Effect of shading and nitrogen application on yield, grain size distribution and concentrations of nitrogen and water soluble carbohydrates in malting spring barley (Hordeum vulgare L.)

European Journal of Agronomy ELSEVIER

European Journal of Agronomy 6 (1997) 275-293

Effect of shading and nitrogen application on yield, grain size distribution and concentrations of nitrogen and water soluble carbohydrates in malting spring barley (Hordeum vulgare L.) C. Grashoff

a,*, L.F. d’Antuono

a DLO-Research

Institute for Agrobiology and Soil Fertility (AB-DLO), b Universitri degli Studi di Bologna, Dipartimento di Agronomia, Accepted

25 November

b

P. 0. Box 14, 6700 AA Wageningen, Netherlands Via Filippo Re 6-8, 40126 Bologna, Italy 1996

Abstract Experiments to investigate the effects of periods of low radiation, combined with different rates of nitrogen fertilization, on growth, grain yield, mean grain weight, grain size distribution and concentrations of nitrogen and water soluble carbohydrates (WSC) were carried out in 1991 and 1993. The low radiation (60% of ambient radiation) was achieved by placing shading nets in fields of barley (cv. Prisma). There was an unshaded control (Sl), plus three shading period treatments during the main phenological phases, with three rates of nitrogen fertilization (unfertilized control, 90 kg ha-’ minus soil nitrogen and this rate plus 60 kg ha-’ at the flag leaf stage). It was found that total grain yield was 5% lower after shading during tillering (S2), 35% lower after shading during stem elongation (S3) and 45% lower after shading during grain filling (S4). Treatments S2 and S3 considerably reduced the concentration of WSC in plant organs and increased the nitrogen concentration and, in general, the concentrations returned to the values of the control after the shading period. The S3 treatment markedly reduced the number of grains m-‘, mainly because there were 35% fewer grains per spike. This treatment resulted in a mean grain weight 14% above that of the control, but only in 1991. Weather was probably responsible for this disparity: in 1991, the spring was cold and wet and the summer was warm and dry, but in 1993 the spring was warm and dry and the summer was cool and wet. The S4 treatment reduced mean grain weight by 40% in 1991 and by 25% in 1993 and shifted the median of the size distribution towards smaller grains in both years. In S4, the N concentration in the grains was markedly increased, resulting in unacceptably high protein concentrations (14-21%) for malting quality. Higher rates of nitrogen fertilization increased leaf area index (LAI), total dry matter production and grain N concentration. Nitrogen had a positive effect in establishing yield potential because it increased grain number per unit area; however, it did not improve assimilate supply during grain filling, and hence grain number and mean grain weight were negatively correlated. Only in 1993 did nitrogen increase grain yield. It is concluded that shading during the growing period and high rates of nitrogen fertilization adversely affect the quality (in terms of grain size, size distribution and grain nitrogen concentration) of malting barley. 0 1997 Elsevier Science B.V. Keywords:

Grain weight; Malting

barley; Nitrogen;

Quality;

* Corresponding author. 1161-0301/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PI1 Sll61-0301(97)00001-4

Radiation;

Shading;

Size distribution

216

C. Grtrshqff; L. F. d’Amuno

111 Europrur~ Jourrrul of’dgronomy

1. Introduction Barley growers and breeders, extension services and the malting and brewing industry are showing increasing interest in the quantitative effects of different weather and soil conditions on the quality of malting barley, because this information is useful when developing appropriate crop management and breeding strategies and when assessing quality and yield in different regions. Mean grain size, size distribution and grain protein concentration are among the most important quality factors of malting barley. It is known that grains of different size behave differently during the malting and brewing processes. Consequently, heterogeneous grain lots must be graded before processing (Burger and Laberge, 1985). Moreover, large grains generally have a higher starch concentration, which increases the efficiency of the extracting process. However, starch concentration per se is not important. With high protein concentrations in the grains, a matrix of storage proteins may prevent the action of diastatic enzymes on the starch granules, thus reducing extract efficiency. The malting industry requires wide grains (with a minimum width of 2.5 mm) with a protein content of 9911.5% (corresponding to a grain nitrogen concentration of 14-l 8 g kg- ‘). The size and protein concentration of the individual cereal grains depend on the relationships between (1) the total sink potential (grain number), (2) the carbohydrate source available for grain filling from current assimilate production and translocation, and (3) the nitrogen accumulation and re-distribution. In cereals, total sink potential depends almost entirely on seed number per unit area (Evans and Wardlaw, 1976) and its components ear number per unit area and grain number per ear. Van Keulen (1982) stated that the formation of a viable plant organ requires a minimum flow of carbohydrates; consequently, each factor that reduces assimilation before heading is likely to affect final grain number and therefore sink potential. Reduced assimilation after heading can adversely affect individual grain size (Fischer and HilleRisLambers, 1978) and grain size distribution since the growth of grains in less favourable positions in the spike is diminished by increased competition among the grains (Bremner.

6 ( 1997) 275-1793

1972; Scott et al., 1983). Finally, grain protein concentration depends on the balance between carbohydrate and nitrogen accumulation in the grains, processes which are at least partly independent (Jenner et al., 1991; Ma et al.. 1995; Dreccer et al., 1996). Because assimilate supply crucially affects yield and quality and depends on radiation level, low radiation levels (hereafter referred to as ‘shading’) have often been used when investigating the implications of a reduced source and/or sink potential in cereals. In general, the effects have confirmed expectations of the impact of reducing photosynthesis. However, the concentration of water soluble carbohydrates (WSC) in the plant, which is an important intermediate factor for understanding the reducing effect of shading on the formation of viable plant organs and/or on organ filling, has been reported in a few studies. Furthermore, the results of shading on final yield have been variable; in some cases the reduction of the number of grains by early shading has been found to be more relevant ( Willey and Holliday, 1971a; Fischer, 1975). In other cases, post anthesis shading has been found to be instrumental in reducing yield (Pendelton and Weibel, 1965; Willey and Holliday, 1971b). This inconsistency may be due to differences in compensation capacity among different yield components determined over quite a long time span, from tillering to kernel filling. Besides radiation, nitrogen supply is certainly one of the major factors that affects assimilation and quality; its effects on barley organogenesis have been shown experimentally (Cannell, 1969; Fletcher and Dale, 1974). It is clear that, although many different processes are involved, nitrogen primarily affects plant growth by enhancing photosynthesis (Van Keulen et al., 1989). Therefore, low nitrogen might strengthen the negative effect of high radiation on organogenesis and grain filling. This paper shows the integrated effects of short periods of shading during different phases of crop development and nitrogen dressings on the dynamics of WSC, organ formation, grain filling, size distribution and grain protein concentration. The following hypotheses were tested: ( 1) That shading will reduce photosynthesis and consequently WSC concentration. This will result

