Manuel Agustí

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.

The synthetic auxin 3,5,6-TPA stimulates carbohydrate accumulation and growth in citrus fruit

The application of the synthetic auxin 3,5,6-TPA at the cell enlargementstage increased hexoses in developing fruit from foliated and fully defoliatedplants of Satsuma mandarin, cv. ‘Okitsu’ (Citrusunshiu Marc.). Although the sucrose concentrations also increased,ingeneral the differences were not statistically significant. The plant growthregulator reduced fruit abscission in defoliated trees while it stimulatedfruitgrowth in the foliated ones. The carbohydrate accumulation induced by 3,5,6-TPAindicates that its stimulatory effect on fruit growth might operate viapromotion of sink strength. The results also suggest the occurrence of pathwaysfor induction of fruit abscission, linked to carbon availability, relativelyindependent of the stage of development.

Girdling increases carbohydrate availability and fruit-set in citrus cultivars irrespective of parthenocarpic ability

Summary The effects of girdling performed at various dates were evaluated during two consecutive years in high- and low-bearing commercial orchards of ‘Fortune’ mandarin and ‘Clausellina’ Satsuma mandarin. The time-dependent response was evaluated through fruitlet abscission, final fruit-set and yield as related to carbohydrate contents in developing fruitlets. A few days after treatment, girdling increased the soluble sugars content (SSC) in fruitlets, reduced the daily fruit drop, and thereby diminished abscission. Application of girdling to low-bearing ‘Fortune’ mandarin orchards was most effective 15 d before anthesis (DBA) and 35 d after anthesis (DAA). It increased yield by 125%. In high-bearing orchards, the best results were achieved by girdling 35 DAA, which increased yield by 28%. In both orchards, the effect of girdling declined gradually from 35 DAA to the end of fruit drop. SSC values were found to be positively correlated with fruit-set at 25 DAA, but this tendency was reversed by 50 DAA. This study showed that the effects of girdling are time-dependent, and that yield can be improved in cultivars of different parthenocarpic ability.

Loquat fruit size is increased through the thinning effect of naphthaleneacetic acid

The thinning effect of naphthaleneaceticacid was examined in loquat trees (Eriobotryajaponica Lindl.). The effect depended on theconcentration applied and on the treatment date. Themost effective treatment found to thin fruit andincrease the average fruit size was the application of20 mg l−1 of naphthaleneacetic acid 10–15 daysafter anthesis. Treatment reduced the fruit resistanceto puncturing and improved total soluble solidsconcentration and coloration and, consequently,harvest time was markedly advanced.

EFFECTS OF RINGING BRANCHES ON FRUIT SIZE AND MATURITY OF PEACH AND NECTARINE CULTIVARS

SummaryRinging branches at pit hardening resulted in larger fruits and enhanced fruit colouring of peaches and nectarines. Because both together determine the time of harvest, fruit from ringed trees were picked earlier. Further, ringing resulted in advanced fruit ripening compare with controls. Results obtained were similar to those reported for girdling. Ringing is easier and quicker than girdling and represents a useful technique to improve fruit quality.

Synthetic auxin 3,5,6-TPA promotes fruit development and climacteric in Prunus persica L. Batsch

SummaryThe application of 3,5,6-TPA advances fruit maturation of peaches and nectarines through different mechanisms, depending on the fruit development stage at treatment. Treatments applied at pit hardening encourage the development of fruit, which grow faster, reach the climateric stage earlier, and can be harvested earlier. The effect is direct, since the number of fruits per tree is not reduced by the treatment and also can be obtained by applying the auxin locally to a small number of fruits per tree. Treatments applied a few days before climateric increase ethylene production without affecting fruit growth.

Effect of the synthetic auxin 2,4-DP on fruit development of loquat

Application of the butylglycol ester of 2,4-DP increased final fruit size in loquat without causing fruit thinning. Its effectiveness depended on the concentration applied and treatment date. When applied at the onset of the cell enlargement fruit stage, 25 mg l−1 was the most effective treatment. Fruit diameter distribution showed a significant shift to the larger size for treated trees; further, fruit colour break and maturation were encouraged and harvest time was earlier than in untreated trees.

