Developmental Stage and Temperature Influence Elongation Response of Petiole to Low Irradiance in Cyclamen persicum

Reduced irradiance promotes shoot elongation depending on developmental stage and environmental factors and decreases plant quality in Cyclamen persicum Mill. To determine the petiole elongation responses to low irradiance, 'Metis Scarlet Red' cyclamen at different developmental stages [juvenile (5-6 unfolded leaves), transitional (1-3 visible flower buds), or mature (1-3 elongating peduncles)] was grown in growth modules at 60 (low light, LL) or 240 (high light, HL) μmolㆍm-²ㆍs-¹ PPFD within the growth chambers at different temperatures [16/12 (low temperature, LT), 22/18 (medium temperature, MT), or 28/24℃ (high temperature, HT) (day/night)]. In Experiment Ⅰ, juvenile plants were either kept in an LL or HL module during the entire treatment of 4 weeks or were transferred to the other module at 1, 2, or 3 weeks after treatment in an MT chamber. In Experiment Ⅱ, juvenile, transitional, or mature plants were moved to the HL module at 0, 3, 6, 9, or 12 days after being placed in the LL module at the MT chamber and grown for 21 days. In Experiment Ⅲ, transitional plants were moved to the HL module at 0, 3, 6, 9, or 12 days after being placed in the LL module at the LT, MT, or HT chambers. As the exposure duration to LL increased from 0 to 4 weeks or from 0 to 12 days, petiole length and plant height increased at all temperatures and developmental stages. In Experiment Ⅰ, the exposure to LL during the latter period, rather than the early period, increased elongation rate. In Experiment Ⅱ, petiole elongation in transitional plants was more sensitive to LL than juvenile or mature plants during the early period of the treatment for 12 days. In Experiment Ⅲ, petiole length increased with increasing temperature and exposure duration to LL. Petiole elongation rate at HT increased rapidly from the beginning of LL exposure as compared to LT. Increase of 6℃ in temperature had the similar effect to LL exposure for 3 days in petiole elongation. To conclude, transitional cyclamen under higher temperatures responds more immediately to low irradiance and elongates its petioles.