Analysis of the Effects of Virus Infection on the Photosynthetic Properties of Peach Leaves

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Abstract
Infection of peach leaves with the viruses causing 'peach rosette and decline disease' reduced their net photosynthetic rate (PN) over a range of light intensities. The Pmax value, derived from the asymptotic value of the light-response curve of young leaves was significantly reduced by 13%. The reduction of PN was partially attributable to increases in both the gas-phase (r1) and residual (rm) resistances. Virus infection, however, had no effect on the PN values of older leaves, as leaf senescence effects predominated over any effects of infection. The 'dark' respiration rate of young leaves was increased by virus infection by c. 50 %, but not significantly so. The constants of the polynomial regressions, relating PN to the environmental variables of ambient CO2 concentration and irradiance, were used as indices of the leaf photosynthetic characteristics. The photochemical efficiency of young infected leaves was 28 % less than that of healthy controls, but infection had no effect on older leaves. The mean total leaf conductance to CO2 transfer was also reduced by 30 % (not significantly) in infected leaves. The CO2 and light compensation points were unaffected. It is concluded that the (previously reported) large, inhibiting effects of virus infection on the growth of young peach trees are not attributable to these comparatively less severe effects on leaf photosynthesis but rather to effects on leaf growth.