Contrasting growth and dry matter partitioning in winter and spring evening primrose crops (Oenothera spp.)

Contrasting growth and dry matter partitioning in winter and spring evening primrose crops (Oenothera spp.)

Field Crops Research 68 (2000) 9±20 Contrasting growth and dry matter partitioning in winter and spring evening primrose crops (Oenothera spp.) Andre...

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Field Crops Research 68 (2000) 9±20

Contrasting growth and dry matter partitioning in winter and spring evening primrose crops (Oenothera spp.) Andrew F. Fieldsenda,*, James I.L. Morisonb a

Scotia Pharmaceuticals Ltd., Plant Technology Centre, Writtle College, Chelmsford CM1 3RR, UK Department of Biological Sciences, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, UK


Received 12 October 1999; received in revised form 30 May 2000; accepted 1 June 2000

Abstract Evening primrose (Oenothera spp.) is a relatively new, high value oilseed crop for temperate regions. Despite a long growing season, seed yields are much lower than the average yields of the major arable seed crops and commercial spring crops can yield as much as winter ones. Biomass accumulation and partitioning and crop canopy development were compared between winter and spring crops of cv. Merlin in 2 years of ®eld trials. A winter crop of cv. Peter was also studied in year 2. Although post-winter growth was slow to restart, the winter crops produced up to 1589 g biomass mÿ2. The harvest index was low (<14%) because the crops grew tall and approximately half of the assimilate was partitioned into stem. Biomass production of the spring crops was lower, but the harvest index was higher (up to 17%) and in year 1, the winter and spring crops produced similar seed yields. The evening primrose leaf canopy was planophile and the peak green area index was in the region of 3±4, except in the year 2 winter crops where it was 7±8. Population density after crop establishment varied substantially between treatments but crops compensated for low plant populations by the production of larger, more branched plants bearing more capsules. Following the start of seed growth, the cultivar Merlin partitioned a greater proportion of new biomass into seed than did cv. Peter. The mean number of seeds per capsule was higher in cv. Merlin, but the seeds were smaller. Machine harvesting resulted in the loss of 19 and 55% of the seed produced by the year 1 winter and spring crops. Improvements in harvest index in the winter crop, an earlier start to growth in the spring crop and an increase the proportion of seed recovered by combine harvesting would lead to substantial increases in the seed yield of evening primrose crops. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Evening primrose; Oenothera spp.; Canopy structure; Biomass partitioning; Seed yield; Yield components

1. Introduction Evening primrose (Oenothera spp.) seed oil contains g-linolenic acid, an essential fatty acid with proven applications as a nutrient and pharmaceutical * Corresponding author. Present address: Semundo Ltd., Abbots Ripton, Huntingdon, PE28 2PH, UK. Tel.: ‡44-1487-773595; fax: ‡44-1487-773532. E-mail address: [email protected] (A.F. Fieldsend).

for humans. In clinical trials, evening primrose oil has also given promising results in the treatment of diabetic complications, cancer, cardiovascular disease and arthritis (Horrobin, 1990). An increasing market for evening primrose seed has given farmers an opportunity to diversify into non-food production and commercial evening primrose production has been established in northern and eastern Europe, North America and Australasia (Simpson and Fieldsend, 1993).

0378-4290/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 4 2 9 0 ( 0 0 ) 0 0 1 0 4 - 0


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Evening primrose may be over-wintered (`winter') or spring-sown (`spring') or occasionally spring transplanted. In eastern England, the winter crop is drilled in August and harvested in October of the following year (Dodd and Scarisbrick, 1989). Despite the long growing season, the recovered seed yield from a good crop of winter evening primrose is rarely as high as 1.0 t haÿ1. This ®gure is much lower than the average yields of the major arable seed crops such as oilseed rape (Brassica napus L.) even though biomass production is similar (Russell, 1988). Spring evening primrose crops are drilled in April and have a much briefer rosette stage. Compared to the winter crop, spring-sown evening primrose more readily ®ts into the crop rotation and input costs are lower, but harvesting is delayed until November. In most temperate crop species, the over-wintered crop will normally outyield the spring-sown equivalent, but in commercial crops of evening primrose, comparable or higher seed yields are frequently obtained from spring sowings. Little research has been conducted on the reasons for low seed yields in evening primrose crops and no comparative study of the growth of winter and spring evening primrose has previously been published. Higher yields leading to a reduction in seed price would allow evening primrose oil to be more widely used in `mainstream' food products, which would in turn substantially increase the demand for seed. Evening primrose partitions comparatively little assimilate into seed. In pot trials, Kromer and Gross (1987) and Collins and Scarisbrick (1994) obtained results of 0.37 and 0.41, respectively, for `reproductive effort', de®ned by Kromer and Gross (1987) as the proportion of crop biomass in reproductive tissues (¯owers, capsules and seeds). From seed yields obtained by hand-harvesting, Russell (1988) calculated harvest index (seed mass as a percentage of total crop biomass) to range from 5 to 14% in winter evening primrose cv. Constable, depending on year, which compares to 21±29% in winter oilseed rape (excluding roots) sown at a similar time (Leach et al., 1994). The seed yields obtained by Russell (1988) were much higher than usual commercial yields. In commercial crops, harvested yield will be reduced by seed losses, both in the ®eld (particularly from older cultivars such as Constable, on which the capsules split as they ripen) and during seed cleaning (Simpson and Fieldsend, 1993).

