Changes in levels of heme a, protoheme and protochlorophyll(ide) in submerged rice seedlings after exposure to air

Rice (Oryza sativa L. cv. Yamabiko) seedlings germinated under water for 5 days contained small amounts of heme a and protoheme but no protochlorophyll(ide) [Pchl(ide)]. Levels of hemes and Pchl(ide) increased rapidly upon transfer to air. When expressed in terms of fresh weight of tissue, hemes reached the levels in aerobic controls after 24 h of contact with air, but Pchl(ide) did not. A comparison of the increases during 24-h adaptation to air in levels of heme a and Pchl(ide), which are specific to mitochondria and plastids, respectively, suggested that the development of mitochondria preceded that of plastids. The rate of synthesis of 5-aminolevulinic acid (ALA) was low in submerged seedlings, as compared to the rate in aerobic controls, but it increased during air adaptation. The sum of the amounts of heme a, protoheme and Pchl(ide) increased in parallel with the amount of porphyrins, equivalent to the amount of ALA synthesized during the experimental period. When submerged seedlings that had been pretreated with levulinic acid were exposed to air, no Pchl(ide) was formed. In contrast, Pchl(ide) accumulated under water when submerged seedlings were fed with ALA. These results indicate that the synthesis of ALA, the limiting step in the synthesis of Pchl(ide), is repressed under hypoxic conditions.