After three weeks of growth under the night chilling or control treatment, several above-ground and root morphological traits were measured, and the plasticity of each trait was calculated. Night chilling decreased plant height, leaf 1 area and collar diameter significantly (Fig.3). Roots responded strongly to night chilling, with a large and significant decrease (-44%) in biomass due to a 53% decrease in root projected area, length and tip number, although the root diameter increased. This result indicated strong repression of primary root growth and induction of secondary root growth.

To identify the main physiological processes of sunflower influenced by low temperatures, as well as developmental changes, we studied physiological processes known to be influenced by low temperatures, such as lipid metabolism, production of ROS and chlorophyll content. Cold stress significantly increased leaf anthocyanin and flavonol contents but decreased chlorophyll and nitrogen contents (Fig. 4A). Leaves produced significantly more ROS (i.e., hydrogen peroxide and superoxide anion) between 8h to 10h (Fig. 4A). These results confirm the intensity and pleiotropic impact of the night chilling stress applied.

One molecular response to cold (including freezing) is to modify fatty acid saturation and length. To describe these characteristics, we measured five fatty acids in the organs: C16:0 (palmitic acid), C18:0 (stearic acid), C18:1 (oleic acid), C18:2 (linoleic acid) and C18:3 (linolenic acid). The largest impact of cold stress was observed in hypocotyls, in which the cold treatment decreased the amount of saturated palmitic (-0.19%) and stearic acids (-48%) and tripled the amount polyunsaturated linoleic and linolenic acids (+206%). Interestingly, the amounts of the two latter fatty acids decreased in leaves (-50%) but did not differ significantly in the roots.