Abstract

The effect of temperature (18/10°C and 26/18°C day/night) and water deficit (50%) was examined in winter rapeseed cv. Samourai from the end of the flowering period to maturity. Yield and the yield components, number of siliques/m² and seed number/m², significantly decreased at high temperature and reduced water. Single seed weight decreased at high temperature and also at low temperature under conditions of water stress. Nitrogen content increased and seed oil content decreased at elevated temperature and with water stress. In oilseed rape, oil is deposited late during seed development. At the onset of oil deposition, the fatty acid profile consisted of 50-70% saturated fatty acids (C16:0, C18:0), and a greater content of C18:2 (15-27%) than C18:1 (6-12%). At seed maturity, high temperature resulted in an increase of the C18:1 content and a decrease of the C18:3 content. At low temperature and under conditions of water stress, the C18:1 content decreased and the C18:3 content increased.

INTRODUCTION

Our objective was to quantify the effect of environment, and more particularly of temperature and water stress during seed development and maturation, on seed quality and especially on the fatty acid composition.

MATERIAL AND METHODS

Plants of winter rapeseed cultivar Samouraï were grown in 2 m² containers under natural conditions. At the onset of flowering, the containers were placed into four enclosed tunnels in which CO₂ concentration and hygrometry were controlled, and a 22/14°C day/night temperature regime was applied (Triboi et al., 1996). Towards the end of flowering, 200 degree days after pollination of the first 20 flowers on the main stem, the first 20 siliques were collected on 10 plants (200 siliques in total) in each container. Four treatments were then applied to the plants: 18/10°C or 26/18°C day-night temperatures with or without water stress (50% of the evapotranspiration of the control). In each treatment, seeds were harvested (3 replicates) every 200 degree days until maturity. Oil content was determined by Nuclear Magnetic Resonance. Fatty acid composition was analysed using gas chromatography of methyl esters. Each fatty acid was expressed as a percent of the total fatty acids. Protein content was estimated by Kjeldahl N determination x 6.25. At harvest, yield and the yield components number of siliques/m², average silique weight, number of seeds per silique, number of seeds/m², and average seed weight were determined for the three replicates (0.5 m²) per treatment.

Results and discussion

Yields and yield components (Table 1)

At harvest, seed yield was significantly affected by temperature and water stress with a major relative reduction at low temperature. The first of yield components to be determined, the number of siliques per m² was affected similarly (as yield) at high temperature or in conditions of water stress. Silique losses were greater on lower branches that formed towards the end of flowering. The last formed siliques aborted when temperature increased or when water was reduced. This allowed better seed survival in the remaining siliques (i.e. that had formed earlier) with a greater number of seeds per silique with the water stress treatments at both temperatures. Nevertheless, average silique weight is only slightly modified by water stress at low temperature, and decreased significantly with an increase of temperature without any effect on seed number per silique. The last yield component, average single seed weight, depended on the rate and duration of seed growth, related mainly to temperature, and depended also on the total number of seeds per plant. In this experiment, single seed weight decreased at high temperature and also at low temperature under conditions of water stress.

Table 1. Yields and yield components (relative values in %) as influenced by temperature (18/10°C and 26/18°C day/night temperatures) and water regime (I= Irrigated and S= Water Stressed) applied from the end of flowering until maturity. Means within a row followed by different letters are significantly different at the P<0.05.

Oil and protein content (Figure 1)

At maturity, in a cool environment and with an adequate water supply (environment I1) the seeds were larger and had a high oil content, as observed in Tasmania compared to Western Australia by Mendham and Salisbury (1995). High temperature (environment I2) reduced oil content. Water stress also reduced oil content at both low and high temperature. Our results were consistent with previous work which demonstrated a strong oil content reduction with a water deficit from anthesis to maturity (Champolivier and Merrien, 1995). Conversely, seeds growing in a cool environment had a low protein content; high temperature and water stress decreased protein content. Thus, oil and protein contents were negatively correlated and we observed a 8 % difference for oil or protein content between the two extreme environments, I1 and S2, i.e. low temperature/adequate water supply and high temperature/water stress.

Figure 1. Oil and protein content of seeds at maturity as influenced by temperature (18/10°C and 26/18°C day/night temperatures) and water regime (I= Irrigated and S= Water Stressed). Data are shown as means ± SE of 3 replicates.

Fatty acid composition (Figure 2)

In oilseed rape, oil is deposited late during seed development. Seeds were sampled every 200 degree days after pollination. We observed 5-10% oil (when the seed size is about 0.5 mg), 200 degree days after pollination when profile consisted of 50-70% saturated fatty acids (C16:0, C18:0), and a greater content of C18:2 (15-27%) than C18:1 (6-12%). The fatty acid composition changed rapidly in subsequent days, and the proportions approached those observed in mature seed. At maturity, high temperature resulted in a significant increase of the C18:1 content and a decrease of the C18:3 content. This is consistent with the results of Trémolières et al. (1982) and Pleines et al. (1987) who showed that low temperature increased C18:1 and C18:2 desaturation, resulting in a higher C18:3 content of mature oilseed rape seeds. At low temperature and under conditions of water stress, the C18:1 content decreased and the C18:3 content increased.

REFERENCES

1. Champolivier L, Merrien A (1996). Effect of water stress applied at different growth stages to Brassica napus L. var. oleifera on yield, yield components and seed quality. Eur J Agron 5, 153-160.

2. Mendham NJ, Salisbury PA (1995). Physiology: crop development, growth and yield. In: Kimber D and McGregor DI ( Eds.), Brassica oilseeds - Production and utilization, CAB, pp 11-64.

3. Pleines S, Marquard R, Friedt W (1987). Recurrent selection for modified polyenoic fatty acid composition in rapeseed (Brassica napus L.). 7^th Int Rapeseed Congress, Poznan (Poland), 140-145.

4. Trémolières A, Dubacq JP, Drapier D (1982). Unsaturated fatty acids in maturing seeds of sunflower and rape: regulation by temperature and light intensity. Phytochem 21, 41-45.

5. Triboi E, Triboi AM, Martignac M, Falcimagne R (1996). Experimental device for studying post-anthesis canopy functioning in relation to grain quality. 4^th ESA Congress, Veldhoven (NL), 68-69.

Figure 2.

A) Oil component (fatty acids), carbohydrate and protein accumulation (mg seed^-1) during seed development (expressed in accumulated degree days after pollination) of control plants (environment I1).

B) Fatty acid composition at 200 degree days and final (>1000 dd) composition of seeds subjected to four treatments involving two temperatures and two water regimes). Means within a row followed by different letters are significantly different at the P<0.05.

Abbreviations: tt= thermal time; dd = degree days after pollination; C16:0 = palmitate ; C18:0 = stearate; C18:1 = oleate; C18:2 = linoleate ; C18:3 = linolenate