Theobroma cacao L. cultivar CCN 51: a comprehensive review on origin, genetics, sensory properties, production dynamics, and physiological aspects

Gas exchange and osmotic adjustment

Cacao is a crop that adapts to shady conditions (Almeida & Valle, 2007), but for CCN 51 it has been found that it has higher CO₂ assimilation rates (A) in open field conditions compared to growing it in shade in agroforestry systems, as reported by Suárez Salazar et al., 2018. For the Amazon region of Colombia, these authors reported maximum rates of A of 8 µmol CO₂ m⁻²s⁻¹ at full solar exposure, while cultivation under trees, where the density of photosynthetic photon flux (DFF) that reaches CCN 51 is 400 µmol CO₂ m⁻²s⁻¹ at times of higher radiation, only A maxima of 3 µmol CO₂ m⁻²s⁻¹ have been measured. However, in shade, the quantum efficiency is significantly higher (3.8 µmol CO₂ m⁻²s⁻¹), as compared to plants grown at full exposure (3.1 µmol CO₂ m⁻²s⁻¹). The highest rates of A in this study at full exposure may be related to the specific leaf area (SLA). In their study, Suárez Salazar et al. reported SLA values of 149 cm² g⁻¹ at high radiation and 185 cm² g⁻¹ at low radiation (Suárez Salazar et al., 2018). Also, in plants grown under full sun exposure, Jaimez et al. (2018) in Ecuador reported A of 8.5 µmol CO₂ m⁻²s⁻¹ and 5.3 µmol CO₂ m⁻²s⁻¹ at DDF of 1000 µmol m⁻²s⁻¹ and 400 µmol m⁻²s⁻¹, respectively (Jaimez et al., 2018). Under these conditions of full sun exposure, they reported an SLA of 116 cm² g⁻¹.

Subsequent trials in CCN 51 seedlings grown under DFF conditions of 2000, 1150 and 630 µmol m⁻²s⁻¹ showed significant decreases of A (4 µmol CO₂m⁻²s⁻¹) under DFF 2000 µmol m⁻²s⁻¹ in comparison to the other two radiation conditions that showed A between 7-8 µmol CO₂ m⁻²s⁻¹. Low and significant A at high radiation is also manifested in other cultivars (ICS 1, ICS 95, Luker 40, and Luker 50). (Suárez Salazar et al., 2021). Decreases in A at high radiation rates are related to also significant decreases in foliar water potentials (Suarez Salazar et al., 2021). However, a characteristic that stands out of CCN 51 is the increase in the non-photochemical quenching of Chla at higher radiation as compared to other cultivars (Suarez Salazar et al., 2021) which allows greater heat dissipation of Chl excitation and protection of the photosynthetic apparatus. CCN 51 has exhibited A less than 2.8 µmol CO₂ m⁻²s⁻¹ in plants of 1.5 years of age, planted in pots and growing under conditions of DFF below 400 µmol m⁻²s⁻¹ (Baligar et al., 2008; Baligar et al., 2021), which coincides with field values reported by Suárez Salazar et al. (2018) for shaded plants in agroforestry systems (Suárez Salazar et al., 2018). The increase in A rates under high radiation conditions in CCN 51 is also reported for other Ecuadorian clones with A values similar to those obtained in CCN 51 (Jaimez et al., 2018). It seems that in environments of high cloudiness that predominate at the Ecuadorian coast and in the Colombian Amazon, growing cacao under shady conditions can limit its maximum photosynthetic capacity due to excess shade. In addition, keeping the cacao under excessive shade can increase the incidence of frosty pod rot infection (Moniliophthora roreri) (Gidoin et al., 2014).

Under dry conditions without irrigation, Tezara Fernández et al. (2020) reported A values of around 5 µmol CO2 m⁻²s⁻¹ with DFF of 1000 µmol m⁻²s⁻¹ in the province of Esmeraldas, Ecuador. Despite the decrease in A rate in the dry period, CCN 51 maintained a higher yield (above 1200 kg ha⁻¹). Like other cacao cultivars in Ecuador, CCN 51 maintains higher rates of stomatal conductance (between 250–350 mmol m⁻² s⁻¹) (Tezara Fernández et al., 2020; Jaimez et al., 2018). In these high conductances in plantations under full sun exposure, CCN 51 exhibits water use efficiency (WUE) around 2.3 mmol mol⁻¹. Concerning other Ecuadorian cultivars, they are intermediate values (Tezara Fernández et al., 2020; Jaimez et al., 2018), Other cultivars of Criollo type in field conditions have shown greater WUE in dry periods (3.5−4.5 mmol mol⁻¹), and in other cultivars, higher WUE has been obtained in periods of rain (Ávila Lovera et al., 2016).

Several cacao cultivars have shown osmotic adjustment (Rada et al., 2005; Almeida et al., 2002), which has been attributed to increases in K and P under conditions of water deficit (Almeida et al., 2002). In cacao, it has previously been shown that maintaining higher osmotic adjustments contributes to a lower number of dead juvenile plants during the dry season (Araque et al., 2012). In the case of CCN 51, using 18-month-old juvenile plants sown in pots and subjected to different amounts of irrigation (varying between 100 to 40% of evapotranspiration), it has been observed osmotic adjustments of 0.60 MPa in plants watered with 60% of evapotranspiration (Arteaga, 2020). Table 2 summarize the osmotic potential at full turgor (Ψ _π100), at the point of turgor loss (Ψ _π0) and, the osmotic adjustment obtained for 6 years old trees (from 3 clones (EETP-800, EETP-801 and CCN 51)) in field conditions during the dry and rainy season in Quevedo, Ecuador, according to methodology reported by Tyree & Jarvis (1982) and Schulte & Hinckley (1985). EETP-800 and EETP-801 are cultivars recently released by the INIAP (Loor-Solórzano et al., 2019) and have as common parents the cross between CCN 51 ×EET-233. Interestingly, the latter has shown a low susceptibility to frosty pod rot disease and a medium incidence of witch’s broom infection (Moniliophthora perniciosa) (Jaimez et al., 2020). CCN 51 showed the highest osmotic setting of 0.46 MPa, while EETP-800 and EETP 801 showed an osmotic adjustment of 0.40 and 0.43 MPa, respectively.

