Phenology of hass avocado in the Andean tropics of Caldas, Colombia

- The objective of this study was to document and analyze the effect of climate on the phenological development of ‘Hass’ avocado in two contrasting zones of the Andean tropics of Caldas, located at altitudes of 1,950m and 2,400m a.s.l., respectively. A completely randomized experimental design was carried out using a five to seven year-old ‘Hass’ tree as experimental unit and 15 replicates on each area. Observations were conducted on lateral sylleptic shoots and their corresponding apical buds located on the eastern and western sides of each tree. The destination of the apical bud of lateral shoots was quantified, floral phenology was documented, heat units were calculated from reproductive bud stage 1 to anthesis and to harvest and root growth was followed for each study area. Air temperature, soil temperature, precipitation and photosynthetically active radiation (PAR) values were recorded over time. Differences were found in the number of apical buds of lateral shoots with reproductive and vegetative destination, flowering phenology, heat units, root growth, PAR and precipitation. Differences were found among variables evaluated at the two contrasting altitudes. There is a clear need for further ecophysiological studies on this fruit tree in tropical areas in particular.


Introduction
Since 2020, world avocado production has shown annual increase of 4.1% (ARIAS et al., 2018). Current estimates indicate that around 5 million tons of avocado are consumed worldwide each year (SOMMARUGA; ELDRIDGE, 2021). In Colombia, the 'Hass' cultivar corresponds to approximately 25% of the total area cultivated with avocado but it is noteworthy that the increase in the planted area is close to 73%, from 11,000 hectares in 2014 to 20,182 hectares in 2019. It is estimated that there are currently more than 30,000 hectares of 'Hass' established in the country at different regions; however, about 26% of the cultivated area is at the development stage; therefore, an increase in the annual fruit production is expected in the coming years (MADR, 2021). Despite these figures, in order to meet market demand, it is necessary to have a consolidated knowledge base to adequately support the increase of new areas and safeguard the current ones (DÍAZ et al., 2020).
The knowledge about the genotype/environment interaction of the 'Hass' cultivar in the conditions of the Andean tropics in which Colombia is located will increase our understanding of how avocado grows and produces in these environments, and is of fundamental to promote sustainable and competitive production systems necessary to address problems such as alternate bearing, a phenomenon that implies a year of very abundant harvests ("on" year) followed by reduced ones ("off" year) and vice versa (ALCARAZ et al., 2013). This behavior results in dramatic variations in fruit volume from year to year and translates into loss of income during low yield years (MICKELBART et al., 2012).
Recognizing that alternate production is closely related to the presence and location of fruits within the canopy and the subsequent flowering over time (ALCARAZ et al., 2013), the study of the architecture and behavior of avocado buds that generates the units that give rise to reproductive organs, and that defines the growth structure of the avocado tree, is of special importance (ARIAS et al., 2021). Thus, the objective of the present study was to follow and evaluate the phenology of 'Hass' avocado in two contrasting zones of the Andean tropics of Caldas.

Plant material
The field stage was carried out between January 2020 and February 2021 in two commercial 'Hass' avocado orchards, located at 5˚18' 40" N and 1,950 m a.s.l. (Aranzazu), and Villamaria at 5˚01' 26'' N and 2,400 m a.s.l., both with loamy textured soils. Trees are 5 to 7 years old grafted on "criollo" rootstocks from seeds planted at 6m x 6m, without irrigation system and with management based on the ICA guidelines (Colombian Agricultural Institute) in resolutions 448 and 30021 for exporting farms (ICA, 2016).
Fifteen trees were selected on each orchard in a completely randomized experimental design (CRD). On each tree, branches of approximately 1.5 m in length were selected and marked from the middle part of the tree canopy with orientation to the eastern and western quadrants. On these selected branches, epicormic shoots and respective lateral shoots were selected and marked (SALAZAR-GARCÍA et al., 2018).

Destination of apical buds of lateral shoots
On each tree and branch with eastern and western orientation, epicormic shoots and their respective lateral shoots were marked, and the destination of their apical buds (vegetative or reproductive) was monitored over time, on each growth flush observed.
Relative intensity of lateral shoot growth.
The relative intensity of both vegetative and reproductive growth of the apical buds of sylleptic lateral shoots was determined using the following mathematical formula (ROCHA-ARROYO et al., 2011

Climatic variables
Climatic variable data were obtained from automatic weather stations located on each orchard. The variables recorded were air temperature, soil temperature, relative humidity and precipitation. Photosynthetically active radiation was periodically measured at one-hour intervals from 8:00 am to 4:00 pm using PAR MQ-303 full spectrum meter from Apogee Instruments.

