Assessment of P700 Redox State of Tomato Plants Under the Combined Influence of Elevated Temperature and Fusarium oxysporum Infection by Differential Absorption Photometry Using Saturating Light Pulse Technology

Diff erential absorption photometry and sequential saturating light pulses of modulated frequency were used to study the kinetics of P700 oxidation/reduction in leaves of the upper tier of tomato plants under the influence of elevated temperature (40°C, 3h) and infection with Fusarium oxysporum and their combined action. Thermal exposure reduced the level of P700 oxidation and the quantum yield of photosystem I (PSI) photochemical reactions and increased nonphotochemical energy dissipation on the PSI acceptor side. At the same time, the contribution of electron flow catalyzed by ferredoxin-plastoquinone reductase (FQR) decreased in heat-treated plants while the proportion of cyclic electron transport dependent on the NADH dehydrogenase-like complex (NDH) increased. Infection of the plants with the pathogenic fungus Fusarium oxysporum caused a decrease in the quantum yield of PSI photochemical reactions and a significant increase in nonphotochemical energy dissipation on the PSI acceptor side. The combined eff ect of elevated temperature and F. oxysporum enhanced the suppressive eff ect of both stress factors on photoinduced oxidation of P700 and activated nonphotochemical energy dissipation on both the acceptor and donor sides of PSI. The suppression of linear and FQR-dependent cyclic electron flows induced by Fusarium wilt and elevated temperature was compensated by activation of NDH-dependent electron transport.