Bud-break promoters for the improvement of the budburst of pecan cultivars

– The objective of this work was to evaluate the effect of different bud-breaking substances on the budburst of the Barton, Desirable, and Jackson pecan ( Carya illinoinensis ) cultivars. The experimental design was a randomized complete block with four replicates. The treatments consisted of spraying different rates of commercial bud-break promoters on the trees of these cultivars, in order to induce budburst. The budburst percentage of axillary buds and the budburst heterogeneity index (BHI) were evaluated. Regardless of the used rate, the application of the hydrogen cyanamide product improved the budburst of the tested cultivars and reduced the BHI. In addition, water soluble N + Ca at 6% and water soluble N + organic C soluble in water at 7% significantly improve the budburst and reduce the BHI of the evaluated cultivars, being, therefore, potential alternatives to replace hydrogen cyanamide.

Pecan is a temperate fruit species that requires the accumulation of low temperatures during the winter to overcome dormancy (Zheng et al., 2021). Chilling accumulation in Southern Brazil ranges from 200 to 1,000 chilling hours (<7.2ºC, CH), depending on the cultivar (Crosa et al., 2021), which is usually sufficient to overcome the dormancy of pecan nuts. However, Southern Brazil is known as a typical warm-Pesq. agropec. bras., Brasília, v.57, e02956, 2022 DOI: 10.1590/S1678-3921.pab2022.v57.02956 winter climate region, which show a wide range of temperature fluctuation during the winter (Lamela et al., 2020), and great variations in CH accumulation throughout years. Budburst and flowering in such conditions are usually deficient, not uniform (Pasa et al., 2018a), and they frequently lead to reduced yields. An alternative to overcome these problems is the use of rest-breaking substances to partially compensate for the lack of CH, which is a common practice widely used in apple orchards to promote budburst (Pasa et al., 2018a;Marques et al., 2022).
The most widely used rest-breaking agent is hydrogen cyanamide (Dormex) showing high efficiency to overcome dormancy in several fruit species, such as apple (Pasa et al., 2018a;Marques et al., 2022), peach (Chen & Beckman, 2019), and grapevine (Sudawan et al., 2016). However, its high toxicity is a limiting factor (Petri et al., 2014), the reason why studies on rest-breaking substances of low toxicity and good efficiency are important (Pasa et al., 2018a). In pecan nuts, studies regarding rest-breaking agents are scarce worldwide and not yet known in Brazil. Recent studies have shown that more environment friendly, restbreaking compounds (based on inorganic nitrogen, glutamic acid, and amino acids) -such as Erger (Petri et al., 2014;Pasa et al., 2018a), Syncron (Petri et al., 2014), Siberio and Bluprins (Petri et al., 2021) -have a similar efficiency in apples.
The objective of this work was to evaluate the effect of different bud-breaking substances on the budburst of the Barton, Desirable, and Jackson pecan nut cultivars.
The experiment was carried out in a commercial orchard of pecan nuts located in the municipality of Encruzilhada do Sul, in the state of Rio Grande do Sul, Brazil (30°37'21.2"S, 52°42'16.8"W, at 485 m altitude), in the 2019/2020 and 2020/2021 growing seasons. The climate of the region, according to Köppen-Geiger's classification is Cfa, that is, humid subtropical, oceanic climate, without a dry season and with a hot summer (Alvares et al., 2013). The accumulation of chilling hours (CH) below 7.2ºC (45°F), from June 15 to September 15 was 475 hours in 2019, and 773 hours in 2020. Chilling units (CU) in the mentioned period were 1,047 and 1,239 in 2019 and 2020, respectively, in the conditions of the experimental site during the experiment (Figure 1) The soil of the experimental orchard is an Argissolo Vermelho-Amarelo distrófico (Santos et al., 2018), i.e., an Ultisol (Soil Survey Staff, 2014. Plant material consisted of pecan trees of the Barton, Desirable, and Jackson cultivars, which were planted in the winter of 2015 and trained to a central leader system. These cultivars complement each other regarding pollination. Trees were spaced 8.5 m between rows and 8.5 m between trees within the row, totaling 138 trees per hectare. The experiment was carried out in a randomized complete block design, with four replicates of three trees in each plot. Only the central tree was used for evaluation, leaving one at each end as border. Trees were chosen by uniformity of size, trunk diameter, and health. Treatments were sprayed to the same trees in both seasons. Orchard management was performed according to commercial growing standards and were similar for all treatments, that is, trees were fertilized according to soil analysis, at the beginning of the growing season, pruned during the winter, and sprayed with fungicides to avoid diseases -mainly pecan scab (Venturia effusa). The orchard was not irrigated.
In both seasons, the percentage of budburst of axillary buds and budburst heterogeneity index (BHI) were evaluated. Eight uniform one-year-old shoots were selected to perform the evaluations. These chosen shoots were 20-30 cm long and had between 15 and 25 axillary buds; water sprouts were avoided. Budburst was obtained by the ratio between buds breaking dormancy and the total number of buds of each shoot, expressed as percentage (%). Budburst was evaluated at 30 days after the application (DAA). Buds breaking dormancy were considered as those which were at least in the stage 009 (green tip clearly visible), according to the BBCH scale (Lancashire et al., 1991). The standard deviation (SD) within each treatment/replicate was determined from the eight oneyear-old shoots selected, and then the coefficient of variation was calculated as CV = [(SD/average axillary bud break) ×100], which was called the budburst heterogeneity index (BHI). The CV ranged from 0 to 100%, expressing the homogeneity of budburst among shoots within the treatment.
