Light absorption and water loss in overwintered and spring–sown evening primrose (Oenothera spp.) crops

European Journal of Agronomy 14 (2001) 275– 291 www.elsevier.com/locate/eja

Light absorption and water loss in overwintered and spring – sown evening primrose (Oenothera spp.) crops Andrew Fieldsend a,*,1, James I.L. Morison b a b

Scotia Pharmaceuticals Ltd, Plant Technology Centre, Writtle College, Chelmsford CM1 3RR, UK Department of Biological Sciences, Uni6ersity of Essex, Wi6enhoe Park, Colchester CO4 3SQ, UK Received 4 July 2000; received in revised form 16 November 2000; accepted 29 November 2000

Abstract Evening primrose (Oenothera spp.) is a high-value oilseed crop for temperate areas which may be either overwintered or spring–sown. Light absorption, light use efficiency, water loss and biomass water ratio were compared between overwintered and spring–sown crops of cv. Merlin in two years of field trials. An overwintered crop of cv. Peter was also studied in year two. 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 photosynthetically active radiation (PAR) interception for long periods but canopy closure occurred much later than in other temperate seed crops. In spring– sown evening primrose, maximum PAR interception did not occur until August, by which time incident light levels were declining and 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 the 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 (water use efficiency) 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 (‘‘normalised’’) and soil water deficit, after canopy closure normalised daily water loss declined with increasing soil water deficit. Earlier canopy closure, particularly in the spring crop, and the avoidance of soil water deficits through irrigation, would lead to substantial improvements in the size and consistency of seed yields of evening primrose crops. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Evening primrose; Oenothera spp; Light interception; Light conversion efficiency; Water loss; Biomass water ratio; Water use efficiency; Soil water deficit

* Corresponding author. E-mail address: [email protected] (A. Fieldsend). 1 Present address: 70 Pickwick Avenue, Chelmsford, CM1 4UR, UK. Tel./fax: +44-1245-442147. 1161-0301/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S1161-0301(01)00090-9

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Nomenclature Symbols and abbre6iations h fractional absorption of PAR (dimensionless) D saturation vapour pressure deficit of the air (kPa) daily rate of water loss (mm day − 1) E E% water loss (mm) ED ‘normalised’ daily water loss (mm kPa − 1 day − 1) Eh energy content of crop biomass at final harvest (MJ m − 2) Ep potential evaporation (mm day − 1) mc efficiency with which intercepted radiation is converted into biomass energy (dimensionless) mi fraction of It intercepted by the crop canopy (dimensionless) f fractional crop ground cover (dimensionless) It integral of incident insolation received over the life of the crop (MJ m − 2) i fractional interception of PAR by the crop canopy (dimensionless) kc crop coefficient (dimensionless) kj energy content of biomass (MJ g − 1) L green leaf area index (dimensionless) q soil water content (mm) qdef soil water deficit (mm) PAR photosynthetically active radiation (mmol m − 2 s − 1) ~p fractional transmission of PAR through the crop canopy (dimensionless) …B biomass water ratio (g kg − 1) …D ‘normalised’ biomass water ratio (g kPa kg − 1) B

1. Introduction In regions where substantial ‘overproduction’ of food crops has occurred, farmers have sought to diversify into non-food production and many plant species have been suggested as possible new crops. Evening primrose (Oenothera spp.) has become established as a non-food crop for temperate areas since it is both technically feasible to grow on an agricultural scale and because there is a market for the product. The seed oil contains g-linolenic acid, which has been shown to produce substantial clinical improvements when administered to patients suffering from diseases such as atopic eczema or breast pain (Horrobin, 1990). Evening primrose oil has gained widespread acceptance as a dietary supplement and pharmaceutical and world production of seed has been

estimated at 3000–5000 tonnes year − 1 (Simpson and Fieldsend, 1993). The plant is naturally a monocarpic biennial, but crops of evening primrose may be either overwintered or spring–sown. In eastern England, the overwintered crop is sown in August and harvested in October of the following year (Dodd and Scarisbrick, 1989) and is therefore exposed to a large amount of incident solar radiation integrated over its long growing season. Spring–sown crops normally emerge during May and have a much shorter rosette stage, but mature during November and seed filling occurs mostly during a period of declining light levels. Despite improvements through breeding and better agronomic practices, evening primrose yields are low and variable (Simpson and Fieldsend, 1993) and con-

