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  • Author or Editor: Hui Cao x
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The irritant effect of Dieffenbachia sap is attributed to protelytic enxymes but calcium oxalate crystals are considered to puncture cells and allow enzyme entrance. To date, no detailed study of the location, type, or frequency of calcium oxalate crystals in Dieffenbachia species or cultivars has been undertaken. To do so, three uniform tissue culture plantlets of Dieffenbachia `Carina',`Rebecca' or `Star Bright' were transpanted into 15 cm pots, grown in a shaded greenhouse under 385 μmol·m-2·s-1 and fertigated with 20 N-8.7 P-16.6 K water-soluble fertilizer at N concentrations of 200 mg·L-1 twice weekly. Ten weeks later, samples of stem, root, and leaves were taken from 4 pots of each cultivar to determine the distribution and type of calcuium oxalate crystals in each plant organ via polarized light microscopy. Two types of calcium oxlate crystals, raphides and druses, were found in the stem, leaves and roots. Druse density increased as leaves andd stems matured while the number of raphide idioblasts remained relatively constant. Crystal density was highest at lateral initation sites of buds and roots. Significant differences were found in crystal density among cultivars even though `Carina' and `Star Bright' are sports selected from `Camille'. This suggests that reduction of calcium oxalate density of Dieffenbachia cultivars is possible through breeding.

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Tissue-culturedexplantsofDieffenbachiamaculate`Exotic Perfection', D.`Snow Flake', and D. × `Tropic Breeze' were grown on ebb-and-flow trays subirrigated with nitrogen (N) at 50, 200, or 800 mg·L-1 using a water-soluble fertilizer 17N–2.1P–15.7K for 10 weeks in a shaded greenhouse under a maximum photosynthetic photon flux density of 285 μmol·m-2·s-1. Plants were then transferred to interior rooms under a light level of 8 μmol·m-2·s-1. Samples of the midrib were taken from the first mature leaf of plants before being placed indoors and also from the first mature leaf of plants 8 months after growing indoors. Counts of calcium oxalate crystal idioblasts in cross-sections of the basal midrib using polarized light microscopy showed that the number of crystal idioblasts was higher in all three cultivars fertigated with 200 mg·L-1 N than those fertigated with either 50 or 800 mg·L-1 N. The number of crystal idioblasts in each cultivar grown under 8 μmol·m-2·s-1 was about 50% of the number detected when plants were grown under 285 μmol·m-2·s-1. `Snow Flake' had the highest number of crystal idioblasts with counts up to 60 per cross-section, whereas `Exotic Perfection' had the lowest with only 30 per cross-section. This study shows that in addition to cultivar differences, light intensity and N can significantly affect calcium crystal formation, and the highest number of crystal idioblasts occurred when Dieffenbachia cultivars were grown under optimum conditions.

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Postbloom fruit drop (PFD) of citrus is incited by the fungus Colletotrichum acutatum J. H. Simmonds and may result in young fruit drop and severe yield losses. Previous studies suggested that imbalance of growth regulators such as auxin, ethylene, and jasmonic acid (JA) plays an essential role in young fruit abscission. In this work, we determined the factors associated with fungal-induced fruit drop by testing compounds inhibitory to hormonal transport or biosynthesis. As assessed on sweet orange (Citrus sinensis Osbeck) and grapefruit (C. paradisi Macf.) for 4 years, we found that many auxin transport and action inhibitors such as 2,3,5-triiodobenzolic acid (TIBA), 2-(4-chlorophenoxy)-2-methyl-propionic acid (clofibrate), or quercetin and JA biosynthesis inhibitors such as salicylic acid (SA) and aspirin (methyl-SA) applied 7 d after C. acutatum infection resulted in higher percentages of young fruit retention compared with the water controls. The commercial products ReZist and Actigard, widely used as systemic acquired resistance (SAR) agents, also improved fruit retention. Furthermore, application of gibberellic acid (GA3) on sweet orange, regardless of C. acutatum infection, significantly increased fruit retention. These commercial products may be very useful in managing this destructive disease of citrus in the field.

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