Thresholds for faba bean (Vicia faba L.) leaf expansion and stomata conductance response to plant available water

Leaf expansion and stomata opening of faba bean (Vicia faba L.) are closely related to the amount of plant water availability (PAW). The study was conducted to determine the critical level of PAW for leaf expansion and stomata opening as affected by the length of soil drying cycle. Soil water content of control treatment was maintained at 100% field capacity (FC), while water stress plants were irrigated back to field capacity when the soil water content decreased to 80, 60, 40, 20 and 0% of field capacity. Daily water content was measured by weighing each pot and expressed as the fraction of plant available water (PAW). Leaf expansion was measured every four days and stomata conductance when the respective drying cycle was reached. Relationship between relative leaf expansion (RLA) and relative stomata conductance (RSC) was described by a logistic function. A 50% reduction (critical threshold level) in RLA and RSC occurred when PAW = 0.42 and 0.27 respectively. The PAW threshold values when each process is affected would be useful in developing criteria for scheduling irrigation.