Chlorophyll-sensitized photooxidation of indoleacetic acid (IAA)-with chlorophyll extracted from Pisum sativum L. cv. Alaska W.R.-was determined in the presence of deuterium oxide and known quenchers of singlet oxygen ('02) to explore the involvement of '02 in the reaction. 02 uptake was measured in light in a buffered aqueous miceliar system containing Triton X-100, KCI, chlorophyll, and IAA. The rate of 02 uptake was zero in darkness. The reaction was stimulated by deuterium oxide and inhibited by sodium azide indicating that '02 participated in IAA photooxidation. Both mannitol and superoxide dismutase failed to inhibit 02 uptake suggesting that neither the hydroxyl radical nor the superoxide anion played a significant role in the reaction.The Chl-sensitized photooxidation of IAA and other indoles in an aqueous micellar system was described in a recent report (18).The presence of indoles, IAA in particular, stimulated 02 uptake in light by a buffered medium containing Triton X-100, KCI, and Chl. Triton X-100 was provided to solubilize Chl (for a discussion on its role, see [18]). More recently, the same system was used to demonstrate photooxidation of other plant growth substances including cytokinins and the gibberellic acid precursor, kaurene (I. A. Tamas, J. L. Germain, and J. L. Koch, unpublished manuscript). These results suggest that IAA, and other plant growth substances as well, may be degraded in leaves through a photooxidative reaction sensitized by Chl.In one type of photooxidation, the sensitizer interacts with the [15]) from leaves of 2-week-old plants as described previously (18). Stock solutions of Chl were adjusted to 0.3 mg ml-1 in 80%1o (v/v) acetone, and kept at -40°C in darkness until used.To measure IAA photooxidation, 02 uptake was monitored with a polarographic probe (Biological Oxygen Monitor, model 53; Yellow Springs Instrument Co.) connected to a strip chart recorder. Unless otherwise noted, the aqueous reaction medium contained 50 mm Mes at pH 5.5, 1% (v/v) Triton X-100, and 800 mM KCI in a volume of 3 ml. IAA was added as a 10% (w/w) solid stock in sucrose. The medium was stirred for 5 min, allowing IAA to fully dissolve and the medium to approach air saturation, then Chl was added at a final concentration of 0.03 mg ml-'. The reaction mixture was kept at 20°C and illuminated at an intensity of 7.5 x I04 ergs cm-2 s-1 with a slide projector using a 500-w General Electric DFR projection lamp (18). 02 uptake rates were expressed as mol 02 mg ' Chl h-.02 uptake without IAA was determined in each experiment (generally 18-20 umol 02 mg' Chl h-'), and subtracted from the rates measured with added IAA.Measurements with rose bengal were performed in an identical manner as described above for Chl.The reduction of NBT by xanthine oxidase was determined according to Beauchamp and Fridovich (1)
