Plant and Soil 52, 553 559 (1979) Ms. 3947

W A T E R P O T E N T I A L G R A D I E N T S OF T A L L A N D S H O R T C U L T I V A R S OF W I N T E R W H E A T

by M. B. KIRKHAM and E. L. S M I T H

Department of Agronomy, Oklahoma State University, Stillwater, Oklahoma 74074, U.S.A.

KEY W O R D S

F ie ld G r a i n H e i g h t O s m o t i c p o t e n t i a l Silt l o a m T h e r m o c o u p l e p s y c h r o m e t e r T u r g o r p o t e n t i a l W a t e r p o t e n t i a l W i n t e r w h e a t Yie ld

S U M M A R Y

Water and osmotic potentials were measured with thermocouple psychrometers, weekly after heading, two times each day at pre-dawn and at noon, in flag leaves and grain o f tall and short cultivars of winter wheat grown in the field under rain-fed conditions.

Water was held with less tension in the grain than in the leaf for both tall and short cultivars. The tall cultivars had lower leaf water potentials, but higher grain water potentials, than the short cultivars. The grain osmotic potential was lower in the short cultivars compared to the tall ones. Grain yield of short cultivars (1810 kg/ha) was more than that of tall cultivars (1730 kg/ha). Apparently higher leaf water potentials o f short cultivars enabled more photosynthates to move into the grain.

I N T R O D U C T I O N

G r o w t h occurs only when the water potential in plants is sufficiently high v. M a n y measurements o f water potential o f wheat leaves have been obtained 3, 5, s, 9, lo, 11,12, ~ 3 However , apparent ly no one has reported water

potential o f wheat grain. Yet up to 50 percent o f grain growth in wheat occurs after leaves senesce 6. Aspina lP , Ward law t4, and Fischer s determined relative

water content o f wheat grain. Aspinall ~ and Ward law ~ 4 found that grain relative water content did no t fall below limiting values, even though plants were grown in droughted soil. Fischer s concluded that ear relative water content was ' o f

dubious value as an indicator o f plant water stress'. Water potentials and their componen t parts appeared to offer more meaningful indices.

M A T E R I A L S A N D M E T H O D S

Eight cultivars o f winter wheat (Triticum aestivum L. em. Thel!.), planted on 27 Oct. 1977 and harvested on 19 June 1978, were studied. Four were tall and four were semidwarf in stature. The tall

554 M. B. KIRKHAM AND E, L, SMITH

ones were ~F23-7Y (from Romania), 'Odesskaya 51' (from Odessa, U.S.S.R.), 'Sadovo 1' (from Bulgaria), 'Scout 66' (from Nebraska, U.S.A.). The short ones were 'David' (from Austria), 'Payne' (from Oklahoma, U.S.A.), 'Plainsman V' (from Kansas, U.S.A.), 'Sturdy' (from Texas, U.S.A.)

Plantings were part of a variety trial comprising a total of 30 cultivars. The experiment was laid out on a silt loam in two fully randomized blocks. Plots carried four rows, 3 m long, at a spacing of 30 cm. Seeding rates for cultivars from the U.S.A. and Europe were 76.3 and 84.1 kg/ha, respectively. European cultivars were planted at a heavier seeding rate because they tend to tiller less than regionally-adapted cultivars. An application of 224 kg/ha of an 18-46-0 fertilizer was given before planting and a nitrogen top dressing of 40 kg/ha was applied in February, 1978. Grain yields were determined by harvesting a 2.4m length of the two center rows from each plot.

Water and osmotic potentials of flag leaves and grain were determined, after heading, before dawn and at about noon, weekly from 6 to 27 May 1978; 191 to 213 days after planting. Pre-dawn values were considered to reflect maxima because plants had time to absorb water during the night. Noon values were considered to represent near minima because environmental stresses were usually greatest during midday.

Water and osmotic potentials were determined with a thermocouple psychrometer designed by Dalton and Rawlins z using the technique described by Ehlig4. In the procedure, after water potenttal is determined, the tissue is frozen, then thawed. Values referring to killed tissue are the sums of osmotic and matric potentials. The matric potential is assumed to be small and the potential of the frozen tissue is considered the osmotic potential. Turgor potential was calculated as the difference between water potential and osmotic potentials (~up = ~,w - ~,s). Plant height was measured both from the ground to the flag leaf and from the ground to the tip of the head, excluding awns. For potential measurements, single samples were drawn from the middle parts of flag leaves. Grain samples were taken from the tips of ears.

