AVENA PHYTOCHROME-A OVEREXPRESSED IN TRANSGENIC TOBACCO SEEDLINGS DIFFERENTIALLY AFFECTS RED FAR-RED REVERSIBLE AND VERY-LOW-FLUENCE RESPONSES (COTYLEDON UNFOLDING) DURING DE-ETIOLATION

Etiolated seedlings of tobacco (Nicotiana tabacum L.) were exposed to single light pulses predicted to establish different proportions of ph ytochrome in its far-red absorbing form (Pfr/P). The angle between the cotyledons was compared in wild-type and transgenic seedling overexpr essing Avena phytochrome A over the range of both very low-fluence res ponses (VLFR) and low-fluence responses (LFR). The unfolding of the co tyledons increased linearly for 24 h after the light pulse. At (his ti me the Pfr/P-response curve showed two linear segments. The segment be low a calculated Pfr/P = 3% (i.e. VLFR) was steeper than the segment a bove 3% (i.e. LFR). In the VLFR range the slope was almost threefold h igher in transgenic than wild-type seedlings. However, in the LFR rang e the difference was less than 50%. From these data we propose that Av ena phytochrome A makes a higher contribution to VLFR than LFR in etio lated tobacco seedlings.