C. Grashofl L. F. d’dntuono / European Journal of Agronomy 6 (1997) 275-293

in a decreased formation of those specific organs which are normally formed during the shading period. However, the final effect on yield and quality will depend on the phenological stage during which the shading occurs and on the duration of the shading period. Situations may occur in which the final effect of a short period of shading will reduce grain number but may enhance mean grain weight and size distribution and thus quality. (2) That the crop may have compensation mechanisms such as adaptation of LAI, or increased translocation of carbohydrates from vegetative parts to the seeds, which counteract the adverse effects of shading on yield and quality. If this compensation is insufficient, shading during grain filling will reduce the WSC supply to the grains and will indirectly increase grain nitrogen concentration, resulting in an adverse effect on malting quality. (3) That interactions between shading and nitrogen fertilization may occur, because high rates of nitrogen fertilization also increase the formation of tillers rne2, grain number and, probably, grain filling and grain nitrogen concentration.

2. Materials and methods 2.1. Design and treatments The malting barley cultivar Prisma was grown under different shading treatments and with different rates of nitrogen fertilization in two field trials on clay soil in Wageningen, Netherlands, in 1991 and 1993. The trials were conducted on the experimental farm ‘De Bouwing’ in Randwijk on an alluvial clay soil containing 60% silt and 3% organic matter. The experiment was of a randomized block split-plot design with four replications, with the shading treatments in main blocks and nitrogen rates in sub-blocks (Table 1). Four shading treatments at different stages of crop development were tested: a no shading control (Sl ); shading from onset of tillering to beginning of culm elongation (S2); shading from beginning of culm elongation to heading (S3); and shading from heading to the end of grain filling (S4).

211

Shading nets of woven plastic were placed in the field 1.5 m above the soil and reduced the level of radiation by about 60%. Nitrogen treatments were: no nitrogen fertilizer (Nl); 90 kg ha-’ N minus soil mineral nitrogen content at sowing (N2); N2 plus a top dressing of 60 kg ha-’ N at flag leaf stage (N3). The soil mineral nitrogen contents at sowing in the two years are shown in Table 1, together with details on crop management and development. Phenological development was recorded using the Z.C.K. decimal code (Zadoks et al., 1974) and the Kirby apex development scale (Kirby and Appleyard, 1982). In 1991, the intermediate treatment N2 was not analyzed, but the combine harvested grain was used for a micro malting and brewing procedure (Duynhouwer et al., 1993). 2.2. Whole-crop harvests Crop growth was analyzed by periodic harvests on six occasions in 1991 and eight in 1993 (Table 1). The crop was cut at soil level, except on the first three harvests, when the whole plants were harvested. On the first two harvests, the number of plants rnw2 was counted. In all subsequent harvests the number of tillers rnp2 was determined. At each harvest after anthesis, the number of spikes rns2, the number of grains per spike and the mean grain weight were determined. At final harvest, grain size distribution was determined by sieving. Sieves with oblong holes of increasing width were used which divided the grains in the size classes (in mm): ~2, 2.0-2.2, 2.2-2.5, 2.5-2.8, 2.8-3.0, 3.0-3.2, 3.2-3.5, >3.5. The weight of grains in each of these size fractions relative to the total was measured. The values were used as ‘weight frequency’ in size distribution histograms. Although mean grain weight varies with size, the histograms with weight frequency did not differ appreciably from those based on number frequency. Therefore, only weight frequency is used in the presentation of the results. 2.3. Calculation of translocation and assimilation contributions to grain growth The contributions of assimilation and translocation of carbohydrates to the biomass of different

C. Gashof;

278 Table I General overview Experimental

of the materials

and methods

details

day)

Growth regulator (Julian day) Active components Herbicide (Julian day) Active components Fungicides, insecticides (Julian Active components

i

Europeun Joumul oj’Agronom_v 6 (1997) 275-293

of the two field experiments

Year

Sowing date (Julian day) Soil Nmin O-60 cm (kg ha-‘) Seed rate (# m-‘) Gross and net plot size Row spacing (cm) Emergence (Julian day) Periodic harvests (Julian

L. F. d’dntuono

day)

1991

1993

16 March (75) 70 300 4 tn’ and 0.96 m’ 12 92 92, 106, 141, 177. 224, 234b 133 Tridemorph, chlormequat/ethephon 134 Bentazone, mecoprop-P. Huroxypyr 175. 191 Propiconazole, pirimicarb, triadimenol

15 March (74) 40 300 4 m’ and 0.96 mZ 12 98.-104” 104, 112. 117. 131. 152, 179, 193, 215

126, 158 (see 1991)

“Emergence was slightly irregular. bS3 secondary tillers were still green on day 224

organs were derived from analyzing growth according to Biscoe and Gallagher ( 1977). The

2.4. Chemical analyses of nitrogen and WSC

weight gain of each organ between two consecutive samplings was considered as potentially determined by two parts, derived respectively from accumulation of current assimilates and translocation from other organs. The calculations assumed that the weight gain of assimilating, non-storage organs is primarily attributable to current assimilates and, conversely, that the sink strength of these organs for translocation was weaker. This resulted in the following ranking of priorities for translocation: leaves
Nitrogen concentration in samples of plant organs of each periodic harvest was measured by the Dumas method on a macro-N analyzer (Foss Heraeus elementar analysesystemen, Hanau, Germany). The total amount of water soluble carbohydrates ( WSC ), consisting of mono- and disaccharides, was extracted by adding demineralized water to a ground sample. Sugars were measured on a Bran and Luebbe Autoanalyser II (Maarssen, The Netherlands, Method NL213-89FT). On this autoanalyser, sucrose and other disaccharides are reduced to monosaccharides. The total reducing sugars are then measured by reaction with ferricyanide which is reduced to colorless ferrocyanide. The reduction in absorbence at 420 nm is used to calculate the amount of sugars as glucose equivalents (Bouwmeester and Kuijpers, 1993).

3. Results 3.1. Phenology and length of shading periods as affected by weather 1991 and 1993 were very contrasting years with respect to temperature, radiation and rainfall

279

C. Grashoft; L.F. d’dntuono 1 European Journal of Agronomy 6 (1997) 275-293

(Table 2). The higher temperatures in the spring of 1993 shortened the tillering phase by 17 days and culm elongation by 9 days; as a consequence the duration of shading in these two phases was also shorter in 1993 than in 1991. In contrast, the cool and wet weather in the summer of 1993 resulted in a l&day longer period of grain filling than in 1991, with a correspondingly longer duration of the latest shading (Table 3). The effects of higher rates of nitrogen fertilization on the duration of phenological stages were negligible. As the air ventilation under the shading nets was very good, the nets had hardly any effect on temperature and/or humidity; the effect of these factors on phenology was also negligible.

