Screening of plant growth retardants for growth control in camellia

In camellia, the most effective plant growth retardants (PGRs) are triazoles and, in particular, paclobutrazol. Recent restrictions, revisions and ban of several chemicals used in agriculture, including PGRs, suggest the necessity to test new molecules able to effectively replace the banned ones. On this basis, we arranged a screening of the effect of several PGRs on potted Camellia japonica ‘Charles Cobb’s’, so as to define the most effective substances and application protocols. On a first experimentation, potted camellias were treated twice with various concentrations of Ca-prohexadione (750, 1500, 2250 ppm foliar spray), flurprimidol (3.75, 7.5, 11.25 ppm foliar spray or 70 ml/pot of 1.87, 3.75 ppm drench) and chlormequat (1150, 2300, 3450 ppm foliar spray or 1150, 2300 ppm drench). Paclobutrazol (250 ppm foliar spray) and water were used as positive and negative control. Results pointed at flurprimidol as the most appropriate substitute for paclobutrazol. During a second experiment, which aimed at a deeper evaluation of flurprimidol application protocols, times and methods of application and concentrations were differently combined for a total of 7 flurprimidol treatments. Shoot elongation was subsequently monitored. The highest reduction in growth was registered by drenching 1.87 ppm of this product. These results clearly suggest the possibility not to rely only on a single chemical for regulating plant growth, nevertheless the future availability of the most promising one (flurprimidol) is doubtful.