Auxin interaction with the vesicular-arbuscular mycorrhizal fungus Glomus intraradices Schenck & Smith improves vegetative growth of two citrus rootstocks

Abstract The vesicular-arbuscular mycorrhizal (VAM) fungus Glomus intraradices Schenck & Smith increased root and shoot growth of the citrus rootstocks sour orange ( Citrus aurantium L.) and ‘Carrizo’ citrange ( Citrus sinensis (L.) Osb. X Poncirus trifoliata (L.) Raf.). The application of indolebutyric acid (IBA) at 2.0 g l −1 as a 10 s root dip to non-mycorrhizal seedlings was ineffective in increasing growth, but application to mycorrhizal seedlings had a positive VAM-IBA effect on growth.

Rootstock influence on the incidence of rind breakdown in ‘Navelate’ sweet orange

Summary The influence of three rootstocks, Carrizo citrange (Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.), Cleopatra mandarin (Citrus reshni Hort. ex Tanaka) and sour orange (Citrus aurantium L.) on the incidence and severity of rind breakdown in ‘Navelate’ sweet oranges was studied over seven years in four orchards. The proportion of affected fruit was greater in trees on Carrizo citrange (60% on an average) followed by Cleopatra mandarin (38%) and then on sour orange (9%). There were also significant differences in the severity of the disorder among rootstocks; more than 35% and 20% of fruit from trees on Carrizo citrange and Cleopatra mandarin, respectively, were rejected for commercialization, whereas only 4% of fruit on sour orange was rejected. Cryo-scanning electron microscopy (SEM) of fruit peduncles showed that average diameter of xylem vessel elements of peduncles of fruits on Carrizo citrange rootstock were 6% and 17% larger than those on Cleopatra mandarin and sour orange, respectively. Defoliation of fruit-bearing branches reduced water loss from fruit, maintained fruit peel water potential at a higher value than that of fruit on undefoliated branches and reduced the proportion and severity of affected fruit. It is concluded that fruit-tree water relationships are related to rind breakdown in ‘Navelate’ oranges and low fruit water content might be responsible for the disorder.

Gibberellin reactivates and maintains ovary-wall cell division causing fruit set in parthenocarpic Citrus species

Citrus is a wide genus in which most of the cultivated species and cultivars are natural parthenocarpic mutants or hybrids (i.e. orange, mandarin, tangerine, grapefruit). The autonomous increase in GA1 ovary concentration during anthesis was suggested as being the stimulus responsible for parthenocarpy in Citrus regardless of the species. To determine the exact GA-role in parthenocarpic fruit set, the following hypothesis was tested: GA triggers and maintains cell division in ovary walls causing fruit set. Obligate and facultative parthenocarpic Citrus species were used as a model system because obligate parthenocarpic Citrus sp (i.e. Citrus unshiu) have higher GA levels and better natural parthenocarpic fruit set compared to other facultative parthenocarpic Citrus (i.e. Citrus clementina). The autonomous activation of GA synthesis in C. unshiu ovary preceded cell division and CYCA1.1 up-regulation (a G2-stage cell cycle regulator) at anthesis setting a high proportion of fruits, whereas C. clementina lacked this GA-biosynthesis and CYCA1.1 up-regulation failing in fruit set. In situ hybridization experiments revealed a tissue-specific expression of GA20ox2 only in the dividing tissues of the pericarp. Furthermore, CYCA1.1 expression correlated endogenous GA1 content with GA3 treatment, which stimulated cell division and ovary growth, mostly in C. clementina. Instead, paclobutrazol (GA biosynthesis inhibitor) negated cell division and reduced fruit set. Results suggest that in parthenocarpic citrus the specific GA synthesis in the ovary walls at anthesis triggers cell division and, thus, the necessary ovary growth rate to set fruit.