -1.0, and -1.5 MPa did not result in differences in speed of germination. Temperature differences in the 5 to 30°C range had a large influence on rate of kernel water uptake and speed of germination. As temperature increased, rate of water uptake increased and median germination time decreased from 6.9 d at 5°C to 0.9 d at 25 and 30°C. This study demonstrated that the effects of temperature on speed of germination are much larger than those of moisture, indicating that seeding of stubbled-in winter wheat should proceed at the optimum date regardless of seedbed moisture conditions.

production of winter wheat on the IUCCESSFUL northern edge of the Northern Great Plains has

been made possible by direct seeding into standing stubble (stubbling-in). Standing stubble maintains snow cover, thereby minimizing the risks of low-temperature injury during winter. Seeding at the optimum date is important in winter wheat production in this region (Fowler, 1982). Seeding into standing stubble is usually accompanied by dry soil conditions, and often the producer is left with the dilemma of either seeding at the optimum date in a dry seedbed or else waiting for some precipitation before seeding. Consequently, successful establishment of winter wheat depends upon the soil temperature and soil moisture content at seeding. Delayed seeding may result in better soil moisture conditions, but the ensuing soil temperature will be lower, thereby reducing the rate of water uptake by the kernel, speed of germination, and speed of emergence. Effects of temperature and soil moisture on kernel water uptake and germination have been well documented. Becker (1960) noted that, as temperature increases, the rate of water uptake increases. It is also G.P. Lafond, Indian Head Exp. Farm, Box 760, Indian Head, Saskatchewan, SOG 2KO; and D.B. Fowler, Crop Develop. Cntr., Univ. of Saskatchewan, Saskatoon, Saskatchewan, S7N OWO, Canada. Supported in part by a grant from the New Crop Develop. Fund of Agric. Canada. Contribution from the Crop Develop. Cntr., Univ. of Saskatchewan. Received 13 May 1988. *Corresponding author. Published in Agron. J. 81:447-450 (1989)

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AGRONOMY JOURNAL, VOL. 81, MAY-JUNE 1989

well known that, as soil water content decreases, the rate of water uptake by the wheat kernel decreases (Ward and Shaykewich, 1972; Ashraf and Abu-Shakra, 1978). Increasing temperatures tend to accelerate germination in wheat up to an optimum temperature of 25°C (Wilson and Hottes, 1927). Decreasing soil moisture content tends to delay germination (Pawloski and Shaykewich, 1972). No studies were found that measured the interaction of different soil temperatures and soil moisture levels on seed water uptake and germination. As well, no reports were found that measured moisture content of the kernel at germination when temperature and soil moisture were varied. The objectives of this study were; (i) to provide some insight into the producer's dilemma of whether to seed early into a dry seedbed or wait for some precipitation before seeding, and (ii) to determine the combined effects of soil temperature and moisture stress on kernel water uptake and germination.

MATERIALS AND METHODS Seed Material Kernels from the hard red winter wheat cultivar Norstar (Lethbridge, Canada) were used for the studies described in this paper. In spring wheat small kernels tend to germinate faster than large kernels (Lafond and Baker, 1986). Therefore, only kernels from a common source that fell through a 3.6- mm sieve, but remained on top of a sieve with hole diameters of 3.2 mm were used. After sizing, the kernels were culled further to ensure that only uniform undamaged kernels were used. Kernel Water Uptake and Germination Studies The effect of temperature and soil moisture potential on kernel water uptake and germination was investigated using six temperatures (5, 10, 15,20,25, and 30°C) and four moisture conditions (free distilled water and soil at water potentials of -0.2, - 1.0, and - 1.5 MPa). Surface soil of a Typic Haploboroll with a pH of 7.4 was collected and sieved using a 2-mm mesh screen. The soil was air dried and put in a plastic bag. The soil moisture content at -0.2, - 1.0, and - 1.5 MPa was determined using a pressure plate. Following equilibration to the desired moisture tensions, soil moisture content was determined gravimetrically. Each bag of air-dried soil was tested to determineactual moisture content. An amount of water required to bring airdried soil to the desired water potential was mixed slowly into large soil volumes using a fine spray while the soil tumbled in a cement mixer. The wetted soil was then sealed in a plastic bag and brought to the desired temperature. Samples from each soil moisture content were tested to make ,certain that the desired soil moisture content had been obtained. Kernels (80 g kg-I moisture) were equilibrated for 12 h at the desired temperature before being subjected to the experimental conditions. A layer of soil 20 mm thick was placed in a plastic con.tainer equipped with a tight-fitting lid. Twenty kernels were .uniformly distributed and covered with 15 mm of soil. For .the treatment involving distilled water, twenty kernels were :placed in a plastic petri dish on two Whatman no. 1 filter papers and 10 rL of distilled water was added. Containers were randomly placed in an incubator set at the desired temperature. Three sensors monitored temperature throughout the course of the experiment and at no 7

