Click on the image below to download my book on evening primrose.


Selected additional papers:

• Fieldsend, A.F. (2005): Interactive effects of light, temperature and cultivar on photosynthesis in evening primrose (Oenothera spp.) crops. Acta Agronomica Hungarica 52 (4), 333-342.

• Fieldsend, A.F. and Morison, J.I.L. (2001): Light absorption and water loss in winter and spring evening primrose crops (Oenothera spp.). European Journal of Agronomy 14, 275-291.

• Fieldsend, A.F. and Morison, J.I.L. (2000): Contrasting growth and dry matter partitioning in winter and spring evening primrose crops (Oenothera spp.) Field Crops Research 68, 9-20.

• Fieldsend, A.F. and Morison, J.I.L. (2000): Climatic conditions during seed growth significantly influence oil content and quality in winter and spring evening primrose crops (Oenothera spp.) Industrial Crops and Products 12, 137-147.

• Fieldsend, A.F. and Morison, J.I.L. (1999): Manipulating light capture and seed yield in winter and spring evening primrose (Oenothera spp.) Aspects of Applied Biology 56, 233-240.

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You are here: Home >> My PhD thesis

A Summary of my PhD Thesis

During my PhD research, biomass accumulation and partitioning, canopy development and light and water use were investigated in three years of field trials and in a controlled environment study. Many key factors determining yield formation in evening primrose were identified. Five papers and a book entitled 'Evening Primrose (Oenothera spp.): Productivity, Canopy Development and Resource Use' were published from this study.

Aims and objectives

Evening primrose (Oenothera spp.) is a relatively new, high value oilseed crop which has become established as a non-food crop for temperate regions since it is both technically feasible to grow on anagricultural scale and because there is a market for the product. The seed is an important source of γ-linolenic acid, a relatively rare fatty acid with value as a pharmaceutical and nutritional supplement. Despite a long growing season, its seed yields are much lower than the average yields of major arable seed crops and hence the seed is expensive. Also, commercial spring-sown crops can yield as much as overwintered ones. The main benefit of spring sowing is a shorter growing season but the crop matures late and harvesting can be difficult, particularly in a wet year.


The aims of this study were, through three years of field trials and a controlled environment study, to:

  • Evaluate the influence of various factors on evening primrose productivity;
  • Compare the productivity of overwintered and spring-sown evening primrose.

The following objectives were set to achieve these aims:

  • To record the pattern of biomass accumulation and partitioning in the crop;
  • To measure the harvest index and components of yield at maturity;
  • To study the pattern of light interception by the crop throughout the season;
  • To calculate the crop photosynthetic efficiency and assess the influence of water stress;
  • To measure the effect of low temperatures on growth of evening primrose rosettes;
  • To record the pattern of seed oil and fatty acid accumulation.

Growth and biomass partitioning

Chapter 2 describes a comparison of biomass accumulation and partitioning and crop canopy development in overwintered and spring-sown crops in two years of field trials, 1995-96 and 1996-97. The widely-grown commercial cultivar Merlin was used in year one. The year two trial, which was of necessity sown before the year one trial was harvested, also included an overwintered crop of cv. Peter to provide a comparison of two cultivars with distinct differences in plant habit. Although post-winter growth was slow to restart, the overwintered crops produced up to 1.6 kg m-2 of biomass. The harvest index was low (<14 per cent) because the crops grew tall and approximately half of the assimilate was partitioned into stem. Biomass production of the spring-sown crops was lower, but the harvest index was higher (up to 17 per cent) and in year one the overwintered and spring-sown 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 two overwintered 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, cv. 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 per cent respectively of the seed produced by the year one overwintered and spring-sown crops.

