Phytoplasma infection affects the leaf and vascular morphology.
We first investigated and compared the effects of phytoplasma infection on the leaf and vascular morphology.
The phytoplasma infections resulted in already known different 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 that was closer analysed with 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 the 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 the 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).