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.
Physiological and metabolic responses to night chilling under
controlled
conditions
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.