Effects of different water regimes on field-grown determinate and indeterminate faba bean (Vicia faba L) .1. Canopy growth and biomass production

Soil water shortage has been identified as a major constraint in increasing faba bean production. However, high water availability could cause excessive vegetative growth in indeterminate faba bean. Therefore, the present study was conducted with the objectives of quantifying canopy growth and biomass production of determinate and indeterminate faba bean under a range of water availability and thereby finding the suitability of different cultivars for different water regimes. Two field experiments were carried out in 1989 and 1990 at Reading, United Kingdom, using the determinate cultivar 'Tina' and the indeterminate cultivar 'Gobo'. Rainshelters and trickle irrigation were used to establish four different water regimes: unsheltered and well-irrigated (W-1); sheltered at flowering and subjected to post-flowering water stress (W-2); sheltered throughout the season and subjected to continuously-increasing water stress (W-3); open rainfed (W-4). Leaf initiation, expansion and biomass production were significantly reduced by soil water deficits. A linear relationship was observed between final biomass and total leaf area duration. The relationship was common for all water treatments and both cultivars. This indicated that for the two cultivars tested, the effects of water stress were more on canopy size than on photosynthetic efficiency. A constant photothermal duration, which was similar for both cultivars, determined the durations from seedling emergence to flowering and maturity. Crops were planted 3 weeks earlier in 1990 than in 1989. Due to lower temperatures and photoperiods during the early part of growth, the 1990 crops took a longer duration to flower and mature as compared to 1989 crops. In the absence of water stress, a longer post-flowering duration allowed the indeterminates to develop a larger canopy and achieve a high final biomass (21 t/ha in 1990). The determinates were unable to produce a high biomass due to cessation of leaf production after flowering. In the shorter 1989 season, the determinate W-1 had a higher biomass (12 t/ha) than the indeterminate W-1 (10 t/ha). With increasing post-flowering water stress, the final biomass was mainly determined by the LAI at flowering. A higher LAI at flowering produced a greater final biomass. Only the indeterminates had the flexibility in phenology to take advantage of the post-flowering irrigation in W-2. It is concluded that if the whole biomass is to be utilized, indeterminate faba bean are more suitable for early-spring plantings with adequate water availability and for both early- and late-spring plantings under increasing water stress. Determinate faba bean are recommended only for late-spring plantings at sites with adequate water availability which could be predicted by analysis of past rainfall data. The determinates had less season-to-season variability in biomass. Efforts should be made to increase their overall biomass production potential while retaining this stability.