Kinetics of stem elongation in light-grown tomato plants. Responses to different photosynthetically active radiation levels by wild-type and aurea mutant plants

In this research, we measured the short-and long-term stem elongation responses of wild-type and arn en (ari) mutant tomato plants to different photosynthetically active radiation (PAR) levels by using linear voltage transducers. Stem elongation was continuously measured in green tomato plants over 2,75 days, under 12 h light/12 h dark photoperiods or in darkness after a 6 h irradiation period, There is no significant difference in stem elongation between wild-type plants pregrown at either 100 or 400 mu mol m(-2) s(-1) and then exposed to 12 h photoperiods, However, in the as mutant there is a very large difference between plant pregrown under 100 or 400 pmol m(-2) s(-1) and then exposed either to 12 h photoperiods or to continuous darkness, Total stem elongation of the wild type appears to be maximal at 100 mu mol m(-2) s(-1), while that of the art mutant appears to be maximal with PAR 400 mu mol m(-2) s(-1). Wild-type plants displayed PAR-dependent (in the range 100-800 mu mol m(-1) S-1) inhibition of growth both during the day and during the night, In contrast, the au mutant showed a fluence-rate-dependent promotion of growth during the dark periods in the range of 10-400 mu mol m(-2) s(-1), Large, fast and opposite changes in stem elongation rate at the light/dark and dark/light transitions were present in both genotypes, Internode elongation rate in the first half of the night was always modest in wild-type tomato, whereas it increased rapidly in the art mutant. Stem elongation rate of wild type starts to increase after about 6 h in darkness, showing the typical time course of escape from Pfr-mediated inhibition of elongation by an end-of-day response, The role of phytochrome level and type in sensing light quantity is discussed.