Stimulus Produced by Stress

The presence of cotyledons is necessary for the long-day flowering of P. nil in response to poor nutrition or low temperature (Shinozaki and Takimoto 1982; Shinozaki 1985). This suggests that a flowering stimulus like florigen, which is involved in photoperiodic flowering, is involved in stress-induced flowering and is produced in cotyledons. If the stress-induced flowering stimulus is transmissible, defoliated scions may flower when grafted onto rootstocks with cotyledons and grown under stress conditions.

P. nil Violet and Tendan were grafted in several combinations, and the grafted plants were grown in tap water under long-day conditions for 20 days (Wada et al. 2010a). The Violet scions grafted onto the Violet rootstocks flowered (Fig. 17.1). The flowering may have been caused by the influence of the rootstocks because all the long-day conditions for 20 days, transferred to nutrient solution, and grown for an additional 2 weeks to score the flowering response; % flowering (open column) and number of floral buds/plant (closed column). n.f., no flowering occurred. Data adapted from Wada et al. (2010a)

Fig. 17.1 Flowering of grafted plants under poor-nutrition stress conditions in Pharbitis nil. Two varieties of P. nil were grafted in the combinations as shown (scion/stock) and grown hydroponically in tap water. The nongrafted control plants were grown on vermicu-lite and fed with tap water. Plants were grown under long-day conditions for 20 days, transferred to nutrient solution, and grown for an additional 2 weeks to score the flowering response; % flowering (open column) and number of floral buds/plant (closed column). n.f., no flowering occurred. Data adapted from Wada et al. (2010a)

Violet Tendan Viaiet/Vialet Violet/T en dan Tendan/Violet

Violet Tendan Viaiet/Vialet Violet/T en dan Tendan/Violet leaves had been removed from the scions and the cotyledons had been maintained on the root-stocks. This suggests that a transmissible flowering stimulus is involved in the stress-induced flowering of P. nil.

Violet scions flowered even when grafted onto Tendan rootstocks, although Tendan plants themselves were not induced to flower by the stress treatment. On the other hand, Tendan scions did not flower when grafted onto Violet rootstocks. It was predicted that Tendan would not produce such a flowering stimulus because Tendan did not flower in response to the poor-nutrition stress conditions. If this were the case, Violet would not be expected to flower when grafted onto Tendan rootstocks. However, defoliated Violet scions grafted onto Tendan rootstocks with cotyledons were induced to flower. The difference in flowering response between the scions grafted onto Tendan and those grafted onto Violet was not statistically significant. Therefore, Tendan may produce almost the same amount of the flowering stimulus as does Violet. Conversely, the Tendan scions grafted onto Violet rootstocks were not induced to flower. These results indicate that Tendan produces a transmissible flowering stimulus but does not respond to it.

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