Successive applications of gibberellic acid in alternate bearing mandarins

Abstract Gibberellins can promote the inhibition of floral induction in citrus in order to reduce alternate bearing. However, not much is known about the responsiveness of Citrus deliciosa to exogenous applications of gibberellic acid. Furthermore, the successive application of gibberellins is barely studied. The main objective of this work was to evaluate the effect of successive applications of gibberellic acid (GA3) on the reduction of spring flowering subsequent to low fruit load periods (off-year) in ‘Montenegrina’ and ‘Rainha’ mandarin trees, in southern Brazil. The treatments consisted of one to four successive applications of GA3 (40 mg L-1) with 21-day interval in off-year trees. Applications began in May and ended in July. There was a significant reduction in sprouting and flowering of ‘Montenegrina’ mandarin with more than two GA3 successive applications compared to the control. To ‘Rainha’ mandarin, only two GA3 applications reduced the sprouting and solely one GA3 application was enough to reduce sprouting in relation to the control. GA3 use reduced sprouting and flowering of Montenegrina and Rainha cultivars. The orchard age may be related to its level of alternate bearing and, therefore, to the greater or lesser sensitivity of gibberellin applications.

The timing of gibberellin application is a decisive factor to obtain the expected result.The best results are achieved at the end of November and mid-December for the northern hemisphere (GUARDIOLA et al., 1982;AGUSTÍ, 2003).The use of gibberellins can reduce flowering even after the start of differentiation (AGUSTÍ, 2003).The differentiation involves anatomical and morphological transition from the vegetative meristem to the floral meristem (DAVENPORT, 1990;ALBRIGO;GALÁN-SAÚCO, 2004).
Several factors affect the flowering management of citrus cultivars with gibberellins including such as varietal characteristics, orchard management, harvest load, tree development stage and climatic conditions.The Montenegrina and Rainha (Citrus deliciosa Tenore) cultivars are late maturing.Even with seeds, they have a flavor and color that ensures the Brazilian market sales, providing higher prices than the other mandarins that have earlier harvest: 'Caí' and 'Pareci'.However, C. deliciosa late maturing cultivars show a great tendency to alternate bearing.Besides that, cultural practices to overcome this problem are aimed at the year-on of production, such as pruning and thinning (RODRIGUES et al., 1998;SARTORI et al., 2007;GONZATTO et al., 2016).Nonetheless, these procedures are not enough to completely overcome the problem.
Hence, experiments directed to specific genotypes and with edaphoclimatic conditions of interest become essential to achieve reproducible results in field conditions.Thereby, the objective of this study was to evaluate the effect of successive applications of gibberellic acid (GA 3 ) on the reduction of spring flowering subsequent to low fruit load periods (off-year) in 'Montenegrina' and 'Rainha' mandarin trees, in southern Brazil.There was a significant reduction in sprouting and flowering of 'Montenegrina' mandarin in which more than two successive applications of GA 3 were performed regarding the control.The reduction in flowering observed in relation to the control was 37.5%, 46.6% and 57.8% for two, three and four applications of GA 3 , respectively.Otherwise, the behavior of 'Rainha' was erratic, because one application was enough to reduce the sprouting in relation to the control.However, for flowering reduction, two GA 3 applications were required in order to achieve a reduction of 28% in comparison with control (Figure 1).The reduction in sprouting and flowering to one or two applications of GA 3 is according to Muñoz-Fambuena et al. (2012) in which 'Salustiana' orange trees (Citrus sinensis (L.) Osbeck) with one application of 40 mg L -1 of GA 3 , during the floral induction period, reduced the flowering and sprouting.The inhibition of flowering by gibberellic acid is due to the repression of the Citrus Flowering Locus T (CiFT) gene expression (MUÑOZ-FAMBUENA et al., 2012).
Shoot types were also influenced by GA 3 applications.In 'Montenegrina', the SLY frequencies were increased from two successive applications of GA 3 regarding control, while in SL the opposite effect occurred.The frequency of MLY and ML were not affected by treatments.The frequencies of VS and STLY were increased from three successive applications of GA 3 in relation to the control.Moreover, the trees that received four successive applications of GA 3 had more than half of their shoots of the STLY type.
To 'Rainha' mandarin submitted to one application, there was an increase in the frequencies of SLY and STLY, while there was a reduction in the frequencies of SL.ML and VS frequencies were not affected by treatments.In 'Rainha', the highest frequency of STLY (20.4%) was observed with one application of GA 3 (Table 1).Successive applications of gibberellic acid in alternate bearing mandarins 5 In report of Sartori et al. (2007), 'Montenegrina' experiment in southern Brazil, under similar edaphoclimatic conditions to this work, the application of 10 mg L -1 of GA 3 in the autumn period did not inhibit flowering.Ramos-Hurtado et al. (2006), in a similar study with 'Montenegrina', using four concentrations of GA 3 (0, 20, 40 and 60 mg L -1 ), but with one single applications on three different dates (April, May and June), did not had flowering inhibition.In our report, a similar result for 'Montenegrina' was found, since a single application of GA 3 did not result in significant differences.On the other hand, to cultivar Rainha, results were similar Muñoz-Fambuena et al. ( 2012), as one application of 40 mg L -1 of GA 3 during the floral induction period in 'Salustiana' sweet orange reduced the frequency of SL.
The increase in SLY (especially in STLY) as well as a higher occurrence of VS, improved the quality of sprouting, as flower buds with the presence of leaves are more likely to fruit set (JAHN, 1973;IGLESIAS et al., 2007).In a study with 'Ortanique' tango tree (C.sinensis × C. reticulata), single flower terminal leafy shoot fruit set almost 40% of the fruits, while mixed buds (SLY + MLY) fruit set ap-proximately 17% of the fruits.The leafless shoots were the ones that least caused fruit set in this tangor, lesser than 10% (CUNHA BARROS; GRAVINA, 2006).
The fruit set (Figure 2) had a significant difference at 107 DAF for 'Montenegrina' and at 32 DAF for 'Rainha'.In 'Montenegrina', the use of four successive applications promoted greater fruit set in relation to the other treatments.In 'Rainha', one application of GA 3 resulted in greater fruit set.Fruit set has a negative correlation with flowering, the lower the energy expenditure with flowering, the greater the fruit tends to be set (GRIEBELER et al.. 2021).
As for the types of shoots and their fruit set, STLY and SL set more fruit in 'Rainha', and STLY had a greater fruit set in 'Montenegrina'.On the other hand, GA 3 applications did not interfere in fruit set frequency of the different types of shoots analyzed individually (Table 2).
Two to four successive applications of 40 mg L -1 of GA 3 promoted a significant reduction in sprouting and flowering, as well as an improvement in the quality of sprouting in 'Montenegrina'.Nevertheless, only four

