Eduardo Primo-Millo

Hormonal regulation of fruitlet abscission induced by carbohydrate shortage in citrus.

Abstract. The hormonal signals controlling fruitlet abscission induced by sugar shortage in citrus were identified in Satsuma mandarin, Citrus unshiu (Mak.) Marc, cv. Clausellina and cv. Okitsu. Sugar supply, hormonal responses and fruitlet abscission were manipulated through full, partial or selective leaf removals at anthesis and thereafter. In developing fruitlets, defoliations reduced soluble sugars (up to 98%), but did not induce nitrogen and water deficiencies. Defoliation-induced abscission was preceded by rises (up to 20-fold) in the levels of abscisic acid (ABA) and 1-aminocyclopropane-1-carboxylic acid (ACC) in fruitlets. Applications to defoliated plants showed that ABA increased ACC levels (2-fold) and accelerated fruitlet abscission, whereas norflurazon and 2-aminoethoxyvinyl glycine reduced ACC (up to 65%) and fruitlet abscission (up to 40%). Only the full defoliation treatment reduced endogenous gibberellin A1 (4-fold), whereas exogenous gibberellins had no effect on abscission. The data indicate that fruitlet abscission induced by carbon shortage in citrus is regulated by ABA and ACC originating in the fruits, while gibberellins are apparently implicated in the maintenance of growth. In this system, ABA may act as a sensor of the intensity of the nutrient shortage that modulates the levels of ACC and ethylene, the activator of abscission. This proposal identifies ABA and ACC as components of the self-regulatory mechanism that adjusts fruit load to carbon supply, and offers a physiological basis for the photoassimilate competition-induced abscission occurring under natural conditions.

Leaf Abscission Induced by Ethylene in Water-Stressed Intact Seedlings of Cleopatra Mandarin Requires Previous Abscisic Acid Accumulation in Roots

The involvement of abscisic acid (ABA) in the process of leaf abscission induced by 1-aminocyclopropane-1-carboxylic acid (ACC) transported from roots to shoots in Cleopatra mandarin (Citrus reshni Hort. ex Tan.) seedlings grown under water stress was studied using norflurazon (NF). Water stress induced both ABA (24-fold) and ACC (16-fold) accumulation in roots and arrested xylem flow. Leaf bulk ABA also increased (8-fold), although leaf abscission did not occur. Shortly after rehydration, root ABA and ACC returned to their prestress levels, whereas sharp and transitory increases of ACC (17-fold) and ethylene (10-fold) in leaves and high percentages of abscission (up to 47%) were observed. NF suppressed the ABA and ACC accumulation induced by water stress in roots and the sharp increases of ACC and ethylene observed after rewatering in leaves. NF also reduced leaf abscission (7-10%). These results indicate that water stress induces root ABA accumulation and that this is required for the process of leaf abscission to occur. It was also shown that exogenous ABA increases ACC levels in roots but not in leaves. Collectively, the data suggest that ABA, the primary sensitive signal to water stress, modulates the levels of ethylene, which is the hormonal activator of leaf abscission. This assumption implies that root ACC levels are correlated with root ABA amounts in a dependent way, which eventually links water status to an adequate, protective response such as leaf abscission.

Growth and gas exchange parameters of Citrus plants stressed with different salts

Summary Valencia orange scions (Citrus sinensis (L.) Osbeck) budded to either Cleopatra mandarin (Citrus reticulata) or Poncirus trifoliata ([L.] Raf.) rootstocks were treated with 60mmol/L of different salts (NaCl, KCl and NaNO3) in order to distinguish the specific effects of each ion on growth and gas exchange parameters. The chloride salts markedly reduced plant growth in both scion-rootstock combinations whereas NaNO3 had very litde effect. Poncirus trifoliata accumulated relatively more Cl− in leaves and less in roots than Cleopatra mandarin. In contrast, the sodium concentration in leaves was higher in Cleopatra mandarin than in Poncirus trifoliata. The chloride and sodium contents in leaves of both scion-rootstock combinations were depressed when 30 mmol/L Ca(NO3)2 was added to the culture solution. Both chloride salts (KCl and NaCl) caused a similar reduction in photosynthesis and stomatal conductance, whereas NaNO3 had no detectable effects on these parameters. Addition of calcium nitrate considerably increased growth and the gas exchange parameters in plants exposed to chloride salts and also reduced ion uptake and transport. Taken together, these results indicate that the salinity effects on growth and gas exchange parameters apparendy are induced by the external chloride supply.

