Influence of paclobutrazol on tomato root architecture

Water scarcity is limiting plant growth and productivity in many regions of the world, including agriculturally managed ecosystems. Improving plant tolerance to drought stress becomes increasingly important to maintain high plant productivity in water‐limiting environments. For example, the use of plant growth regulators (PGRs) is an important agricultural tool for increasing plant productivity and growth under water deficit conditions. PGRs increase the drought tolerance of plants through both morphological and physiological changes above and below ground. Paclobutrazol (PBZ), for example, inhibits the synthesis of gibberellins while promoting abscisic acid, auxin and cytokinin. However, information about the morphological changes among root systems induced by PBZ and other PGRs is still scarce although root morphology holds important information about root systems' capacity for water and nutrient uptake. Considering the importance of root systems for drought tolerance, we examined the fine root architecture, morphology and biomass of two Lycopersicon esculentum (tomato) cultivars, Ikram (drought tolerant) and Mose (drought sensitive). Root architecture and morphology were analyzed through root orders which represent the root branching hierarchy. Two days after seeding, three different rates of PBZ were applied. Two soil moisture treatments [well watered and water deficit (i.e. 60% of well watered)] were initiated six weeks after seeding and were kept for five weeks. Plants were sampled before the start of the soil moisture treatments and at its finish. PBZ enhanced the specific root area, especially of first order roots (i.e. root tips). The diameter of tap roots was larger, and PBZ treated tomato root system had finer side branches. Changes in the numbers of root orders and their frequency, morphology and anatomy indicate major modification of tomato root systems under PBZ treatment. As most of the roots were concentrated in the wet zone under drip irrigation, higher specific root areas would improve the efficiency of water and nutrient uptake. The increased root/shoot ratios and decreased leaf areas are additional indicators for the improved drought tolerance of these two tomato cultivars after PBZ treatment.