An evening primrose breeding programme at Writtle College, Chelmsford, UK has produced several cultivars which are better suited to agricultural production. For example, seed dormancy has been greatly reduced, although unreliable establishment is still a problem in this small-seeded crop (Dodd and Scarisbrick, 1989). In addition to genotype, seedling emergence as a percentage of viable seed sown is in¯uenced by several factors including the vigour of the seed stock used and soil temperature, and lower establishment rates are usually obtained from spring sowings. Hence, whilst populations of 50±200 plants mÿ2 are optimal (Scotia, unpublished data), it is still not possible to accurately predict the population of plants which will be obtained from a particular seeding rate. The cultivars Merlin and Peter bear non-splitting capsules and thereby offer improved seed retention at maturity and greater ¯exibility in harvest date (Simpson and Fieldsend, 1993). This paper reports a comparison using these newer cultivars of the growth, canopy development and yield characteristics of winter and spring evening primrose crops grown in 2 years in the south-east of England. It identi®es aspects of yield formation in evening primrose, where signi®cant improvements may be possible and contributes to the information available on the relative performance of over-wintered and spring-sown temperate crops. 2. Materials and methods 2.1. Plant material, site and establishment A replicated-plot ®eld experiment of evening primrose was established in 1995 on a commercial farm at Hat®eld Peverel, near Chelmsford, Essex, UK (latitude 518470 N, longitude 08310 E, altitude 50 m). The soil type was a deep silty loam, pH 7.9, containing 61 mg P lÿ1, 246 mg K lÿ1 and 44 mg Mg lÿ1. The previous crop was winter wheat harvested in 1994. No fertiliser was applied to the trial at any stage as ®eld trials over several years have shown that evening primrose is highly unresponsive to applied nitrogen fertiliser (Stobart and Simpson, 1997). A four-replication, randomised complete block design was adopted with two treatments, over-wintered and spring-sown. Plot length was 9.85 m, the spacing between rows was 0.42 m and all plots were drilled in a south-east to

A.F. Fieldsend, J.I.L. Morison / Field Crops Research 68 (2000) 9±20

north-west alignment. Over-wintered plots consisted of 25 rows. Seed of cv. Merlin was drilled on 10 August 1995 at a rate of 5.62 kg viable seed haÿ1, or 1304 viable seeds mÿ2. To assist establishment, irrigation was applied on 17 August 1995 and on 29 August 1995. Pendimethalin herbicide was applied prior to crop emergence. To control Septoria oenotherae, tebuconazole fungicide was applied on 9 October 1995 and iprodione was applied on 16 January 1996. Spring-sown plots consisted of 20 rows. The same seed stock of cv. Merlin was drilled on 15 April 1996, again at a rate of 5.62 kg viable seed haÿ1, in the expectation that a plant population similar to the post-winter population in the August-sown plots would be obtained, and pendimethalin was applied on 30 April 1996. In fact, few plants emerged from the spring-sown seed, but some seed which had been sown in error in these plots in August 1995 and had remained dormant over winter did germinate in the spring. Consequently, an adequate stand of plants was obtained, evenly distributed throughout the plots rather than con®ned to rows. No irrigation or fungicide was applied to these plots. The second-year experiment was established in the neighbouring ®eld with a similar soil type with pH 7± 7.3, containing 27 mg P lÿ1, 142 mg K lÿ1 and 69 mg Mg lÿ1. The previous crop was winter wheat harvested in 1995. Trial design was similar to year 1 except that there were three treatments, as described below, and all plots were drilled in a south-west to north-east alignment. All plots consisted of 15 rows. Over-wintered plots were drilled on 14 August 1996 and there were two treatments, cultivars Merlin and Peter. Cv. Merlin was drilled at a rate of 2.62 kg haÿ1 viable seed (or 670 viable seeds mÿ2). Cv. Peter was drilled at a rate of 3.12 kg haÿ1 viable seed (or 862 viable seeds mÿ2). Pendimethalin and paraquat herbicides were applied prior to crop emergence. To control S. oenotherae, tebuconazole was applied on 28 November 1996 and iprodione was applied on 29 January 1997. The spring plots of cv. Merlin were drilled on 15 April 1996. Since the plant population obtained from the August-sowing of cv. Merlin was low, a rate of 8.60 kg viable seed haÿ1, or 2150 viable seeds mÿ2, was used for the spring sowing to ensure that an adequate plant population was obtained. Pendimethalin and glyphosate were applied on 30 April 1997. No fertiliser was applied to the trial but all plots received


