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HAYING-OFF IN WHEAT: ENDURING MYTH OR CURRENT PROBLEM?

A.F. van Herwaarden1,2

1CSIRO Division of Plant Industry, GPO Box 1600, Canberra, ACT 2601
2
Research School of Biological Sciences, ANU, PO Box 475, Canberra, ACT 2601

Summary. Nitrogen (N) fertiliser applied to wheat growing at three sites in southern NSW produced contrasting yield responses, ranging from a large positive response to a large negative response. In all cases additional N led to increased tillering and greater biomass at anthesis. However, at each site, water soluble carbohydrate reserves (WSC) at anthesis were negatively correlated with anthesis biomass. This finding contrasts with previous studies which estimated greater remobilisable reserves with greater biomass. In the absence of post-anthesis water stress, grain yield increased with additional N while remobilisation of WSC and protein did not change. When there was severe post-anthesis water stress, additional N led to reduced yields accompanied by reduced remobilisation of WSC and protein. A model for haying-off in wheat is presented which takes account of WSC, N status and post-anthesis water stress.

INTRODUCTION

Most generally, haying-off is regarded as the phenomenon in which cereal crops fail to yield grain in accordance with their vegetative potential (16). Early research attributed the negative yield response to exhaustion of soil water by the vigorous vegetative growth stimulated by high soil nitrogen levels (15). Haying-off is also induced by the use of nitrogen (N) fertiliser (2, 11, 15). The belief that water stress is an essential factor contributing to haying-off has persisted (14). However, studies in the field (15) and controlled environments (9) showed reductions in kernel weight with the addition of N fertiliser in the absence of grain filling water stress. Clearly factors in addition to water stress are also involved in haying-off (8).

The aim of the field experiments presented in this paper was to determine the physiological responses to increasing soil mineral N which contribute to, or predispose a wheat crop to, haying-off. The study is based on field experiments at sites with contrasting mean temperatures, evaporative demand and water-stress conditions during grain filling.

MATERIALS AND METHODS

Experiments were sown at three sites in late May or June of 1991 at CSIRO Experiment Station, Ginninderra, ACT, at Harmon’s Tank, Pucawan, NSW and at Charles Sturt University, Wagga Wagga, NSW. The experimental sites followed a breakcrop (3) to minimise the detrimental effects of soil-borne disease on N response (11). All experiments were 4-replicate, randomised complete block designs. Nitrogen fertiliser as urea (46% N) was topdressed at sowing or late tillering. Time of N application had minimal impact on yield and N uptake, so data for the two application times were combined for each level of N.

At anthesis, quadrats were harvested at the soil surface to estimate total above-ground dry matter production (biomass). Samples were counted for fertile shoots, oven dried at 70C and ground in a Wiley mill to pass a 1-mm sieve. These subsamples were used to determine WSC levels (7). N concentration was determined using a semi-micro-Kjeldahl method. At physiological maturity ten random grab samples were taken from each plot to estimate harvest index, and quadrats, as at anthesis, were harvested to calculate biomass at maturity. Grain yield was calculated from a machine harvest. Harvest index samples were separated into spikes and straw, oven dried at 70C and weighed. Spikes were threshed, and the glumes, awns and rachis put with the straw, the grain redried at 70C and weighed. Kernel weight was calculated from the mean weight of 3 lots of 100 kernels. Grain was ground using a Cyclotec mill with an 0.5 mm sieve and the non-grain dry matter ground in a Wiley mill to pass a 1-mm sieve. WSC remaining in the non-grain biomass were also estimated. N concentration in the grain and non-grain biomass was analysed by near infrared reflectance spectroscopy using locally determined calibrations.

RESULTS AND DISCUSSION

Maximum and minimum temperatures were comparable between Wagga Wagga and Pucawan while Ginninderra was the coldest. April to December rainfall was below the mean at Pucawan and Wagga Wagga. The proportion of sowing to maturity rain which fell after anthesis was 23% at Ginninderra, 13% at Pucawan and only 7% at Wagga Wagga. In the present study, greater biomass production to anthesis resulted in vastly different grain yield responses at the three sites (Table 1).

Table 1. Effect of N fertiliser on the growth, water soluble carbohydrate (WSC) and grain yield of wheat at sites in southern NSW and ACT.

 

Anthesis

 

Maturity

N rate

Biomass

Spike

WSC

 

Biomass

WSC

Grain

Kernel

   

density

       

yield

weight

(kgN/ha)

(g/m2)

(g/m2)

(g/m2)

 

(g/m2)

(g/m2)

(g/m2)

(mg)

Ginninderra (15.6.91, Janz)a

0

989

515

244

 

1366

31

607

40.1

40

1089

603

222

 

1578

25

723

39.1

80

1167

627

227

 

1615

25

729

37.7

120

1134

606

210

 

1619

19

765

36.8

160

1166

633

204

 

1683

15

781

37.4

200

1197

646

181

 

1670

15

777

36.3

240

1190

644

172

 

1676

13

798

37.1

(l.s.d., P = 0.05)

97

70

20

 

131

9

78

1.6

Pucawan (19.6.91, Janz)

0

653

372

122

 