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sequently the seed is expensive. Significant improvements in the productivity of evening primrose might be expected if resource capture and utilisation by the crop can be improved. Monteith (1977) defined the efficiency of crop production in thermodynamic terms as the ratio of energy output (mainly in the form of carbohydrate) to energy input (solar radiation), described by the equation: Eh =Itmimc/kj, where Eh is the energy content of the crop at final harvest (MJ m − 2), It is the integral of incident photosynthetically active radiation (PAR) received over the life of the crop (MJ m − 2), mi is the fraction of It absorbed by the crop canopy, mc is the efficiency with which the absorbed radiation is converted into crop energy (‘‘radiation use efficiency’’) and kj is the energy content of the biomass (kJ g − 1). Water stress can cause a reduction in mc (Russell et al., 1989). In East Anglia and Essex, annual rainfall in the range 400– 500 mm is of the same order as potential evaporation (Ep, mm day − 1) and the soil water balance runs into substantial deficits in most summers (Monteith, 1977). Such deficits reduce actual evaporation (E, mm day − 1) considerably below Ep. The saturation vapour pressure deficit (D, kPa) of the air influences water loss from a crop (E%, mm) and, to allow comparison of crops in different conditions, E should be ‘‘normalised’’ (E D, mm kPa − 1 day − 1) by dividing E’ by the mean D over the period (Monteith, 1993). Above- and below-ground biomass production per unit of water loss, frequently called ‘‘water use efficiency’’, is more correctly (Monteith, 1993) termed biomass water ratio (…B, g biomass kg H2O − 1). ‘‘Normalised’’ biomass water ratio, −1 …D ) can be calculated as the B (g kPa kg product of …B and D; equivalent to biomass divided by E D (e.g. Beale et al., 1999). Little research has been conducted on resource capture in evening primrose crops. This paper reports a comparison of light absorption, light use efficiency, water loss and biomass water ratio in overwintered and spring– sown crops and presents the first published values of It, mi and mc for evening primrose.

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2. Materials and methods

2.1. Site, treatments and weather This study was conducted over two seasons, 1995–1996 and 1996–1997, on four replication, randomised complete block trials grown on a silty loam of the Hamble 2 series, on a commercial farm at Hatfield Peverel, near Chelmsford, Essex, UK (latitude 51°47%N, longitude 0°31%E, altitude 50 m). The plot length was 9.85 m, each plot was 15 rows wide (except the overwintered plots in year one which were 20 rows wide) and the spacing between rows was 0.42 m. The year one trial consisted of overwintered and spring– sown plots of evening primrose cv. Merlin sown on 10 August 1995 and 15 April 1996, respectively. The year two trial consisted of three treatments, overwintered plots of cvs. Merlin and Peter sown on 14 August 1996 and spring–sown cv. Merlin sown on 15 April 1997. Other details of the site, trial design and crop establishment were reported by Fieldsend and Morison (2000). Daily temperature, precipitation, humidity, incident solar radiation and wind data were obtained from the Meteorological Office approved climatological station at Writtle College, Chelmsford, approximately 10 km from the experimental site. Daily maximum vapour pressure deficit was calculated from the vapour pressure recorded by a screened, unaspirated psychrometer at 0900 GMT and the maximum daily air temperature, by assuming that vapour pressure was constant during the day.

2.2. Biomass yield and energy content From each plot 1 m2 samples of plant material (including tap roots and most fibrous roots and, where appropriate, abscised material) were harvested by hand at intervals of two to three weeks during periods of rapid plant growth as detailed by Fieldsend and Morison (2000). The last harvest used in this study was carried out when capsules on 95 percent of the spike length contained non-white (i.e. cream to dark brown) seeds, defined by Simpson (1994) as growth stage (GS) 5,95. During year one kj of roots, leaves

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and, where available, stems plus capsules and seed was estimated on four occasions for the overwintered crop and three occasions for the spring– sown crop. 1.5 g plant subsamples were combusted in oxygen in an adiabatic bomb calorimeter (CB040, Gallenkamp, Loughborough, UK) which had been calibrated with standard quantities of pure benzoic acid. Average values of total crop kj were calculated by weighting the average kj of each plant part according to its proportion of the biomass. At other harvest dates kj was estimated using values for the constituent plant parts obtained by interpolating between the measured values. These values were assumed to be correct for the equivalent harvests in year two. Asymmetric logistic functions were fitted to the mean results for each date (SigmaPlot v.3, Jandel Scientific, Erkrath, Germany) to describe the time course of kj.

