The risk of vegetative water deficit in early-sown faba bean (Vicia faba L.) and its implications for crop productivity in a Mediterranean-type environment

Farmers in Mediterranean-type environments should plant annual crops as early as possible to maximise yield. Greater rainfall variability at the start of the growing season potentially exposes early-sown crops to water deficit, which may be severe enough to cause crop mortality or to reduce potential productivity. This paper shows that in a typical Mediterranean-type environment at Merredin, Western Australia, much longer dry periods between rainfall events are likely in April rather than in May or June, but with a sowing rule based on farmer behaviour the likelihood of damaging water deficit is small. Soil water at sowing is a good indicator of this likelihood. The implications of early water deficit for crop productivity were investigated for faba bean in two experiments at Merredin in 1997 and 1998. In 1997 simulated plant available water in the top 40 cm (PAW40) at sowing was 24 mm and 8-week-old plants displayed severe wilting after 6 weeks without rain. There was no crop mortality even after 8 weeks without rain and plants recovered quickly when rewatered. Water deficit reduced grain yield through lower evapotranspiration since withholding water reduced total supply but also because severely stressed plants could not extract water from as deep in the soil as less stressed plants. In 1998 simulated PAW40 at sowing was 41 mm and no wilting was observed when water was withheld for 8 weeks. Apparent transpiration efficiency was not affected by mild water deficit in either year, but was reduced by 35% by delaying sowing in 1998. This was due to higher atmospheric vapour pressure deficit during reproductive growth of the later-sown crop. These results suggest that delaying sowing in faba bean is more likely to reduce faba bean grain yield unless there is a strong likelihood of severe water deficit soon after sowing. This likelihood can be judged from the amount of soil water at sowing.