Salicylic Acid-Mediated Stress-Induced Flowering - Salicylic Acid: Plant Growth and Development - page 171

conditions. However, it is unlikely that the photosynthetic deficiency induced
flowering because the photoassimilate is a flower-inducing factor (Bernier and
Périlleux 2005 ; Seo et al. 2011 ). In fact, the application of sucrose induced
flowering in P. frutescens plants cultured in vitro under LD conditions (Purse
1984 ). We treated the red-leaved P. frutescens plants with several other stress
factors. None of the tested factors induced flowering, although a reduction in
vegetative growth was observed. This indicates that not all stresses induce

flowering.
Young red-leaved P. frutescens plants respond to low-intensity light treatment,
even immediately after their cotyledons were expanded. They were induced to
flower with the 3 week treatment, and 100 % flowering was obtained under the
4 week treatment. Flowers were formed even at the cotyledonal nodes. The plants
induced to flower under the low-intensity light conditions reached anthesis and
formed seeds. The seeds germinated, grew normally and were induced to flower in
response to SD treatments.

6.2 Effect of PAL Inhibitors on the Flowering Induced
by Low-Intensity Light

It was observed that the leaves of red-leaved plants were deep green under low-
intensity light (Wada et al. 2010a ). The greening might be associated with stress-
induced flowering. Accordingly, we extracted the leaves with methanol and
obtained absorption spectra for the extract. The color of the red-leaved P.
frutescens plants resulted from the presence of anthocyanin, which has an
absorption peak at 529.5 nm. With decreased light intensity, the absorbance at
529.5 nm (A 529.5 ) decreased and that at 652.5 nm increased, indicating that the
greening of the leaves increased with decreasing anthocyanin content and
increasing chlorophyll content. Although the greening of the leaves partially
resulted from the increase in chlorophyll content that might not be directly related
to flowering because an increase in chlorophyll content under low-intensity light
was observed even in green-leaved plants, which exhibited only a slight flowering
response. The relationship between anthocyanin content (A 529.5 ) and flowering
was examined in experiments concerning the effect of different light intensities on
flowering. There was an apparent negative correlation between these variables
(r =-0.71). The absence of flowering was also observed when the threshold
reaction reached an A 529.5 of greater than 0.8. Therefore, the metabolic pathway
related to anthocyanin synthesis might be involved in the regulation of flowering.
Namely, low-intensity light stress could reduce anthocyanin synthesis and induce
flowering. However, this conflicts with previous reports. Stress generally promotes
anthocyanin biosynthesis through increasing PAL activity (Chalker-Scott 1999 ;
Christie et al. 1994 ; Dixon and Paiva 1995 ), and stress-induced flowering in P. nil
was accompanied by an increase in PAL activity. Thus, two hypotheses have been
proposed: low-intensity light stress could either decrease or increase PAL activity