Statistical analyses were performed using the R software (R Core Team, 2014). Data of 'Desirable' pecan budburst in 2019/2020 and of BHI of 'Jackson' in 2020/2021 were transformed by arcsin [square root (n + 1)] to meet the assumptions of the analysis of variance. Data were analyzed for statistical significance by the F-test and, when means were significant, Scott-Knott's test was performed to group the treatment means.
In the 2019/2020 growing season, a significant effect of treatments was observed for all parameters evaluated and cultivars. The greater budburst was induced by HC at 1%, in 'Barton'; by HC (1, 2, and 3%), WSN+Ca (4 and 6%), and WSN+OCS (7%) in 'Desirable'; and by HC (1, 2, and 3%), in 'Jackson' (Table 1). Hydrogen cyanamide at all rates resulted in the lower BHI of all cultivars, but other treatments had similar effect, such as OM+FAA+ON (4%), WSN+CaO (8%), and  WSN+OCS (7%) in 'Barton'; WSN+Ca (4 and 6%) in 'Desirable'; and WSN+Ca (4 and 6%), WSN+CaO (6 and 8%) and WSN+OCS (7%) in 'Jackson' (Table 1). Our results show that the budburst of all tested cultivars was improved by hydrogen cyanamide, and that BHI reduced, irrespectively of the rate applied. These results agree with those found for 'Maxi Gala' and 'Fuji Suprema' apples (Petri et al., 2014;Pasa et al., 2018aPasa et al., , 2018b, which showed consistent improvement of budburst with hydrogen cyanamide at 0.7%. Given the limited information on the subject, rates from 1 to 3% were evaluated, and the results show that in future studies lower rates could be tested to check this agent efficiency. Considering the 2020/2021 growing season, budburst and BHI were significantly affected by treatments in all cultivars, except for BHI in Jackson (Table 2). Budburst and BHI were not improved by any treatment in 'Barton' in this growing season. Indeed, budburst was reduced for all rates of OM+FAA+ON and WSN+CaO, and WSN+OCS at 3%, while BHI increased in all rates of OM+FAA+ON. However, budburst of 'Desirable' was improved by all treatment application, in comparison with the control, and the best results were observed in the application of HC (1, 2, and 3%), whereas BHI was not significantly reduced by any treatment, in comparison with the control. For 'Jackson', all treatments, except for OM+FAA+ON (all rates) improved budburst, in comparison with the control. The greatest budburst in this cultivar was observed with the application of HC (at all rates) and WSN+OCS (7%), followed by WSN+Ca (2, 4, and 6%).
The effect of the greater chilling accumulation from the first to the second season on treatment effect can be observed by the fact that any treatment reduced BHI relative to control, i.e., chilling supplied was capable to fulfill even control trees requirement, regardless the cultivar. Similar results were observed in a three-year study of 'Fuji Suprema' apple trees, in which the authors observed differences only in the BHI induced by promoters in the year of lower chilling accumulation (Pasa et al., 2018b). Cultivar responses to treatments were somehow similar, and hydrogen cyanamide induced greater budburst and lower BHI, but there were positive cultivar responses to WSN+Ca (6%) and WSN+OCS (7%). However, in the second growing season, there was no budburst improvement in 'Barton', even in response to hydrogen cyanamide.
Even though hydrogen cyanamide is still the most widely applied substance to induce bud break (Marques et al., 2022) in fruit trees with proved efficiency, other substances (mainly nitrogen-based compounds have shown to be approximately or equally efficient, such as WSN+Ca, which has been extensively studied in apples, with optimum rates varying from 2 to 4% combined with either CaN or mineral oil (Pasa et al., 2018a). A recent study showed that the bud release from dormancy of ' Anna' apple trees was correlated with increased levels of soluble nitrogen, total nitrogen, and polyamines (small aliphatic nitrogenous compounds) (El-Yazal & Rady, 2012), which partially explains the positive effect of nitrogenous substances, such as WSN+Ca on budburst, as observed in the present study, in which WSN+Ca 6% + CaN 3% had an efficiency like hydrogen cyanamide, irrespectively of the cultivar tested. However, rates greater than 7% in apples are not recommended, since negative impacts may occur on fruit set, resulting in higher operational issues and costs given the greater substance volume needed for application, according to Petri et al. (2021). (1) Different letters within columns indicate significant differences, by Scott-Knott's test, at 5% probability. The following promoters were applied to the cultivars on 9/18/2020: HC, hydrogen cyanamide (52% a.i, w/v) (Dormex); WSN+Ca, water soluble nitrogen (18.7%, w/v) and calcium (4.2%, w/w) (Erger); OM+FAA+ON, organic matter (80%, w/w), free amino acids (2%, w/w), and organic nitrogen (0.3%, w/w) (Syncron); WSN+CaO, water soluble nitrogen (15%, w/v) and calcium oxide (4.5%, w/v) (Siberio); and WSN+OCS, water soluble nitrogen (8% w/w) and organic carbon soluble in water (5%, w/w) (Bluprins); MO, mineral oil; CaN, calcium nitrate.