Correlation of phytoplasma concentration in Hydrangea macrophylla with green-flowering stability

The interaction between phytoplasma concentration and green-flowering stability was studied in hydrangea cultivars. Three green and 18 nongreen cultivars were subjected to polymerase chain reaction (PCR) analysis to determine Japanese hydrangea phyllody (JHP) phytoplasma infection. The results showed that JHP-phytoplasma was detected only in ‘Midori’ plants, which have green sepals. ‘Midori’ plants were propagated, and from 29 rooted cutting plants, they were grouped into three types on the basis of sepal color, that is, green (75.9%), blue-green (13.8%) and blue (10.3%) sepals. To clarify the variability in the sepal color of ‘Midori’ plants, JHP-phytoplasma concentration in the sepals and leaves of green-, blue-green- and blue-flowering plants was determined by PCR analysis. The semiquantitative PCR comparisons of 370 bp DNA fragments showed that the JHP-phytoplasma concentrations in green sepals were 16 times higher than that in blue-green sepals. JHP-phytoplasma could not be identified by PCR analysis in blue sepals and leaves. These results showed that JHP-phytoplasma concentration correlated with green sepal stability in ‘Midori’ plants. A histological observation of sepals showed that epidermal cells of blue and blue-green sepals had a dome shape. Otherwise, green sepals were leaflike with flat epidermal cells, and palisade parenchyma cells with numerous chloroplasts.