Feedback regulation of irrigation via soil moisture monitoring and its effects on plant growth and physiology

Currently, 80% of the world's available fresh water resource is used for irrigated agriculture. Growers usually try to ensure profitable crop production rather than considering possible environmental impacts, which can lead to misjudgement of the plant's actual needs and result in under- or over-irrigation. To understand how manual watering influences soil water status, soil moisture sensors were deployed in a representative commercial nursery. By automatically scheduling irrigation according to soil moisture thresholds, this work tries to understand the effects of deficit irrigation (under-irrigation - applying less than full water requirements) on possible growth regulation through changes in leaf water status, gas exchange and plant hormone balance. Tomatoes (Solanum lycopersicum Mill. 'Ailsa Craig') were grown in a peat-based substrate in a controlled environment, and exposed to different irrigation treatments by varying the number of drippers in the pot (1 water deficit, 2 control). Hand-watering in a commercial nursery resulted in significant variation in soil water content that may compromise crop uniformity and quality. In contrast, feedback regulation of irrigation via soil moisture monitoring provides a useful tool to water plants according to their actual water needs, with significantly less variation in soil moisture. Under-irrigating containerised tomato plants significantly reduced evapotranspiration, crop fresh weight, height and leaf area and decreased leaf water potential and stomatal conductance. Although foliar concentrations of the phytohormone abscisic acid (ABA) varied throughout the treatment period, ethylene emission from shoot increased. However, the role of these plant hormones during water deficit requires further experimentation using mutants deficient in these hormones. Although deficit irrigation as applied here limited tomato vegetative growth, further long-term experiments (that measure fruit yield) are needed to evaluate whether automatic irrigation scheduling can be used as a water-saving technique.