Characterization of a differential low-temperature growth response in two species of Lycopersicon: the plastochron as a tool

Abstract

The plastochron model was used to evaluate the differences in the growth response of two Lycopersicon spp. grown under two temperature regimes (25/18 and 12/5°C). Two altitudinal accessions of L. hirsutum Homb. et Bnpl., from low and high altitude, a breeding line of L. esculentum (L.) Mill. and the hybrid between the latter and the high-altitude L. hirsutum were studied. The plastochron (P) values were estimated directly according to the formula of R.O. Erickson and F. Michelini (1957, Am. J. Bot. 44, 297–305), and indirectly through a linear model estimating the exponential rates of leaf elongation (r) and the ln of the plastochron ratios (q). The P values were obtained as P=q/r, and with one exception values obtained with both methods were comparable. Low temperature significantly decreased r in all genotypes, but the extent of this reduction depended on the genotype. The hybrid exhibited the least reduction, followed by the high-elevation L. hirsutum, L. esculentum and the lowelevation L. hirsutum. While the q values of the L. hirsutum accessions were significantly reduced by low temperature, those of L. esculentum and the hybrid were not. With the exception of the low-altitude L. hirsutum, low temperature significantly increased P, however the extent of the increase was significantly greater in L. esculentum. Analysis of temperature dependent changes of r, q and P indicate that L. esculentum extended its P by approximately the same factor its r was reduced. On the other hand, the L. hirsutum accessions increased P to a lesser extent, therefore having the ability to produce, comparatively, more leaves at lower temperatures than the cultivated tomato. The linear model of the plastochron is proposed as a tool for comparative studies of environmental growth responses of different genotypes. Plant size was reduced by low temperature. Considering plant size attained at high temperature as 100%, at low temperature sizes were reduced to 73% for the hybrid, 61% for the high-altitude L. hirsutum, 39% for L. esculentum and 30% for the low-altitude L. hirsutum. The low-temperature regime delayed flowering by two, three and nine plastochrons in the hybrid, the high-altitude L. hirsutum and L. esculentum, respectively, while the low-altitude L. hirsutum did not flower for the duration of the experiment. When artificially pollinated, L. esculentum yielded parthenocarpic fruits, while the high-altitude L. hirsutum and the hybrid produced fruits with viable seeds.