Responses of Avena and Pisum Tissues to Phytochrome Conversion by Red Light

Since the phytochrome in seedlings grown in (larkness is entirely in the red-absorbing (Pr) form, it is reasonaible to suppose that the morphogenetic effects of red light are due to formation of the farred-absorbing (Pfr) form (4). This hypothesis can be studied by briefly illuminating dark-grown tissues, replacing them in darkness, and determining the relationship between the percentage of the total phvtochrome initially converted to Pfr and the subsequent physiological response. It is importaint to note, however, that there are 2 distinct ways of establishing a given percentage conversion to Pfr. The mllost obvious is to vary the dosage of essentially pure red light. The other is to give saturating exposures to sources emitting both red and far-red in ratios maintaining various photostationary states (2,3,4). One might expect results obtained by the 2 mlethods to agree, but this expectation has not so far been carefully tested. Reports that the growth responses of segmilents of both Pisuimii stems (2) and Avenza coleoptiles (3) require a substantial phytochrome conversion (at least 50 % Pfr) for saturation depend on photostationary state miiethods, though with some indicationis that in Pisitini the results with red light would fall in the same range. On the other hand, the effects of red light pretreatment oln Zea phototropism saturate at least 2 orders of magnitude below the energy required to give detectable phytochrome conversion, though the phenonmenon is far-red-reversible (1 and Briggs, private communication). Since all far-red filters leave at least 1 % Pfr (2, 3, 4) the Zea results represent a complete contradiction betweeni the results of the red (losage and photostationary state methods. Hence, it seemed desirable to re-examine the Pisuit and Avenia results previously obtained, this time by the re(l dosage method. \V'ith the exception of the red light source, all materials and procedures were as described previously (2, 3). This includes the convention on calculating percentage Pfr, in which a correction is made for incomplete conversion by far-red, but n,ot for in-