The 2012 ASHS Annual Conference

Plants of Ligustrum japonicum from 11.4 L containers were transplanted into elevated rhizotrons.  Moisture levels were maintained at 30% or 70% of plant available water (PAW) by an automated system based on a single capacitance probe per rhizotron.  Independently determined saturation volumetric water content (100% PAW) for each rhizotron caused two distinctive irrigation frequencies within each PAW level, resulting in four distinct moisture ranges.  All plants in both moisture ranges in the 70% PAW level grew rapidly, with no differences in biomass accumulation or allocation.  However at the 30% PAW level, the higher Moisture Limiting-Marginal (ML-Marginal) moisture range reduced total biomass 22%, while the lower Moisture Limiting Fatal (ML-Fatal) range reduced biomass by 42%, compared to plants in the 70% PAW level.  Plants within the 30% PAW treatment disproportionally limited allocation to shoot mass, particularly limiting leaf growth.  Relatively smaller reductions were measured in root biomass.  Plant responses to 30% PAW moisture differed greatly with small variations in irrigation set points.  A 3.4% absolute difference in percent volumetric water content (%VMC) was the difference between plant survival and death.  ML-Marginal plants received minimal but regular irrigation.  This resulted in similar allometric relationships to plants at the lower moisture range of the 70% PAW level.  In contrast ML-Fatal plants endured slow dehydration resulting from 0 to 2 irrigation events over 10 weeks which severely restricted new leaf expansion and eventually stopped root elongation toward moist substrate. Only the extreme stress imposed by little to no irrigation for over 10 weeks increased root-to-shoot ratios compared with well-irrigated plants.