Abscisic acid reduces leaf abscission and increases salt tolerance in citrus plants

AbstractThis paper describes the physiological effects of abscisic acid (ABA) and 100 mM NaCl on citrus plants. Water potential, leaf abscission, ethylene production, photosynthetic rate, stomatal conductance, and chloride accumulation in roots and leaves were measured in plants of Salustiana scion [Citrus sinensis (L) Osbeck] grafted onto Carrizo citrange (Citrus sinensis [L.] Osbeck × Poncirus trifoliata [L.] Raf) rootstock. Plants under salt stress accumulated high amounts of chloride, increased ethylene production, and induced leaf abscission. Stomatal conductance and photosynthetic rates rapidly dropped after salinization. The addition of 10 mM ABA to the nutrient solution 10 days before the exposure to salt stress reduced ethylene release and leaf abscission. These effects were probably due to a decrease in the accumulation of toxic Cl- ions in leaves. In non-salinized plants, ABA reduced stomatal conductance and CO2 assimilation, whereas in salinized plants the treatment slightly increased these two parameters. The results suggest a protective role for ABA in citrus under salinity.

Hydrogel substrate amendment alleviates drought effects on young citrus plants

Water deficits affect citrus physiology, yield, fruit size and quality. Citrus can respond to drought stress conditions through endogenous hormonal regulation of water status and leaf abscission. In this work, we assayed the efficiency of an amendment to soilless media in delaying the drought stress effect in young citrus seedlings and trees. Substrate amendment promoted plant survival of citrus seedlings subjected to several cycles of drought stress and rehydration. In budded trees, the amendment increased substrate water content, leaf water potential, leaf number, root biomass, CO2 assimilation and stomatal conductance over that of control plants growing in non-amended substrates. We conclude that the substrate amendment reduced the damaging effects of drought stress in citrus plants. The longer survival of seedlings in the amended treatment together with the reduction in leaf abscission and the improvement of physiological parameters, can account for a higher vigour of citrus grown under water stress conditions.

Defoliation increases fruit abscission and reduces carbohydrate levels in developing fruits and woody tissues of Citrus unshiu

Abstract Carbohydrate levels in fruitlets of control and defoliated trees of dwarf Citrus unshiu (cvs. Clausellina and Okitsu) were determined from bloom up to the end of fruit set. Full and partial (50%) defoliations were carried out at anthesis and at the beginning of the cell enlargement period. Carbohydrate reserves in woody storage tissues were also analyzed soon after defoliation. In control fruits, sucrose, glucose, fructose and starch showed a transient increase at anthesis and remained low during the cell division phase. Soluble sugars accumulated at the onset of the cell enlargement phase. Defoliation did not modify carbohydrate status in either fruits or woody tissues during the cell division period, although the first fruit abscission wave, which takes place at this stage, was increased. Fruit growth was not altered either. However, at the onset of the second phase of growth, soluble sugars were reduced in fruits and sucrose and starch were lower in woody tissues from defoliated plants. In these plants, fruit abscission at this stage (June drop) was also increased, while fruit growth was arrested or delayed. Once the June drop was overcome, fruits remaining on either defoliated or control plants were similar in size and contained analogous high levels of sugars. Taken together, the results suggest that developing fruitlets are utilization sinks during the cell division period and act as storage sinks during the cell enlargement stage. At this critical transition, sucrose levels correlated positively with fruit growth and negatively with fruit abscission. These results are compatible with the proposal that sucrose supply is a major factor of the regulatory mechanism for citrus fruit abscission during the June drop.

Pollination Increases Gibberellin Levels in Developing Ovaries of Seeded Varieties of Citrus

Reproductive and vegetative tissues of the seeded Pineapple cultivars of sweet orange (Citrus sinensis L.) contained the following C-13 hydroxylated gibberellins (GAs): GA53, GA17, GA19, GA20, GA1, GA29, and GA8, as well as GA97, 3-epi-GA1, and several uncharacterized GAs. The inclusion of 3-epi-GA1 as an endogenous substance was based on measurements of the isomerization rates of previously added [2H2]GA1. Pollination enhanced amounts of GA19, GA20, GA29, and GA8 in developing ovaries. Levels of GA1 increased from 5.0 to 9.5 ng/g dry weight during anthesis and were reduced thereafter. The amount of GA in mature pollen was very low. Emasculation reduced GA levels and caused a rapid 100% ovary abscission. This effect was partially counteracted by either pollination or application of GA3. In pollinated ovaries, repeated paclobutrazol applications decreased the amount of GA and increased ovary abscission, although the pattern of continuous decline was different from the sudden abscission induced by emasculation. The above results indicate that, in citrus, pollination increases GA levels and reduces ovary abscission and that the presence of exogenous GA3 in unpollinated ovaries also suppresses abscission. Evidence is also presented that pollination and GAs do not, as is generally assumed, suppress ovary abscission through the reactivation of cell division.

