Plasticity of biomass allometry and root traits of two tomato cultivars under deficit irrigation × chemically induced drought hardening by Paclobutrazol

Tomato yield is seriously affected by water stress. Paclobutrazol, a fungicide and plant growth retardant, has previously shown the potential to improve drought tolerance of crops. However, knowledge on the impact of Paclobutrazol on root system traits is scarce. Seeds of two tomato cultivars were primed with three different rates of Paclobutrazol. After an establishment phase of 60 days, greenhouse-grown plants were partially subjected to deficit irrigation (60% ET). Subsequently, biomass and surface area allometry between plant organs and within the root system were determined. The morphology and architecture of the root system were studied in detail. Changes in root system traits under deficit irrigation and after Paclobutrazol treatments were largely based on basal roots’ plasticity. The proportion of basal roots significantly increased with increasing Paclobutrazol concentration; deficit irrigation resulted in both cultivars in increased branching of basal roots. Cultivars differed in their plastic response of root system biomass, enhancing tap root and lateral root growth in one cultivar but basal root proportions in the other. Paclobutrazol priming of seeds facilitated basal root versus tap root system growth in tomato. It is hypothesized that basal roots, positioned in the topsoil, are most advantageous to maximize water uptake under (deficit) irrigation.