2.3. Fractional light interception and absorption During the rosette and early bolting phases fractional crop ground cover, ( f, dimensionless), measured using a portable red/near infrared spectral ratio meter (Macam Photometrics Ltd, Livingston, Scotland) was used as an estimate of fractional interception of PAR (i ) at approximately weekly intervals. To compensate for changes in the spectral quality of the incident light the measurements were expressed relative to a grey reference card (Eastman Kodak Co., Rochester, USA). The meter was supported 1.02 m above the crop giving a field of view of two crop rows. Within each plot four measurements were taken with the meter located directly above the row, and a further four with the meter located above the mid-point between the rows so that the biases with respect to over-sampling the centre of the field of view were cancelled out. The same sites were assessed on each date and f was calculated by the equation: f = 41.8 ln(r)+ 4.08, where r is the ratio of the meter reading from the grey reference card to that from the crop. This equation is similar to those reported for other crops including sugar beet (Steven et al., 1983). To

determine the values of the constants vertical overhead photographs of the crop were taken at the same sites using the method described by Biscoe and Jaggard (1985). A 35 mm camera with a lens of focal length 24 mm, high-speed infra-red sensitive film (2481 HIE, Eastman Kodak) and a Wratten 87 filter (Eastman Kodak) were used. Plots were photographed on three occasions in year one between day of year 104 and day 136, and on four occasions in year two between day 104 and day 183. Cloudy days were selected to minimise the effects of shadows. A wire ring was used to demarcate the field of view of the meter. The photographs were scanned into a computer and the percentage of pixels occupied by the plants was calculated with a purpose-designed image analysis program. Later in the growing season, i was estimated from fractional PAR transmission (~P) using an 80 cm line photometer (Sunfleck Ceptometer, Delta-T Devices Ltd, Cambridge, UK) positioned horizontally and perpendicular to the rows, where possible at the sites previously used for spectral ratio meter measurements. On almost all occasions, measurements were made within one hour of solar noon. Daily estimates of i were obtained by interpolation. PAR intercepted by the crop was calculated on a daily basis as the product of i and incident PAR. The difference between i and the fraction absorbed (h) is the fraction reflected by the canopy (Russell et al., 1989). To estimate canopy reflectance, measurements were taken at full crop ground cover in pairs, with the line photometer facing vertically upwards then vertically downwards above the canopy.

2.4. Pattern of light interception through the canopy Tube solarimeters (TSL, Delta T Devices Ltd) were placed at ground level, 375 and 750 mm above ground level in each of two overwintered and two spring–sown plots. A solarimeter located above the crop recorded incident solar radiation and the daily mean results from this were cross-checked against the results from a quantum sensor (SKP 215, Skye Instruments, Llandrindod Wells, Wales) (year one) or the climatological

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station at Writtle College (year two). Sensors were connected to a data logger (21X, Campbell Scientific Ltd, Shepshed, UK) recording at one minute intervals and averaged over 30 min. In year one, data from the solarimeters placed at ground level were also used to estimate late-season fractional interception. Tube solarimeters are sensitive to the whole solar spectrum and thus the data obtained were a measure of transmitted solar radiation ~T. ~P was calculated using the formula ~P =~T 1.35 (Russell et al., 1989).

2.5. Determination of soil water content and calculation of soil water deficit Changes in the volumetric soil water content (q, mm) in the plots were measured with a capacitance probe (Didcot Capacitance Soil Moisture Probe, model IH1, Didcot Instrument Company Ltd., Abingdon, UK) positioned within PVC access tubes. The tubes were installed soon after crop emergence. In year one, two access tubes were installed in each plot, one within the row and one mid-way between the rows and the mean result from each pair of tubes was calculated. In year two, one tube was installed in each plot, located within the row. Soil water content was measured at 60 mm intervals from a depth of 1 to 0.4 m, and at 20 mm intervals to a depth of 0.1 m. Measurements taken above 0.1 m depth were felt to be unreliable as the probe is responsive to a soil layer of several centimetres. To calibrate the capacitance probe, the water content of soil samples removed during the installation of the access tubes was determined gravimetrically on three occasions, day 310 of 1995 and days 177 and 319 of 1996. The relationships between these data and capacitance probe assessments carried out on the day following the installation of the access tubes were examined. In the first-year experiment, close linear relationships were obtained between the water content of soil samples and the values calculated from the capacitance probe readings (r 2 = 0.85− 0.92). Correlations between data sets for the second-year experiment were poorer, particularly for one tube (r 2 =0.61 and −0.52), probably due to soil heterogeneity (e.g. presence of stones) within the volume of measurement of the