Table 2 Temperature, radiation and rainfall in Wageningen (Netherlands) (30-year averages) (Normal) and deviations from normal in 1991 and 1993 in the months May-August

Temperature (“C)

Radiation (kJ cm-*)

Rain (mm)

Month

Normal

Deviation 1991

Deviation 1993

May June July August May

12.2 15.1 16.8 16.7 50.5

-2.4 -2.4 f2.2 +1.1 -1.8

f2.2 f0.7 -0.7 - 1.7 + 5.4

June July August May June July August

51.0 49.2 43.6 55.8 69.0 77.5 71.3

- 12.2 +8.3 +8.1 - 17.8 +45.0 -31.0 -64.8

+I.6 -2.7 -2.9 -7.3 -25.0 + 56.0 -35.8

3.2. Analysis of growth and yield Significant (P~0.05) main effects of shading and nitrogen were found in many growth and yield characteristics but there were no statistically significant shading x nitrogen interactions, except for the number of late (secondary) spikes (Table 4). Therefore, in the following sections the effects of shading on growth and yield are presented separately from the effects of nitrogen. Biomass and grain yield. Shading during tillering (S2) had little effect on total biomass production, whereas shading during culm elongation (S3) reduced final biomass by about 30% in 1991 and 25% in 1993; shading during grain filling (S4) reduced total biomass by about 35% in both years (Table 4). In both years, final grain yield was reduced in S2 (by 5%), S3 (by 35%) and S4 (by 45%). Higher rates of nitrogen fertilization slightly increased total biomass in 1993 but had no effect in 1991. In 1993, higher nitrogen also increased grain yield, by 20% in N2 and by 30% in N3 (Table 4), but in 1991 the grain yield from N3 was 10% lower than from Nl. Leaf area dynamics. Shading, especially the S2 and S3 treatments, resulted in thinner Leaves (with a greater SLA; specific leaf area) so that the LA1 of these shaded treatments was equal to or even greater than that of the unshaded control Sl (Fig. IFig. 2), despite less leaf biomass. Shading during grain filling did not affect leaf area dynamics in 1991, but slightly increased SLA and LA1 later in the season of 1993. In all cases, a recovery of SLA to values close to the control was observed after the end of the shading period.

Table 3 Start and end (Julian day)” of the shading periods in 1991 and 1993 Treatment code

Sl s2 s3 s4

Description of shading period

Control From onset of tillering to onset of culm elongation From start of culm elongation to start of grain filling From start of grain filling to end of grain filling

1991

1993

Start

End

Length

Start

End

Length

101 141 178

139 115 217

38 34 39

112 133 159

133 158 214

21 25 55

“It took l-3 days to move the shading nets from one treatment to the other.

Sl s2 s3 S4 Sign. shading LSDS% NI N3b Sign. nitrogen LSDS% S x N int Sl s2 s3 s4 Sign. shading LSDS% Nl N2 N3 Sign. nitrogen LSDS% S x N int.

1991

Grains

11 242 11381 8447 7428 ** 1010 8031 9977 10 866 ** 614 ns

11s

12760 12 346 8841 8486 ** 1396 I0784 10432 ns 658

-

-

ns 881 ns 4592 4378 2550 2434 **

(*) 2.8 II, 60.8 58.3 46.8 52.0 **

(*)

375

1.1

379 ns *

1001 2802 3543 4121 **

3.3 56.0 54.0 53.4 **

758 4552 5483 5908 **

494 ns 6833 6636 3944 3845 **

749 3434 3237

3.6 50.5 47.8

684 5535 5030 *

4671 3955 2965 1751 **

Assim. 1 (kg ha- ‘)

52.0 52.3 50.7 41.5 **

HI (%)

6612 6411 4507 3540 **

(kg ]la I) (kg ha i)

Biomass

(*)

(*) 7.7

(*I 8.5

IX?

"S

12.4 39.4 36.3 30.7

715 1771 1968 1792 ns 276 ns

66.6 66.1 63.5 64.0 ns 13.4 61.2 64.5 69.4

15.0 41.2 42.1 ns 10.8 ns 33.4 34.3 37.3 36.7

2241 2277 1424 1433 *

15.0 58.8 57.9 ns 10.8

35.2 40.0 37.0 54.6 *

(%o)

(S) 64.8 60.0 63.0 45.4 *

Rel. transl.

Rel. ass.

ns

‘1”

11s

(*) 867 2322 2143 ns 629

2473 2626 1728 2103

Transl. (kg ha

65 904 947 ns SY nb 720 674 628 570 *

2702 17 082 18839 **

58 ns

854 ns

85 533 648 763 **

1204 9817 12 223 14 252 **

1126 ns 13 827 14 285 9869 10 410 **

891 982 1034 793 **

21091 20018 12 636 18 372 **

46 **

111 107 117 306 **

202 83 282 141 *

83 *

2.2 20.8 21.6 21.1 ns 1.1 ns

(*)

1.2

2.8 45.7 44.0 40.7 **

**

0.6

0.9 18.1 19.3 21.7 **

0.9 ns 17.3 17.6 22.3 21.6 **

1.o ns 49.6 47.0 40.2 37.1 **

10.5 21.6 28.5 *

1.2 19.2 19.7 ns 2.4 ns 22.6 23.9 15.5 22.6 **

163 311 472 **

20.5 20.5 26.3 33.0 *

_

(gkgg’) _

Gr. N cont.

3.7 32.8 27.4 **

32.0 32.6 36.4 19.3 **

21.1 20.0 14.5 21.3 **

501 348 542 175 **

Grain wt. (mg)

(#mm’) (# mm2 ) -

Cr. per sp. (# gr. per. # SP.)

(#m 7

L. spike No.

Spike No.

Grain No.

0.9 ns

1.7 32.9 32.1 33.7 **

(*) 31.8 32.3 32.2 35.1 *

34.9 36.0 34.8 37.7 ns 6.0 35.1 36.5 ns 2.4

(gkg

‘)

Gr. WSC cont.

LSDS% =least significant difference at 5% level; (*). * and *** indicate significant at lo%, 5% and I% level, respectively; ns=not significant, “Contribution of assimilation and translocation sometimes more than final grain yield, due to grain yield losses between penultimate and final harvest and independent calculation of mean values from original data. % 1991. the intermediate treatment N2 was not analysed, but the combine harvest was used for a micro-malting and brewing procedure (Duynhouwer et al., 1993).

