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

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.