Seasonal uptake of 15N-nitrate and distribution of absorbed nitrogen in peach trees

The absorption and distribution of N was measured monthly throught a calendar year in 3-year old peach trees (Prunus persica (L) c.v. Maycrest) grafted on Nemaguard rootstock. Plants were grown on siliceous sand in 500-L pots and fertilized with a solution containing 15N enriched KNO3. During flowering and fruit set (March) approximately 7% of N found in new growth came from the fertilizer and the remainder came from the N stored in the old organs. Maximum N absorption took place during the periods of fruit ripening and maximal vegetative growth (May to August). This nitrogen was relocated from leaves to woody tissues and stored as reserve-N before leaf fall. In the following growth season reserve-N was used for flower development and new shoot growth. The N absorbed during plant dormancy was quite low and remained in the stem bark and roots mainly as soluble-N.

Kinetics of nitrate uptake by Citrus seedlings and inhibitory effects of salinity

Abstract Nitrate absorption by roots of both Troyer citrange (hybrid of Citrus sinensis×Poncirus trifoliata ) and Cleopatra mandarin ( C. reshni ) seedlings was studied. The ion depletion in the external nutrient solutions was used to estimate NO 3 − net uptake. In both rootstocks the kinetics of nitrate uptake showed a biphasic pattern in response to increasing external NO 3 − concentrations up to 10 mol m −3 . The low concentration transport system followed Michaelis-Menten kinetics reaching saturation at 0.5 mol m −3 NO 3 − in both rootstocks. The apparent K m s of nitrate uptake were 282 and 281 mmol m −3 respectively, for Troyer citrange and Cleopatra mandarin. A linear system was also found at higher external NO 3 − concentrations (between 1–10 mol m −3 ). Double reciprocal plots showed that Cl − inhibited NO 3 − uptake competitively in the low concentration system. In contrast, the high concentration system was insensitive to external Cl − , but was inhibited when plants were pretreated with salt solutions. Net NO 3 − uptake in both systems was severely affected by Cl − ions more in Troyer citrange than in Cleopatra mandarin. Since Cl − accumulation in tissues was higher in Troyer citrange than in Cleopatra mandarin, a relationship between the ability to absorb Cl − and the inhibition of NO 3 − uptake is suggested for these plants.

The influence of nitrogen concentration and ammonium/nitrate ratio on N-uptake, mineral composition and yield of citrus

In short-term water culture experiments with different 15N labeled ammonium or nitrate concentrations, citrus seedlings absorbed NH4+ at a higher rate than NO3−. Maximum NO3− uptake by the whole plant occurred at 120 mg L−1 NO3−-N, whereas NH4+ absorption was saturated at 240 mg L−1 NH4+-N. 15NH4+ accumulated in roots and to a lesser degree in both leaves and stems. However, 15NO3− was mostly partitioned between leaves and roots.Adding increasing amounts of unlabeled NH4+ (15–60 mg L−1 N) to nutrient solutions containing 120 mg L−1 N as 15N labeled nitrate reduced 15NO3− uptake. Maximum inhibition of 15NO3− uptake was about 55% at 2.14 mM NH4+ (30 mg L−1 NH4+-N) and it did not increase any further at higher NH4+ proportions.In a long-term experiment, the effects of concentration and source of added N (NO3− or NH4+) on nutrient concentrations in leaves from plants grown in sand were evaluated. Leaf concentration of N, P, Mg, Fe and Cu were increased by NH4+ versus NO3− nutrition, whereas the reverse was true for Ca, K, Zn and Mn.The effects of different NO3−-N:NH4+-N ratios (100:0, 75:25, 50:50, 25:75 and 0:100) at 120 mg L−1 total N on leaf nutrient concentrations, fruit yield and fruit characteristics were investigated in another long-term experiment with plants grown in sand cultures. Nitrogen concentrations in leaves were highest when plants were provided with either NO3− or NH4+ as a sole source of N. Lowest N concentration in leaves was found with a 75:25 NO3−-N/NH4+-N ratio. With increasing proportions of NH4+ in the N supply, leaf nutrients such as P, Mg, Fe and Cu increased, whereas Ca, K, Mn and Zn decreased. Yield in number of fruits per tree was increased significantly by supplying all N as NH4+, although fruit weight was reduced. The number of fruits per tree was lowest with the 75:25 NO3−-N:NH4+-N ratio, but in this treatment fruits reached their highest weight. Rind thickness, juice acidity, and colour index of fruits decreased with increasing NH4+ in the N supply, whereas the % pulp and maturity index increased. Percent of juice in fruits and total soluble solids were only slightly affected by NO3−:NH4+ ratio.