Table 1. Water (~uw), osmotic (~us), and turgor potentials (~up = ~uw -- ~'s) of leaves of four tall and four short winter wheat cnltivars*

Day and

time * *

Tall cultivars Short cultivars

~uw ~us ~up ~uw ~us ~up (bar)

191

p -- 14.5 -- 28.5 + 14.0 -- 12.8 -- 26.6 + 13.8 n -- 12.1 --30.2 + 18,1 -- 11.1 --32.5 + 214

198

p --26.1 -33 .2 + 7.1 --18.8 --33.6 +14_8 n --29.1 --44.4 + 15.3 --23.0 --41.0 + 18 0

205

p -- 14.0 --26.8 + 12.8 -- 14.0 --33.0 + 19.0 n --18.2 --26.9 + 8.7 --17.5 --24.1 + 66

213

p -- 13.4 --24.7 + 11.3 -- 13.0 --25.2 + 12.2 n --41.8 --53.9 + 12.1 --40.0 -- 52.5 + 12.5

* Average coefficients of variation were as follows: water and osmotic potentials, 0.09; turgor potential, 0,13 ** Number of days after planting and time of sampling; p = pre-drawn; n = noon.

W A T E R P O T E N T I A L OF W H E A T G R A I N 555

R E S U L T S

Measured to the flag leaf, tall and short cultivars grew to average heights of about 55 and 45 cm, respectively. Heights to the tips o f the heads of all cultivars were 16 to 17cm more.

At any date and time, both tall and short cultivars showed higher water potentials in the grain than in the flag leaf(Tables i and 2). Water potentials were usually lower at noon than in the morning for both leaf and grain. Grain water and osmotic potentials were lower in the short cultivars than in the tall cultivars (Table 2). Grain turgor potentials were usually higher in the short than in the tall cultivars. Flag leaves o f the tall cultivars had a lower water potential than the flag leaves of the short cultivars. Daily fluctuations in grain water potential were usually less than those in leaf water potentials.

Differences between grain and leaf water potential were less for the short than for the tall cultivars (Fig. l). In general, differences were similar at pre-dawn and at noon. An exception occurred on the last sampling day when leaves became

Table 2. Water (~uw), osmotic (~ts), and turgor potentials (q/p = ~ w - ~us) of grain of four tall and four short winter wheat cultivars*

Day and

time * *

Tall cultivars Short cultivars

~uw ~us ~up ~Uw ~ s ~up

(bar)

191

p -- 6.8 - - 1 4 . 8 + 8.0 - - 1 0 . 8 - - 2 1 . 3 + 1 0 . 5

n -- 5.0 - - 2 2 . 2 + 1 7 . 2 -- 9.2 - - 2 8 . 4 + 1 9 . 2

198

p -- 12.3 - - 2 7 . 0 + 14.7 -- 18.1 - - 3 3 . 5 + 15.4

n -- 14.8 - - 3 0 . 0 + 15.2 - - 2 2 . 7 - - 3 9 . 8 + 17.1

205

p -- 5.9 - - 2 0 . 9 + 1 5 . 0 - - 1 1 . 4 - - 2 7 . 4 + 1 6 . 0

n -- 5.5 -- 9.8 + 4.3 - - 1 6 . 0 - - 2 2 . 0 + 6.0

213

p -- 8.2 - - 1 3 . 3 + 5.1 -- 8.2 - - 1 6 . 3 + 8.1

n -- 17.1 - -35 .3 + 18.2 - - 2 0 . 2 - - 3 4 . 2 + 14.0

* Average coefficients of variation were as follows: water and osmotic potentials, 0.I0; turgor potential, 0.13 ** Number of days after planting and time of sarnpling; p = pre-dawn; n = noon.

556 M. B. KIRKHAM AND E. L. SMITH

z5 SA.PLiHGT,.~ CULTI;A.S I ' ' / ' uJ ~PRE-DAWN �9 TALL I

�9 SHORT I ~: ~ :20 _--NOON // T / ) T /

..... s ' / .

z 5

(.9 0 I J . ~ I

! 90 200 210 2 20 DAYS AFTER PLANTING

Fig. 1. Difference in wa te r po ten t i a l between gra in and leaves, sampled before d a w n and at noon, o f four ta l l and four shor t cu l t ivars of win ter wheat . Vert ical l ines indica te the average of the s t andard er ror o f the gra in po ten t i a l + leaf potent ia l . On ly h a l f the s t andard -e r ro r l ine is d r awn to avoid

c lu t te r ing the figure.

Tab le 3. Yie ld of four ta l l and four shor t win ter whea t cu l t ivars

Ta l l cu l t ivars Yie ld Shor t cul t ivars Yie ld (kg/ha) (kg/ha)

F23-71 1760 D a v i d 1800 Odesskaya 51 1890 Payne 1940 Sadovo 1 1670 P l a i n s m a n V 1810 Scout 66 1610 S turdy 1700 Average 1730 1810 L.S.D.* (0.5) 150 90 C.V.** 0.18 0.10

* Least Significant Difference. The L.S,D. for tall cultivars was higher than that for short cultivars because the short cultivars had stiffer straw and were more uniform in size. This resulted in less fluctuation between yields from replicated plots of short cultivars than tall cultivars. The L.S.D. for intercomparison of tall and short means was 130 kg/ha at the 5~ level and 78 kg/ha at the 20% level. ** Coefficient of variation.

