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.