Development of a Rapid and Effective Screening Method for Basal Stress Tolerance of Petunia x Hybrida

High temperature and drought stress are two of the greatest impediments to bedding plant growth and development. The objective of this study was to develop a rapid and effective protocol for screening Petunia x hybrida for basal heat or drought tolerance. A practical growth system for measuring seedling growth, or seedling growth sensitivity test (SGST), was first established.  Based on this protocol radicle growth rate was chosen over hypocotyl growth as the most reliable and accurate measurement for the SGST. Nineteen petunia cultivars from three plant classes (floribunda, grandiflora, or spreading) were previously evaluated, where cultivars within the same plant class and series were grouped as either best or worst overall landscape performance and then subjected to the SGST. Seeds were germinated in Petri dishes at 26°C for 4 days and then subjected to 5-h heat shock at a temperature of 40°C or 6 d drought stress at an osmotic pressure of -0.8MPa stimulated by PEG 6000 to determine heat or drought tolerance, respectively. The results indicated heat or drought stress significantly affected the relative radicle growth rate of seedlings. However, the imposed stress did not affect all cultivars similarly. ‘Avalanche Lilac’ and ‘Dreams Burgundy Picotee’ had the greatest radicle growth rate but were considered as more heat susceptible cultivars due to a significant reduction in radicle growth rate under heat stress. ‘Avalanche Lilac’ was also regarded as the most drought sensitive cultivar. The results from this study indicate that the SGST may be used to determine heat or drought tolerance but further research should be conducted to determine greenhouse and landscape performance under heat or drought stress.