The effect of water stress and subsequent rehydration on 1-aminocyclopropane-1-carboxylic acid (ACC) content, ACC synthase activity, ethylene production, and leaf abscission was studied in Cleopatra mandarin (Citrus reshni Hort. ex Tan.) seedlings. Leaf abscission occurred when drought-stressed plants were allowed to rehydrate, whereas no abscission was observed in plants under water stress conditions. In roots of water-stressed plants, a high ACC accumulation and an increase in ACC synthase activity were observed. Neither increase in ACC content nor significant ethylene production were detected in leaves of water-stressed plants. After rehydration, a sharp rise in ACC content and ethylene production was observed in leaves of water-stressed plants. Content of ACC in xylem fluid was 10-fold higher in plants rehydrated for 2 h after water stress than in nonstressed plants. Leaf (2,4,24). The relationship between ethylene production and abscission, both induced by water stress, has also been well established in cotton (11,15) and citrus (4). However, it has been indicated recently that these findings may be artifactual results obtained when working with detached tissues (19,20). These authors concluded that water stress alone cannot induce ethylene synthesis in intact plants.Citrus trees present a characteristic behavior of water stress-induced leaf abscission. When trees are subjected to water stress, leaves are injured but are not abscised. They remain attached until water stress is relieved (by rain or irrigation) and, soon afterwards, they are abscised (1). Similar behavior has been reported in cotton plants (15), which has led us to suggest that during water stress an abscissioninducing factor is synthesized in roots of citrus trees, and later it is transported to the aerial parts of the plant where it induces abscission. Ethylene is the natural regulator of abscission, and due to its gaseous nature, it may be that this abscission-inducing factor is ACC, the metabolic precursor of ethylene. Xylem transport of ACC from roots to shoots was first demonstrated by Bradford and Yang to induce epinasty in tomato plants subjected to anaerobic root stress (5, 6). Later, interorgan ACC transport was shown in several systems (3,9,12,18,21,23,28,29). However, no studies to test the relationship between water stress-induced abscission and xylem transport of ACC have been made. The aim of this work is to study this hypothesis. MATERIALS AND METHODS Plant MaterialEight-to 10-month-old Cleopatra mandarin (Citrus reshni TUDELA AND PRIMO-MILLO flux density, 120 Mmol-s-1 m2. RH was 95% during dark periods and oscillated between 60 and 95% during light periods. Plants were watered three times per week with 300 mL of half-strength modified Hoagland solution. Water Stress TreatmentsWater stress began at the start of the 16-h light period. Well-watered plants were taken from the pots and their roots washed with water. Then, the plants were placed in pots with dry sand for different periods under continuous light. In rehy...

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MiUo, E. 1990. Stimulation of ACC-dependent ethylene production in citrus leaf discs by light. -Physiol. Plant. 80: 89-94.The influence of light and darkness incubation on in vivo ethyiene forming enzyme (EFE) activity in citrus {Citrus sinensis L. Osbeck cv. Salustiana) mature leaf discs was studied. Leaf discs incubated in light produced higher amounts of ethylene than in darkness. Transfer of discs from light to the dark resulted in a marked inhibition of EFE activity, whereas transfer of discs from the dark to light enhanced ethylene forming activity considerably. Light did not affect 1-aminocyclopropane-l-carboxylic acid (ACC) uptake. Incubation in a OO2-eniiched atmosphere enhanced EFE activity both in light and in darkness, but light stimulation of I5FE activity was apparently not affected by CO,. Effects of 3-(3,4-dichiorophenyI)-i.l-dimethylurea (DCMU, inhibitor of photosynthetic electron flow) and KCN (inhibitor of cytochrome oxidase) were studied. DCMU at 0.2 mM inhibited EFE activity in light, whereas no effect was detected in the dark. On the other hand 1 mM KCN stimulated EFE activity in the Hght, and no significant effect was observed in the dark. C0CI2 at 1 mM inhibited ACC-dependent ethylene production, suggesting that ethylene production from ACC is mediated by EFE in citrus leaf discs both in light and in the dark. Cycloheximide also inhibited EFE activity in the light and no effects were detected in the dark. Therefore protein synthesis in light (perhaps EFE synthesis) could be required for the light stimulation of the in vivo EFE activity.