10 mm irrigation (to encourage emergence in the spring plots) on 21 April 1997 and again on 1 May 1997. In all plots, post-emergence weed control was by hand. 2.2. Meteorological data Daily temperature, precipitation, wind and incident solar radiation data were obtained from a Meteorological Of®ce-approved climatological station at Writtle College, Chelmsford, approximately 10 km from the experimental site. 2.3. Harvest methods Destructive harvests of above- and below-ground biomass (including, where appropriate, abscised material) were carried out by hand. In the ®rst year, 16 and 12 destructive harvests were taken from each winter and spring plot, respectively, whilst in the second year, 12 and 8 harvests were taken. A 1.25 m0.8 m area was sampled from each ®rst-year spring plot on each occasion. In all other treatments, the sample unit consisted of two adjacent 1.19 m lengths of row (equating to 1 m2). Within the plots, harvested areas were separated by at least two guard rows and sites were staggered, leaving 3.19 m of unharvested row between samplings. Below-ground material was removed to a depth suf®cient to ensure recovery of the entire tap root and the roots were carefully washed to recover as great a proportion of the ®brous roots as possible. Any dead material (mainly leaf and ¯owers) was collected from the soil surface, or if appropriate, removed with the living plants. A representative proportion of the above-ground material was separated into green leaf, stem, ¯ower and ¯ower bud, capsule and seed. This proportion declined from the entire plot in harvests prior to stem extension (bolting) to not less than 10 plants in late-season harvests. After separation, all harvested material was dried at 808C in a forceddraught oven and then weighed. The penultimate harvest was carried out as close as practicable to when capsules on 95% of the spike length contained non-white (i.e. cream to dark brown) seeds, de®ned by Simpson (1994) as growth stage (GS) 5.95. The ®nal harvest was designed to simulate the commercial practice of leaving the crop to lie in the swath to dry before combine-harvesting. The plants


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were cut by hand several centimetres above the ground, on the same date as the penultimate harvest. The roots with the bases of the stems attached were dug up in the normal way and all harvested plant material was laid out on the trial ground and covered by a net to prevent bird damage. After several weeks, these plants were collected by hand and analysed as described above. In year 1, a 14.9 m2 area from each plot was used to provide an estimate of the seed yield (and seed losses) obtained from machine harvesting. In each winter plot, four rows were cut by hand on 4 September 1996 when at GS 5.95 and harvested with a plot combine harvester on 7 October. In the spring plots, a 1.68 m wide strip was cut by hand on 16 October and combine-harvested on 22 November. Post-harvest, all seeds were dried to approximately 10% moisture content in a fan-assisted ¯ow of unheated air, then cleaned using sieves and a winnower. Results were corrected for the measured moisture content of a sub-sample.

separated into its constituent parts. On samples taken during plant senescence, the percentage area of nongreen material was estimated. After separation, the measured material was dried in the oven and Lt was calculated. Speci®c leaf area was calculated from the projected area and dry weight of the large leaf fraction. Leaf angle distribution in evening primrose cv. Merlin was directly measured in situ using a simple protractor. All large leaves were assessed on 24 representative over-wintered plants (total 650 leaves) and 24 spring plants (total 309 leaves). The leaves were allocated to six 108 leaf angle classes, covering from ‡158 to ÿ358, and greater than ÿ358, relative to the horizontal. After angle measurement, each leaf was removed and the total projected area of each size class was measured using the leaf area meter. Stems were assumed to be vertical.

2.4. Canopy structure

At each harvest date, the total number of plants was recorded. After the onset of stem extension, the numbers of bolted and non-bolted plants were recorded. Subsequently, the numbers of plants in ¯ower (de®ned as bearing at least one open ¯ower), no longer ¯owering, and productive (de®ned as bearing at least one capsule containing viable seed) were recorded. A fuller range of assessments were carried out on plants from the penultimate harvest in each treatment. On all plants in each treatment, the number of basal primary branches, upper primary branches (branches attached to the main stem just below the lowest capsules) and secondary branches were counted, except in the dense year 2 spring plots where 25 productive plants per plot were used. On 10 plants selected to represent the range of material present (25 plants in the year 2 spring plots), the number of capsules on the main stem and each class of branch was recorded. A corrected number (ct) of capsules per plant for each entire plot was calculated from the mean number of capsules per plant in the subsample (cs) by the formula: ctˆcs(wt/ws), where wt is the mean weight of all productive plants in the plot and ws is the mean plant weight of the subsample. The mean result for (wt/ws) of a treatment ranged from 0.90 to 1.12. Capsules were removed from the 10 representative plants, pooled according to origin, counted, and dried. The seed was removed by hand from a proportion (between 20 and 100%

Stem height and the height at which the lowest leaf was attached to the stem were recorded on 25 plants per plot. These were selected to represent the range of sizes and growth stages present, excluding non-bolted plants (where present). For most treatments, crop height and canopy depth were calculated as the mean for all 25 plants. However, to avoid substantially underestimating canopy height in treatments with high populations (i.e. the year 1 winter crop and the year 2 spring crop), crop height and canopy depth were de®ned as the mean results for the tallest 50 plants mÿ2 in the crop. Total green area index (Lt) was estimated from the 25 plants or a further subsample (normally 10 plants) of them. The projected area of the above-ground green material was measured using a leaf area meter (Mk 2, Delta-T Devices, Cambridge, UK). Large leaves, which normally accounted for 80% or more of total green leaf weight (except for late in the season when green leaf area was low), were measured separately from other materials. Non-green leaves readily abscise from the plant and were not included in the area assessments. Many small leaves were clustered around the stem apex, so the projected area of all other materials, which depending on growth stage could consist of stem, small leaves and bracts, buds and capsules, was measured prior to the material being