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probe. Since the pattern of results for each tube were consistent throughout the sampling period, it was felt that, although some of the absolute figures obtained for the second-year experiment may be suspect the data gave a reasonable estimation of changes in soil water content. In both years the data indicated that the soil was still at field capacity at day 90, i.e. before the onset of rapid crop growth. Soil water deficit values (qdef, mm) were calculated from the difference between the treatment mean water content of the soil at field capacity (qFC) and the recorded mean q of each treatment. Following the onset of rapid crop growth, the plots were never at field capacity except for a very brief period in year two. Thus, E% for any period was calculated as the sum of precipitation plus any irrigation plus qdef of the soil between 0.1 and 1.0 m depth, ignoring both evaporation from the soil and from rainfall intercepted by the canopy. To estimate …B, the increase in crop biomass as derived from a curve fitted to the data reported by Fieldsend and Morison (2000) was divided by E% over the same interval. Values of Ep were estimated according to the procedures recommended by the Food and Agriculture Organisation (INSTAT v.6, Statistical Services Centre, Reading, UK)

3. Results

3.1. Crop energy content For most tissues kj was approximately 17 kJ g − 1 (Table 1), but the oil-rich seeds contained approximately 24 kJ g − 1. In both years, the energy content of the overwintered crop biomass at day 90 was B1 MJ m − 2 (Fig. 1), but then increased rapidly. Rapid accumulation of biomass energy in the spring–sown crops did not commence until approximately day 165. Despite this late start, the fitted functions estimated the energy content of the year one spring–sown crop at GS 5,95, Eh, to be 88% of that of the overwintered crop (Fig. 1a). By contrast, in year two although Eh in the overwintered crops was slightly higher than the previous year, Eh in the spring–sown crop was very low (Fig. 1b).

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3.2. Light interception and absorption All crops achieved full canopy closure and maintained high i for long periods (Fig. 2a and Fig. 3a). In year one, i by the overwintered crop increased rapidly after day 100 to reach 0.97 by day 177 and almost full i was maintained during the mid-summer period. The spring– sown crop emerged on day 137 and reached full i (0.96) on day 213, by which time levels of incident PAR were declining. In year two, i in the overwintered crops was already increasing by day 90 and in cv. Merlin had reached 0.93 on day 155. For cv. Peter the equivalent figure was 0.82. In both cultivars i declined markedly after day 211. The otherwise rapid increase in i in the spring– sown crop from day 161 was delayed by necrosis in the older, lower leaves caused by the pathogen Septoria oenotherae. After peak i was recorded on day 232, incident PAR levels rarely exceeded 6 MJ m − 2 day − 1. Differences were observed in the pattern of PAR penetration through the crops. The overwintered crops grew tall and, at the time of peak flowering and end of stem extension, leaves had already abscised from the lower 0.4– 0.5 m of the stem (Table 2). Even so, almost all PAR was intercepted by the leaf canopy and very little additional interception occurred below a height of 0.38 m. The spring– sown crops were much shorter. In year one, very little leaf abscission had occurred in the spring– sown crop by day 241, whereas in year two leaves had abscised from the lowest 0.38 m of stem by day 223. 14% of incident

PAR was intercepted between a height of 0.38 m and ground level by the year one crop compared with just 5% in the year two crop. The fraction of incident PAR reflected from the overwintered crop canopy was measured on four occasions between day 167 and day 232. The mean results of the four sets of data were 0.039 for cv. Merlin and 0.041 for cv. Peter. The equivalent figure for four sets of measurements made on the spring–sown crop between day 211 and day 269 was 0.042. Thus, for all treatments on all dates, absorbed PAR was taken to be 0.96 of incident PAR. The proportion of area covered by petals was estimated by computer image analysis of photographs taken from above the year two spring–sown crop during full flowering. The result obtained was 0.029 B0.01 and hence the effect of petal shading was disregarded.

3.3. Efficiency of photosynthetically acti6e radiation con6ersion For overwintered crops radiation use efficiency, mc, was calculated for the period between day 91 and GS 5,95 and for spring–sown crops mc was calculated from emergence to GS 5,95 (Table 3). The amount of PAR absorbed was remarkably consistent between years, with the overwintered crops absorbing approximately 35% more PAR than the spring–sown crops. In year one, mc of the spring–sown crop was 18% higher than the overwintered crop, but in year two, mc of the spring– sown crop was 14% lower. These figures mask differences in mc calculated for individual growth

Table 1 Energy content, kj, of different components of overwintered and spring–sown evening primrose plants cva Treatment

Day of year

Root (kJ g−1)

Leaf (kJ g−1)

Stem (kJ g−1)

Seed (kJ g−1)

Winter (sown 10.08.95)

319 102 191 246

15.8 9 0.2 16.4 9 0.1 17.3 9 0.1 17.2 9 0.2

16.6 9 0.2 16.6 9 0.1 17.3 90.1 17.0 90.2

17.3 9 0.1 17.6 9 0.2

24.4 9 0.1

171 226 288

16.2 9 0.1 17.4 9 0.2 18.0 9 0.2

15.8 90.1 16.8 9 0.1 16.2 9 0.2

17.4 90.1 17.8 90.1

23.1 9 0.1

Spring (sown 15.04.96)

a

Merlin harvested at different dates in 1995 and 1996. Values are means of four samples 9 1 SE.

