Kinetics of Stem Elongation in Light-Grown Tomato Plants. Responses to Different Photosynthetically Active Radiation Levels by Wild-Type and aurea Mutant Plants

Abstract— In this research, we measured the short- and long-term, stem elongation responses of wild-type and aurea(au) 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 LOO or 400 μmol m^?2 s^?1 and then exposed to 12 h photoperiods. However, in the au mutant there is a very large difference between plants pregrown under 100 or 400 umol 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 umol m^?2 s^?1, while that of the au mutant appears to be maximal with PAR 400 umol m^?2 s^?1. Wild-type plants displayed PAR-dependent (in the range 100-800 umol m^?2 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 umol 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 au 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.