1-Aminocyclopropane-1-Carboxylic Acid Transported from Roots to Shoots Promotes Leaf Abscission in Cleopatra Mandarin (Citrus reshni Hort. ex Tan.) Seedlings Rehydrated after Water Stress

Abstract

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...

Physiology of citrus fruiting

“…In citrus, both growth attenuation and abscission activation have been primarily associated with increases in ABA content (Takahashi et al, 1975;Talon et al, 1990b;Sagee and Erner, 1991;Zacarias et al, 1995). Moreover, it is well established that leaf abscission in water-stressed citrus is also regulated by ethylene (Tudela and Primo-Millo, 1992), although again previous ABA accumulation is required for abscission to occur (Gómez-Cadenas et al, 1996). Interestingly, it has been shown that the hormonal sequence triggering fruitlet abscission under water deficit is the same that operates during natural ovary and fruitlet abscission, as explained above.…”

“…It has been mentioned above that water stress is a strong inducing factor of flowering but aside from this stimulating effect many other physiological parameters are negatively affected by water stress. Characteristic leaf injuries and abscission are typical consequences of water deficit conditions (Tudela and Primo-Millo, 1992). After long exposure to water deficit, gas exchange parameters, CO 2 assimilation, water potential and stomatal conductance are progressively affected (Gómez-Cadenas et al, 1996).…”

“…In citrus, the involvement of ethylene in abscission has also been recognized for a long time . It has also been shown that ethylene is the pivotal hormonal factor promoting the processes of leaf abscission (Tudela and Primo-Millo, 1992).…”

Plant Responses to Salt Stress: Adaptive Mechanisms

“…Different authors have associated the decrease in FW or DW with a reduction in the number of leaves or in leaf abscissions (Table 1). To offer an explanation, some experiments have shown that a specific Cl − build-up in the leaves of salt-stressed plants triggers 1-aminocyclopropane-1-carboxilic acid (ACC) synthesis and its conversion to ethylene with high efficiency, releasing enough hormone to trigger leaf abscission as occurred in citrus leaves and other plants [58][59][60][61]. Salt and osmotic stress also promoted the conversion of ACC to ethylene in the halophyte Allenrolfea occidentalis [62].…”

Floral organ abscission is regulated by a positive feedback loop

“…Plants can abscise organs as part of a developmental program or in an inducible manner in response to stimuli like abiotic or biotic stress. For example, tomatoes can abscise leaves and flowers in response to drought or insect feeding (1)(2)(3). In order for abscission to occur, a layer of small and cytoplasmically dense cells, known as an abscission zone, must be laid down during development at the boundary between the organ to be abscised and the body of the plant (4).…”