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Fig. 1. Time course of crop energy above and below ground (MJ m − 2) in (a) year one (1996) and (b) year two (1997) in overwintered cv. Merlin (closed circles), overwintered cv. Peter (triangles) and spring – sown cv. Merlin (open circles). The curves are fitted asymmetric logistic functions. Error bars represent 91 SE. Dotted lines denote crop energy of biomass excluding seed.

increments which occurred during the season. In the year one overwintered crop mc was within the range 3–4% for most of the period of rapid growth, but declined as the crop matured (Fig. 2b). A high result during the period ending on day 191 coincided with a period of low incident PAR. In the year one spring– sown crop mc was high from day 213 to 275, but lower for the final

13 days before harvest. The error bars represent the errors combined in quadrature from the crop biomass data and the PAR interception data. The errors associated with energy content were negligible. More variation in mc with time was evident in year two (Fig. 3b). From day 97 to 197 mean mc was 4.6 for both overwintered cultivars. A low result for the period ending on day 153 coincided

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with a period of high PAR while from then until day 197 incident PAR was low. After day 197 there was a very rapid decline in mc. Similarly, in

the spring–sown crop mc declined rapidly after day 223 and averaged just 0.89% for the 29 days prior to harvest.

Fig. 2. Time course in overwintered (closed circles) and spring – sown (open circles) cv. Merlin in year one (1996) of (a) fractional light interception. Error bars represent 9 SE. Upward arrows indicate the approximate dates of peak flowering and end of stem extension. Also shown is incident daily photosynthetically-active radiation (five day moving averages). (b) PAR conversion efficiency (%). The data points represent the average efficiency for the period represented by the preceding lines. Error bars represent 9 the combined SE from the crop biomass data and the PAR interception data.

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Table 2 Crop height, the height of the leaf canopy base above the ground and fractional PAR interception at different heights in the canopy at the end of stem extension for overwintered and spring–sown crops of two cultivars of evening primrosea Treatment

Day of year

Crop height (m)

Canopy base (m)

Solarimeter height (m) Year 1 Winter Merlin Spring Merlin

206 241

1.109 0.02 0.74 9 0.02

Year 2 Winter Peter Winter Merlin Spring Merlin

197 197 223

1.319 0.01 1.399 0.02 0.839 0.04

Fractional interception of PAR

0.75

0.38

0

B0.40 0.04 90.01

0.63 90.03 0.14 90.09

0.94 90.01 0.82 90.01

0.98 9B0.01 0.96 9 0.01

0.51 90.01 0.47 90.02 0.38 90.04

0.83 90.07 b 0.85 0.16 90.06

0.96 9B0.01 0.97 9B0.01 0.89 9 0.02

0.98 9B0.01 0.99 9B0.01 0.94 90.01

a Heights are means of four samples 9 1 SE. Interception data are means of two samples 91 SE, each sample being the average value over five consecutive days. b Result from one solarimeter only.

3.4. Soil water status The period April to June 1996 was drier than normal for this location. During June, soil water deficit values, qdef, increased rapidly in the overwintered plots (Fig. 4a) whilst in the spring– sown plots, qdef increased steadily to reach 42 mm on day 290. Almost all water loss occurred from the upper 0.6 m of soil (data not shown, but see Fieldsend (2001)). In 1997 April and May were dry but June was very wet and precipitation was distributed throughout the month (Fig. 4b). In the year two overwintered plots, after day 137 qdef increased rapidly but the soil had returned to field capacity by day 180. Very little rain fell during July, August and September with the exception of 20.4 mm on day 224 and 22.2 mm during days 236–241. During July qdef again increased rapidly in the overwintered plots and after day 193, a rapid and prolonged increase in qdef also took place in the spring–sown plots, with a significant loss of water occurring in all plots from below a depth of 0.6 m (data not shown).

