Response of linseed (Linum usitatissimum L.) to irrigation, nitrogen and plant population

A field trial was carried out on a Wakanui silt loam from December 2003 to April 2004 at Lincoln University, Canterbury to determine the influence of irrigation (nil and full), nitrogen (N) at 0 and 150 kg/ha N and a range of plant populations (540, 1080, 1,620 and 2,160 plants/m²) on yield and yield components of linseed (Linum usitatissimum (L.) Griesb.). The design was a split plot with irrigation as main plots and factorial combinations of N and plant population as sub-plots. Linseed seed, straw and total dry matter (TDM) yield responded well to irrigation. Total dry matter production was increased by irrigation from 509 g/m² to 763 g/m². The main effects of plant population on seed and straw yield were also significant. However, N and plant density did not influence TDM production. The fibre yield/plant in irrigated plots was three times more than in rainfed plants. Without irrigation, fibre yield/plant did not respond to N. In irrigated plots, plants given 150 kg N/ha produced nearly twice as much fibre (0.8 g/plant) as plants which received no N (0.5 g/plant). Plant population influenced the production of primary branches more in irrigated plants than in unirrigated plants. Seed yield components except for the thousand seed weight (TSW), responded positively to irrigation. Nitrogen had no effect on seed yield components except for seed yield/plant. Increasing the plant population reduced capsule production/plant. Under dryland conditions plant population had little effect on capsule number/plant and seed yield. However, there was a large effect in plants from irrigated plots. Seed oil yield was increased by 77% in irrigated plants (87 g/m²) compared with rainfed plants (49 g/m²). Mean seed oil content decreased 2% in N fertilised plants (372 g/kg) compared with unfertilised plants (363.8 g/kg). At the highest plant population oil yield was 22% less than in plants sown at medium populations. The mean total unsaturated fatty acids (oleic, linoleic and linolenic) in the oil was 92.3% while the mean total saturated fatty acids (palmitic and stearic) was 7.7%. Throughout the growing season, irrigated plants accumulated significantly more DM than rainfed plants. Irrigated linseed produced a maximum yield of 800 g/m² with a maximum growth rate of 17 g/m²/day while rainfed plants produced a maximum yield of 505 g/m² with a maximum growth rate of 12.7 g/m²/day. Leaf area index (LAI) ranged from 0.61 to 2.2 in rainfed crops and 0.75 to 3.20 in irrigated crops. The higher LAI in irrigated crops was translated into greater leaf area duration (LAD). Water stress suffered by the plants before flowering reduced LAD by 83%. Dry matter production was linearly correlated with LAD from emergence to maturity. The canopies of unirrigated linseed crops intercepted 42 to 81% of incident radiation during the season while irrigated crops intercepted 56 to 91%. At early growth stages (26 to 44 days), the high population plots had a higher LAI and greater light interception than the low population plots. The extinction coefficient (k) ranged from 0.74 - 0.82 throughout the growing season with an overall k of 0.79. Total intercepted PAR was increased by irrigation by up to 25% from 505 to 633. The relationship between linseed DM and radiation interception was linear (r² = 0.88) and the efficiency of conversion of solar radiation to DM (RUE) ranged from 1.0 g DM MJ/m² (rainfed) to 1.2 g DM MJ/m² (irrigated crops). The yields obtained compare well to those obtained in other studies and confirm the potential for growing linseed in Canterbury. The positive yield responses to irrigation suggest that irrigation is necessary to maximise linseed yield especially since this study showed the sensitivity of the crop to water stress. If the crop is to be grown mainly for oil, N application may not be necessary in moderate to high fertility soils as high N reduced seed oil content and hence affected oil yield.