Improving drought tolerance of Leucophyllum frutescens through the application of paclobutrazol and cycopcel plant growth regulators

Leucophyllum frutescens is an evergreen shrub renowned for its drought tolerance. Paclobutrazol (PBZ) and cycocel (CCC) growth retardants were applied as foliar spray on L. frutescens plants cultivated in pots to assay the possibility of increasing these plants’ tolerance to water stress while maintaining high quality. The experiment was accomplished under full sun in the open field of the Nursery of Ornamental Plants and Landscape Gardening Res. Dept., Hort. Res. Inst., ARC, Egypt during the 2022 and 2023 seasons. Different concentrations of PBZ at 50, 100 and 150 ppm and CCC at 1000, 2000 and 3000 ppm were combined with 100, 75 and 50% pot capacity (P.C.) irrigation water levels, and some morphological, chemical and tolerance indices were examined. The obtained results showed a great reduction in all studied parameters except for proline content due to reduced irrigation levels. All growth retardants applied increased the values over control except for plant height and fresh weight of 10 flowers. Regarding the interaction treatments, the highest concentration of PBZ (150 ppm) and CCC (3000) produced the highest values in most cases when combined with both 75 and 50% irrigation water levels. Such treatments increased the number of main and lateral branches/plant, fresh and dry weights of vegetative growth and roots and root length, while mediated values were obtained for chemical constituents. Water use efficiency, relative stress index and stress tolerance index are also, greatly enhanced by such treatments. Cycocel at 3000 ppm could be recommended to treat Leucophyllum frutescens plants cultivated in 30 cm pots and subjected to only 50% irrigation water level as this treatment demonstrated good performance with high stress tolerance index to drought.