Oilseed crops are frequently subject to contamination by mycotoxins produced by Aspergillus spp., particularly aflatoxin (AF) and to a lesser extent sterigmatocystin (ST). Several studies have suggested that metabolites generated from the plant lipoxygenase (LOX) pathway may either decrease or increase mycotoxin production by Aspergillus spp. We tested the possibility that the occurrence of seed LOX isozymes that produce distinct hydroperoxy fatty acids may account for these different effects on AF biosynthesis. For example, soybean LOX1 catalyzes the addition of O2 to the C13 position of linoleic and linolenic acids while maize embryo LOX catalyzes the addition of O2 to the C9 position. In vitro experiments showed that 13S-hydroperoxy fatty acids at concentrations of 10 and 100 μM repressed AF and ST pathway gene expression and significantly (P = 0.05) reduced AF and ST production in both A. parasiticus (AF producer) and A. nidulans (ST producer). Treatment with 1 μM 13S-hydroperoxy linoleic acid also significantly decreased AF production when introduced into growth media at continuous 24-h intervals. In contrast, the same concentrations of 9S-hydroperoxy linoleic acid did not reduce AF or ST production but extended the length of time AF and ST transcripts were detectable. These results show that 13S-hydroperoxy fatty acids directly or indirectly repress AF and ST biosynthesis and provide in vitro evidence that specific seed lipoxygenase activity could provide resistance to mycotoxin contamination by Aspergillus spp.

ABSTRACrSupr content and composition are major criteria used in judging the quality of netted muskmelon (Cwucmis melo L. var reticulatus) fruit. Sugar composition and four enzymes of sucrose metabolism were determined in 'MNum 45' muskmelon fruit at 10-day intervals binig 10 days after pollination (DAP) until full-slip (35 DAP). Sugar content increased in both outer (green) mesocarp and inner (orange) mesocarp between 20 and 30 DAP. The major proportion of total increase in sugar was attributed to sucrose accumulation. The large increase in sucrose relative to glucose and fructose was accompanied by a dramatic decrease in acid invertase activity, which was highest in both tissues at 10 and 20 DAP, and increases in sucrose phosphate synthase (SPS) and sucrose synthase activities. The green tissue had a lower propordto of total sugar as sucrose, greater invertase activity, and less SPS activity than the orange tissue. Changes in relative sucrose content were highly correlated with changes in enzyme activity. The results strongly suggest that increases in the proportion of sucrose found in melon fruit were associated with a decline in acid invertase activity and an increase in SPS activity approximately 10 days before full-slip. Therefore, these enzymes apparently play a key role in determining sugar composition and the quality of muskmelon fruit. Ethylene. Tissue cylinders were taken along the equatorial plane of each fruit with a 0.8 cm diameter cork borer and cut to a uniform length of 2.5 cm. The tissue cylinders were divided into a 0.5 cm portion including the rind and the remaining 2.0 cm segment made up of the inner orange tissue. Each segment was placed in a 12 x 75 mm glass vial and capped with a rubber serum cap. The headspace gas was allowed to accumulate for 0.5 to 2 h; 1 ml was then analyzed for ethylene by FID-gas chromatography on an alumina column (2.46 m x 0.32 cm).Sugar extraction and analysis. Four cylinders of tissue were taken from each melon as desibed above using a 1.9 cm diameter cork borer. The netting was removed, and the cylinders of mesocarp were divided into the outer 0.5 cm (green) and the inner 2.0 cm (orange). Tissue was weighed and frozen at -80°C

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Seed size was examined as a possible explanation for variation in the size of containerized loblolly pine (Pinustaeda L.) seedlings. Under laboratory conditions of minimal environmental stress, larger seed were found to germinate more quickly and produce a larger germinant after 28 days of growth. In a similar experiment conducted under greenhouse growing conditions, large seed produced the largest seedlings. Size differences among seedlings in both experiments resulted from differences in the rate of germination unique to each seed size class. Consequently, seedling size and possibly uniformity of growth were considered to be a function of germination patterns which were strongly influenced by seed size and weight.