445 Successive applications of gibberellic acid in alternate bearing mandarins 3 experiment
(ALVARES et al., 2013;VALÉRIO et al., 2018) a 5-year-old orchard, located in the municipality of Montenegro -RS (29º 37' 47" S and 51º 28' 34" W, at 23 m altitude).The soil of the orchard is classified as an Argissolo Vermelho distrófico espessarênico(SANTOS et al., 2018).In both experiments the plants were grafted on Poncirus trifoliata.According to the Köppen-Geiger's classification, the climate of the region is Cfa type, with anual mean temperature below 18°C in the coldest month and greater than 22°C in the warmest month.Moreover, a well distributed rainfall, between 100 and 170 mm is also a feature of Cfa type(ALVARES et al., 2013;VALÉRIO et al., 2018).
(GRAVINA, 2007)exação: Fitorreguladores, citros, Citrus deliciosa Tenore, florescimento.Griebeler et al. (2023)Rev.Bras.Frutic.2023;45:e-The selected trees were in the year-off, (i.e. with zero or almost zero fruit load) Gibberelic acid was employed in a concentration of 40 mg L -1 (GA 3 , ProGibb 400, 40% m/m, Sumitomo Chemical do Brasil Representações LTDA., São Paulo, SP, Brazil, manufactured in the U.S.A.)(GRAVINA, 2007).One to four successive applications were performed, with intervals of 21 days.Applications began in May and ended in July or early August.'Montenegrina'; and in May, beginning of June, end of June and July for 'Rainha').A spray mixture provided plant coverage until the point of runoff.In addition, a non-ionic spreader sticker was used (polyether copoly-

Griebeler et al. (2023) Rev. Bras. Frutic. 2023; 45: e-445 Successive applications of gibberellic acid in alternate bearing mandarins 7 successive
applications of GA 3 increased fruit set in 'Montenegrina'.Two successive applications of 40 mg L -1 of GA 3 reduced sprouting and flowering in 'Rainha' mandarin, while only one application of GA 3 increased sprouting quality.The age of the orchard may be related to its level of al-ternate bearing and to the greater or lesser sensitivity to gibberellin applications in order to reduce flowering.GA 3 application did not affect the fruit set rate of specific shoots.Single flower terminal leafy shoot set more fruit compared to the other types of shoots evaluated.