Journal of Plant Physiology 162 (2005) 281—289 High temperature effects on photosynthetic activity of two tomato cultivars with different heat susceptibility Daymi Camejo a , Pedro Rodrı ´guez a ,M % a Angeles Morales b,c , Jose´Miguel Dell’Amico a , Arturo Torrecillas b,c,d ,JuanJose´Alarco´n b,c,à a Instituto Nacional de Ciencias Agrı ´colas (INCA), Gaveta Postal 1, 32700 San Jose´ de Las Lajas, La Habana, Cuba b Centro de Edafologı ´a y Biologı ´a Aplicada del Segura, CSIC, P.O. Box 164, E-30100 Murcia, Spain c Unidad Asociada al CSIC de Horticultura Sostenible en Zonas A ` ridas (UPCT-CEBAS) d Dpto. Produccio ´n Agraria, ETSIA, Universidad Polite´cnica de Cartagena (UPCT), Paseo Alfonso XIII s/n, E-30203 Cartagena, Murcia, Spain Received 3 May 2004; accepted 15 July 2004 Summary The functional activities of the photosynthetic apparatus of two tomato cultivars of different thermotolerance were investigated after a short period of high temperature treatment. Seedlings of two tomato genotypes, Lycopersicon esculentum var. Campbell-28 and the wild thermotolerant Nagcarlang, were grown under a photoperiod of 16 h at 25 1C and dark period of 8 h at 20 1C. At the fourth true leaf stage, a group of plants was exposed to heat stress of 45 1C for 2 h. The heat shock treatment caused important reductions of the net photosynthetic rate (P n ) of Campbell-28 plants due to non-stomatal components. These non-stomatal effects were not evident in Nagcarlang-treated plants. This reduction in the CO 2 assimilation rate observed in Campbell-28 was generated by affections in the Calvin cycle and also in the PSII functioning. No changes in these parameters were observed in the thermotolerant genotype after the stress. Injury to the plasma membrane because of the heat stress was evident only in the Campbell-28 genotype. Heat led to a sun-type adaptation response of the photosynthesis pigment apparatus for the Nagcarlang genotype, but not for Campbell-28, and thus an increase in chlorophyll a/b ARTICLE IN PRESS www.elsevier.de/jplph KEYWORDS Carotenoid; Electron transport; Fluorescence; Photosynthesis; Rubisco; Stomata; Thermo tolerance; Tomato 0176-1617/$ - see front matter & 2004 Elsevier GmbH. All rights reserved. doi:10.1016/j.jplph.2004.07.014 Abbreviations: C i , intercellular CO 2 concentration; F o , initial chlorophyll fluorescence; F m , maximum chlorophyll fluorescence; F v , variable chlorophyll fluorescence; F v /F m , maximum photochemical efficiency of Photosystem II in dark adapted leaves; P n , net photosynthetic rate; PSII, photosystem II; FPSII open , efficiency of the open reaction centre in light; J max , maximum electron transport rate contributing to Rubisco regeneration; LHCPs, light harvesting chlorophyll proteins; V c , max , maximum carboxylation velocity of Rubisco à Corresponding author. Centro de Edafologı ´a y Biologı ´a Aplicada del Segura,(CEBAS-CSIC), P.O. Box 164, Espinardo, Murcia E-30100, Spain. Tel.: +34 968 396303; fax: +34 968 396213. E-mail address: jalarcon@cebas.csic.es (J.J. Alarco ´n).