Rapid Metabolism of Propylene by Pea Seedlings 1

Propylene uptake by intact pea seedlings (Pisum sativum L. cv. Alaska) was easily detected using standard gas chromatographic techniques suggesting rapid metabolism. Comparative studies with highly purified 14C3Hs and 14C2H4 under aseptic conditions verified that propylene was rapidly metabolized and indicated that some aspects of its metabolism were similar to that of ethylene since 14C3H6, like 14C2H4 (Beyer, Nature 1975, 255: 144-147), was oxidized to 14CO2 and incorporated into water-soluble tissue metabolites. However, 14C3H6 was metabolized at a substantially faster rate and unlike 14C2H4 the rate of 14C3H6 tissue incorporation exceeded its rate of oxidation to 14C02. In addition the neutral 14C-metabolites derived from 14C3H6 were chromatographically distinct from those formed from 14C2H4. In contrast to ethylene (2, 3, 5, 7), the metabolism of propylene has not been investigated even though it can clearly mimic ethylene action if applied at a concentration about 100 times that of ethylene (1). Because of its structural similarity, propylene undoubtedly attaches to the same receptor site as ethylene. Therefore, it was of interest to determine if propylene, like ethylene, is also actively metabolized by plant tissues. A preliminary report of this work has been published (6). MATERIALS AND METHODS Initial experiments were conducted with nonradioactive propylene. Ten healthy, or hot-water-killed (70 C, 2 min), aseptically grown 3-day-old etiolated pea seedlings were enclosed in each of several 240-ml Lucite acrylic resin chambers and gassed with either 1.3, 12.3, or 104 ,Il/l of nonlabeled propylene for 96 hr. Each chamber contained a beaker with 4 ml of 3 N NaOH to trap evolved CO2. Chambers without seedlings were also included to evaluate sampling losses. 02 was added to each chamber after each 24-hr propylene sampling period to restore chambers to atmospheric pressure and prevent 02 deficiencies. In subsequent work the nonlabeled propylene was replaced with [1_-4C]propylene purified by preparative gas chromatographic techniques (4) using a Porapak N column rather than the standard Porapak T column. Following a 24-, 48-, 72-, or 96-hr incubation period at 1 and 9 ,tl/ 1 of 14C3H6, the seedlings and NaOH were removed and counted for radioactivity as previously described (2, 3). For comparative purposes purified 14C2H4 (4) was also run in parallel with "C3H6. RESULTS Attempts to measure the uptake of nonlabeled ethylene by ' Contribution No. 2536 from Central Research and Development Department, Experimental Station, E. I. du Pont de Nemours and Company, Wilmington, Delaware 19898. conventional gas chromatographic techiques have failed (1) primarily since this technique is not precise enough to detect the small amount of uptake that has been shown to occur by '4Ctracer studies (2, 3, 5). Such measurements are further complicated by the fact that they must be made against a background of continuous endogenous ethylene production. In contrast to ethylene no previous attempt has apparently been made to measure the uptake of propylene by plant tissues. This is unfortunate since, as seen in Figure 1, such measurements would have revealed a very rapid uptake of propylene that could have been readily detected by conventional gas chromatographic techniques and, unlike ethylene, would not have required the use of labeled material. Starting with an initial propylene concentration of 1.3 ,tl/l (Fig. IA) or 7.9 qmol of propylene for each g dry wt of pea seedling tissue, over half of the propylene in the chamber had disappeared (4.2 ijmol/g dry wt) at the end of the first 24 hr. By the end of 72 hr, 97% (7.7 qmol/g dry wt) of the propylene was gone from the gas phase in the chamber. Increasing the propylene concentration to 12.3 ,ul/l (Fig. 1B) or 73 tqmol/'g dry wt did not significantly change the percentage of total propylene taken up at various times, clearly indicating that the uptake rate increased with increasing concentration. The rate of propylene uptake at both 1.3 and 12.3 ,il/l (Fig. I A and B) was fairly linear during the first 48 hr of incubation but fell off quickly thereafter because of the rapidly diminishing amount of propylene left in the chamber. When high levels of propylene were applied (104 ,ul/ 1 or 619 vqmol/g dry wt), and propylene never became limiting, the rate of propylene uptake was constant over the entire 96-hr incubation period (Fig. IC). The uptake was clearly dependent on living tissue since no uptake was observed in those chambers containing the hot-waterkilled seedlings (Fig. 1, A, B, and C). Microorganisms were not involved in the uptake of propylene since no uptake occurred even when nonaseptic conditions prevailed in the chambers containing the killed seedlings and massive contamination was apparent. Studies with radioactive propylene confirmed the uptake results obtained with nonlabeled propylene (Fig. 2A). Like ethylene (2, 3, 7; Fig. 2B), [1-_4C]propylene applied at I ,ul/l was oxidized to 14CO2 and incorporated into tissue metabolites. The rate for total propylene metabolism during the first 48 hr, when uptake was linear, was approximately 25 times faster than wtih 14C2H4 (Fig. 2, upper curve A versus B). Unlike 14C2H4, the rate of 14C3H6 tissue incorporation exceeded the rate of oxidation to 14C02. (Compare lower two curves in A and B.) Similar results were obtained when 9 uI/1 of 14C3H6 was applied (data not shown). By comparing the total metabolism curve in Figure 2A with the uptake curve in Figure IA, where a similar propylene concentration was applied, it can be seen that tissue incorporation plus oxidation completely account for the nonlabeled propylene lost in Figure IA. Because 14C3H6 and 14C2H4 were oxidized and incorporated into pea tissues at substantially different rates at I ,ull I (Fig. 2) a direct comparison was made between 14C3H6 and 14C2H4 over a wide concentration range using gases of equal specific radioactivity. As 893 www.plantphysiol.org on July 15, 2017 Published by Downloaded from Copyright © 1978 American Society of Pla t Biologists. All rights reserved. Plant Physiol. Vol. 61, 1978