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.