Morphological and physiological aspects of phase transition in radicchio (Cichorium intybus L. var. silvestre Bisch.): Influence of daylength and its interaction with low temperature

A series of experiments was undertaken to study daylength control of transition to flowering, and interaction between daylength and low temperature in 'Rosso di Chioggia' radicchio (Cichorium intybus L. var. silvestre Bisch.). In the experiments, short day (SD) and long day (LD) treatments were variously combined with vernalizing (5°C) and non-vernalizing (> 15°C) temperatures to investigate: (1) the stage at which photoinducibility is achieved; (2) the number of LD cycles needed for flowering; (3) changes in LD requirements caused by duration and time of cold treatment. Experiments were conducted in controlled (growth chambers) and partially controlled environments (heated glasshouses). Data collected throughout the growth period included percentage of bolting, and number of days to both bolting initiation and anthesis of the first capitulum. Additional observations regarded: height and number of nodes of flower-stalk; number and length of lateral shoots, and number of capitula produced by each plant. Some characteristic data (leaf number, leaf area, fresh and dry weight of foliage) at the start of LD exposure were also collected, to study the relationship between developmental stage and sensitivity to daylength. Results indicate: (1) radicchio can be induced to flower merely by LD, but low temperature hastens and enhances bolting and flowering. (2) The vernalizing effect of low temperature is quantitatively related to the duration of cold treatment. (3) Sensitivity to both low temperatures and daylength increases with plant age. (4) The stage at which plants acquire maximum sensitivity to cold precedes that at which they achieve maximum sensitivity to LD. (5) In early growth stages, there is a period of full insensitivity to the photoperiod, i.e., plants must reach a certain size before acquiring photoinducibility. (6) Low temperature increases plant sensitivity to LD, also accelerating photoinducibility and anticipating the stage at which plant acquires maximum sensitivity to daylength. As vernalization proceeds, fewer LD are required for induction-plants exposed to 5°C for 15 or 30 days required more than 10 and 5 LD cycles to flower, respectively. (7) Thermoinduction cannot altogether suppress the LD requirement in radicchio since, whatever the vernalizing treatment, flowering fails in SD-treated plants. (8) Once the stage of maximum sensitivity to LD is overcome, photoperiodic sensitivity may diminish gradually, plants reverting to a non-photoinductive state. This was particularly evident in plants in which vernalization was suboptimal.