2.5. Yield components

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depending on number of capsules) of each set of capsules and 500 seeds were counted and weighed following a further period of drying. From this, individual seed weight and the mean number of seeds per capsule were calculated. 3. Results 3.1. Weather The 2-year period starting from July 1995 was generally much drier than normal (Fig. 1a). During


the period December 1995±February 1996, rainfall was similar to the 30-year average for this locality and this coincided with cooler than average temperatures which persisted into April (Fig. 1b). A return to more normal temperatures with close-to-average rainfall during July and August was followed by a cold, mostly dry period at the end of 1996. February, March and early April 1997 were warm whilst June was wet with low incident radiation. The latter part of the summer of 1997 was very warm and dry. Although August rainfall appears close to average, 20 mm fell on 12 August and 15 mm fell during the period 27±29 August.

Fig. 1. Seasonal variation in (a) total monthly rainfall (mm, vertical bars) and 30-year mean rainfall and in (b) mean monthly PAR (MJ mÿ2, vertical bars), mean monthly temperatures (8C, closed circles) and 30-year mean monthly temperatures (8C, open circles).


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3.2. Crop biomass production: year 1 Crop emergence was noted in the over-wintered plots on day 249 of 1995 and a population of 361 plants mÿ2 (28% of viable seed sown) was recorded on day 277. Biomass increased to a peak of 47 g mÿ2 on day 361, then declined to 26 g mÿ2 on day 102 of 1996 (Fig. 2a). Although partly attributable to a reduction in plant size, the decline was primarily due to a substantial reduction in plant numbers, probably caused by S. oenotherae. The decline in plant population continued rapidly until day 150 (144 plants mÿ2), then slowed considerably. Nonetheless,

plant growth resulted in a steadily more rapid increase in biomass after day 102. From day 150 to day 206, crop biomass increased linearly at a rate of 15.6 g mÿ2 per day. At GS 5.95 on day 246, the crop biomass was 1424 g mÿ2 and a similar result was obtained from the plots harvested on day 282. Crop emergence in the spring plots was ®rst noted on day 136 and a second ¯ush of seedlings emerged (following rain) around day 153. From their random distribution (rather than in rows) it was evident that most of the 56 plants mÿ2 which had established by day 171 were from August-sown seed. From day 182 until day 226, biomass increased at an almost constant rate of 14.9 g mÿ2 per day (Fig. 2a). The biomass on day 288 (GS 5.95) was approximately 93% of that of the winter crop at the same growth stage, but appeared to have declined by about 10% by day 330. 3.3. Crop biomass production: year 2

Fig. 2. Seasonal variation in crop biomass above and below ground (g mÿ2) in (a) year 1 (1996) and (b) year 2 (1997) in winter cv. Merlin (closed circles), winter cv. Peter (triangles) and spring cv. Merlin (open circles). Solid lines denote biomass to GS 5.95. Isolated points denote biomass of plants cut at GS 5.95 and left to lie in the swath. Dotted lines denote biomass excluding capsules and seed. Error bars represent 1 S.E. Downward arrows indicate approximate date of onset of ¯owering in winter and spring crops.

Crop emergence in the over-wintered plots was recorded on day 250 of 1996. On day 296 the cv. Merlin and cv. Peter plots contained 56 and 38 plants mÿ2, respectively, i.e. 8 and 4% of viable seed sown. Growth continued until day 327 when biomass of 8 and 9 g mÿ2 were recorded for the two cultivars and biomass then remained almost constant until day 54 of 1997 (Fig. 2b). The difference in the winter biomass of the year 1 and year 2 winter crops is largely attributable to the difference in plant population. In both cases, average plant weight during the winter was approximately 0.15 g. In year 2, very little crop mortality occurred during the winter, 52 and 44 plants mÿ2 being recorded in the cv. Merlin and cv. Peter plots, respectively, on day 130 (data not shown). Crop growth had recommenced by day 96, and over a period of 67 days from day 130, the mean biomass increase was 17.2 g mÿ2 per day in cv. Merlin and 16.0 g mÿ2 per day in cv. Peter. On day 239 (GS 5.95), cv. Merlin and cv. Peter biomass were very similar, but while the biomass of cv. Merlin was maintained in the plots harvested on day 266, the biomass of cv. Peter was about 30% lower. The result for cv. Peter is explained by the failure of the crop to compensate for a very low mean population (22 plants mÿ2) in these plots. Crop emergence in the spring-drilled plots was observed on day 129. Warm soil and irrigation resulted in the emergence of 19% of viable seed sown and 404

