Changes in light scattering properties of healthy and infected by Xanthomonas vesicatoria tomato leaves as indicator for the initial stage of the development of bacterial spot disease.
B. Ivanova1, D. Stefanov2 and N. Bogatzevska3
1Institute of Genetics, 1113 Sofia, Bulgaria
2Institute of Plant Physiology, 1113 Sofia, Bulgaria E-mail: detelin@obzor.bio21.bas.bg
3Institute of Plant Protection, 2230 Kostinbrod, Bulgaria
Bacterial spot of tomato caused by Xanthomonas vesicatoria has become a very important disease of tomato in Bulgaria. In our country, the population of X. vesicatoria belongs to T (consisting of races T1 and T3) and PT pathotypes (Bogatzevska and Sotirova, 1992; Bogatzevska and Sotirova, 2001-2002).
X. vesicatoria penetrates by wounds or by stomata openings. It was established that the number of stomata on leaf surface and the time for stomata opening correlated to the number of leaf spots during development of the infection (Ramos and Volin 1987). After penetration water-soaked lesions are formed on the leaves, which later become necrotic.
The changes in photosynthesis induced by plant pathogens are characterized with a complex nature (Tesci et al., 1996). The development of leaf symptoms changes leaf anatomy and is related to the appearance of spatio-temporal heterogeneity in photosynthetic responses to different bio-stresses (Berger et al., 2005). The application of chlorophyll fluorescence imaging techniques revealed that inhibition of photosynthetic electron transport was restricted to the direct vicinity of the infection site, which was surrounded by a circle of increased photosynthetic activity. The photosynthesis of the remaining leaf was not affected at this stage. The availability of necrotic zones on leaf surface and spatio-temporal heterogeneity make studies on photosynthetic responses to bio-stress difficult.
The aim of this investigation was to examine the effect of X. vesicatoria leaf infection on far red light (FR) induced photo-oxidation of the first electron donor of Photosystem I (PSI), P700, immediately after penetration of the pathogen before formation of water-soaked lesions and necrotic spots. The changes in leaf absorbance were used as an indicator for bacteria induced alterations in tomato leaves.
Tomato plants susceptible to X. vesicatoria at the 5-6 true leaf phase were vacuum infiltrated with a bacterial suspension obtained from a 36h culture at a concentration of 108 cfu/ml of strain 42 of race T1 and strain 56 of T3 (Bogatzevska and Sotirova, 1992). The redox state of P700 was investigated in vivo with a dual wavelength (810/860 nm) unit (Walz ED 700DW-E) attached to a PAM101E main control unit (Klughamer and Schreiber, 1998) of a PAM fluorometer (Walz, Effeltrich, Germany, model PAM 101-103). P700 was oxidized by irradiation with FR (13.4 Wm-2) provided by a photodiode (FR-102, Walz, Effeltrich, Germany) that was controlled by the PAM 102 unit.
The investigation of leaf absorbance in the FR region (DA830) excited by FR light (>715 nm) reflects P700 oxidation because PSII is not activated by FR light and linear electron transport in thylakoid membranes of chloroplasts was not induced. Water infiltration of non-inoculated tomato leaves showed a great increase in DА830 signal (Fig. 1A). Leaf infiltration caused mechanically induced changes in leaf anatomy that influences global absorption properties of a leaf. Light scattering in leaves is largely determined by the intercellular air spaces (Evans et al. 2004). Scattering is determined by changes in refractive index between air and cells. Water infiltration of the leaves leads to changes in the mesophyll tissue that induces a decrease in light scattering and thence an increase in probability for light capture by P700, i.e. increased FR leaf absorption. The inoculation with X. vesicatoria caused a significant decrease in A830. The entrance of bacteria in intercellular spaces and the subsequent increase in bacterial concentration enhanced light scattering and therefore decreased leaf absorption (Fig. 1B and C). This effect was further expressed after inoculation with race T3 of X. vesicatoria (Fig. 1B and C). All these changes are immediately observed after inoculation. When above-mentioned infiltration-induced changes disappear, the differences between leaf absorbance between non-inoculated and inoculated leaf are thereafter not observed (data not shown).
On the basis of these results, it could be concluded that changes in PSI photochemical activity as evaluated by changes in FR induced P700 oxidation are not a direct effect of bacteria on PSI activity. Decreased leaf absorbance in infiltrated X. vesicatoria tomato leaves caused enhanced light scattering and hence a decrease in A830. We suggest that efficiency for penetration of X. vesicatoria in intercellular spaces is the reason for decreased A830.
Literature cited:
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Bogatzevska, N., and V. Sotirova (1992) Occurrence of two pathotypes of Xanthomonas vesicatoria on tomato in Bulgaria. TGC Report, 42: 11-12.
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Evans J., Vogelmann T., Williams W. and H. Gorton (2005) Sunlight capture; chloroplast to leaf. In: Photosynthetic adaptation: chloroplast to landscape. Smith W, Vogelman T and Critchley C (eds.) Springer, 15–41 pp.
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Ramos, L., and R. Volin (1987) Role of stomatal opening and frequency on infection of Lycopersicon spp. by Xanthomonas campestris pv. vesicatoria. Phytopathology, 77, 1311-1317.
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Figure caption:
Figure 1. The onset of photo-oxidation of P700 by far-red light and the subsequent re-reduction of P700+ (decay signal) on cessation of illumination. Far-red light (13.4 W m-2) was turned on at time zero and off after 10 s FR illumination. Each trace is the average for 8 separate leaf discs. Leaf discs were infiltrated with water (non-inoculated leaves, n.i.) and inoculated with bacteria and measured after different periods after inoculation.
OP = output signal in volts.