Interaction between bulb cold treatment and plant genotype regarding flowering and metabolism in cut flower tulip

A period of exposure of bulbs to low temperature before planting is required to break bulb dormancy and to promote stem elongation in tulip (Tulipa gesneriana L.). The choice of the thermal level and duration depends on the genotypes’ sensitivity to temperature; however little is known about the metabolic responses underlying the different plant responses in the numerous genotypes. We evaluated the influence of the two preparation procedures of bulbs, at 9 and 5°C, in two tulip hybrids, ‘Royal Virgin’ and ‘Ad Rem’, grown hydroponically, in terms of plant growth, flowering earliness and characteristics, and metabolic profile. Plant growth and flower stem characteristics were similar in the hybrids and were unaffected by the bulb preparation procedure. Conversely, the two genotypes responded differently to the bulb cold treatment in terms of flowering earliness, as this was unaffected by the preparation procedure in ‘Royal Virgin’, while it was earlier after treatment at 9°C compared to 5°C in ‘Ad Rem’. The different plant sensitivity may be related to the diverse metabolic responses enacted by the bulbs for cold acclimation. In fact, while ‘Royal Virgin’ accumulated polyphenols in bulbs to recover from cold related oxidative stress, ‘Ad Rem’. particularly when pre-treated at 9°C, was able to invest more in structural macromolecules. Moreover, the high levels of soluble sugars and polyphenols in leaves confirmed the presence of a still active photosynthesis able to synthetise carbon skeletons to be exported to the bulbs for the conversion into starch in both types of bulbs independently of the thermal pre-treatment.