3.5. Crop water loss In year one water loss, E%, from the overwintered crop during the main growing period was almost 20% higher than from the spring– sown crop (Table 3) but average actual evaporation, E,

were similar (1.3 and 1.4 mm d − 1, respectively) due to the shorter growing period of the latter. In both crops normalised evaporation, E D, remained below 2.0 mm kPa − 1 d − 1 (Fig. 5a) and the apparently high E D for the overwintered crop for the period between days 47 and 121 can be attributed to water running-off or draining from the soil prior to day 90 (data not shown). In the overwintered crop, the crop coefficient (kc, the ratio of E% to Ep, dimensionless) was low while in the spring–sown crop kc was close to 1 (Fig. 5b) and kc did not decline as the crops matured. In the year two crops E% was much higher than in year one (Table 3) and average E for the overwintered and spring–sown crops were 1.8 and 2.1 mm d − 1, respectively. The latter value is probably an overestimate due to water draining from the soil following heavy rain during days 157–178. In the overwintered crops, E D values were low prior to canopy closure (Fig. 6a) but then exceeded the rates recorded in year one as the crops attained very high values of L (\ 6). At the time of peak L in the spring–sown crop, E D was similar to the values obtained in year one, but declined rapidly as the crop matured. In all year two treatments, kc increased with increasing crop ground cover to exceed 1 at peak L, during a period of very low potential evaporation, but then declined as the crops matured and as qdef increased (Fig. 6b).

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Table 3 Incident PAR, PAR absorbed and biomass energy accumulated, PAR conversion efficiency, water loss, biomass water ratio and normalised biomass water ratio from the start of active growth to GS 5,95 in two cultivars of evening primrose Treatment

Year 1 Winter Merlin Spring Merlin SED P-value Year 2 Winter Peter Winter Merlin Spring Merlin SEDe P-value

a

Incident PAR to GS 5,95 (MJ m−2)

a

Absorbed PAR to GS 5,95 (MJ m−2)

a

Biomass energy to GS 5,95 (MJ m−2)

a

PAR conversion efficiency (%)

b

Water loss (mm)

c

Biomass water ratio g (kg−1)

c

Normalised biomass water ratio (g kPa kg−1)

1109 983

765ad 574b 11.22 B0.01

25.2a 22.7b 0.50 0.03

3.29a 3.93b 0.14 0.02

198a 162b 4.37 B0.01

7.7a 8.1a 0.49 0.48

8.9a 9.3a 0.56 0.58

1091 1091 1008

741a 785b 579c 15.88 B0.01

27.0a 27.8a 17.1b 0.94 B0.01

3.68a 3.55a 2.97b 0.07 B0.01

261a 270a 258a 23.15 0.86

5.7a 5.4a 4.5a 0.50 0.28

6.3a 6.0a 5.3a 0.54 0.46

a

Calculated from day 91 (overwintered crops) or from crop emergence (spring–sown crops). Calculated from day 91 (overwintered crops), day 174 (spring–sown crop, year one) or day 151 (spring–sown crop, year two). c Calculated from day 102 (overwintered crop, year one), day 171 (spring–sown crop, year one), day 96 (overwintered crops, year two) and day 159 (spring–sown crop, year two). d Data with the same letter(s) within a column and year are not significantly different at P =0.05. e SED is the standard error of the difference between means for a one-way ANOVA for each year. b

Table 4 Biomass water ratios and normalised biomass water ratios of overwintered and spring–sown crops of two cultivars of evening primrosea Treatment

Overwintered cv. Peter

Period (day of year)

Biomass production (kg m−2)

…B g (kg−1)

Overwintered cv. Merlin …D B (g kPa kg−1)

Year 1 121–149 150–174 175–203 204–244 245–277 278–290 Year 2 115–151 152–180 181–210 211–236 237–252 253–269

366 547 395 125

8.5 5.4 5.8 2.7

8.8 4.5 7.4 4.3

Spring cv. Merlin

Biomass production (kg m−2)

…B g (kg−1)

…D B g kPa (kg−1)

157 333 505 344

5.9 7.3 13.4 4.8

4.1 10.3 17.4 6.7

431 649 340 72

6.7 7.8 4.3 1.8

7.0 6.5 5.5 3.0

Biomass production (kg m−2)

…B …D B −1 (g kg ) (g kPa kg−1)

312 586 250 37

8.5 6.5 9.1 5.2

11.0 8.9 7.8 3.5

103 395 318 73 27

1.0 7.4 4.3 2.9 4.7

0.9 9.5 7.0 3.3 4.5

a Biomass water ratios (…B) calculated as biomass production per unit of water loss. Normalised biomass water ratios (… D B) calculated as the product of …B and the mean vapour pressure deficit (D). Biomass production values were derived from fitted asymmetric logistic functions

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Fig. 3. Time course in overwintered cv. Merlin (closed circles), overwintered cv. Peter (triangles) and spring – sown cv. Merlin (open circles) in year two (1997) of (a) fractional light interception. Error bars represent 91 SE. Upward arrows indicate the approximate dates of peak flowering and end of stem extension. Also shown is incident daily photosynthetically-active radiation (five day moving averages). (b) PAR conversion efficiency (%). The data points represent the average efficiency for the period represented by the preceding lines. Error bars represent 9the combined SE from the crop biomass data and the PAR interception data.

