Biophysical characteristics of peach trees infected with phony peach disease

Abstract

The effect of phony peach disease (PPD) on leaf gas exchange, several growth parameters, and plant water relations was assessed on Prunus persica (L.) Batsch “Flordaking” peach. The presence of xylem-limited bacteria (XLB) in roots and stems of trees with PPD symptoms was confirmed by phase-contrast microscopy and successful culture on periwinkle wilt medium. Mean predawn leaf conductance (gl) was 0 mmol m −2s −1 and shoot water potential ( Ψ sw) was −0·15 MPa for both PPD and control trees. The only difference between treatments occurred at midday where gl and Ψ sw of PPD trees were reduced by 25 and 18%, respectively. Reductions in terminal growth, gl, transpiration rate, net CO 2 assimilation rate and Ψ sw were poorly correlated with XLB concentration in roots. Turgor pressure ( Ψ p) of shoots was not reduced with PPD. Dwarfing associated with PPD was not consistently related to the bulk modulus of elasticity (ε) of terminal growth. Stomatal closure of PPD and control leaves occurred at similar leaf water potentials ( Ψ Lw) although in neither case was a distinct threshold Ψ Lw for stomatal closure apparent. Trees severely infected with PPD (8cm terminal growth/year) manifested a 12–18% reduction in midday net CO 2 assimilation rate, a 15–25% reduction in midday gl, and a 0·2-0·6 MPa decline in midday Ψ sw throughout the spring and summer. The decline in A appeared to be due to stomatal closure associated with increased water stress. Significant increases in Mn, Na, and Al were diagnosed in leaves of PPD-trees. The hydraulic conductance ( L) of root terminals and root and stem pieces was not significantly affected by PPD. Root L, whether determined with vacuum or pressure, was not related to extractable XLB cm −3 xylem. In contrast to earlier reports, we conclude that the relatively minor reductions in midday Ψ sw may be insufficient to entirely explain PPD symptoms. The relevance of these findings is discussed.