The translocation situation and phytohormone distribution are different in the individual host-pathogen-systems.
We next examined the consequences of the morphological changes (Figures 2-4) on the physiological situation within the sieve elements. In apple leaves the phloem mass flow velocity and the calculated volumetric flow rate decreased significantly (p<0.05) in infected leaves in comparison to healthy ones by - 25% and -58%, respectively (Figure 5a). In pear leaves the phloem mass flow velocity and the volumetric flow rate increased significantly by +32.6% and +46.6%, respectively (Figure 5b). In peach leaves the phloem mass flow velocity was not affected, but the volumetric flow rate decreased significantly (-30.8%; Figure 5c).
The varying effects for the phloem mass flow indicated changes in the flow properties of the phloem sap. Thus, the dynamic viscosity, density and refractive index of phloem sap obtained by bark tissue centrifugation were measured (Table 2). No changes of the refractive index for apple and peach were found. In contrast, the dynamic viscosity in infected pear plants was doubled (+104%) and the relative density increased strongly (+97.7%). Unfortunately, the peach plants did not deliver enough phloem sap volume for a complete analysis. For this reason, only the relative density was determined without any significant changes. Furthermore, a comparative analysis of the phloem’s relative density between apple, pear and peach revealed significant differences, illustrating a plant specificity of the phloem sap composition regarding total sugar content. The measured/calculated phloem mass flow parameters again exhibit heterogeneous effects of a phytoplasma infection (Figure 5, Table 2) and confirmed the variability of previously shown anatomical/morphological results (Figures 1 to 4).
To obtain indications for the changed mass flow translocation of the individual plant-phytoplasma variations, the callose deposition in the SEs was visualized and its intensity analysed (Figure 6). No differences of callose depositions were found for the phytoplasma infection, in comparison to healthy apple trees (Figure 6a). In contrast, an increase of callose deposition was found in peach (+300%) and pear (+67%; Figures 6b+c), showing a stronger impact on the anatomical and physiological balance in comparison to apple trees.
The stress level of the plants was explored by the measurement of salicylic acid (SA), jasmonic acid-isoleucine (JA-Ile), jasmonic acid (JA), abscisic acid (ABA), 12-oxo-phytodienoic acid (cis-OPDA) and indole-3-acetic acid (IAA) in leaves (Figures 7 and S1; Table S8). Upon an infection with the virulent accession 3/6, in apple, SA (+109%), ABA (+55%) and JA-Ile (+78%) increased significantly (p<0.05) whereas a significant decrease of cis-OPDA (-45%) and no changes of JA and IAA were observed (Figures 7a and S1). No significant changes for the several measured phytohormones were detected in pear trees due to infection (Figure 7b). In peach trees, SA (+192%), JA-Ile (+345%) and IAA (-40%) were significantly changed in infected plants, whereas JA and ABA did not show any significant changes (Figures 7b and S1). Moreover, the basic level of SA, ABA and cis-OPDA differed among healthy apple, pear and peach plants. For example, ABA was 6-fold higher in pear and 3-fold higher in peach compared to apple.
In accordance with the obtained morphological and functional results, also the effect of phytoplasma infections on the stress-related phytohormone contents revealed different patterns among particular host-pathogen combinations.