EFFECT OF END-OF-DAY FAR RED LIGHT ON THE RESPONSES TO FLUCTUATIN DAY/NIGHT TEMPERATURES IN CUCUMBER AND ARABIDOPSIS

Day (DT) and night temperature (NT) fluctuations influence stem elongation and leaf morphology of many species, but can be modified by an end-of-day (EOD) far-red (FR) light treatment. We have carried out experiments with phytochrome mutants in cucumber and Arabidopsis as well as with cryptochrome mutants in Arabidopsis to investigate the involvement of specific light receptors in this thermoperiodic response. The WT and the phytochrome B deficient, long hypocotyl (lh) mutant of cucumber (Cucumis sativus) were grown under alternating DT and NT (DT/NT) of 25/19°C and 19/25°C combined with a 30 min. exposure to EOD-red (R) or EOD-FR light in growth rooms. WT and two phytochrome (phyA and phyB) and two cryptochrome deficient mutants (cry1 and cry2) of Arabidopsis thaliana ‘Landsberg Erecta’ were grown at DT/NT combinations of 25/17°C and 17/25°C. In WT cucumber grown under EOD-R light, stem, internode and hypocotyls were shorter under a low DT/high NT (negative DIF) compared to a high DT/low NT regime (positive DIF). EOD-FR light treatment completely eliminated the growth retarding effect of negative DIF on hypocotyl elongation and strongly reduced the negative DIF response on stem length. With respect to hypocotyl length, the lh mutant did not respond to the EOD treatment or alternating DT/NT. In Arabidopsis, DT/NT alternation showed similar responses on flower stem elongation and leaf morphology for the phyA, cry1 and cry2 mutants compared to the WT. However, the phyB mutant showed significantly less response to alternating DT/NT than the WT. These results suggest that phytochrome B is required for a maximum effect of daily temperature alternations on stem elongation and leaf morphology in cucumber and Arabidopsis. The EOD-FR treatment, which almost eliminated the effect of negative DIF on stem elongation, indicates that a high amount of phytochrome in the active form (P_fr) is needed for a strong response to DIF in those species.

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