TEMPERATURE SENSITIVITY OF THE PHOTOCHEMICAL FUNCTION OF PHOTOSYNTHESIS IN POTATO (SOLANUM-TUBEROSUM) AND A CULTIVATED ANDEAN HYBRID (SOLANUM X JUZEPCZUKII)

The responses of the photochemical apparatus of photosynthesis to low and high temperatures were compared in leaves of the frost-sensitive S olanum tuberosum (cv. Haig) and of a frost-tolerant Andean potato, Sol anum x juzepczukii (cv. Lucki). The main observations and conclusions of this study are that: (i) Photosystem II (PSII) is noticeably more h eat-resistant in S. x juzepczukii than in S. tuberosum, indicating an enhanced generalized stress tolerance of the former genotype to extrem es of temperature. (ii) The higher thermostability of PSII in S. x juz epczukii leaves is not associated with any enhancement of the sensitiv ity of PSII photochemistry to chilling temperature. In both species, t he chilling-induced inhibition of electron transport through PSII is c losely correlated with the inhibition of the PSII-to-PSI electron flow , the rate of which is determined by the reoxidation of reduced plasto quinone. A slowdown of the latter reaction at low temperature can be a ttributed to the accumulation of protons in the thylakoid lumen associ ated with the inhibition of the Calvin cycle activity in chilled leave s, as suggested by the strong non-photochemical quenching of chlorophy ll fluorescence. (iii) The photochemical activities of both species ar e similarly impaired by chilling treatments in the light, indicating t hat frost resistance does not preclude susceptibility to photoinhibiti on damage at chilling temperature. (iv) A striking difference between S. tuberosum and S. x juzepczukii is the high plasticity of the PSII t hermotolerance in the latter species, with low (8 degrees C) and high (35 degrees C) temperature treatments respectively decreasing and incr easing the heat-tolerance of PSII. These changes are not observed or a re very limited in the Haig variety of S. tuberosum. (v) In contrast t o the constitutive thermotolerance of PSII (measured in 23 degrees C-g rown plants), 35 degrees C-induced thermotolerance has a dramatic effe ct on the photochemical activity at chilling temperature. When placed at 5 degrees C, the intersystem electron flow of 35 degrees C-treated leaves is dramatically inhibited as compared with non-treated leaves w hereas Delta pH-related quenching of chlorophyll fluorescence is uncha nged. These findings indicate independent control of non-acclimated he at-tolerance and thermally induced heat-tolerance of the photosyntheti c membranes. Taken together, the presented data show that the photosyn thetic apparatus of the cultivated Andean hybrid, S. x juzepczukii, th ough sensitive to chilling injury in the light, is adapted to the chan ging temperature conditions prevailing in the natural habitat of its w ild progenitor where night frosts are associated with warm and sunny d ays.