The translocation situation and phytohormone distribution are different in the individual host-pathogen-systems.
We 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 of - 25% and -58%, respectively (Figure 5a). In pear leaves the phloem mass flow velocity and the volumetric flow rate increased significantly for +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 variating effects for the phloem mass flow indicated changes in the flow properties of the phloem sap. Thus, the dynamic viscosity, absolute and relative densities of phloem sap obtained by bark tissue centrifugation were measured (Table 2). For apple and peach, no changes were found. In contrast, the dynamic viscosity in infected pear plants was doubled (+104%) and also the relative density increased strongly (+97.7%) supporting any effects for the phloem mass flow (Figure 5b). Unfortunately, the peach plants did not deliver enough volumes of phloem sap for a complete analysis. For this reason, only the relative density could be determined without any significant changes. A comparative analysis of the phloem’s -relative density among 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 showed again heterogeneous effects of a phytoplasma infection (Figure 5, Table 2) and confirmed the variability of previous 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 comparably analysed (Figure 6). No differences of callose depositions were found for the phytoplasma infection, in comparison to healthy apple trees (Figure 6a). In contrast to apple, a rise of callose was found in peach (+300%) and pear (+67%; Figures 6b+c) showing a stronger impact into the anatomical and physiological balance in comparison to apple trees.
The stress situation of the plants was explored with 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 acetic acid (IAA) (Figures 7 and S1; Table S8) in leaves. 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 following an infection with the virulent accession 3/6 (Figures 7a and S1). No significant changes for the several measured phytohormones could be detected in pear trees (Figure 7b). In peach trees, SA (+192%), JA-Ile (+345%) and IAA (-40%) were significantly affected whereas JA and ABA did not show any significant changes (Figures 7b and S1). Moreover, the fundamental 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 morphological and functional results, the effect of phytoplasma infections on the phytohormone contents revealed different patterns among particular host-pathogen combinations.