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plants mÿ2 were counted on day 159. June was exceptionally wet and this may have served both to maintain the total plant population and to maximise the number of bolted plants. Between day 179 and day 223, the rate of increase in bolted plant biomass was 14.5 g mÿ2 per day (Fig. 2b), very similar to the result for the spring crop in year 1. On day 223, the total population was 289 plants mÿ2, of which 206 plants mÿ2 had bolted. Whereas, in all other treatments, the rate of increase in biomass was almost maintained until GS 5.95, a decline in the rate was more evident in the April 1997-drilled plots. On day 273, when most plants had reached GS 5.95, the biomass was just 63% of that of winter Merlin at the same growth stage. The productive plants accounted for over 99% of bolted plant biomass. 3.4. Crop biomass partitioning In year 1, the difference in biomass at GS 5.95 between treatments was not statistically signi®cant (Table 1), but it should be noted that one of the spring crop plots yielded 1612 g mÿ2 whilst the yields of the other three plots ranged from 1194 to 1280 g mÿ2. In year 2, the winter crops produced signi®cantly more biomass than the spring crop. However, a greater proportion of the biomass was partitioned into stems


in the winter crops and in both years the spring crops achieved a higher harvest index (Z). The year 1 treatments produced similar seed yields (Table 2) but in year 2, the higher Z of the spring crop was not suf®cient to compensate for the lower biomass. 3.5. Canopy structure The leaf canopy of evening primrose is strongly planophile. In the winter crop of cv. Merlin assessed near peak Lt, 52% of leaf area was angled 58 to the horizontal and 76% of the leaf area was angled 158 to the horizontal. The equivalent ®gures for the spring crop were 41 and 70%. Leaves sampled at the top of the canopy were usually horizontal. Bolting had commenced in the year 1 winter crop by day 130 and the crop had attained maximum height by day 206 (Fig. 3a). The spring crop was much shorter. Bolting was ®rst observed on day 171 and full height was not reached until day 241. Leaf abscission in the winter crop was ®rst recorded on day 178 when the crop was 0.72 m tall, but Lt did not peak until 13 days later, the speci®c leaf area being 15.0 m2 kgÿ1 on this date. In contrast, leaf abscission was ®rst recorded in the spring crop at the same time as peak Lt, when the speci®c leaf area was 14.3 m2 kgÿ1. Peak Lt in the spring crop was only slightly less than in the winter

Table 1 Biomass of winter and spring crops of two cultivars of evening primrose at GS 5.95 and proportion of biomass in different plant parts. Also proportion of reproductive effort (i.e. capsule‡seed) in seed and the mean length of main stem bearing capsules Biomass (g mÿ2)



Leafa and flower


Seed (harvest index)

Seed as fraction of reproductive effort

Length of capsulebearing stem (m)

Year 1 Winter Merlin Spring Merlin

1424ab 1320a

0.063a 0.061a

0.404a 0.357b

0.227a 0.198a

0.167a 0.214b

0.139a 0.170b

0.454a 0.442a

0.336a 0.287b

S.E.D.c p-Value

145 0.191

0.002 0.527

0.007 0.008

0.011 0.072

0.006 0.003

0.004 0.004

0.006 0.163

0.002 0.020

Year 2 Winter Peter Winter Merlin Spring Merlin

1572a 1589a 997b

0.048a 0.052a 0.059b

0.512a 0.510a 0.445b

0.222a 0.225a 0.206a

0.115b 0.100a 0.167c

0.102a 0.112ab 0.124b

0.469b 0.528a 0.426c

0.724a 0.426b 0.198c

S.E.D. p-Value

53 <0.001

0.002 0.019

0.013 0.003

0.010 0.211

0.005 <0.001

0.005 0.020

0.010 <0.001

0.018 <0.001



Includes both green and dead leaf. Data with the same letter(s) within a column and year are not signi®cantly different at pˆ0.05. c Standard error of the difference between means for a one-way ANOVA for each year. b


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Table 2 Seed yield and yield components at GS 5.95 in winter and spring crops of two cultivars of evening primrose. Reproductive plants were those which produced viable seed. Stems include main stems and basal primary branches only Treatment

Plants mÿ2

Reproductive plants mÿ2

Reproductive stems mÿ2

Capsules per plant

Capsules mÿ2

Seeds per capsule

1000 seed weight (g)

Seed yield (g mÿ2)

Year 1 Winter Merlin Spring Merlin

124aa 49b

108a 41b

125a 126a

29a 82b

3080a 3367a

183a 178a

0.354a 0.374b

198a 224a

S.E.D.b p-Value

9.01 0.004

8.10 0.004

15.22 0.928

3.55 <0.001

203 0.251

6.72 0.541

0.006 0.034

14.9 0.171

Year 2 Winter Peter Winter Merlin Spring Merlin

37a 33a 252b

37a 33a 186b

44a 73a 186b

100a 83a 15b

3102a 2606a 2743a

127b 196a 135b

0.408b 0.348a 0.338a

161a 178a 123b

S.E.D. p-Value

26.89 <0.001

16.86 <0.001

17.05 <0.001

22.85 0.012

214 0.135

10.95 <0.001

0.018 0.013

8.70 0.002

a b

Data bearing the same letter(s) within a column and year are not signi®cantly different at pˆ0.05. Standard error of the difference between means for a one-way ANOVA for each year.

