SUSCEPTIBILITY OF TOBACCO-LEAVES TO PHOTOINHIBITION FOLLOWING INFECTION WITH 2 STRAINS OF TOBACCO MOSAIC-VIRUS UNDER DIFFERENT LIGHT AND NITROGEN NUTRITION REGIMES

Sensitivity to photoinhibition under high light stress (2000 mu mol ph otons m(-2) s(-1) for 2 h in air) and recovery from this stress were e xamined in leaves of control, uninfected tobacco (Nicotiana tabacum cv Xanthi) leaves and in leaves of tobacco plants infected with tobacco mosaic virus (TMV) when grown under low light (150-200 mu mol photons m(-2) s(-1)) or high light (1200 mu mol photons m(-2) s(-1)) with high (8.0 mM) or low (0.5 mM) nitrate supply. Photoinhibition was monitore d using the dark-adapted fluorescence parameters variable fluorescence /maximum fluorescence, an indicator of photosynthetic efficiency that correlated well with the quantum yield of photosynthetic oxygen evolut ion, and initial fluorescence, potentially an indicator of photoinhibi tory damage. Susceptibility to photoinhibition was greater in low ligh t- and low nitrogen-grown control plants than in high light- or high n itrogen-treated plants. Compared with uninfected controls, infection w ith the masked strain PV42 increased susceptibility to photoinhibition only in plants grown under low tight/low nitrogen conditions. In expa nding leaves, infection with severe strain TMV PV230 markedly accelera ted photoinhibition under these conditions and under high light/low ni trogen conditions, even before visible symptoms were evident. High nit rogen levels during growth protected against this accelerated photoinh ibitory response to virus infection during light stress and generally promoted recovery, at least prior to symptom development. As symptoms developed, the yellow regions provided evidence for chronic photoinhib itory damage, prior to and during the stress treatment, irrespective o f growth conditions. Green regions of leaves showing visible symptoms were generally indistinguishable from control, uninfected plants durin g photoinhibitory stress and recovery. In developed leaves that remain ed free of visible symptoms during the experiments, in spite of the ac cumulation of about the same amounts of virus protein (S. Balachandran , C.B. Osmond, A. Makino [1994] Plant Physiol 104: 1043-1050) infectio n led to an acceleration of photoinhibition during stress treatments,e specially in row light/low nitrogen treatments, in which chronic photo inhibitory damage was evident. These studies suggest a role for photoi nhibitory damage in the acceleration of visible symptom development fo llowing TMV PV230 infection of expanding leaves, as well as in acceler ation of senescence in developed leaves without visible symptoms.