Pretreatment of hydroponically-grown cotton and corn with lithium (Li + ) and lanthanum (La 3+ ) chlorides, respectively, inhibited 2,4-dichlorophenoxyacetic acid (2,4-D) and metribuzin [4-amino-6-tert-butyl-3-methylthio-1,2,4-triazin-5(4H)-one (MTZ)]-induced hormetic growth in both plant species. Lithium chloride proved to be a more effective inhibitor of hormetic growth. This may be due to the fact that the Li + ion readily crosses cell membranes and interferes with calcium (Ca 2+ ) metabolism. Lanthanum, on the other hand, has a relatively high affinity for specific sites in various types of Ca 2+ channels and can displace Ca 2+ from the cell wall, but cannot penetrate the cell membrane via the channel pore. Thus it acts as an inhibitor of Ca 2+ channel activity and regulation. This work provides indirect evidence that a Ca 2+ influx is involved in a hormetic (growth stimulation) response by cotton and corn plants when exposed to sublethal dosages of herbicide.

A linear relationship (r2 = 099) was found between the concentration of acephate in the nutrient solution applied to gerberas (Gerbera jamesonii H. Bollus ex Hook. f) grown in perlite and the mortality of nymphs of greenhouse whitefly, Trialeurodes oaporariorum Westwood. The LD,, for this method of application was 84 mg litre-'. A single irrigation with 180 mg litre-acephate killed all nymphs in 10 days without apparent damage to plants. At 120 mg litrel , acephate accumulated readily in leaves, reaching a maximum concentration of 11 mg kg-' dry matter on the third day of treatment. Concentrations in leaves then fell gradually, reaching 50% of the maximum concentration seven days later. Methamidophos was first detected in leaves approximately a day later than acephate. A maximum concentration of 9 mg kg-' dry matter was recorded on day 5 after which the concentration slowly fell. The half-life of methamidophos was estimated to be 11 days. The systemic use of acephate for the control of greenhouse whitefly and its relevance to Integrated Pest Management (IPM) programmes for pests of ornamental plants are discussed.