time did the measured temperature depart from the set point by more than 0.2"C. At each reading two petri plates and two containers of each soil moisture treatment wen: removed. Kernels were removed from soil, weighed, dried.,and weighed again. Kernels from disiilled water treatments were blotted before being weighed, then dried to constant weight and weighed again. Moisture contents were calculated as g of water kg-I of dry kernel. The: number of kernels germinated was also recorded at each reading. Germination was considered to have occurred when the radicle pierced the coleorhiza and was approximately 2 to 3 mm long. Thirteen samples were taken at each temperature considered. The time intervals between temperaiiures sampled were shorter at the start of each experiment because of the initial rapid rate of uptake and longer for the lower temperature treatments.

Data Analysis The kernel water uptake data were analyzed by fitting b (x1I2)where y is the kt:rnel curves of the form y = a water content (g water kg-1 dry kernel) at time x in days (Becker, 1960). The mean of two observations per reading for each combination of temperature and moisture potential was fitted to this equation. Median germination time was calculated by fitting the germination data to the logistic equation. Because of the slcewness in the data, it was necessary to use the logarithm of time when fitting the logistic equation. The equation used exp(-a - b(ln(t)))], where P = has the form P,= 1 / [ l n, / N is equal to the number of kernels germinated by time t (n,)divided by the final number of kernels germinated (N), t is the time in days from initiation of the experiment, and a and b are constants estimated for each germination-time curve. Median germination time was estimated as exp(-a 1 b). A detailed description of this method can be found in Lafond and Baker (1986). Median germination times were subjected to an analysis of variance. Since only one median germination time was estimated for each temperature-moisture potential combination, the main effects (temperature and moisture potential) were tested against the temperature X moisture potential interaction. The amount of water present in the seed at medium germination time was determined for each temperature-soil moisture potential combination. This was done by solving the kernel water uptake equations with median germination time. From these results, four curves were generated and fitted to a quadratic equation, one for each moisture potential using temperature as the independent variable and kernel water content at median germination time as the dependent variable.

+

+

RESULTS AND DISCUSSION The equation used to describe kernel water uptake provided a good fit to the observed data, i.e. rZ values ranged from 0.96 to 0.99 (P-0.01) : (Table 1). Temperature had a large effect on kernel water uptake. As temperature increased (5 to 3O"C), the rate of water uptake increased dramatically regardless of water potential. The rate of water uptake was similar at 10 i3nd 15°C with the kernels placed in soil at moisture potentials of - 1.0 and - l .5 MPa. At -0.2 MPa, kernel water uptake rate was slightly higher at 15"C than at 10°C. When kernels were placed on wet filter papers, the rate of water uptake was substantially faster at

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LAFOND & FOWLER: SOIL TEMPERATURE & MOISTURE EFFECTS ON WHEAT

Table 2. The effects of temperature and soil moisture content on the time it takes to reach 50% germination of winter wheat.