crop and the maximum canopy depth (the distance between the top of the crop and the lowest leaves) was also very similar in both treatments. Canopy depth declined as the plants matured and the lower leaves abscised. Bolting in the year 2 crops commenced on approximately the same dates as in year 1, but the winter crops grew much taller (Fig. 3b). As in year 1, the attainment of peak Lt coincided with the onset of ¯owering in all treatments. Lt in the winter crops reached extremely high levels owing to a combination of high leaf mass and high speci®c leaf areas (19.8 and 21.7 m2 kgÿ1 for cultivars Merlin and Peter, respectively). However, leaf abscission was already occurring by day 153, and following the completion of stem extension, Lt declined rapidly owing to the loss of leaves from the base of the canopy. By GS 5.95, almost all leaves below the capsule-bearing part of the stem had abscised (Table 1). At its maximum height, the year 2 spring crop was still at least 20% shorter than the winter crops in either year. Lt (calculated from all bolted plants) peaked at a low ®gure of 3.1, despite the high plant population and the high speci®c leaf area (23.8 m2 kgÿ1) recorded at this time. Speci®c leaf area in the spring crop eventually declined to 14.4 m2 kgÿ1, a ®gure comparable to that obtained in year 1. Again, few leaves remained below the capsule-bearing part of the stem at maturity.

3.6. Seed growth In year 1, seed growth continued for approximately 40 days in the winter crop and approximately 60 days in the spring crop (Fig. 4a). This longer duration was accompanied by a lower growth rate, resulting in similar seed yields. In year 2, winter crop seed growth commenced slightly earlier than in year 1 and again continued for approximately 40 days (Fig. 4b). At GS 5.95, the difference in yields between the two cultivars was signi®cant only at the 10% level. Seed growth in the spring crop commenced on virtually the same date as in year 1, but the duration of growth was only 50 days. The ®nal seed yield was signi®cantly lower than that of the winter crops, the markedly lower rate of yield accumulation in the spring crop more than offsetting the longer duration of seed ®lling. The duration of seed growth was more consistent across treatments when measured in thermal time, assuming a base temperature of 28C. In the winter crop, the duration was 6318C days in year 1 and 7328C days in year 2. The values for the spring crops were 759 and 7548C days, respectively. Following the start of seed growth only the seeds and capsules grew and the weight of the other plant parts remained almost constant (Fig. 2). In both year 1 treatments seed consisted of slightly less than half of the total reproductive effort at GS 5.95 (Table 1). In

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Fig. 3. Seasonal variation in canopy development in (a) year 1 (1996) and (b) year 2 (1997) in winter cv. Merlin (closed circles), winter cv. Peter (triangles) and spring cv. Merlin (open circles). The ®tted curves indicate plant height (m) as estimated from asymmetric logistic functions and dotted lines indicate position of the leaf canopy base (m). Also total green area index (Lt). Error bars represent 1 S.E. Downward arrows indicate approximate date of onset of ¯owering in winter and spring crops.


Fig. 4. Seasonal variation in capsule (dotted lines) and seed (solid lines) biomass (g mÿ2) in (a) year 1 (1996) and (b) year 2 (1997) in winter cv. Merlin (closed circles), winter cv. Peter (triangles) and spring cv. Merlin (open circles). Lines denote biomass to GS 5.95. Isolated circles and triangle denote seed biomass of plants cut at GS 5.95, left to lie in the swath, then hand-harvested. Squares denote seed biomass of plants treated in the same way, but machine-harvested. Error bars represent 1 S.E.

year 2, the combined (capsule‡seed) yields of the two winter treatments were very similar, but cv. Merlin partitioned more assimilate into seed than did cv. Peter. In year 2, spring crop seed accounted for a very low proportion of the reproductive effort.

winter and spring plots yielded 81 and 45%, respectively, of the equivalent sub-plots hand-harvested at GS 5.95 (Fig. 4a).

3.7. Machine harvesting losses

3.8. Yield components and distribution

In year 1, the subplots which were cut at GS 5.95, left to dry in the ®eld and hand-harvested on day 282 and day 330 produced similar seed yields to the subplots harvested at GS 5.95 (Fig. 4a), suggesting that no seed was lost from these plants whilst they were lying in the ®eld. By comparison, the combine-harvested

The pattern of yield components and distribution was in¯uenced by both plant population and cultivar. In year 1, the winter crop population had declined to 124 plants mÿ2 by GS 5.95, most of which had produced seed (Table 2), and 78% of seed was produced on main stems (Fig. 5). In the spring crop, the


A.F. Fieldsend, J.I.L. Morison / Field Crops Research 68 (2000) 9±20

Fig. 5. Seed yield (g mÿ2) of main stems, basal and upper primary branches and secondary branches at growth stage 5.95 for winter and spring crops of cv. Merlin in 2 years and a winter crop of cv. Peter in year 2. Error bars represent ‡1 S.E.

