A chilling episode of a few hours damaged root ammonium absorption in a cultivated tomato (Lycopersicon esculentum cv. T-5), but not in a wild congener from high altitudes (Lycopersicon hirsutum LA1778). In the cultivar, ammonium influx was strongly temperature dependent and showed the residual effects of chilling, whereas ammonium efflux was nearly temperature invariant and showed no persistent effects. A 2 h exposure to 5°C significantly depressed subsequent ammonium absorption at 20°C, and about 12 h at 20°C was required for recovery. For both the cultivated and wild species, rerooted cuttings were slightly less sensitive to chilling than seedlings. The relative inhibition (mean ± SE) of ammonium absorption before and after chilling was 58·4 ± 2·5% for the cultivated species and 29·0 ± 9·1% for the wild species. The F 1 hybrid between the species showed a relative inhibition of 52·4 ± 3·6%, suggesting that chilling sensitivity may be dominant. In a backcross of the hybrid to L. esculentum, the phenotypic distribution of the relative inhibition of ammonium absorption indicated that this trait is segregating.Key-words: root ammonium uptake; tomato chilling tolerance. INTRODUCTIONTomato (Lycopersicon esculentum Mill.) is a classic example of a chilling-sensitive plant. One or two nights of temperatures below 10°C severely inhibit the growth and development of tomato at all life stages, and one or two nights of temperatures below 6°C inflict significant injury (Geisenberg & Stewart 1986). By contrast, Lycopersicon hirsutum, an interfertile wild species that inhabits the Peruvian Andes at altitudes up to 3300 m, may encounter temperatures near or below freezing every night during its growing season (Patterson, Paull & Smillie 1978). Comparisons between these species show that L. hirsutum accessions from high altitudes thrive at chilling temperatures that prove detrimental to L. esculentum (Patterson et al. 1978;Dalziel & Breidenbach 1982;Vallejos & Tanksley 1983;Wolf et al. 1986; Vallejos & Pearcy 1987). The physiological bases for this differential chilling sensitivity remain uncertain although several hypotheses have been put forward (Bowers 1994;Guy 1994;Li 1994;Kaye & Guy 1995;Nishida & Murata 1996).One hypothesis is that chilling causes transitions in membrane lipids from a fluid phase to a gel phase that impair membrane function (Lyons & Raison 1970). Not only has this hypothesis been subject to much debate (Martin 1986;Raison & Lyons 1986;Nishida & Murata 1996) but, in this specific case, phase transitions in leaf membrane lipids from chilling-sensitive and -tolerant genotypes of L. esculentum and L. hirsutum occur at similar temperatures (Dalziel & Breidenbach 1982;Low et al. 1984;Marangoni & Stanley 1989;Raison & Brown 1989). X-ray diffraction measurements on tomato fruits showed only trace amounts of gel phase lipid even after 20 d at 5°C (Sharom, Willemot & Thompson 1994). These results indicate that bulk lipid-phase transitions are not a major factor in the short-term chilling injury of tomato.Other po...
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