Senescence of petals isolated from flowers of Hibiscus rosa-sinensis L. (cv Pink Versicolor) was associated with increased ethylene production. Exposure to ethylene (10 microliters per liter) accelerated the onset of senescence, as indicated by petal in-rolling, and stimulated ethylene production. Senescence was also hastened by basal application of 1-aminocyclopropane-l-arboxylic acid (ACC). Aminooxyacetic acid, an inhibitor of ethylene biosynthesis, effectively inhibited ethylene production by petals and delayed petal in-rolling. In marked contrast to these results with mature petals, immature petals isolated from flowers the day before flower opening did not respond to ethylene in terms of an increase in ethylene production or petal in-rolling. Furthermore, treatment with silver thiosulfate the day before flower opening effectively prevented petal senescence, while silver thiosulfate treatment on the morning of flower opening was ineffective. Application of ACC to both immature and mature petals greatly stimulated ethylene production indicating the presence of an active ethylene-forming enzyme in both tissues. Immature petals contained less free ACC than mature, presenescent petals and appeared to possess a more active system for converting ACC into its conjugated form. Thus, while the nature of the lack of responsiveness of immature petals to ethylene is unknown, ethylene production in hibiscus petals appears to be regulated by the control over ACC availability.

Use of arsenicals for insect control has resulted in an appreciable accumulation of arsenic in the surface soil of Annapolis Valley apple orchards. Levels of total arsenic in 25 representative orchards ranged from 9.8 to 124.4 p.p.m., whereas the content of comparable samples taken in non-orchard land ranged from a trace to 7.9 p.p.m. In some orchards the arsenic accumulation is of sufficient magnitude to be detrimental to the growth of such arsenic sensitive crops as peas and beans.

Increased lead and arsenic concentrations in the surface soil (0–15 cm), resulting from applications of lead arsenate (PbHAsO4), increased both lead and arsenic levels in crops grown on treated plots. The lead levels in some crops approached or exceeded the Canadian residue tolerance of 2.0 ppm. Lead arsenate soil treatments did not affect copper absorption by crops. On areas such as old orchard land contaminated with lead arsenate residues it may be advisable to ascertain the soil lead status before planting certain crops, and also to determine the lead affinity and arsenic sensitivity of the plants to be grown.

ABSTRACT

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