bolted plants compensated for the low population by forming basal primary branches which contributed 48% of the seed produced. The number of reproductive stems mÿ2 (i.e. main‡basal primary) was not signi®cantly different from the winter plots. Similarly, there were no signi®cant differences between year 1 treatments in the number of capsules mÿ2 and the number of seeds per capsule, but the thousand seed weight of the spring crop was 7% higher. Plant populations at GS 5.95 were low in both year 2 winter treatments and all plants produced seed, but cv. Peter gave a different yield component pro®le and a different pattern of seed distribution from cv. Merlin. The number of capsules per plant was higher, although the difference was only signi®cant at the 10% level, and the seeds were larger, but there were substantially fewer seeds per capsule. In cv. Merlin, 67% of yield was derived from the main stem. By contrast, in cv. Peter, not only was a very small percentage (21%) of the yield produced by the main stem, the yield contributed by the basal primary branches was negligible and 76% of yield was produced by the upper primary branches. In the majority of plants of cv. Peter, the main stem appeared to have aborted after producing relatively few ¯owers, allowing one or more branches to become dominant. The reason for this is unknown, but is similar to the condition reported by Collins and Scarisbrick (1994) which they termed `stunt'. As

growth stage was determined on plants not showing main stem abortion, the ®gure in Table 2 of 0.71 m for length of capsule bearing stem is an overestimate for the population as a whole. In the year 2 spring crop high plant populations suppressed branching and 94% of seed was produced on the main stems. Individual spring plants were very low yielding and on average produced few, small capsules. Despite the large population difference, the number of capsules mÿ2 and the thousand seed weight in the spring crop were not signi®cantly different from the winter cv. Merlin crop. In all treatments there was little or no contribution to seed yield from secondary branches. 4. Discussion Across our four sowings of cv. Merlin, the percentage of viable seed sown which emerged varied widely, with a maximum of 28%. Russell (1988) ascribed the variation between years in evening primrose crop biomass to differences in plant population, but did not quote population data. In our study, in each of the over-wintered and spring-sown treatments there was one high population year and one low population year, and in contrast to Russell (1988), lower plant populations did not result in lower crop

A.F. Fieldsend, J.I.L. Morison / Field Crops Research 68 (2000) 9±20

biomass. In spite of differences in population, we were able to show that the spring crops were consistently shorter, later and accumulated less biomass than winter crops. Despite the slow post-winter start (Fig. 2) in winter evening primrose, the duration of the growing period and the growth rate are suf®cient to bring about a high biomass yield. The peak rate of biomass accumulation in our winter crops was comparable with the rates reported for both evening primrose (Russell, 1988) and oilseed rape (Mendham et al., 1981). This phase coincided with stem extension, resulting in tall plants with thick stems which retain moisture and are bulky to combine harvest. In the spring crop, the rate of biomass accumulation did not peak until early July. The shorter growing period leads to a more manageable crop with shorter, thinner stems and a lower biomass. However, in both years, GS 5.95 was not reached until October, by which time plant material is slow to dry after swathing and combine harvesting is dif®cult. Our winter crops were more consistent with regard to biomass and harvest index than those of Russell (1988). The year 2 crops compensated for substantially reduced populations through the production of larger, more branched plants (Fig. 5) bearing more capsules (Table 2). The higher productivity of the year 2 crops may be attributable to the much warmer early February to mid-April temperatures in 1997. For example, on 9 May 1996, the biomass of the year 1 crop was 92 g mÿ2, 6% of ®nal yield (Fig. 2), but on the same date in 1997 the biomass of the year 2 Merlin and Peter crops was over twice that, representing 13 and 11%, respectively, of ®nal yields and Lt in the year 2 crop was 0.3, compared with 0.15 in the year 1 crop (Fig. 3). The reason for the particularly high peak Lt in our year 2 winter crops is not known but in their natural environment evening primrose plants are subjected to ``substantial levels'' of leaf defoliation as a result of herbivory (Morrison and Reekie, 1995) and perhaps produce an excess of leaf area to compensate. In the year 1 spring crop, biomass production approached that of the winter crop despite a low plant population. The spring crop was much shorter than the winter crop and partitioned much less assimilate into stem. Consequently, more was partitioned into reproductive effort and the seed yields of the two crops were


not signi®cantly different. In cv. Merlin spring crops can produce as many capsules mÿ2 as winter crops (Table 2). The relative performance of these crops re¯ects frequent commercial experience such as in eastern England in 1992 when winter crops of cv. Merlin yielded 0.73 t haÿ1 compared to 0.79 t haÿ1 for spring crops (Scotia, unpublished data from 220 ha). Bouwmeester et al. (1995) reported comparable results for caraway (Carum carvi), where biomass production of biennial crops was almost twice that of annual ones but the mean Z of annual caraway was ca. 57% compared to 36% for the biennial crop. The relative performance of our two spring crops was similar in several respects, indicating that the plants in year 1 were typical of a spring sowing. Emergence occurred at nearly the same time, i.e. day 136 in year 1 and day 129 in year 2. Despite large differences in plant population, the crops were of similar height and had similar ¯owering dates (Fig. 3). Seed growth was ®rst recorded on day 226 in year 1 and on day 223 in year 2. Biomass excluding reproductive effort was also similar, but Z in the year 2 spring crop was only slightly higher than in the winter crops and the seed yield was much lower (Fig. 4). July, August and particularly September 1997 were unusually dry and water shortage may have caused the low rate of increase in biomass during seed ®lling. The data suggest that the number of seeds per capsule was low, but it may be that the very smallest seeds were lost during sieving and that, although the true number of seeds per capsule may be higher, the mean thousand seed weight should be lower. In a low yielding evening primrose crop, Russell (1988) reported that leaves continued to be produced until harvest, possibly as a consequence of higher rates of applied nitrogen, and they may have been competing with the capsules and seeds for assimilates. In our crops, the capsule was the only signi®cant competitor to the seed for assimilate as the growth of other parts had stopped (Fig. 2). Cv. Merlin appears to put a greater proportion of assimilate into seed than did cv. Peter. Better partitioning of assimilate between seed and pod has been suggested as a breeding objective in white lupin (Lupinus albus, Huyghe et al., 1998), another novel agricultural crop, and we suggest this is also appropriate for evening primrose. The year 1 trial con®rmed that both winter and spring evening primrose crops are able to yield


