Readers' Rating:

Rate this article

Brazil is the world’s biggest producer of oranges, producing 400 million boxes (16.3 million mT), 80% of which are destined for orange juice processing and 20% to fresh fruit. Consumers prefer fresh fruits that are large and sweet, without seeds or spots on the peel.

To improve orange productivity and quality, plant hormones or growth regulators such as cytokinins and gibberellins are of commercial interest (El-Otmani et al., 2000).

The cytokinins are involved in several physiological processes, such as cellular division and elongation, apical dominance, enzymatic activities and protein synthesis. They exert their effects on RNA and nucleic acid metabolism. They promote cellular elongation, although the specific means by which they do so have not been completely clarified (Metivier, 1979; Agustí and Almela, 1991; Taiz and Zeiger, 1991, Mok and Mok, 2001).

The application of cytokinins in citrus production can partially substitute for lack of seeds in parthenocarpic varieties (seedless); and constitutes a complementary mechanism that parallels hormone synthesis, which is performed by the ovarian wall (Kriedemann, 1968; Monselise, 1977).

The increased carbohydrate drain by the fruits can result in higher fixing and bigger sizes. (Kriedemann, 1968; Powell and Krezdorn, 1977; Moss, 1972; Primo- Millo et al., 1977; García-Martínez and García-Papi, 1979).

The application of 20 mg/L of benziladenin increased harvest between 30 and 60% in Late Navel variety oranges (Hernández-Miñana et al., 1988). The best results obtained from benziladenin or kinetin application are observed in concentrations close to 25 mg/L and the best period for spraying is during the natural petal drop (Agustí and Almela, 1991).

Gibberellins are involved in branch growth through stimulation of cellular division and elongation.

Application of gibberellic acid increases the size of the subapical region and the percentage of cells in division, reducing the cellular cycle by about 30% (Agustí and Almela, 1991; Davies, 1987).

Several scientific trials show that the gibberellins increase the cell wall flexibility by stimulating the synthesis of new cellulose polymers (Richards et al., 2001). Gibberellins could also reduce the senescence of leaves and fruits (Davies, 1987).

Some natural extracts are rich in cytokinins and when sprayed, produce similar results to those obtained with synthetic molecules. High levels of cytokinins are usually found in the meristem tissues, which have high rates of cellular division (Metivier, 1979; Taiz and Zeiger, 1991).

Gibberellins have been used in citrus production with several objectives including bloom reduction, increased fruit setting, improvement of fruit quality and improved maturation control (Agustí and Almela, 1991). The application of gibberellic acid soon after flowering at doses between 10 and 15 mg/L can result in delayed abscission and increased fruit set, mainly in Clementine tangerines (Fornes et al., 1992; El-Otmani, 1992).

However, the increase in fixing and productivity does not happen frequently and depends on factors including variety, plant status and time of application (Davies, 1987; Talón et al., 1997). In addition, when an increase in fruit set is observed, there is a reduction in fruit size as a result of increased competition for carbohydrates necessary for development.

Besides the hormones, other factors are involved in citrus fruit setting, such as the presence of seeds, carbohydrate reserves, photosynthetic rate and mineral nutrition. (Augustí and Almela, 1991; Goldschmidt, 1999; Ruiz et al., 2001). Positive results have been obtained with potassium nitrate supplementation even with good plant nutritional status, which is possibly related to the lower ability to mobilize mineral reserves necessary for the period (Ruiz et al., 2001).

The objective of this study was to evaluate the effect of Crop-SetTM (Improcrop, Alltech Inc.), a plant extract comprised of cytokinins and trace minerals, gibberellic acid and potassium nitrate, on the vegetative development, productivity and fruit quality of orange trees from the Navelate (Late Navel) group.


Material and methods

Two trials were conducted in a 7-year old Navel group orange orchard (Washington Navel and Late Navel) located near Limeira, in São Paulo state, Brazil. At the end of blooming (October, 2001), when fruits were 8.0 ± 2.0 mm in diameter, the plants were subjected to one of the following treatments:

1 Potassium nitrate (2%)
2 Gibberellic acid (10 mg/L)
3 Crop-SetTM (600 ml/ha)
4 Crop-SetTM (900 ml/ha)
5 Control

The treatments were sprayed at a rate of 8 L solution/plant. After 45 days, when the fruits had grown to a diameter of 14 ± 2 mm, all treatments were reapplied. The treatments were distributed into a completely randomized design, with 10 replications of one plant each.