Table 1. Regression coefficients and coefficients of determination for kernel water uptake at six temperatures and four water potentials.? Temperature

Water potential

a

b

?

"C 5

MPa Distilled water -0.2 - 1.0 - 1.5 Distilled water -0.2 - 1.0 - 1.5 Distilled water -0.2 - 1.0 - 1.5 Distilled water -0.2 - 1.0 - 1.5 Distilled water -0.2 - 1.0 -1.5 Distilled water -0.2 - 1.0 - 1.5

0.144 0.172 0.165 0.168 0.05 1 0.108 0.109 0.118 0.096 0.146 0.143 0.147 0.094 0.102 0.109 0.108 0.067 0.125 0.122 0.122 0.086 0.128 0.137 0.141

0.259 0.193 0.190 0.184 0.376 0.263 0.257 0.243 0.433 0.292 0.279 0.269 0.48 1 0.399 0.378 0.370 0.631 0.432 0.420 0.410 0.664 0.506 0.463 0.458

.99 .96 .96 .96 .99 .99 .99 .99 .97 .96 .96 .97 .99 .99 .99 .99 .99 .97 .99 .99 .98 .99 .98 .99

10

15

20

25

30

t The fitted equation has the form y

Soil moisture content (MPa) Temperature

+

a b (XI'*) where y is the kernel water content (g water X lo-' kg-' dry kernel), x is the time in days, and a and b are the estimated coefficients. =

"C

Distilled water

7.3 3.9 2.0 1.5 0.9 1.o 2.8a

5 10

15 20 25 30 Meant

-0.2

- 1.O

- 1.5

Meant

days to 50% germination 6.1 3.5 1.7 1.4 0.8 0.9 2.5a

6.7 3.5 2.2 1.5 1.0 0.8 2.6a

6.9 3.9 2.3 1.4 1.0 0.9 2.7a

6.9a 3.7b 2.lc 1.5d 0.9e 0.9e

t Means followed by the same letter are not significantly different (LSD(0.05) =

0.3).

1000

+

m (I) o (2) A (3) A (4)

t

: l-

900

ax' Distilled watery = 0.683~' + y = 0.628~' + -0.2 MPa y = 0.284~' + -1.0 MPa -1.5 MPa y = 0.426~'

+

+ c + 987 + 782

bx -28.9~ -25.0~ -13.7~ + -12.8~

+

r' 0.92 0.78 711 0.90 734 0.97

0

A

Em

.- ,

s

? cn 800

'0

15°C than at 10°C. It would seem that, in the temperature range of 10 to 15"C, the ability of the soil at - 1.0 and - 1.5 MPa to supply water was more limiting than temperature. Shaykewich and Williams (197 1) have reported that the hydraulic properties of a soil affected the water uptake by rapeseed. In contrast, the rate of water uptake increased as temperature increased for the remaining temperatures considered in the present study. It is also important to note that the rate of water uptake was still increasing at 30°C. Soil water potential had an effect on rate of water uptake, regardless of temperature used. As moisture content decreased, the rate of water uptake decreased. The differences in rate of water uptake between kernels placed on wet filter papers and kernels placed in soil was much greater than between kernels placed in soil at -0.2 and - 1.5 MPa. Differences in rate of water uptake between kernels placed in soil at -0.2 MPa and at - 1.5 MPa were small considering the large differences in moisture potential encountered by the kernel. This supports the view of Phillips (1968) and Hadas (1970) that water difisivity in kernels is several orders of magnitude smaller than that in soils in the available water range. Speed of germination, as measured by median germination time, was strongly influenced by temperature, but not by the moisture potentials used in this study. As temperature increased the median germination time decreased (Table 2). Effects of changes in temperature from 10 to 5°C resulted in an increase in median germination time of 3.2 d or 0.64 d/"C. At the higher temperatures, going from 25 to 20°C resulted in an increase of 0.6 days or 0.12 d /"C. The relationship between median germination time and temperature was best described by expressing the reciprocal of median germination time as a linear function of

