Effect of DIF and end-of-day light quality on stem elongation in Cucumis sativus

Abstract

A long-hypocotyl phytochrome B deficient mutant ( lh) and its isogenic wild type (WT) of cucumber ( Cucumis sativus L.) were used to study interacting effects between thermo- and photomorphogenetic stem elongation in long- and short-term experiments. The plants were grown under a 12 h high light intensity period and day/night temperature (DT/NT) of 25/19 °C (positive DIF, difference between DT and NT) or 19/25 °C (negative DIF) in combination with 30 min end-of-day (EOD)-red (R) or far red (FR) light. Thirty minutes EOD-FR, compared to EOD-R, significantly increased the length of stem, hypocotyl and internodes, and the number of visible internodes in WT. Positive DIF enhanced stem elongation compared to negative DIF under an EOD-R treatment, while the effect of negative DIF was strongly reduced by EOD-FR. For instance, an exposure to EOD-FR increased the plant height 68 and 41% under negative and positive DIF, respectively. In contrast, the lh mutant showed little difference in stem elongation in response to EOD-R and EOD-FR treatments, and small difference between positive and negative DIF treatments. These results indicate that DIF interacts significantly with EOD light quality on stem elongation, and that phytochrome B may play a key role in the interaction between thermo- and photomorphogenetic responses. A study on the kinetics of hypocotyl elongation showed that DIF interacted with EOD treatments in manipulating hypocotyl elongation rate (HER). In the photoperiod, positive DIF resulted in greater average HER than zero DIF and negative DIF, and the effect of DIF was stronger under EOD-R than EOD-FR. In the dark period, negative DIF showed larger HER compared to positive and zero DIF, and the effect was enhanced by EOD-FR compared to EOD-R. An EOD-FR increased HER both in photoperiod and dark period, in comparison to an EOD-R. Further, under EOD-R, negative DIF postponed a distinct peak of HER early in the dark period by approximately 4 h compared to positive DIF. A sharp peak was only found in the photoperiod under positive DIF. Under EOD-FR, a sharp peak of HER appeared at the end of the dark period or early in the photoperiod under negative DIF. This may indicate that the phytochrome is involved in thermoperiodic control of the kinetics of hypocotyl elongation.