1993

Treatment

Year

Table 4 Main effects of shading (SllS4). nitrogen rate (Nl-N3) and significance of main effects and shading x nitrogen interaction (Sign. shading; Sign. nitrogen; S x N int.) for total above-ground biomass (Biomass), grain yield (Grains), harvest index (HI), absolute and relative contribution of assimilation (Assim.) and translocation (Transl.) to grain yield. number of grains (Grain No.), number of primary spikes (Spike No.), number of secondary (‘late’) spikes (L. spike No.), number of grains per spike (Gr. per sp.), mean grain weight (Grain wt.), grain nitrogen concentration at final harvest (Cr. N cont.) and grain WSC concentration at final harvest (Gr. WSC cont.) for 1991 and 1993

281

C. Grashofi L. F. d’dntuono / European Journal of Agronomy 6 (1997) 275-293 GREEN

1991 Nl

SLA (cm’/g)

500-

s2

J

1991 N3

SLA (cm?g)

s3

s4

-

400

LEAVES

500

-

400

-

s2

,

0

-*

_f

i

s4

0

/*

300

,

s3

,

300

4

0

-o

‘4’

8 0

* 8”

I

200 100

-m

8

.=

0 s2

-

Sl s3

l

i A

‘a 0

-

200 100

S4

140

1993 Nl ,

SLA (cm*/g)

s2,

500

160

s3

180

I

.= -

0

120

OS1

200 220 Julian day

I

1

;

OS3

~ A s4 1-

LSD.05

~~‘(‘(‘~‘~. 100

120

500

140

160

180

200 220 Julian day

1993 N3

SLA (cm*/g) s4

]OS2

‘b

LSD.05

““*~.~~~* 100

- 0

-7

400

100

l

-

100

-_

I.-

T-

__

-_

s3

AS.4

C

0

‘-

t

- LSD.05 ‘~.‘~~(~~~* 120

140

160

180

200 220 Julian day

100

120

140

160

180

200 220 Julian day

Fig. 1. Specific leaf area (SLA: mZ g _ ‘) during the growing season for four shading treatments (S 1-S4). (a) 1991, no nitrogen fertilizer applied (Nl); (b) 1991, high nitrogen fertilization (N3); (c) as (a) for 1993; (d) as (b) for 1993. LSD.05 =least significant difference at P=O.O5. Horizontal bars: timing and duration of the shading treatments. See text for explanation of treatment codes.

Higher rates of nitrogen fertilization resulted in faster growth of leaf dry weight (not shown) and LA1 (Fig. 2) but hardly affected SLA (Fig. 1). Nitrogen and WSC. The shading treatments S2 and S3 significantly increased the nitrogen concentration in the leaves and in the stems, during the shading (Fig. 3Fig. 4). After the end of the shading period, the N concentration in the stems, but not in the leaves, returned to the level of the unshaded control Sl. The S4 treatment hardly affected N concentration in stems and leaves, but the nitrogen

concentration of the filling grains was markedly increased and reached a maximum at final grain harvestof33gkg-‘in1991and22gkgg1in1993 (Table 4). In the latter year, S3 also had higher N concentrations than Sl (Table 4). Lower rates of nitrogen fertilization lowered the nitrogen concentrations in the plant organs in all shading treatments (Figs. 3 and 4, Table 4). Treatments S2 and S3 considerably reduced the concentration of WSC in stems (Fig. 5) and leaves (not shown) during the shading period, but after

C. Grushofj, L.F. d’Anruono i European Journal of’ Agronomy 6 (1997) 275-293

282

GREEN LEAVES LAI (mz/m2)

1991

s2

6 4

1

LAI(m21m2)

Nl

s3

,

s4

5 0

4 -

, I

N3

s3

-

4

d

m

l

'd

1 .a 0

S1

l

S2

-

s3 LSD05

l

2 \

*"~'~~~.'_ 100

120

3 2

-

160

1993

LSD05

0

1 -& .d

\,

Sl

0

s2

0 -

53 LSD.05

'i

it

o-*a".'.".g

140

LAI (m*/m’)

s4

'$\

3

i 2 -

1 I 0 0

0

:

l

3 -

s2

6$

5 -

1991

180

200 220 Juhan day

Nl

100

120

140

LAI(m2/mz)

l

3-

F F

LSD05

-

2

-

1

-ci

160

180

1993

N3

200 220 Julianday

;

B

%E 4 0 ‘,O

Oil 100

120

140

160

180

200 220 Julianday

-

100

dpr

120

,

140

,

160

180

.E,

_I

200 220 Julianday

Fig. 2. Leaf area index (LAI: m2 mm’) during the growing season for four shading treatments (SI-S4). (a) No nitrogen fertilizer applied (NI); (b) high nitrogen fertilization (N3); (c ) as (a) for 1993; (d) as (b) for 1993. LSD.OS=least significant difference at P=O.O5. Horizontal bars: timing and duration of the shading treatments. See text for explanation of codes.

the end of the shading the WSC concentration increased to the level of the unshaded control or became even higher. In S4, the level of WSC was decreased in the grains at the start of the filling period (not shown), but at final harvest the level of WSC decreased in all treatments and no differences between treatments were detectable (Table 4). The rate of nitrogen fertilization had hardly any effect on WSC concentration. In general, the effects described above were similar for 1991 and 1993. However, not all effects

were significant in 1991 (Figs. 3-5). This was mainly because the N2 treatments in 1991 were not analyzed (see Section 2) resulting in fewer degrees of freedom and a lower discriminating power of the statistical analysis for that year. Organ jbrmation, grain Jilling and grain size distribution. In the treatment with shading during

tillering (S2) the number of tillers mm2 was much lower at the end of the shading period. However,

after the shading nets were removed, tiller formation recovered (Fig. 6) and at final harvest, the

283

C. Grashoft; L.F. d’dntuono / European Journal of Agronomy 6 (1997) 275-293 GREEN LEAVES N concentration (g/kg)

1 ggq

~1

N concentration (g/kg)

N3

1991

70 60 50

i

40

0 52 8

40 30

‘m

Sl

I I

;

30

0 cl

s2 Sl

‘0

a

120

140

20 10 0 100

120

N concentration 70

s2

140

160

(g/kg)

,

1993 s3

,

180 200 Julian day

N1

100

N concentration s4

1 gg3

(g/kg)

,

s2

I

70-

160

-

50

:T

40

-

30

-

s4 OS1

0” \\ ‘Q ‘b

~3

,

s3

Q 60

180 200 Julian day

‘, b

._: CT

0

s2

0

s3

A

S4

y.0:

‘q

“\ 6 “$

20

-

lo.d

-

TI-

11

I

I

0 160

180 200 Julian day

100

120

140

160

180 200 Julian day

Fig. 3. Nitrogen concentration (g kg-‘) in green leaves during the growing season for four shading treatments (Sl-S4) in 1991 and 1993. (a) No nitrogen fertilizer applied (Nl); (b) high nitrogen fertilization (N3). LSD.OS=least significant difference at P=O.O5. Horizontal bars: timing and duration of the shading treatments (NB: at the end of treatment S4 in 1991, no green leaves were left; therefore, the N concentration in S4 leaves was not measured).