A.F. Fieldsend, J.I.L. Morison / Field Crops Research 68 (2000) 9±20

2 t haÿ1 or more of seed. Such yields are rarely achieved in commercial practice. The highest recorded yields of commercial evening primrose crops are 1.95 t haÿ1 in New Zealand in 1990 and 2.22 t haÿ1 in OR, USA in 1992, both quoted at approximately 10% moisture content and both from spring sowings of cv. Merlin (Scotia, unpublished data). In many instances, low yields can be ascribed to agronomic problems such as poor crop establishment or weed or disease pressure. However, our results suggest that a signi®cant proportion of the yield potential even of well-grown crops may be lost during or after harvest. Non-splitting capsules should lead to improved seed retention by the plant after swathing, but seed may be lost during combining through incomplete threshing of the capsules, or in damp conditions, by seeds sticking to plant material discarded from the combine harvester, particularly in spring crops owing to their later time of harvest. During cleaning, light seeds will normally be discarded as part of the process of weed seed removal. 5. Conclusion Winter evening primrose crops produce large amounts of biomass and seed yields could be increased by at least 25% if harvest index was improved to the level achieved by the spring crop. In the spring crop, the late start to growth results in lower biomass production, but harvest index can be much higher and seed yields can match those of winter crops. Both winter and spring crops can produce in excess of 2 t haÿ1 of seed but a high proportion of this can be lost during combine-harvesting. Therefore, substantial improvements in evening primrose yields could be achieved if (a) the size of the stem was reduced in the winter crop; (b) the spring crop could be made to emerge earlier; (c) the proportion of assimilate partitioned into the capsule during seed ®lling could be increased, or seed losses during combine harvesting of either crop could be reduced.

Acknowledgements The assistance of the staff of the Scotia Plant Technology Centre is gratefully acknowledged. References Bouwmeester, H.J., Smid, H.G., Loman, E., 1995. Seed yield in caraway (Carum carvi). 2. Role of assimilate availability. J. Agric. Sci. (Camb.) 124, 245±251. Collins, C.D., Scarisbrick, D.H., 1994. The effect of the plant growth retardant RSW 0411 on assimilate distribution in evening primrose. Field Crops Res. 36, 59±67. Dodd, M., Scarisbrick, D.H., 1989. Evening primrose. Biologist 36, 61±64. Horrobin, D.F., 1990. GLA: an intermediate in essential fatty acid metabolism with potential as an ethical pharmaceutical and as a food. Dev. Contemp. Pharmacotherapy 1, 1±45. Huyghe, C., Harzic, N., Papineau, J., 1998. The genetics of crop architecture in white lupin. In: Anon. (Ed.), The Manipulation of Crop Architecture. Association of Applied Biology, Warwick, UK, 2 pp. Kromer, M., Gross, K.L., 1987. Seed mass, genotype, and density effects on growth and yield of Oenothera biennis L. Oecologia (Berlin) 73, 207±212. Leach, J.E., Darby, R.J., Williams, I.H., Fitt, B.D.L., Rawlinson, C.J., 1994. Factors affecting growth and yield of winter oilseed rape (Brassica napus), 1985±89. J. Agric. Sci. (Camb). 122, 405±413. Mendham, N.J., Shipway, P.A., Scott, R.K., 1981. The effects of delayed sowing and weather on growth, development and yield of winter oil-seed rape (Brassica napus). J. Agric. Sci. (Camb). 96, 389±416. Morrison, K.D., Reekie, E.G., 1995. Pattern of defoliation and its effect on photosynthetic capacity in Oenothera biennis. J. Ecol. 83, 759±767. Russell, G., 1988. Physiological restraints on the economic viability of the evening primrose crop in eastern Scotland. Crop Res., Hort. Res. 28, 25±33. Simpson, M.J.A., 1994. A description and code of development of evening primrose (Oenothera spp.). Ann. Appl. Biol. 125, 391± 397. Simpson, M.J.A., Fieldsend, A.F., 1993. Evening primrose: harvest methods and timing. Acta Hort. 331, 121±128. Stobart, R.M., Simpson, M.J.A., 1997. The effect of nitrogen rate on yield and oil content in evening primrose. In: Smartt, J., Haq, N. (Eds.), Domestication, Production and Utilisation of New Crops. International Centre for Underutilised Crops, Southampton, UK, p. 282.