Chemical Forms of Manganese and Zinc in Phloem Exudates

Investigations were performed to study the chemical form in which manganese and zinc are transported in sieve tubes. As a test plant Ricinus communis was used. From this plant phloem sap can be obtained in a rather pure state. The plants received labelled manganese or zinc. In the experiments on manganese translocation a radioactive phosphorus isotope was added to the nutrient solution to test if complex compounds of the cation contain also phosphorus. Components of the phloem exudate were determined by means of physicochemical separation methods. Almost all zinc but only a part of the manganese were bound to organic compounds. The major part of the manganese was in ionic form. The molecular weight of the complexing com-pound(s) of manganese was estimated to be between 1000 and 5000 and of zinc between 1000 and 1500. The complexes probably contain some phosphorus. The charge of the zinc complex is negative.     

Hormonal Regulation of Phototropism in the Light-grown Sunflower Seedling, Helianthus annuus L.: Immobility of Endogenous Indoleacetic Acid and Inhibition of Hypocotyl Growth by Illuminated Cotyledons

Light-grown sunflower seedlings contain 10^–7–3 x10^–7 M indoleacetic acid (IAA). The even distributionof this endogenous IAA in straight hypocotyls does not changeduring phototropic curvature. The diffusate from hypocotylscontains substance(s) influencing the elongation rate of Avenacoleoptile segments but hardly any IAA. Phototropic curvature of the hypocotyl requires the presenceof illuminated cotyledons. Illumination of cotyledons inhibitshypocotyl growth. It is concluded that the phototropic response of the sunflowerhypocotyl is regulated by factors promoting and inhibiting cellelongation other than IAA.     

Cell Wall Solubilization in Pedicel Abscission of Begonia Flower Buds

Effects of metabolic inhibitors and growth regulators on the course of abscission and on the activities of cell wall solubilizing enzymes were studied in pedicel explants of Begonia flower buds. Actinomycin D, chloramphenicol and 2,4-dinitrophenol slightly retarded abscission, whereas cycloheximide exerted a strong inhibition if applied until 10.5 h after explant excision. Indoleacetic acid retarded and ethylene promoted abscission and cell wall solubilization. However, the activities of cell wall solubilizing enzymes did not correspond with the course of abscission. No polygalacturonase and pectic acid and pectin transeliminases could be detected in the abscission zone during abscission, whereas a low pectin methylesterase activity did not change. Endo- and exocellulase activities did not increase until about 10 h after the onset of abscission, indicating that they are the result rather than the cause of abscission.     

Hormonal Regulation of Pedicel Abscission in Begonia Flower Buds

In order to analyse the hormonal regulation of flower bud shedding in Begonia, levels of indoleacetic acid (IAA), abscisic acid (ABA) and ethylene were determined in buds and pedicels. The translocation and metabolism of ¹⁴C-labeled IAA in pedicel segments were also studied. In a monoecious Begonia fuchsioides hybrid, abscising male flower buds contain about 1% of the IAA present in non-abscising female flowers. In a male Begonia davisii hybrid, the seasonal variation in bud drop coincides with changes in the IAA content of the buds, while also the release of IAA from the bud to the pedicel is hampered. Abscission zones of these pedicels always contain abscission promoting ethylene concentrations. The tissue is prevented from responding with abscission by IAA from the flower buds. The buds also contain ABA but without influencing abscission considerably. Pretreatment with ethylene or ABA does not affect IAA transport in pedicel segments. The rate of this transport is 4–6 mm × h^–1:; the capacity increases with the transverse area. In young segments, IAA is decarboxylated and also otherwise metabolized.     

Respiration Rate and Mitochondrial Activity in Iris Bulbs

The oxygen uptake of iris bulbs (Iris×hollandica‘Wedgwood’) which had been stored dry at 30 C (“retarded’ bulbs) was low (10 μmol O₂ per h and bulb), the oxygen uptake of the intact bulb, the three outer fleshy scales and the remaining central part of the bulb increased three- to fourfold, nearly twofold and fourfold, respectively. Mitochondria were isolated from the scales of retarded and activated bulbs and their oxygen consumption with succinate, l -malate (plus pyruvate). x-ketoglutarate and NADH as substrate was measured polarographically. The oxidative capacity of mitochondria isolated from the scales of activated bulbs was only slightly higher than that from retarded bulbs when calculated on a tissue basis. No difference was found between the phosphorylation efficiency, respiratory control, cytochrome c deficiency, succinate dehydrogenase, malate dehydrogenase, succinate-cytochrome c rductase, NADH-cytochrome c reductase and cytochrome oxidase activity of the retarded and activated bulbs. The increase in the in vitro oxygen uptake of the scales after transition from 30 to 13 C was not accompanied by an equal increase in the oxidative capacity of their mitochondria suggesting that they are not responsible for this rise in oxygen uptake.     

Community structure and abundance of insects in response to early‐season aphid infestation in wild cabbage populations

1. Changes in the arthropod community structure can be attributed to differences in constitutively expressed plant traits or those that change depending on environmental conditions such as herbivory. Early‐season herbivory may have community‐wide effects on successive insect colonisation of host plants and the identity of the initially inducing insect may determine the direction and strength of the effects on the dynamics and composition of the associated insect community. 2. Previous studies have addressed the effect of early infestation with a chewing herbivore. In the present study, the effect of early infestation was investigated with a phloem‐feeding aphid [Brevicoryne brassicae L. (Hemiptera, Aphididae)] on the insect community associated with three wild cabbage (Brassica oleracea L.) populations, which are known to differ in defence chemistry, throughout the season in field experiments. 3. Aphid infestation had asymmetric effects on the associated insect community and only influenced the abundance of the natural enemies of aphids, but not that of chewing herbivores and their natural enemies. The effect size of aphid infestation further depended on the cabbage population. 4. Aphid feeding has been previously reported to promote host‐plant quality for chewing herbivores, which has been attributed to antagonism between the two major defence signalling pathways controlled by the hormones salicylic acid (SA) and jasmonic acid (JA), respectively. Our results show no effects of early infestation by aphids on chewing herbivores, suggesting the absence of long‐term JA–SA antagonism. 5. Investigating the effects of the identity of an early‐season coloniser and genotypic variation among plant populations on insect community dynamics are important in understanding insect–plant community ecology.