final spike number m-’ and grain number per spike (Table 4) in S2 were similar to those in the control (Sl). Shading during culm elongation (S3) did not reduce the spike number me2 in 1991 and reduced it only slightly in 1993 (Table 4). In both years, the number of grains m-’ in S3 was about 40% lower than in Sl. This difference was mainly attributable to the number of grains per spike, S3 being 35% lower than Sl: the number of spikes mm2 was not appreciably affected. Shading during

grain filling (S4) had no effect on the number of grains per spike, but resulted in fewer spikes m-’ and thus in a lower grain number rne2, especially in 1993. In general, the grains in all shading treatments (except S3) were 50-90% heavier in 1993 than in 1991. Shading during tillering (S2) had no effect on mean grain weight in either year (Table 4), but there was a slightly higher proportion of smaller grains (Fig. 7(a,b)). In 1991, shading during culm

C. Grashqff;L.F. d'Antuono

284

i Eurr,peanJournrrlofAgronom?l6 (1997) 275-293 STEMS

N concentration

50 b-1

(g/kg)

s2

1991

N1

N concentration

s4

50

s3

i

(g/kg)

s2

s3

,

N3

1991

,

s4

I

0

0

8

s1

40

-

l

s2

l

s3

30

-m -

100

40

tS4 8

120

140

N concentration

(g/kg)

,

s2

160

s3

180

100

N1

4 8 Sl

120

OS2

50

- I

120

140

140

40

-

180

180

0

s3

A

s4

s4

0

~

l

s3

A

54

-

LSD.05 /

30

_ 0

20

-

0

200 220 Julian day

100

120

0

140

200 220 Julian day

N3

1993

s3

II

160

160

(g/kg)

52

mp

100

0 s2

c!

N concentration

s4

0

Sl

LSD 05

200 220 Julian day

1993

0

-

30 -0

-

0

0

I

0

Sl

0

s2

0

s3

A

S4

-

LSD.05

0

160

180

200 220 Juhan day

Fig. 4. Nitrogen concentration (g kg ‘) in stems during the growing season for four shading treatments (SI-S4) in 1991 and 1993. (a) No nitrogen fertilizer applied (N 1); (b) high nitrogen fertilization ( N3 ). LSD.05 = least significant difference at P= 0.05. Horizontal bars: timing and duration of the shading treatments.

elongation (S3) resulted in a greater mean grain weight and the grain size distribution showed a higher frequency of larger grains than in the control Sl (Fig. 7(a)). However, in 1993, S3 had a lower mean grain weight than the control and a higher frequency of small grains (Fig. 7(b)). Shading during grain filling (S4) resulted in the largest reduction of mean grain weight (40% in 1991 and 25% in 1993) and resulted in a very high frequency of smaller grains, especially in 1991 (Fig. 7(a,b)).

Higher rates of nitrogen fertilization increased total tiller number in both years (Fig. 6); in 1993, many of these tillers succeeded in forming ears, but in 1991 a large proportion was still green and non-productive at final harvest (Table 4). Higher rates of nitrogen fertilization had no effect on the number of grains per spike and increased the number of grains me2 by 10% in 1991 and by 45% in 1993. It reduced grain weight by 10-l 5%, however, and shifted the size distribution of the grain population to smaller grains (Fig. 7).

285

C. Grashoff L.F. d’Antuono J European Journal of Agronomy 6 (1997) 275-293 STEMS

WSCWg)

wsc(mu

1991 Nl

s2

s3

s4

,

500

t

400

-

1991 N3

s2

53

54

',Sl

o-51

~ .

s2

+

s3

t

s4

/ -~

0

300 -

--- LSD.Of -

? , p.;

.b

0 140

WSC Wkg)