WATER POTENTIAL OF WHEAT GRAIN 557

251 ' , , I I ;-"~'~ I SAMPLING TIME CULTIVARS I I I

zlJJ

Z

(.9 O / I I ~ I I I I I 190 200 2 I0 220

DAYS AFTER PLANTING Fig. 2. Difference in osmotic potential between grain and leaves, sampled before dawn and at noon, of four tall and four short cultivars of winter wheat. See legend of Fig. 1 for explanation of

vertical lines.

severely stressed by noon due to high temperatures and strong winds. Grain potentials did not fall as much as leaf potentials.

In all ears, osmotic potentials of grain were higher than of leaves. Differences between grain and leaf osmotic potentials were larger for tall cultivars than for short cultivars (Fig. 2). Except for day 191, differences in osmotic potential were larger at noon than at pre-dawn for both the tall and short cultivars.

Most of the short cultivars yielded more than the tall ones (Table 3) with two exceptions. The exceptionally high yield of the tall cultivar Odesskaya 51 might be explained by its mildew resistance. Conversely, the short cultivar Sturdy may have yielded less as a result of its low mildew resistance.

DISCUSSION

The tall cultivars had a higher water potential, but lower turgor potential, in the grain compared to short cultivars. The grain of both tall and short cultivars had a higher water potential than that of the leaf, even though water had to travel

558 M. B. KIRKHAM AND E. L. SMITH

farther to get to the grain than to the leaf. Usually, the more distant a plant part is f rom the ground, the lower the water potential is. There is no direct xylem connection with the grain 1 s, 16. Water must diffuse into the grain. Once it moves into the grain, it is more likely to stay in the grain than in the flag leaf which loses water through stomata. Apparently, because the water potential of the flag leaves of the short cultivars was higher than that of the flag leaves of the tall cultivars, photosynthesis in the leaves of the short cultivars was less inhibited. Thus, short cultivars could produce more photosynthates to be grain-incorpo- rated than tall cultivars. The higher solute concentration of the grains of the short cultivars is reflected in their lower osmotic potential.

A C K N O W L E D G E M E N T

We thank George H. Morgan for help at planting and at harvest.

Received 19 December 1978

R E F E R E N C E S

1 Aspinall, D. 1965 The effects of soil moisture stress on the growth of barley. II. Grain growth. Aust. J. Agric. Res. 16, 265-275.

2 Dalton, F. N. and Rawlins, S.L. 1968 Design criteria for Peltier effect thermocouple psychro- meters. Soil Sci. 10g, 1~17.

3 Denmead, O. T. and Millar, B. D. 1976 Water transport in wheat plants in the field. Agron. J. 68, 297-303.

4 Ehlig, C. F. 1961 Measurement of energy status of water in plants with a thermocouple psychrometer. Plant Physiol. 37, 288-290.

5 Fischer, R. A. 1973 The effect of water stress at various stages of development on yield processes in wheat, p. 233 241. In R. O. Slatyer (ed.) Plant response to climatic factors. U.N. Educ. Sci. Cult. Organ., Paris.

6 Fischer, R. A. and Kohn, G. D. 1966 The relationship of grain yield to vegetative growth and post-flowering leaf area in the wheat crop under conditions of limited soil moisture. Aust. J. Agric. Res. 17, 281-295.

7 Gardner, W. R. 1973 Internal water status and plant response in relation to the external water r~gime, p. 221-225. In R. O. Slatyer (ed.) Plant response to climatic factors. U.N. Educ. Sci. Cult. Organ., Paris.

8 Kaul, R. and Crowle, W. L. 1971 Relations between water status, leaf temperature, s tomatal aperture, and productivity of some wheat varieties. Z. Pflanzenzuecht. 65, 233-243.

9 Kirkham, M. B. and Smith, E. L. 1978 Water relations of tall and short cultivars of winter wheat. Crop Sci. 18, 227-230.

10 Millar, B. D. and Denmead, O.T. 1976 Water relations of wheat leaves in the field. Agron. J. 68, 303 307.

11 Sands, R. and Correll, R. L. 1976 Water potential and leaf elongation in radiata pine and wheat. Physiol. Plant. 37, 293 297.

12 Simmelsgaard, S. E. 1976 Adaptat ion to water stress in wheat. Physiol. Plant. 37, 167-174. 13 Skidmore, E. L., Hagen, L. J., Naylor, D. G. and Teare, I. D. 1974 Winter wheat response to

barrier-induced microclimate. Agron. J. 66, 501-505.

WATER POTENTIAL OF WHEAT GRAIN 559

14 Wardlaw, I. F. 1971 The early stages of grain development in wheat: response to water stress in a single variety. Aust. J. Biol. Sci. 24, 1047-1055.

15 Zee, S. Y. and O'Brien, T. P. 1970 A special type of trachery element associated with 'xylem discontinuity' in the floral axis of wheat. Aust. J. Biol. Sci. 23, 783-791.

16 Zee, S.-Y. and O'Brien, T. P. 1971 Vascular transfer cells in the wheat spikelet. Aust. J. Biol. Sci. 24, 35-49.