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 differ ent photosynthetically active radiation (PAR) levels by using linear v oltage 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 significan t difference in stem elongation between wild-type plants pregrown at e ither 100 or 400 mu mol m(-2) s(-1) and then exposed to 12 h photoperi ods, However, in the as mutant there is a very large difference betwee n plant pregrown under 100 or 400 pmol m(-2) s(-1) and then exposed ei ther to 12 h photoperiods or to continuous darkness, Total stem elonga tion 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 mo l 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-de pendent 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 elon gation 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 rapi dly in the art mutant. Stem elongation rate of wild type starts to inc rease after about 6 h in darkness, showing the typical time course of escape from Pfr-mediated inhibition of elongation by an end-of-day res ponse, The role of phytochrome level and type in sensing light quantit y is discussed.