Statistical analysis
Completely randomized design (CRD) was used, considering the tree as the experimental unit and replicates and subreplicates corresponding to branches marked at each cardinal point. Results were analyzed by analysis of variance and Tukey's mean comparison (p≤0.05). The SAS statistical software (SAS Inst, Cary N.C. Version 9.4) was used.

Results and Discussion
The variables evaluated, such as destination of apical buds of lateral shoot, flowering phenology and accumulated heat units (AHU), show significant differences between locations, which confirms the fact that environmental conditions have an effect on tree growth processes. Likewise, the behavior of climatic variables such as air temperature, precipitation and solar radiation show significant changes over time in each evaluation zone.

Destination of apical buds of lateral shoots.
Shoot growth is generally monopodial and axillary buds can be proleptic (formed after a rest period of their apical meristem) or sylleptic (formed without rest period) (ALCARAZ et al., 2013;WANG et al., 2018). All lateral shoots evaluated on this research were sylleptic (Figure 1 and 2). These shoots are considered highly productive due to their flowering intensity (GARNER; LOVATT, 2016) and are more commonly observed in tropical conditions than in temperate climates (DÍAZ et al., 2020).
Observations made in this study indicate that the Aranzazu area (1,950 m) may be more productive in terms of number of flowers and fruits than the Villamaría area (2,400 m), due to the presence of statistically significant differences in the destination of reproductive apical buds of sylleptic lateral shoots in both areas under study ( Figure 3).
Photo: Arias-García, 2021.      These results are probably related to the difference in average temperature between locations, with Aranzazu having higher average temperature (21.4 ºC), compared to Villamaría (17.8 ºC).

Root growth
Root growth can occur throughout the year but be reduced at times of vegetative flush or flowering (ROCHA-ARROYO et al., 2011;MICKELBART et al., 2012), or even growth dormancy can occur (GARNER; LOVATT, 2016). The results of this research show continuous roots growth over time in both locations ( Figure 5); however, higher growth is evident in the Villamaría area (2,400 m). Likewise, it is observed that for the Aranzazu area (1,950 m), the most significant growth was observed on the second semester of the year (20.7 gr), while in the Villamaría area, the highest growth was registered in the first semester (23 gr).
Regarding the relationships between root growth, vegetative flush and flowering, it is important to mention that flowering events occurred in February and August in the Aranzazu area, and in March and September in the Villamaría area. For both areas, root growth was reduced after flowering in the first half of the year; however, in the second half of the year in the Villamaría area, root growth increased after flowering, while in Aranzazu, root growth was reduced. As for the initial vegetative flush, the apical growth of lateral shoots began in December 2020 in both areas, and in the second semester of 2021, vegetative flush occurred near the month of June.

Climate conditions in both areas under study
The results obtained in this study show climatic differences between areas, which confirm the fact that climate has high influence on the vegetative and reproductive growth of avocado trees (CHUNG et al., 2022), since at level of these variables, the two areas under study also show important differences.

Air temperature
According to the Caldas-Lang climate classification, the Aranzazu area is classified as semi-humid temperate climate, while the higher elevation area of Villamaría is classified as humid cold climate (IDEAM, 2015). In this regard, differences in minimum, mean and maximum air temperatures recorded in both areas over time are evident ( Figure 6).
While the three ecological avocado cultivars offer a wide range of adaptation to air temperature (WEIL et al., 2022), for the 'Hass' cultivar in particular, Garner and Lovatt (2016) found that under the southern Australia conditions, only the coldest (17/10°C) and hottest (37/30°C) treatments restricted growth and dry matter accumulation, while Rocha-Arroyo et al. (2010) found that in California, the 'Hass' cultivar did not flower when maintained at temperatures of 30/25, 25/20 or 20/5°C (day/night), but flowered when exposed to 3-4 months of 15/10, 18/15, 20/15 and 23/18°C (day/night) respectively. Air temperature data during this research do not seem to restrict tree growth in either of the two areas under study. Research carried out in Brazil determined that constant night temperatures below 10ºC can alter avocado flowering development (SOARES et al., 2002) and in California (USA), evaluations carried out on 'Hass' avocado cultivar showed that maximum air temperatures below 15 ºC reduce bee movement in the orchard (GARNER; LOVATT, 2016); however, in this area, inflorescence development is correlated with night temperature ≤15 ºC (SALAZAR-GARCÍA et al., 2018). In the case of Aranzazu and Villamaría, night temperatures were rarely above 15ºC or below 10ºC, which indicates that floral development should not be affected; however, differences between one location and the other can accelerate or reduce the time of the different phenological phases, specifically the flowering time and the flower-to-fruit period.