Resource use efficiency

Chapter 3 reports a comparison of light absorption, light use efficiency, water loss and biomass water ratio in these trials. The energy content of evening primrose plant material was shown to be similar to other crops. Both overwintered and spring-sown crops can achieve full canopy closure and maintain high fractional PAR interception for long periods but canopy closure occurred much later than in other temperate crops. In spring-sown evening primrose, full PAR interception did not occur until August, by which time incident light levels were declining. Consequently, the proportion of incident light energy captured during the main growing season was low. Most light was intercepted by green leaves and very little shading by senescent tissue and flowers occurred. Light conversion efficiencies for the main growing period were comparable with other temperate C3 crops, but in year two a steep decline in light conversion efficiency was observed as the crops matured and the soil water deficit exceeded 60 mm. In year one, water loss from both the overwintered and spring-sown crops were low and the soil water deficit increased relatively slowly. By contrast, in year two crop water loss was high and the soil water deficit built up very rapidly between the end of June and crop maturity. No significant differences in biomass water ratio were recorded between overwintered and spring-sown crops but ratios were 50% higher in year one than in year two. Although no relationship was detected between biomass water ratio corrected for vapour pressure deficit and soil water deficit, after canopy closure corrected daily water loss declined with increasing soil water deficit.

Low temperature effects on rosette growth

The first two years of field trials showed that the overwintered crop grew very slowly in the early spring. Post-winter growth in year two was recorded approximately 50 days earlier than in year one. The major environmental difference between the two years was temperature. A controlled environment experiment set up to assess the effect of temperature on the growth of evening primrose rosettes is described in Chapter 4. Relative growth rate was positively correlated with temperature but in apparent contrast to the results from the field trials the rosettes grew at constant temperatures as low as 6.5ēC. However, following transfer to warmer temperatures an increase in relative growth rate did not occur until 7-10 days later whilst a change to a cooler environment caused an immediate reduction in relative growth rate. Partitioning of biomass between root and shoot was independent of temperature but at 6.5ēC the relative rate of leaf area increase was very low. Consequently, the specific leaf area was lower in rosettes growing at lower temperatures.

Manipulating light capture to increase seed yield

The year three (1997-98) field trial, described in Chapter 5, investigated whether the spring-sown crop could be manipulated to intercept more light and the consequent effects on biomass and seed yield. Using cv. Merlin, an overwintered crop was compared with early (March) and late (May) spring sowings. The early-sown crop achieved a better light interception efficiency than the later sowing by developing a canopy before the time of peak PAR. This resulted in a higher biomass, higher seed yields (comparable to those of the overwintered crop) and earlier maturity. The spring-sown crop partitioned almost 50 per cent of biomass energy into reproductive effort (i.e. capsules + seeds). In all crops, values for the biomass radiation ratios were comparable to those reported for other crop species.


Seed oil content and quality

Chapter 6 reviews the effect on oil content and quality of climatic conditions during seed growth in all three years of field trials. At the onset of oil accumulation, palmitic acid, linoleic acid and α-linolenic acid were the predominant fatty acids in the seeds and γ-linolenic acid was hardly present. At maturity, linoleic acid constituted 70-75 per cent of the oil, γ-linolenic acid content ranged from 8.0 to 9.9 per cent and α-linolenic acid was almost undetectable. In all years, seeds from the overwintered plants of cv. Merlin contained more oil than did seeds from the equivalent spring-sown plants, but the γ-linolenic acid content of the oil was lower. The rate of increase in seed oil content was faster in the overwintered crops but the duration of oil accumulation was shorter. Oil content at seed maturity in cv. Merlin was positively correlated with both mean daily temperature (r2=0.59) and mean daily incident solar radiation (r2=0.71) during the main period of seed filling. Strong negative correlations existed between the final γ-linolenic acid content of the oil and both climatic variables during the final phase of oil accumulation (r2=-0.78 and -0.83 respectively). Temperature was probably the primary determinant of the final γ-linolenic acid content but it was unclear which variable most influenced final seed oil content. Differences in oil content and seed size also existed between seeds harvested from different parts of the same plant.


Overwintered and early spring-sown trial crops of evening primrose (Oenothera spp.) produced seed yields in excess of 2 t ha-1. The overwintered crop absorbed more photosynthetically active radiation (PAR) and produced proportionately more biomass but this was offset by a lower harvest index. Substantial improvements in the size and consistency of seed yields of evening primrose crops could be achieved by an improved harvest index in the overwintered crop, an earlier start to growth and earlier canopy closure, particularly in the spring-sown crop, the avoidance of soil water deficits through irrigation and an increase the proportion of seed recovered by combine harvesting.

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