3.6. Biomass water ratio In both years, similar figures for biomass water ratio, …B, and normalised biomass water ratio, …D B , over the main growing period were recorded

for overwintered and spring–sown crops but the values obtained in year one were 50% higher than those recorded in year two (Table 3). In all crops …B and … D B varied during the growing period and declined as the season progressed (Table 4). High

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values of … D B were achieved by the year one overwintered crop during the period of peak biomass accumulation, particularly during the 29-day period to day 203 when light levels were low. This was also a period of peak … D B in the spring –sown crop, although … D B remained high until day 277. In the overwintered crops in year

−1 for two … D B was approximately, 7 g kPa kg much of the period from day 115 to day 221, i.e. the main growing period. For the 29 days to day 180 … D B was low, particularly in the crop of cv. Peter. This coincides with the period of heavy rainfall, when some run-off of water may have occurred. Similarly, water draining from

Fig. 4. Seasonal variation in soil water deficit (mm) in (a) year one (1996) and (b) year two (1997) in overwintered cv. Merlin (closed circles), overwintered cv. Peter (triangles) and spring –sown cv. Merlin (open circles) plots. Error bars represent 9 1 SE (for clarity, only one error bar is shown for each overwintered treatment in Fig. 4b). Also daily precipitation (mm).

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Fig. 5. Time course in overwintered (solid lines and closed circles) and spring – sown (dotted lines and open circles) cv. Merlin in year one (1996) of (a) normalised crop water loss (mm kPa − 1 d − 1) and leaf area index and (b) crop coefficient and potential evaporation (mm d − 1, five day moving averages). The data points represent the average values for the period represented by the preceding lines. Error bars represent 91 SE.

the soil contributed to an extremely low … D B for the spring–sown crop at this time. From day 181 to day 236, the … D B values for the spring– sown crop were similar to those for other treatments.

There appears to be no relationship between …D B and qdef (data not shown). However, after canopy closure, E D declined with increasing qdef (Fig. 7a). A decline in mc is also evident as qdef exceeded 60 mm (Fig. 7b).

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4. Discussion Canopy closure in evening primrose is slower than in other temperate seed crops such as oilseed rape and wheat and evening primrose is later to

mature. Consequently in overwintered crops the values of PAR received, It, and fraction absorbed, mI, for evening primrose (approximately, 1100 MJ m − 2 and 0.7, Table 3) are comparable to simulated figures from day of year 105 to maturity for

Fig. 6. Time course in overwintered cv. Merlin (solid lines and closed circles), overwintered cv. Peter (dashed lines and triangles) and spring– sown cv. Merlin (dotted lines and open circles) in year two (1997) of (a) normalised crop water loss (mm kPa − 1 d − 1) and leaf area index and (b) crop coefficient and potential evaporation (mm d − 1, five day moving averages). The data points represent the average values for the period represented by the preceding lines. Error bars represent 91 SE.

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oilseed rape (829 MJ m − 2 and 0.8) and wheat (1034 MJ m − 2 and 0.7) in the Netherlands (Habekotte´ , 1997). However, mi was poor (0.58, Table 3) in our spring– sown evening primrose crops because canopy closure was not completed until after day 200, by which time incident light levels were declining. Earlier establishment in the spring–sown crop should lead to a significant increase in mi coupled with earlier maturity. At peak L, almost all incident PAR was intercepted by the leaf canopy (Table 2) and in the overwintered crops a substantial length of stem below the canopy was therefore shaded and presumably had become reliant on the rest of the canopy for photosynthate. The reason for the very high peak L in the year two overwintered crops is unclear. In year one light capture at peak L by the spring crop occurred over a canopy depth of 0.8 m, whilst in year two a canopy depth of 0.45 m was sufficient to accomplish full light interception. Unlike in oilseed rape (Mendham et al., 1981), very little light is intercepted by the flowers of evening primrose. The energy content, kj, of evening primrose vegetative biomass is close to 17.3 kJ g − 1, the heat of combustion of carbohydrate (Penning de Vries et al., 1989) whilst the results obtained for seeds (Table 1) are consistent with a content of approximately, 25% oil with a heat of combustion of 37.7 kJ g − 1. Overwintered and spring– sown crops gave similar values of kj, except that a lower kj for seed from the latter reflected its lower oil content. Taking kj as 17.3 kJ g-1 a value for mc of 4.8% would be expected if the biomass radiation ratio for evening primrose was 2.8 g MJ − 1 PAR, a typical value for unstressed crops (Monteith, 1977). Although this value was not achieved for the entire period from the start of active growth to GS 5,95 by any crop in this study (Table 3), in all crops values of mc close to 4.8% were obtained during the period of most rapid growth (Fig. 2b and Fig. 3b) showing that the efficiency of light use by evening primrose is similar to that of other crop species. Lower mc were recorded during periods of high incident PAR (Figs. 2 and 3) indicating that radiation saturation of even well-