Soil temperature
The 3 avocado cultivars, Mexican, Guatemalan and Antillean, were cultivated in different climatic regions and may show different responses to soil temperature (SILVA et al 2017). Furthermore, it has been shown that there is root growth restriction at soil temperature below 13.5ºC (WEIL et al., 2022). Likewise, soil temperature ranging from 21º to 27° was better for avocado seedling growth, but temperatures above 27° reduce growth (SILVA et al 2017).
Soil temperature levels in this study did not show strong variations in any of the areas under study (Figure 7), which results are not in agreement with those found by Rocha- Arroyo et al., (2011), where in different growing areas of Michoacán, Mexico, soil temperature variation was from 13 ºC to 21 ºC. The minimum temperature recorded in Villamaría was 16.4ºC in November, while the highest temperature was 18.1ºC in July. In Aranzazu, the minimum temperature recorded was 17ºC in January, while the maximum temperature was 18.4ºC in April. This information allows inferring that the soil temperature in these two areas was never below the critical minimum reported and also did not reach levels relevant to stimulate root growth. Furthermore, the maximum root growth in Villamaría occurred in April, while in Aranzazu, it occurred in July, and considering this information, there seems to be no relationship between maximum root growth and maximum soil temperature.  The two peak flowering periods recorded during the year in the two locations correspond to February-March and August-September, contrasting with high rainfall in the first semester of the year in the Villamaría area, and high precipitation in the third semester of the year in the Aranzazu area. Silber et al. (2012) have explained which is the environmental factor that can induce flowering in avocado and reported that flower initiation can be induced by water stress. Salazar-García et al., (2018) concluded that the environmental stimulus for flowering induction was photoperiod and/or air temperature. In contrast to the above, research carried out in the Andean tropics of Colombia showed that flowering induction occurred in some areas only at times when precipitation was reduced, while in others, flowering occurred after high precipitation (DÍAZ et al., 2020).
The results of this study have shown that several environmental factors may be associated with the floral induction of avocado under the conditions of the Andean tropics. Cloud cover may play an important role since the presence of high precipitation increases cloud cover and therefore there is a drop in photosynthetically active radiation (PAR). In this scenario, a hypothesis that may be viable is that the avocado tree could stop vegetative growth if radiation decreased constantly, and switch to a reproductive phase that would begin with flowering induction.

Photosynthetically Active Radiation (PAR)
For both locations, the light intensity observed at different times of the year showed tendency to concentrate the highest PAR values between 11:00 am and 02:00 pm (Figure 9 A-B); however, the Villamaría area shows significant reduction in radiation after 02:00 pm, which might be due to greater presence of cloud cover if we consider that at that altitude, the presence of cloud cover in the afternoon hours is frequent. According to these results, it is important to consider that incident PAR varies according to latitude, season, planting date and plant phenology (CRUZ et al., 2021). In areas close to the equator, the length of the day does not seem to change significantly (YEANG, 2007), and as for solar radiation, Renner (2007) reported that when the sun passes directly over the equator twice a year, a bimodal cyclic change in the incoming solar radiation (insolation) is observed on these regions. point (WEIL et al., 2022). In this research, maximum PAR values above 2,000 µmol/m-2/s-1 were recorded and the average values observed were slightly less than 1,400 µmol/m-2/s-1 (Figure 9 A-B). This indicates very good PAR supply in areas under study and shows the opportunity to study the effect of this high solar radiation on tree physiology and productivity.

Conclusion
The Aranzazu area showed higher values of shoots with reproductive destination in their apical buds on both semesters of the year. Also, in Aranzazu (AHU 1,096.55), anthesis occurred 21 days earlier than in Villamaría (AHU 862.7), and in Aranzazu (AHU 2,530.9), harvest occurred 76 days earlier than in Villamaría (AHU 2,092.72). For both locations, there were two important root growth peaks, and soil temperature remained within maximum and minimum ranges reported. There were differences in precipitation and air temperature in the different areas.
This study reveals the need for further research on the ecophysiology of 'Hass' avocado under the conditions of the Andean tropics, since the results obtained show differential behavior of trees compared to other latitudes where this fruit is also grown.