In this study, exogenous factors affecting the elongation growth in the short day plant, Pharbitis nil, was investigated. Theobroxide inhibited stem elongation in Pharbitis nil both under short day (SD) and long day (LD) conditions. Salicylhydroxamic acid (SHAM), an inhibitor of jasmonic acid (JA) biosynthesis, and GA3 recovered the inhibitory effect of theobroxide on stem elongation. Quantitative analysis of JA showed that the level of endogenous JA increased significantly in theobroxide treated plants, while exogenously applied GA3 and SHAM suppressed JA biosynthesis stimulated by theobroxide. The activity of lipoxygenase (LOX, the key enzyme of JA biosynthesis) also was stimulated by theobroxide and this stimulation was nullified by SHAM and GA3. Quantitative analysis of GA1 showed that theobroxide suppressed GA1 biosynthesis. In non theobroxide treated Pharbitis nil, SD conditions stimulated JA biosynthesis and LOX activity, while GA1 biosynthesis was suppressed. All these results suggest that JA probably is involved negatively in the control of stem elongation, and the balance between JA and gibberellin might determine the stem growth in Pharbitis nil

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Life cycles of plants including tuberization and flowering are strongly related to environmental factors such as photoperiod and temperature. Theobroxide induces potato tuber formation and flower bud formation of morning glory under non-inductive conditions and stimulates the activity of lipoxygenase (LOX). In this study, to understand the LOX activity more systematically, the relationships between LOX activity and light and temperature, which effects potato tuber and flower-bud formation, have been investigated. The results showed that LOX activity in morning glory was greatly enhanced up to 30 min and then declined after switching from the light to the dark condition, while the activity did not vary when switching from the dark to the light condition. In addition, the temperature profile of measured LOX activity in the potato and morning glory plants was nearly consistent with the time taken to form potato tubers and flower buds in morning glory, respectively, at different growing temperatures. These results strongly suggest that LOX activity is directly connected with light and temperature to regulate the formation of tubers and flowerbuds.

In the current study, we investigated the influences of theobroxide on stem elongation in spinach (Spinacia oleracea). Our results showed that stem elongation and flower formation were inhibited by spraying spinach plants with theobroxide under inductive, long day conditions (16 h light/8 h dark), while application of exogenous applied GA3 prevented the effect of theobroxide. Quantitative analysis showed that theobroxide suppressed GA1 biosynthesis, whereas the endogenous content of jasmonic acid was unchanged. However, under short day conditions (10 h light/14 h dark), there were no differences in stem length between treated and untreated plants. These results suggest that the inhibition of stem elongation by theobroxide is probably due to the suppression of gibberellin biosynthesis.

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