160

180

200 220 Julianday

1993 Nl

-

,\\\&

100 -

120

s4

A

LSD.05 200

100

s2

*- s3

= /, . 0% \

~~~~...~..W

100

120

140

WSCk&g) 500 -

IS2

160

180

200 220 Julianday

1993 N3 : s3

;

1

s4

I cl

400 -

300 -

300

-

200

200

-

-

9

Sl

e

52

4

s3

-:

J 100

120

140

160

180

200 220 Julianday

100

120

140

160

180

200 220 Julianday

Fig. 5. Concentration of water soluble carbohydrates (WSC; g kg-‘) during the growing season for four shading treatments (Sl-S4) in 1991 and 1993. (a) No nitrogen fertilizer applied (Nl); (b) high nitrogen fertilization (N3). LSD.05 =least significant difference at P=O.O5. Horizontal bars: timing and duration of the shading treatments.

The only interaction between shading and nitrogen which was significant in both years was the number of secondary (late) spikes (Table 4); more fertilizer nitrogen increased the formation of late spikes, but only in the shading treatments Sl, S2 and S3; in S4 this effect was absent in both years (data not shown). Urigin of dry matter for grain @ing. No translocation to leaves and chaff was detected and there was little translocation to culms after heading.

Shading during culm elongation (S3) and shading during grain filling (S4) markedly reduced the absolute amount of dry matter allocated to the grains both by assimilation and translocation (Table 4). Shading during grain filling decreased the absolute contribution of assimilation and consequently increased the relative contribution of translocation by 45%, but onIy in 1991. in 1993, the relative contributions were constant in all treatments, with 65% for assimilation and 35%

C. Grushofl L.F. d’iinruono

286

Tiller number per m2 2800 s2 2400

-’ t

2000

-

S

. 0 l

90

1991

900

,

1991

,

N3

,

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s4

s2 s3

130

150

Tiller number per m2 1993 1800 s2 s3

1200

Tiller number per m2 2800 s2

s4

1

110

1500

Nl

s3

,

I European Journul of Agronomy 6 ( 1997) 275-293

l

Sl

0

s2

l

s3

A

s4

-

LSD 05

170

190 210 Julian day

N1

90

110

130

s4

s2

I

190 210 Julian day

N3

s3

s4

I

1500

B A

1200

pa

e

900

600

m-e-

_ -_

-7 _ .b

0

; hJ

“i

0

8

0

Sl

0

s2

0

s3

A

S4

-

LSD05

0 0

--

0

0

600

.a

170

1993

Tiller number per m2 1800

0

300

150

00

--

300 cl

B--B -

-T---

-

_

0 90

110

130

150

170

190 210 Julian day

90

110

130

150

170

190 210 Julian day

Fig. 6. Total number of tillers m ~’ during the growing season for four shading treatments (Sl -S4) in 1991 and 1993. (a) No nitrogen fertilizer applied (Nl ): (b) high nitrogen fertilization (N3). LSD.05 =least significant difference at P=O.O5. Horizontal bars: timing and duration of the shading treatments.

for translocation. Higher rates of nitrogen fertilization hardly affected the relative contribution of assimilation and translocation to grain filling (Table 4). EfSects of shading period on the relation between grain number and grain weight. Mean grain weight was inversely related to grain number per unit area (Fig. S), indicating that assimilate supply was not sufficient to fill a large number of grains completely (source limitation); however, two distinct relations were detectable: Sl and S2 on the one hand and

S3 and S4 on the other hand, since the grain weight of the last two treatments was about 19 mg lower than Sl and S2 over a wide range of grain numbers (Fig. 8). There is an obvious reason for this for the S4 treatment, in which radiation was reduced during grain filling, but the cause is less clear for the S3 treatment. Within each treatment, higher nitrogen dressings determined the increase of seed number; Fig. 8 shows, however, that this nitrogen effect was always accompanied by a lower mean grain weight.

C. Grashofi L. F. d’rlntuono / European Journal of Agronomy 6 (1997) 275-293

4. Discussion 4.1. EfSects of shading on organ formation and grain yield Shading during early growing stages (before anthesis) reduced the proportion of developing organs which survived during this stage. There were no important interactions between shading and rate of nitrogen fertilization and most effects were consistent over the two contrasting growing seasons, one with a cold, wet spring and hot summer (1991), and the other with a warm spring and a cool, wet summer (1993). The general reduction of organ survival in combination with lowered WSC concentrations found is in line with the contention of Van Keulen (1982) and Van Keulen and Seligman (1987) that the number of viable organs decreases when assimilate supply is restricted. Results obtained by Willey and Holliday ( 1971a,b), Fletcher and Dale ( 1974) and Fischer (1975) also showed a reduction of organ survival with shading before anthesis. Our finding that shading after anthesis mainly reduced mean grain weight is in line with the results obtained by Willey and Holliday ( 1971b), Bremner (1972), Gifford et al. (1973), Fischer (1975), Fischer and HilleRisLambers (1978) and Jenner (1979). Our results indicate that this reduction is due to the reduced direct photosynthesis and the insufficient (1991) or absent (1993) compensation by increased translocation from other organs. We hypothesize that the variable effects of shading on grain yield reported in the literature are attributable to differences in compensation capacity among different yield components determined over a long time span, from tillering to kernel filling. Our experiments show that relatively short periods of shading caused a large reduction in the WSC concentration in leaves and stems, presumably due to a decreased assimilation rate. Biomass production and grain yield showed a greater reduction when the shading was applied later in the development of the crop. The crop’s ability to compensate for the adverse effects of shading decreased with ageing. With early shading (S2), thinner leaves were produced, an effect which is

281

often observed with shading (Ellen and Van Oene, 1989a). This resulted in a faster increase in LA1 and light interception and consequently compensated for the reduced light intensity after the removal of the shading nets. In combination with the second compensation mechanism, the recovery of tiller production after removal of the early shading, meant that there were no effects of shading on final biomass, ear and grain number rne2, mean grain weight and grain yield in S2. However, because of the small delay in tiller formation, fewer tillers were able to fill the grains completely and hence quality suffered slightly, because of a small negative shift in grain size distribution. With shading during culm elongation (S3), WSC level declined sharply and even partial compensation through an increase of SLA could not occur; new leaves hardly emerged during this period. It is during this period that the grains are formed, therefore the lack of assimilates, measured by the WSC concentration in stems and leaves, reduced the number of grains per spike considerably. After the shading nets were removed, development had advanced too far to enable the yield losses caused by a low number of kernels per spike to be compensated for by a higher number of spikes rnm2. This is in line with the results of Fischer and Stockman (1980), who showed that shading before anthesis reduced the number of viable florets in wheat. Our results showed that the compensation mechanisms after shading are effective, but only after early shading. In this way, our results provide experimental confirmation of the simulation results of Van Keulen and Seligman (1987), who explored the possible effect of low carbohydrate availability on organ formation and yield performance. Their simulations showed that when tiller or spike numbers mP2 are reduced, sufficient time remains for compensatory growth of grain-forming organs; the grain formation stage, however, is critical and a restriction of assimilate during this period reduces grain number per ear. Shading during grain filling (S4) reduced mean grain weight considerably and, as the crop had no possibilities of compensation, the lowest grain yield was obtained in this treatment. The reduction brought about by shading was greater in 1991 than in 1993 because of the short period of grain filling.

