For field-grown ‘Valencia’ sweet orange (Citrus sinensis) affected by Huanglongbing [HLB (Candidatus Liberibacter asiaticus (CLas)], trees that displayed more severe HLB symptoms (severe trees) had 74% fruit drop before harvest; however, the drop rate for less symptomatic trees (mild trees) was 45%. For mature fruit (3 weeks before harvest) still attached to the branches, 60% of them from severe trees were “loose fruit” [fruit detachment force (FT) < 6 kgf]. In contrast, only 13% of the attached fruit from the mild trees were loose. Overall, fresh weight and size of loose fruit were lower than “tight fruit” (FT > 6 kgf). Irrespective of the symptom levels of trees, the concentrations of glucose, fructose, and inositol in juice of loose fruit were the same or larger than those of tight fruit, suggesting that the shortage of carbohydrates is not the dominant cause of HLB-associated preharvest fruit drop. Expression levels of the cell wall modification genes encoding cellulase (endo-1,4-β-glucanase), polygalacturonase, and pectate lyase were greater in the calyx abscission zones of loose fruit compared to tight fruit, indicating that cell separation was occurring in the former at the time of collection. No differences in the expression levels of genes encoding the ethylene biosynthesis enzymes, including 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC oxidase (ACO), and an ethylene-responsive transcription factor 1 (ERF1) were observed in tissues of loose and tight fruit. Interestingly, ACS, ACO, and EFR1 expressions were lower in calyx abscission zones and in leaves of the severe trees compared with those of mild trees, suggesting an ostensible, HLB-dependent reduction in ethylene biosynthesis and/or signaling close to harvest time. However, the role of ethylene in HLB-associated preharvest fruit drop remains to be determined. The results leave open the possibility of early ethylene production and action before the initiation of fruit abscission.
Lisa Tang, Shweta Chhajed and Tripti Vashisth
Wayne Brown, Theo J. Blom, George C.L. Chu, Wei Tang Liu and Lisa Skog
The sensitivity of easter lilies (Lilium longiflorum) to either ethylene or methane (products of incomplete burning in gas-fired unit heaters) was tested during rooting [3 weeks at 18 °C (65 °F)], vernalization [6 weeks at 6 °C (43 °F)] and subsequent greenhouse forcing (15 weeks at 18 °C). Starting at planting, easter lilies were exposed for one of seven consecutive 3-week periods (short-term), or for 0, 3, 6, 9, 12, 15, 18, or 21 weeks starting at planting (long-term) to either ethylene or methane at an average concentration of 2.4 and 2.5 μL·L-1(ppm), respectively. Short- or long-term exposure to ethylene during rooting and vernalization had no effect on the number of buds, leaves, or plant height but increased the number of days to flower. Short-term exposure within 6 weeks after vernalization reduced the number of buds by 1 bud/plant compared to the control (no ethylene exposure). However, extensive bud abortion occurred when plants were exposed to ethylene during the flower development phase. Long-term exposure to ethylene from planting until after the flower initiation period resulted in only two to three buds being initiated, while continued long-term exposure until flowering caused all flower buds to abort. Short-term exposure to methane at any time had no effect on leaf yellowing, bud number, bud abortion, or height and had only a marginal effect on production time. Long-term exposure to methane from planting until the end of vernalization increased both the number of buds, leaves and height without affecting forcing time, leaf yellowing or bud abortion.