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Paclobutrazol increased the tolerance of soybean (Glycine max L. cv. AC Bravor) seedlings to elevated levels of ultraviolet-B radiation (UV-B: 280 – 320 nm). Within 3 days of exposure to elevated UV-B, nontreated seedlings were photosynthetically less efficient, and by the end of the 11th day of treatment their leaf areas and fresh masses were, respectively, 55 and 44% smaller than their counterparts not irradiated with UV-B. In contrast, paclobutrazol-treated seedlings maintained full photosynthetic efficiency throughout the duration of UV-B irradiation, and their leaf areas and fresh masses were only reduced by 30 and 22%, respectively. No changes in flavonoid composition occurred as a result of the paclobutrazol or UV-B radiation treatments, but UV-B tended to increase the quantities of photosynthetic pigments. The leaves from paclobutrazol-treated seedlings were significantly thicker than those of nontreated seedlings, and SEM analyses showed that the treated seedlings had much denser epicuticular wax layers than the controls. No significant changes in protein expression occurred during exposure to elevated UV-B and (or) paclobutrazol. Further studies with paclobutrazol-treated plants may lead to a better understanding of one or more of the mechanisms employed by plants to tolerate elevated levels of UV-B. Key words: leaf fluorescence, paclobutrazol, pigments, protein synthesis, SEM, UV-B radiation.

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Abstract.Previous studies have shown that uniconazole inhibits ethylene synthesis and protects plants from various stresses. The present research was conducted to delineate the mechanism of ethylene inhibition by uniconazole [(E)-(p-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)-l-penten-3-ol]. Following heat stress of 48~ for 3 h, the shoots of the control wheat seedlings became desiccated, and the seedlings lost 23% of their fresh mass 8 h after stress. The control soybean seedlings had epinastic unifoliate leaves 5 h after foliar application (4.4 g.a.i./ha) of the herbicide triclopyr [(3,5,6-trichloro-2-pyridinyl)oxyacetic acid]. Soil drench applications of uniconazole, a potent member of the triazole family, reduced these symptoms associated with heat and herbicide stress in wheat (5.0 mg/L) and soybean (0.4 mg/L) seedlings, respectively.Basal ethylene production was inhibited 32 and 48% by uniconazole in the wheat and acotyledonous soybean seedlings, respectively. Following a 48~ heat stress, 1-aminocyclopropane-1-carboxylic acid (ACC) levels increased 40% in both the control and uniconazole-treated wheat seedlings. After triclopyr application, ACC levels increased 400% in botl?, the control and uniconazoletreated soybean seedlings: The increased ACC levels, following stress, Were accompanied by increased ethylene production from the control, but not from the uniconazole-trea~ed wheat and acotyledonous soybean seedlings. ~iconazole treat-" ment did not significantly chang,e the basal or stress-induced N-malonyl-1-aminoc~r~lopropane-1-carboxylic acid (MACC) levels compa~ to controis. These results suggest that uniconazo"le inhibits ethylene synthesis by interfering with the conversion of ACC to ethylene in wheat and acotyledonous soybean seedlings. Ethylene production and ACC conversion were not inhibited by uniconazole in excised soybean cotyledons. These results indicate that different ethylene-forming enzyme (EFE) systems operate in the soybean acotyledonous seedling and cotyledon, and the system in the former is inhibited by uniconazole.

The characteristics of ethylene production and ACC conversion in 8-day-old soybean seedlings were examined and a relationship between cytochrome P-450 activity and ethyleneforming enzyme (EFE) activity was found. An atmosphere containing 10% carbon monoxide (CO) significantly inhibited ethylene production and ACC conversion in control soybean seedlings, but had only a slight effect on soybean seedlings treated with uniconazole. Foliar application of triclopyr, a pyridine analogue of the phenoxy herbicides, significantly increased ethylene production and ACC conversion in control, but not in uniconazoletreated seedlings. Triclopyr treatment also resulted in a three-fold increase in extractable cytochrome P-450 of 5-day-old etiolated soybeans. At equimolar concentrations tetcyclacis was more effective than uniconazole in reducing shoot elongation and endogenous ethylene production. Although uniconazole and tetcyclacis did not inhibit ACC conversion in nonherbicide-treated soybean seedlings, they did prevent the observed increase in ACC-dependent EFE activity following triclopyr application. However, the rate of ACC conversion in etiolated soybean segments was sensitive to uniconazole, and tetcyclacis inhibited the rate of ACC conversion by 2.6-fold in etiolated soybean segments within 4 h after treatment. Microsomal membranes were isolated from 5-day-old naphthalic anhydride-treated etiolated wheat shoots as this tissue contains much higher cytochrome P-450 levels than soybean shoots. Optical difference spectroscopy demonstrated that ACC generated binding spectrum characteristic of a reverse-type-I cytochrome P-450 substrate when combined with reduced microsomes. In vitro conversion of ACC to ethylene by microsomal membranes was NADPH-dependent, inhibited by CO, and had an apparent K m and Vr~ of 45 p,M and 0.345 nl/mg protein/h, respectively. These results suggest that cytochrome P-450-mediated monooxygenase reactions may be intimately involved in the conversion of ACC to ethylene in young soybean and wheat seedlings.