C. Grushoff; L. F. d’Antuono i European Journnl of Agronom?, 6 i IYY7) 275-293

288

Weigt Frequency (%)

2.2-2.5

2.5-2.8

2.8-3.0

2.0-2.2

2.2-2.5

2.5-2.8

2.8-3.0

2.0-2.2

2.2-2.5

2.0-2.2

2.2-2.5

> 3.0 Size class

Weight Frequency (%)

<2.0 Weight Frequency (%)

<2.0

>3.0

Size class

Weight Frequency (%) SO 40 30 20 10

s4

0 <2.0

Fig. 7. Frequency distribution (on weight basis) of grain rates (Nl-N3). (a) For 1991; (b) for 1993.

2 5-2.8

size classes of four shading

2.8-3.0

treatments

(SI-S4)

with three N fertilization

289

C. Grashofi L.F. d’dntuono / European Journal of Agronomy 6 (1997) 275-293

Weight Frequency

<2.0 Weight Frequency

< 2.0

(%)

2.0-2.2

2.2-2.5

2.5-2.8

2.8-3.0

3.0-3.2

3.2-3.5

>3.5

2.2-2.5

2.5-2.8

2.8-3.0

3.0-3.2

3.2-3.5

>3.5

Size class

(%)

2.0-2.2

Si2 !e class

Weight Frequency

(%)

40

s3

O <2.0 Weight Frequency

<2.0

2.0-2.2

2.2-2.5

2.5-2.8

2.8-3.0

3.0-3.2

3.2-3.5

‘3.5

3.2-3.5

>3.5

Size class

(%)

2.0-2.2

2.2-2.5

2.5-2.8

2.8-3.0

3.0-3.2

Size CkSS

In 1991, the grains demanded large amounts of carbohydrates, as evident from the increase in the relative contribution of translocation to grain filling. However, this could not compensate for the low availability of assimilates, and led to the lowest

grain weight obtained in our experiments. This confirms that most of the carbohydrates needed for grain growth are provided by current photosynthesis (Wardlaw, 1990). Our results indicate that the variable and sometimes inconsistent effects of shad-

C. Grashofl L.F. d’dntuono 1 European Journal of Agronowy 6 c 1997) 275-293

290

Mean grain weight (mg) 60 CL 50

o-

~

40

_

_~91Sl Nl N,n-.-.‘w-

9152

omm91s3 -*-

-9lS4

- - 0

- .93Sl

I: : z

1

. _

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NZ“.... ’ e.

N& i%_<...

: :z;i;

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Nl

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- - i;A;-;3 Nl N3

I

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-

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_\

N’ FL..

‘..

-.g3s4 R (Sl+SZ)

9

i ‘1. a kN3 N3

R (S3+S4)

5000

10000

15000

20000

25000

Grain number per m2 (SI--S4) with three rates of nitrogen Fig. 8. Mean final grain weight versus final grain number m ’ for four shading treatments fertilization (Nl-N3) in 1991 and 1993. Within each shading treatment the increasing nitrogen dressings are connected by lines. Furthermore, two regression lines are shown: R.(Sl + S2) = regression for the treatments S 1 and S2 (r2= 0.85); R.( S3 + S4) = regression for the treatments S3 and S4 (r’=0.81).

ing on grain yield and quality as reported in the literature are due not only to a reduction of organ formation or of grain filling, but also to the crop’s decreasing ability during development to deploy mechanisms which compensate for yield losses. 4.2. EfSects ofshading on grain quality In addition to the above analysis in terms of grain yield, our results provide information on grain quality. Ellen and Van Oene (1989a,b) showed in pot trials that shading from the end of spikelet initiation until the end of grain filling reduced WSC concentration, tiller number mP2, number of grains per spike and mean grain weight and, consequently, adversely affected quality in terms of mean grain weight and size distribution. Due to low rates of nitrogen fertilization, the grain N concentration in their trials was 8-15 g kg-‘. As we used shorter periods of shading in the field, combined with different rates of nitrogen fertilization, the conclusions of Ellen and Van Oene

( 1989a,b) can be extended: our experiments showed that the number of grains per spike was especially reduced by shading during the period shortly after the end of spikelet initiation (i.e. during the period of culm elongation: treatment S3). The reduced number of grains in S3, followed by removing the shading during grain filling, may even improve quality (heavier and bigger grains with acceptable N concentration) compared to the control. However, this hypothesis must be treated with care for three reasons. First, the phenomenon occurred after a cold and wet spring ( 1991) during which the control produced very many grains m *. The individual grain weight of the control, however, was relatively low, because high temperatures in the hot summer of 1991 appreciably shortened the grain filling period and only slightly increased the rate of grain filling (Sofield et al., 1977; Wiegand and Cuellar, 198 1). Second, even the greater grain weight in S3 in 1991 was insufficient to compensate for the grain yield being less than that of the control. Third, in both years S3

C. Grashofl L.F. d’dntuono /European Journal of Agronomy 6 (1997) 275-293

and S4 showed the same relation between grain number and grain weight which was suboptimal compared to Sl and S2 (Fig. 8). As S3 grains never reached the ‘absolute maximum’ grain weight of about 50 mg but had a constant weight of about 40 mg in the two contrasting seasons, it is difficult to attribute the higher grain weight in S3 compared to Sl simply to sink limitation caused by a lower grain number per unit area. Sink strength is probably related to the number of growing cells in a particular organ (Gifford and Evans, 1981). A possible explanation would be that shading during culm elongation reduced the potential number of cells per grain. As S3 produced thinner leaves, source limitation is also possible; thinner leaves may have a reduced photosynthetic capacity per unit area, or a stronger carbohydrate inhibition of photosynthesis as a result of a lower capacity for intermediate carbohydrate storage. The largest adverse effect on quality was induced by shading during grain filling (S4) and is attributable to a strong reduction of assimilate supply to the filling grains. Concomitantly, the nitrogen concentration in the grains reached values up to 33 g kg-‘, corresponding to unacceptably high protein concentrations up to 21%, in contrast to the low concentrations (5-9%) obtained by Ellen and Van Oene (1989b). Our results show that the combination of low assimilate availability during grain filling and ample N fertilization is the worst combination for malting quality. The grains can hardly control the C/N ratio of the storage material, which is in line with results of Ma et al. (1995) and Dreccer et al. (1996). 4.3. Eflect of nitrogen fertilization on grain quality High nitrogen dressings increased the number of culms rnm2, resulting in a positive correlation between nitrogen and grain number. This may be partly mediated through the positive effect of nitrogen on carbohydrate availability for tiller production in the first part of the growing season (Pinthus and Millet, 1978; Van Keulen and Seligman, 1987), but it is known that nitrogen also has a direct positive effect on tiller formation (Yoshida and Hayakawa, 1970). In 199 1, however, high rates of nitrogen fertilization reduced the final

291

