S23 - Session O8 - The role of plant growth regulators during onion dormancy break and sprout elongation

Authors: Maria Del Carmen Alamar Gavidia *, Zoltan Kevei, Andrew J Thompson

Dormancy break and premature sprouting can cause up to 10% wastage during onion postharvest storage, whilst there is increasing pressure to reduce the use of chemical sprout suppressants. The control of onion dormancy and sprouting are therefore important areas for research. Phytohormones have been associated with dormancy break, both as inhibitors (abscisic acid [ABA]) and promotors (cytokinins [CKs], gibberellins [GAs]). However, the generally accepted hypothesis that sprout elongation is triggered by a reduction in ABA concentration has been recently questioned since there is no decline in ABA in baseplates of cold stored onions prior to sprouting; and ABA does not increase in baseplates during ethylene-induced dormancy [1]. In the same study, dormancy-related hormones were differentially distributed vertically within the baseplate: trans -zeatin concentration was higher in the top baseplate of control bulbs at dormancy break stage compared to ethylene-treated ones. However, the level of spatial and temporal resolution provided a limited picture of how these gradients might affect the progression of dormancy break and sprout elongation. Here, we investigate whether dormancy break correlates with an increase in biological active CKs, rather than by the decrease in ABA in the top section of the baseplate, where the primary thickening meristem is found. Seven commercial cultivars (Alpha, Jiggings, Lorient, Red Radiant, Rumba, TGB and Troy) were screened for dormancy break and sprout elongation to reveal differences between those traits. The spatial hormone profile was also studied in onion baseplates. The results will be discussed to elucidate the spatial and temporal role of the plant growth regulators during dormancy transition. The outcomes will further help us to select cultivars with extreme dormancy phenotypes; these will be crossed to create segregating populations suitable for mapping genes that influence dormancy break and that can give further insight into the mechanism dormancy break and sprouting.