For fruit set analysis, semimonthly fruits of four 1-meter long selected branches were counted. For fruit development evaluation the average diameters of 20 fruits per replication were measured. Fruit quality evaluation involved fruit size at harvest and the numbers of fruits of various sizes per 28 kg box.

Treatment effects on vegetative development were determined by the relative growth rate measured by calculating semimonthly length averages of four branches selected per plant, according to the model proposed by Magalhães (1979).

The environmental temperature, vapor pressure deficit (VPD) and precipitation were measured every 20 minutes by the automated meteorological station of Centro Avançado de Pesquisa Tecnológica ‘Sylvio Moreira’.
The results were subjected to analysis of variance and the means were compared using Tukey’s test.


Results and discussion

WASHINGTON NAVEL ORANGES

Crop-SetTM (at 600 and 900 ml/ha) and gibberellic acid similarly stimulated plant vegetative growth, allowing major branch development (Figure 1). This could be explained by the higher relative growth rate observed in the first 60 days.

Crop-SetTM may stimulate vegetative development by cytokinins, which are related to the increase in cellular division and elongation, as well as the break of apical dominance (Agustí and Almela, 1991; Bangerth et al., 2000; Mok and Mok, 2001). The gibberellic acid effect can be explained by the stimulation of cellular division and cellular elongation (Davies, 1987).

Potassium nitrate application did not influence branch final size, probably due to good initial nutritional status of the plant (data not presented). However, the relative growth rate was enhanced following the second application of this product.

Crop-SetTM (600 ml/ha), gibberellic acid and potassium nitrate benefited fruit setting during the first 15 days following the first application (Figure 2).

The increase was approximately 20% higher than that of the control. However, the fruit drop rate was higher in those treatments in the following period, reducing the advantage over the control to 10%. This can be explained by the increase in carbohydrate competition during this period, exacerbated by the occurrence of temperatures exceeding 32ºC, and high vapor pressure deficit (>4KPa).

These factors led to a reduced photosynthesis rate and consequently reduced carbohydrate availability, which resulted in an increased drop rate (Medina and Machado. 1998; Ruiz et al., 2001). Fifteen days after Crop-SetTM (900 ml/ha) application, a trend toward reduction in fixing was observed, probably related to overdosing with the product.

However, the lower competition due to lower initial fixing resulted in a similar number of fruits at 30 days compared to other treatments.

Final production was unaffected by the treatments (Figure 3).








Figure 1. Effect of treatments on branch length and growth rates of Washington Navel orange trees subjected to two applications (arrows indicate time of application).




Figure 2. Relative fruit numbers per branch of Washington Navel orange trees for 30 days following application of potassium nitrate, gibberellic acid or Crop-Set™ .



In spite of the similarity among treatments in fruit production, there were differences among them in terms of fruit development. In all treatments, a higher number of fruits between 7.0 and 8.5 cm was observed (Figure 4). Potassium nitrate and gibberellic resulted in 38 and 25% of the total fruits, respectively, in the 7.0 and 7.5 cm diameter sizes, while Crop-SetTM (600 ml/ha) had 15% of fruits in this size range.

On the other hand, Crop-SetTM (600 ml/ha) resulted in a higher percentage of fruits in the 8.0 and 8.5 cm category. When applied at 900 ml/ha, Crop-SetTM resulted in higher number of fruits at larger diameters.

Although there was no significant difference in total production, the orange trees treated with Crop-SetTM produced bigger fruits. The higher development of fruits treated with cytokinins is related to their greater ability to drain carbohydrates (Kriedemann, 1968; Powell and Krezdorn, 1977; Moss, 1972; Primo-Millo et al., 1977; García-Martínez and García-Papi, 1979).