grain yield, as in that year nitrogen did not increase the number of normal spikes but only the number of secondary (late) spikes, which rarely contain harvestable grains. Even worse for grain quality was the fact that nitrogen had a large effect on the formation of yield potential (high grain number) but hardly improved assimilate supply during the grain filling phase (Table 4), resulting in lower mean grain weights, suboptimal size distributions, unacceptably high protein concentrations and in some years even lower grain yields (Table 4, Figs. 3-8). 4.4. The consequences of shading and nitrogen fertilization in the malting and brewing process Duynhouwer et al. (1993) demonstrated the importance of the effect of shading on general malting quality. They micro-malted the grains from our shading treatment during grain filling (S4), grown under standard nitrogen fertilization (N2) in 1991, and analyzed the quality of the wort (the liquid which forms the basis for the yeasting process in the beer industry). They demonstrated that it was impossible to make a wort of acceptable quality, even after selecting the bigger grain fractions. The yield of (protein-free) extract and the amount of fermentable sugars were too low and the protein concentration was far too high. These unacceptably high concentrations were already reached with the standard nitrogen dressing in the S4 treatment (S4N2) with a grain N concentration of 30 g g-l. As the nitrogen concentration in N3 grains was even higher, it is evident that S4N3 will produce an even poorer malting quality. In general, our results confirmed that higher N dressings in barley may cause problems with malting quality, both through higher nitrogen concentrations and suboptimal size distributions (see above).

5. Conclusions First, our experiments showed that the adverse effects of shading (especially in combination with high nitrogen dressings) on yield and malting quality of the crop are mediated through the effect

292

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1Europeun Journul

of low WSC concentrations in stems and leaves on organ survival. Second, the adverse effects increase when shading occurs later in the growing season. Third, optimum quality and yield require the, partly contradictory, combination of conditions: (a) high and stable radiation levels, and (b) moderate temperature levels and nitrogen dressings (and ample water supply). Our quantifications of the effects of these environmental factors on organ dynamics, grain filling and grain protein content will be used to validate a mechanistic simulation model which will in turn be used to analyze the optimum combinations of climatic regions and nitrogen dressings, required for high and stable barley yields with high malting quality.

Acknowledgment The authors wish to thank Mr. H.G. Smid and Mrs. ing. M. Marinissen, who conducted the field experiments and collected the data, and Dr. ir. A.J. Haverkort for helpful criticism of the manuscript.

References Biscoe, P.V. and Gallagher, J.N., 1977. Weather, dry matter production and yield. In: J.J. Landsberg and C.V. Cutting (Editors), Environmental Effects on Crop Physiology. Academic Press, New York, pp. 75-100. Bouwmeester, H.J. and Kuijpers, A.M., 1993. Relationship between assimilate supply and essential-oil accumulation in annual and biennial caraway (Cururn carvi L.). J. Essenl. Oil Res., 5: 143-152. Bremner, P.M., 1972. Accumulation of dry matter and nitrogen by grains in different positions of the wheat ear as influenced by shading and defoliation. Aust. J. Biol. Sci., 25: 657-668. Burger, W.C. and Laberge, D.E., 1985. Malting and brewing quality. In: D.C. Rasmussen (Editor), Barley. Agronomy Monograph 26. American Society of Agronomy, Madison. Cannell, R.Q., 1969. The tillering pattern of barley varieties. I. Production, survival and contribution to yield by component tillers. J. Agric. Sci., Cambridge, 72: 405-422. Dreccer, M.F., Grashoff, C. and Rabbinge, R., 1996. Source-sink ratio in barley (Hordeurn vulgure L.) during grain filling: effects on senescence and grain protein concentration. Field Crops Res. (in press). Duynhouwer, I.D.C., Grashoff, C. and Angelino, S.A.G.F., 1993. Kernel filling and malting barley quality. In: Proc. 24th

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Congr. European Brewery Convention, Oslo, 1993. Oxford University Press, Oxford, pp. 121-128. Ellen. J. and Van Oene, H., 1989a. Effects of light intensity on yield components. carbohydrate economy and cell-wall constituents in spring barley (Hordeurn distichum L.). Neth. J. Agric. Sci., 37: 83-95. Ellen, J. and Van Oene, H., 1989b. Effects of light intensity on nitrogen economy of spring barley (Hordeum distichum L.). Neth. J. Agric. Sci.. 37: 205-211. Evans, L.T. and Wardlaw, I.F., 1976. Aspects of the comparative physiology of grain yields in cereals. Adv. Agron., 28: 301-359. Fischer, R.A., 1975. Yield potential in a dwarf spring wheat and the effect of shading. Crop Sci., 15: 607-613. Fischer. R.A. and HilleRisLambers, D.. 1978. Effect of environment and cultivar on source limitation to grain weight in wheat. Aust. J. Agric. Res., 29: 443-458. Fischer, R.A. and Stockman, Y.M., 1980. Kernel number per spike in wheat (Triticum aestivum L.): responses to preanthesis shading. Aust. J. Plant Physiol., 7: 169-180. Fletcher, G.M. and Dale, J.E., 1974. Growth of tiller buds in barley: effect of shade treatment and mineral nutrition. Ann. Bot., 38: 63-76. Gifford, R.M., Bremner, P.M. and Jones, D.B., 1973. Assessing photosynthetic limitation to grain yield in a field crop. Aust. J. Agric. Res., 24: 297-307. Gifford, R.M. and Evans, L.T., 1981. Photosynthesis, carbon partitioning, and yield. Annu. Rev. Plant Physiol., 32: 485-509. Jenner, C.F., 1979. Grain filling in wheat plants shaded for brief periods after anthesis. Aust. J. Plant Physiol., 6: 629-641. Jenner. C.F., Ugalde, T.D. and Aspinall, D., 1991. The physiology of starch and protein deposition in the endosperm of wheat. Aust. J. Plant Physiol., 18: 21 l-226. Kirby, E.J.M. and Appleyard, M., 1982. Cereal Development Guide. Cereal Unit, National Agricultural Centre, Stoneleigh, 96 pp. Ma, Y.Z., MacKown, C.T. and Van Sandford, D.A., 1995. Kernel mass and assimilate accumulation in wheat: cultivar responses to 50% spikelet removal at anthesis. Field Crops Res.. 42: 93.-99. Pendelton, J.W. and Weibel, R.O., 1965. Shading studies on winter wheat. Agron. J., 57: 292-293. Pinthus, M.J. and Millet, E.. 1978. Interactions among number of spikelets, number of grains and grain weight in the spikes of wheat (Triticum aestivum L.). Ann. Bot., 42: 839-848. Scott, W.R., Appleyard, M., Fellowes, G. and Kirby, E.J.M., 1983. Effect of genotype and position in the ear on carpel and grain growth and mature grain weight of spring barley. J. Agric. Sci., Cambridge, 100: 383-391. Sofield, I., Evans, L.T., Cook, E.G. and Wardlaw, LF., 1977. Factors influencing the rate and duration of grain filling in wheat. Aust. J. Plant Physiol., 4: 785-797. Van Keulen, H., 1982. A deterministic approach to modelling of organogenesis in wheat. In: F.W.T. Penning de Vries and H.H. Van Laar (Editors), Simulation of Plant Growth and Crop Production. Pudoc, Wageningen, pp. 151-155.

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Van Keulen, H. and Seligman, N.G., 1987. Simulation of water use, nitrogen nutrition and growth of a spring wheat crop. Simulation Monographs, Pudoc, Wageningen, 310 pp. Van Keulen, H., Goudriaan, J. and Seligman, N.G., 1989. Modeling the effects of canopy development and growth. In: G. Russell, B. Marshall and P.G. Jarvis (Editors), Plant Canopies: Their Growth, Form and Function. SEB Seminar Series 31. Cambridge University Press, Cambridge, pp. 83-104. Wardlaw, I.F., 1990. The control of carbon partitioning in plants. New Phytol., 116: 341-381. Wiegand, C.L. and Cuellar, J.A., 1981. Duration of grain filling

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and kernel weight as affected by temperature. Crop Sci., 21: 95-101. Willey, R.W. and Holliday, R., 1971a. Plant population and shading studies in barley. J. Agric. Sci., Cambridge, 77: 445452. Willey, R.W. and Holliday, R., 1971b. Plant population and shading studies in wheat. J. Agric. Sci., Cambridge, 77: 453461. Yoshida, S. and Hayakawa, Y., 1970. Effects of mineral nutrition on tillering of rice. Soil Sci. Plant Nutr., 16: 1866191. Zadoks, J.C., Chang, T.T. and Konzak, C.F., 1974. A decimal code for the growth stages of cereals. Weed Res., 14: 4155421.