Y,

s -+ z E

0

E z

700

8 a W c

5 600

0

p W W

v)

0

500

0

20 TEMPERATURE ("C)

10

$0

Fig. 1. Water content of kernels placed in distilled water and in soil at three different water potentials at the time it takes to reach 50% germination as influenced by temperature ("C).

temperature (S > 0.95 using the means for the six temperatures in Table 2). The strong temperature effect on germination supports earlier observations with spring wheat (Lafond and Baker, 1986). Soil moisture content in the range studied here did not have a significant effect on median germination time (Table 2). Kernels placed on wet filter papers germinated at the same speed as kernels in soil at -0.2, - 1.O, or - 1.5 MPa. The average median germination time for the four moisture potentials was 2.65 d (range 2.5 to 2.8 d). The lack of a soil moisture effect on speed of germination for this winter wheat cultivar is differ-

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AGRONOMY JOURNAL, VOL. 81, MAY-JUNE 1989

ent from most observations reported for wheat. Spring wheat germination has been shown to be more sensitive to the changes in water potential considered in this study, whether the potentials were created using matric or osmotic potentials (Pawloski and Shaykewich, 1972; Blackshaw et al., 1981; Lafond and Baker, 1986). However, measurements of speed of emergence with this same winter wheat cultivar corroborate the present findings (unpublished results). Given the estimated median germination times and the fitted water uptake curves, it is possible to calculate the amount of water present in the kernel by solving the fitted equations with the estimates of median germination time. The results of these2 calculations were fitted to a quadratic function and r > 0.90 were obtained in all but one equation (Fig. 1). The resulting seed water content curves demonstrate that, as temperature increased, the amount of water present in the kernel at germination decreased up to 25°C and then increased again. The reason for the increase in kernel moisture content at median germination time with temperatures greater than 25°C is because 25°C represents the optimum temperature for the germination of wheat (Wilson and Hottes, 1927). In this case, the median germination times were the same for 25 and 30 °C. It was also shown earlier that the rate of water uptake was still increasing at 30°C regardless of the moisture potential used, which explains why the moisture content of the kernel at median germination time decreased until 25°C and then increased again with temperatures greater than 25°C. The moisture content at germination was always higher for kernels placed on wet filter papers compared with soil regardless of temperature. With kernels placed in soil, the moisture content at germination tended to decrease as the moisture potential of the soil decreased, but the relative differences were small. The amount of moisture present in the kernel at germination was therefore dependent on the temperature and soil water potential to which the kernel was exposed. Kernels were capable of germinating at moisture contents as low as 512 g water kg-1 kernel dry weight. Results from this study provide some insight into the winter wheat producer's dilemma of whether to direct seed into a dry seedbed at the optimum seeding date or wait for some precipitation before seeding. The moisture conditions that kernels are exposed to have been shown to affect rate of water uptake. However, wide differences in soil moisture were not reflected in

differences in speed of germination (Table 2). Kernels showed similar median germination times whether they were germinated in distilled water or in soil at —1.5 MPa. In contrast, soil temperatures on the Canadian Prairies drop rapidly in the fall (Fowler, 1982), and the large influence that temperature has on speed of germination (Table 2) implies that delaying the seeding operations can have a large influence on germination, seedling establishment, and seedling growth. Lindstrom et al. (1976) have also reported large temperature effects on winter wheat emergence. Dry weight accumulation by seedlings before freeze-up is very important for proper cold acclimation, and the accumulation of energy reserves necessary to tolerate sublethal low temperature injury and ensure winter survival under Northern Great Plains conditions (Fowler and Gusta, 1977). It then follows that, because the effects of temperature can be expected to be much larger than those of moisture, seeding should proceed at the optimum date, regardless of the dry seed-bed moisture conditions.