Cytokinins play a role in the relationship between carbohydrate source and drain due to their action in regulating activity of key enzymes such as acid invertase and activating hexose transporters. The induction of this enzyme is essential for transport or release of the apoplast and can be an important mechanism for the effects observed with cytokinin application (Roitsch and Ehneb, 2000).


LATE NAVEL ORANGES

Response in branch development was similar to that obtained with Washington Navel orange trees (data not presented). Gibberellic acid tended to enhance fruit fixing in the first 15 days following application; however it subsequently increased abcision. At the end of the experimental period, fruit set was similar to that observed with application of Crop SetTM (600 ml/ha) treatment, which were both superior to the control (Figure 5).

The positive effect of cytokinin applications in Late Navel oranges was reported by Furió (1991), who compared endogenous cytokinin levels in different organs of Late Navel and Washington Navel cultivars. It was concluded that there are lower levels of cytokinins in the Late Navel reproductive organs.

However, there were no significant differences in endogenous gibberellin levels between the cultivars. This seems to justify the better response of this cultivar to cytokinin applications when compared to Washington Navel. Final production was improved by Crop-SetTM 600 ml/ha and gibberellic acid treatments (Figure 6).



Figure 3. Production of Washington Navel orange tress subjected to two applications after blooming and spaced 45 days apart with potassium nitrate, gibberellic acid or Crop-Set™.
















Figure 4. Percentages of fruits in different size classifications produced by Washington Navel orange trees subjected to two applications after blooming and spaced 45 days apart with potassium nitrate, gibberellic acid or Crop-Set™ .



A fruit size increase was observed in trees treated with Crop-SetTM at 600 and 900 ml/ha, while there was no significant change in response to the gibberellic acid treatment.

Normally, a lower fruit size and higher fixing should be expected, due to the higher competition for carbohydrates (Agustí and Almela, 1991) (Figure 7). However, plant productivity was not high considering the potential, development and vigor of the top, which probably indicated that there were sufficient carbohydrates for fruit growth in all treatments.

Distribution of fruits into commercial classes was affected by treatments. Comparing the most productive treatments, gibberellic acid and Crop- SetTM 600 ml/ha, it was observed that Crop-SetTM increased fruit size.
These results are in accordance with those obtained with Washington Navel oranges and can be related to the better draining capacity of fruit treated with natural cytokinins (Crop-SetTM).

However, in addition to biochemical studies concerning the relationship between treated fruit source and drainage, other aspects must be evaluated, since positive effects on vegetative development and also improved plant capacity to provide carbohydrates due to an increase in foliar area were observed.


Conclusions

The application of natural cytokinins and gibberellins enhanced productivity of Late Navel orange trees.
The production quality as measured by fruit size was improved by the use of cytokinins in Crop- SetTM. This response can be influenced by the cultivar and product dose.







Figure 5. Relative fruit setting at different dates (A and B) of Late Navel orange trees subjected to two applications after blooming and spaced 45 days apart with potassium, gibberellic acid or Crop-Set™.




Figure 6. Late Navel orange production in response to two applications after blooming and spaced 45 days apart with potassium nitrate, gibberellic acid or Crop-Set™ .




Figure 7. Late Navel orange classification following two applications after blooming and spaced 45 days apart of gibberellic acid or Crop-Set™ .



References

Agustí, M.F and V.O. Almela. 1991. Aplicación de fitorreguladores em citricultura. Barcelona, Aedos., pp. 261.

Bangerth, F., C.J. Li, J. Gruber. 2000. Mutual interaction of auxin and cytokinins in regulation correlative dominance. Plant Growth Regulation 32:205-217.

Davies, P.J., 1987. Plant hormones and their role in plant growth and development. Martinus Nijhoff Publishers, Dordrecht, p. 681.

El-Otmani, M. 1992. Principal growth regulator uses in citrus production. Proc. 2nd. Seminar on Citrus Physiology. Bebedouro. Ed. Cargill. pp. 55-69.

El-Otmani, M., C.W. Coggins, M.F. Agustí and C.J. Lovatt. 2000. Plant growth regulators in citriculture: world current uses. Critical Reviews in Plant Sciences 19:395-447.

