PGRs Improve the Postharvest Performance, but Not Ethylene Sensitivity, of Potted Ornamental Plants and Plugs

Plant growth regulators (PGRs) are commonly used for growth control of ornamental plants, but may also offer the added benefits of increased postharvest performance and reduced ethylene sensitivity.  With an increase in paclobutrazol dose or ancymidol/uniconazole concentration, plant growth (height and diameter, shoot fresh weight, or dry weight) control also increased for all species tested. With the postharvest study, use of 1.0 – 2.0 mg paclobutrazol per 15.2 cm pot produced 21-28% shorter plants with 12-15% smaller plant diameter, 13-19% less shoot fresh weight, and 15-21% less dry weight, and darker green foliage color of potted sunflower (Helianthus annuus L.) than untreated plants. Treatment with 1.0 – 4.0 mg paclobutrazol per pot delayed first wilting signs by 0.7 – 1.4 d compared to plants without treatment. Plants subjected to simulated shipping at 15°C in the dark for 4 d wilted more quickly than plants kept in greenhouse or held at 15 °C for 2 d. For zinnia (Zinnia violacea Cav.), 0.5 – 1.0 mg paclobutrazol per 11.4 cm pot controlled plant growth, produced dark green foliage, and extended shelf life by delaying first wilting by 2.6-3.9 d and second wilting by 1.4-2.0 d than untreated plants. For marigold (Tagetes erecta L.) and petunia (Petunia ×hybrida Vilm.) plugs, 40-80 mg L^-1 ancymidol with a volume of 0.21 L m² as a foliar spray provided ample growth control with greener foliage; however, postharvest longevity was extended only when sprayed with 160 mg L^-1. For postharvest ethylene sensitivity, while the use of paclobutrazol drenches controlled excessive plant growth of both potted cuphea (Cuphea hyssopifolia Kunth) (0, 1.0, 2.0, or 4.0 mg L^-1 per 11.4 cm pot) and petunia (0, 2.0, 4.0, or 8.0 mg L^-1 per 15.2 cm pot), postharvest ethylene sensitivity was unaffected as most flowers abscised from all plants exposed to exogenous ethylene. However, use of uniconazole at 4 mg L^-1 produced shorter plant height with less shoot fresh and dry weight, lowered the number of abscised leaves or flowers when not exposed to ethylene, and improved foliage color of portulaca (Portulaca oleracea L.) plugs treated with 0, 2.0, 4.0, or 8.0 mg L^-1 as a foliar spray. In summary, use of plant growth retardants effectively controlled excessive plant growth and extended shelf life of potted plants and plugs, but had no significant effect on ethylene sensitivity of tested species.