Phytoplasma infection affects leaf and vascular morphology.
We first investigated and compared the effects of phytoplasma infection
on leaf and vascular morphology. The successful infections resulted in
known visible disease symptoms: witches’ broom and enlarged stipules in
apple trees, premature foliar reddening in pear trees and chlorosis and
suberization in peach trees (Figure 1). The symptoms indicated
impairments in the leaf development, which was further examined by a
comparison of the leaf lamina, midrib sizes and their ratios among
infected and healthy plants. It was found that leaves of AP-infected
apple trees were significantly (p<0.05) smaller (length -17%
and width -22%) and the diameter of midribs were significantly reduced
(-27%) compared to those of healthy plants (Table 1). The phytoplasma
infection in apple trees did not affect the leaf size ratio and the
midrib ratio (Figure 1a). In pear, basing upon a significant increase of
the leaf width (+8.5%), a significant decrease of the leaf size ratio
of nearly 9% was observed, but no changes for the midrib ratio were
found (Figure 1b). In contrast to apple and pear plants, phytoplasma
infected peach trees exhibited a significant rise of the leaf size ratio
of +13% and the midrib ratio of +16% (Figure 1c). No significant
changes were found for leaf length, width and midrib diameter (Table 1).
All morphological results demonstrated the heterogeneity of the symptoms
and indicated differences in the individual host-pathogen interactions.
The specific impact of the phytoplasma infection on the vascular
morphology was investigated by analysing the areas of vascular bundle,
xylem, phloem and SEs as well as the ratios of xylem to phloem and SE to
phloem (Figures 2-4). For apple, the phytoplasma infection exhibited
significantly (p<0.05) degraded areas of the vascular bundle
(-39.1%), xylem (-49.8%), phloem and SE (-33.7%) in comparison to
healthy plants, whereas the ratio of SE to phloem was not affected
(Figure 2b). Phytoplasma infected pear trees did not show any changes
(Figure 3), whereas in peach trees infected with ESFY, the mean sieve
element area (-26%) and the ratio of SE to phloem (-46.9%) decreased
significantly (Figure 4b). Confirming the heterogeneity of the
morphological results (Figure 1), different disease patterns were also
found on the cellular level of the vascular system for apple, pear and
peach (Figures 2-4).