Fornes, F., P.J.J. Van-Rensburg, M. Sánches- Perales and J.L. Guardiola. 1992. Fruit setting treatments effects on two Clementine mandarin cultures. Proc. in. Soc. Citric. 1:489-492.

Furió. J. 1991. Citoquininas en Agrios. Actividad endogéna, efectos fisiológicas y aplicacienes. Tesis Doctoral. Universidad de Valência, 143 p.

García-Martínez, J.L. and M.A. García-Papí. 1979. The influence of gibberellic acid, 2,4-dichlorpophenoxyacetic acid and 6-benzylaminopurine on fruit-set of Clementine mandarin. Scientia Hotic. 10:285-293.

Goldschmidt, E.E. 1999. Carbohydrate supply as a critical factor for citrus fruit development and productivity. HortSience 34:1020-1024.

Hernández-Miñana, F.M., J. Furio, R. Ibáñez and E. Primo-Millo. 1988. Efecto de la aplicación de benciladenina sobre el cuajado del fruto y la productividade en el naranjo Navelate. Lev. Agrícola 258/286:171-174.

Kriedemann, P.E. 1968. An effect of kinetin on the translocation of 14C-labelled photosynthate in Citrus. Aust. J. Boil. Sci. 21:569-571.

Magalhães, A.C.N. 1979. Análise quantitativa do crescimento. In: Fisiologia Vegetal (M.G. Ferri, ed). São Paulo, Edusp 1:331-50.

Medina, C.L. and E.C. Machado. 1998. Trocas gasosas e relações hídricas em laranjeira‘Valência’ enxertada sobre limoeiro ‘Cravo’ e Trifoliata e submetida à deficiência hídrica. Bragantia 57:15-22.

Metivier, J.R. 1979. Citocininas. In: Fisiologia Vegetal 2 (M.G. Ferri, ed). São Paulo, EDUSP. pp. 90-127.

Mok, D.W.S. and M. Mok. 2001. Cytokinin metabolism and action. Annu. Rev. Plant. Physiol. Plant Mol. Biol. 52:89-118.

Monselise, S.P. 1977 Citrus fruit development: endogenous systems and external regulation. Proc. Int. Soc. Citriculture 2:664-668.

Moss, G.I. 1972. Promoting fruit-set and yield in sweet orange using plant growth substances. Aust. J. Exp. Agric. Anim. Husb. 12:96-102.

Powell, A.A. and A.H. Krezdorn. 1977. Influence of fruit-setting treatment on translocation of 14Cmetabolites in citrus during flowering and fruiting. J. Amer. Soc. Hortic. Sci. 102:709-714.

Primo-Millo, E., R. Ibáñez and A. Navarrro. 1977. Nota previa. Efecto de las citockininas sobre el cuajado de la variedade de naranjo Navelate (Citrus sinensis, l., Osbeck). Rev. Agroquim. Tecnol. Aliment. 17:388-390.

Richards, D.E., K.E. King, T. Ait-ali and N. Harberd. 2001. How gibberellin regulates plant growth and development: A molecular genetic analysis of gibberelin signaling. Annu. Rev. Plant. Physiol. Plant. Mol. Biol. 52: 67-88.

Roitsch, T, and R. Ehneb. 2000. Regulation of source/sink relations by cytokinins. Plant Growth Regulation 32:359-367.

Ruiz, R., A. García-Luiz, C. Monerri and J.L. Guardiola. 2001. Carbohydrate availability in relation to fruit-set abscission in Citrus. Annals of Botany. 87:805-812.

Taiz, L. and E. Zeiger. 1991. Plant Physiology. Redwood City, Benjamin/Cummings P. Co., p. 559.

Talón, M., F.R. Radeo, A. Gómez-Cadena and E. Primo-Millo. 1997. Regulación hormonal de la caída de frutos inducida por defoliaciones en mandarinos Satsuma. Agric. Vergel 186:334-350.


Author: CAMILO LÁZARO MEDINA
Centro Avançado de Pesquisa Tecnológica do Agronegócio de Citros ‘Sylvio Moreira’, Instituto Agronômico and GCONCI, Citrus Consultants Group, Cordeirópolis – SP, Brazil

Readers' Rating:

Rate this article