995

34

420

34.2

40

-

-

-

 

1052

-

446

31.4

80

787

459

96

 

1074

17

458

29.3

120

836

485

98

 

1097

16

463

27.4

160

-

-

-

 

1084

-

444

25.9

200

843

525

86

 

1069

15

432

25.4

(l.s.d., P = 0.05)

58

43

13

 

ns

4

22

1.9

Wagga Wagga (25.5.91, Matong)

0

984

377

214

 

1086

20

374

28.6

40

-

-

-

 

1158

-

366

24.4

80

1075

421

167

 

1148

20

345

22.2

120

1092

416

151

 

1163

18

328

20.9

160

-

-

-

 

1132

-

283

17.6

200

1097

420

123

 

1106

18

284

17.6

(l.s.d., P = 0.05)

ns

31

21

 

ns

ns

30

1.9

a Sowing date and variety.

At Ginninderra the yield response to N was positive with decreasing returns of yield to applied N. At Pucawan, the yield response was positive to low rates of N but negative at high rates. At Wagga Wagga addition of N led to reduced yield. At all sites grain-yield responses to N fertiliser were associated with increased spike density and more kernels per square metre. However, kernel weight fell in response to N at all sites, varying from an 8% decrease at Ginninderra to 39% at Wagga Wagga. Harvest index increased by 9% at Ginninderra, but decreased by 5% at Pucawan and by 25% at Wagga Wagga (data not shown).

Although additional N promoted leaf area development (data not shown) and hence the photosynthetic potential, the amount of WSC in the crop at anthesis fell with increasing N (Table 1). This reduction in WSC with increased biomass is most likely due to assimilates being used to a greater extent for structural materials (13) and increased respiration rates due to higher N concentration (1). Mean WSC reserves at anthesis over the three sites decreased from 13.5 mgWSC/kernel for the control treatment to 6.5 mgWSC/kernel for the highest rate of N.

Retranslocation from the non-grain biomass to grain was estimated by two methods. The simpler, termed apparent retranslocation, assumes weight loss between anthesis and maturity equates with retranslocation (10). This method takes no account of leaf fall (4) or saprophytic decay of lower leaves (6) during grainfilling, and is therefore prone to overestimating the pre-anthesis contribution to grain yield. However, in the absence of water stress, continued growth of the stem after anthesis (7) and cell-wall thickening and lignification (12) can lead to an increase in structural biomass thereby reducing apparent retranslocation. A more complete estimate, termed estimated retranslocation, is the decrease in WSC and protein in the non-grain biomass between anthesis and maturity. This calculation assumes that once laid down WSC is not turned over (18) and that post-anthesis respiration is supplied by current rather than stored assimilates (5). Both estimates of retranslocation are presented in Table 2.

Table 2. Comparison between apparent and estimated retranslocation (g/m2) to the grain from non-grain biomass.

Rate of N

 

Apparent retranslocation

 

Estimated retranslocation

(kg N/ha)

     

WSC

Protein

Total

   

Ginninderra

0

 

230

 

214

53

267

240

 

312

 

159

113

272

   

Pucawan

0

 

78

 

88

28

116

200

 

206

 

71

68

139

   

Wagga Wagga

0

 

272

 

194

36

230

200

 

275

 

105

55

160

Apparent retranslocation increased in response to N with moderate or no post-anthesis water stress, while under severe water stress at Wagga Wagga it remained constant. In contrast, estimated retranslocation remained relatively constant at Ginninderra and Pucawan in response to N but decreased in association with haying-off at Wagga Wagga.

CONCLUSIONS

The results of these experiments lead to an improved model of the development of haying-off. Firstly, high N status leads to decreased WSC reserves at anthesis. Provided there is little or no water stress, a high N crop can then fill grain from current photosynthesis and call on WSC reserves during the periods of peak assimilate demand. However, in the event of water stress, reduced current photosynthesis and the lack of WSC reserves leads to haying-off. A low N crop does not face the same water stress due to lower anthesis biomass and is able to achieve a higher yield through greater current photosynthesis and its greater reserves of pre-anthesis WSC.

This model also accounts for high levels of WSC in low plant density canopies in the accompanying poster (17). It proposes that the worse reputation for haying-off by tall varieties is explained by low pre-anthesis reserves of WSC due to the greater assimilate demand by larger stems. It also leads to speculation that while haying-off is a problem in the closed-canopy crops of the south-eastern wheatbelt, it is not recognised as a problem in the north-eastern or Western wheatbelt where the open-canopy crops are likely to contain higher levels of pre-anthesis stored WSC.

In the south-eastern wheatbelt, haying-off is likely to be a continuing problem when crops of a high N status reach anthesis with low levels of soluble carbohydrate and subsequently encounter water stress. The problem could be reduced by breeding low-tillering cultivars which would not produce excessively large biomass with low reserves of WSC when grown at high N status. A fine balance would need to be maintained so that yield potential is not sacrificed when water supply is adequate.

ACKNOWLEDGMENTS

This project was largely supported by the Grains Research and Development Corporation. The technical assistance of Geoff Howe and Eric Koetz is gratefully acknowledged.

REFERENCES

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