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developed evening primrose canopies can occur. The significant decline in all treatments in mc following canopy closure may in part reflect the increasing respiratory burden imposed by the stem and the developing seeds. In year one, which was drier, crop water loss was lower and …B was higher than in year two (Table 3). The greater total water loss in overwintered crops can be attributed mainly to the longer growing season but …B was similar in overwintered and spring–sown crops (Table 3). Crop water loss values for C3 crops, when ‘‘normalised’’ to take into account atmospheric demand, typically range between 2.5 and 5 g kPa kg − 1 (e.g. Beale et al., 1999). In this study, … D B values in evening primrose exceeded this range, particularly in year one, and this may be attributed partly to our inclusion of roots (representing between 5.2 and 8.4% of biomass at GS 5,95). Also, the crops may have used water from lower in the soil profile but this is more likely to have occurred in year two, when … D B values were lower. By contrast, the obvious uncertainties in our method (not estimating evaporation from the canopy surface or soil) and possible drainage would contribute to an underestimate of … D B . Values of kc were low (Figs. 5 and 6) even though these were relatively small plots and might have been expected to be susceptible to advection. A decline in … D B with increasing qdef (Fig. 7a) suggests stomatal control of crop water loss. This, plus the fact that evening primrose roots were able to extract water from a depth in excess of 0.6 m (Fieldsend, 2001), may explain why populations of evening primrose plants frequently occur on sites with low water-holding capacity such as sand dunes (Dodd and Scarisbrick, 1989). No relationship between … D B and qdef was evident, indicating a matching of carbon uptake and growth rate with water loss. This is also consistent with the observation that evening primrose plants growing on sand dunes may take several years to reach a critical rosette size and flower (Dodd and Scarisbrick, 1989). Whilst the late-season decline in mc in year one (Fig. 2b) can be attributed to ontogenic changes such as an ageing leaf canopy the much sharper reduction in mc in year two (Fig. 3b) was caused by

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water stress. As qdef exceeded 60 mm a clear decline in mc occurred (Fig. 7b) indicating stomatal closure and reductions in CO2 uptake. In this study, the high qdef coincided with the seed growth phase of the year two spring– sown crop and caused a significant loss of yield. This suggests that irrigation should be used to prevent the

occurrence of large, late-season soil water deficits in spring–sown evening primrose crops. 5. Conclusion Evening primrose crops can achieve full canopy closure and can maintain high fractional PAR

Fig. 7. The relationship between soil water deficit (mm) and (a) normalised biomass water ratio (g kPa kg − 1) and (b) PAR conversion efficiency (%) in evening primrose. Circles: (closed) overwintered and (open) spring – sown cv. Merlin in year one; squares: (closed) overwintered and (open) spring –sown cv. Merlin in year two and triangles: overwintered cv. Peter in year two.

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interception for long periods. The efficiency with which light energy was converted into biomass energy was comparable to other C3 crop species, as was the energy content of crop biomass. However, the proportion of incident light captured by spring –sown evening primrose crops was low, because canopy closure occurred late and more rapid canopy closure should result in higher biomass yields or earlier crop maturity. Water loss per unit of carbon gain (expressed as normalised biomass water ratio) was relatively high for a C3 crop species. Moderate soil water deficits caused a reduction in light conversion efficiency and consequently biomass yield and in such instances irrigation is likely to be justifiable in order to ensure maximum crop yield.

Acknowledgements We thank Dr. Kevin Oxborough of the Department of Biological Sciences, University of Essex for the development of the image analysis software and Dr. Clive Beale of Writtle College for assistance with the soil water probe.

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