Another important physiological trait that has been recently reported for CCN 51 is its low Cd intake (Argüello et al., 2019; Barraza et al., 2019; Arevalo-Hernandez et al., 2021). For example, an average of 1.4 times higher concentration of Cd has been found in beans of National types when compared to CCN 51 (Argüello et al., 2019). Similar trends have been also reported by Barraza et al. (2019). Remarkably, Arevalo-Hernandez et al. (2021) in an evaluation of 53 genotypes have included CCN 51 between the top 11 genotypes showing a low accumulation of Cd.

Regarding its agronomical management, it seems convenient for CCN 51 in the seedling stage to avoid high radiation due to decreases in A and possibly leading to lower growth rates. This aspect must be taken when planning new plantations in regions with high radiation throughout the year. However, the ability of photoprotection mechanisms associated with increased xanthophyll’s cycle exhibited by CCN 51 allows it to avoid drastic damage to the photosynthetic apparatus due to the excess radiation received in the juvenile stages. Already in the productive stage CCN 51 has shown higher A at full sun exposure but in conditions where there is low evaporative demand due to the predominance of high cloudiness (Suarez Salazar et al., 2018, Jaimez et al., 2018). It would be appropriate to evaluate the effect of partial shade conditions for at least the first two years on growth and production in regions with high DFF throughout the year. The irrigation and fertilization conditions that occur in large commercial plantations undoubtedly offer less stressful conditions, avoiding important reductions in water potentials that lead to stomatal closures and consequently lower A rates. Its osmotic adjustment capacity and its median WUE allow the CCN 51 to tolerate periods of water deficit and probably fewer juvenile plant deaths. For small producers that mostly lack irrigation services, this trait offers a great advantage as compared to other more susceptible cultivars.

Production dynamics

In Ecuador, the highest yields of CCN 51 occur between October - December, regardless of the amount of water supplied to the plantation. For example, in the province of Los Ríos, where the average rainfall is 1800 mm and plants received irrigation, the highest yield in the period 2013-2016 was observed between the months of October-November as shown in Fig. 3. The yield values with irrigation range from 33 to 50 quintals (equivalent to 1500 and 2300 kg ha⁻¹, respectively), while the average yields without irrigation reach average values of 1050 kg ha⁻¹ in the localities of Quevedo, Tenguel, and Esmeraldas, Ecuador (Sánchez-Mora et al., 2015; Tezara Fernández et al., 2020). This is a high yield considering that these areas are experiencing 5 months without rain. The high-yield Nacional -type clones from the Centro de Cacao Aroma Tenguel-CCAT collection, in the same trial showed yield averages between 200 and 590 kg h⁻¹ which is between 80 to 44% lower compared to those obtained with CCN 51 (Sánchez-Mora et al., 2015). In other evaluations with 5-year records for the Quevedo region, CCN 51 showed superior yields between 50 to 70% higher, compared to Nacional-type clones (Boza et al., 2014). In Colombia, average yields between 1000 and 2000 kg ha⁻¹ have been registered. It seems that the high variation depends on the fertilization program (Puentes-Páramo, Menjivar-Flores & Aranzazu-Hernández, 2014).

It is known that the amount of precipitation is a factor that notably influences cacao production (Zuidema et al., 2005). However, the higher production at the end of the dry period of CCN 51 in the Ecuadorian coast implies changes in the distribution of assimilate between the vegetative and reproductive parts. Very little is known regarding the distribution of assimilates in cacao (Lahive, Hadley & Daymond, 2018). Therefore, is important to understand why despite water limitations pods grow. This period is also characterized by a low incidence of DFF in this region. It seems the low DFF and the lower vapor pressure deficit (DPV) is a result of high relative humidity (RH) and cloud cover, thus creating a favorable condition of less evaporative demand and less stress, which may favor the continued productivity of CCN 51. It is likely that in the dry period, the growth rate of the pods is lower due to lower rates of A, but it seems that a greater amount of assimilates are destined for the growth of pods. The characteristic of the fruit ripening period during the dry season influences a lower incidence of fungal diseases such as frosty pod rot disease (Ploetz, 2016) or pod infections by witch’s broom M. perniciosa (Sánchez-Mora et al., 2015). Environmental conditions in the dry period thus increase the growth of fruits in CCN 51. The physiological mechanisms that induce high flowering during the late rainy/early dry period are unknown and it has usually been reported that the largest numbers of flowers occur during the rainy period (Adjaloo, Oduro & Banful, 2012). Besides, a lower amount of rain leads to a lower number of cacao pollinating insects (Arnold et al., 2018) which would influence a lower number of fruits, but this is not what happens with CCN 51. Branco et al. (2018) have classified CCN 51 as a self-compatible and cross-compatible clone. Such characteristics could partially explain its higher production, as it is less dependent on pollinators. Moreover, it has been previously shown that its number of retained flowers is higher as compared to other clones cultivated in other countries after 15 days of self-pollination (Branco et al., 2018).

Floral biology

The flower of CCN 51 is pinkish, with staminodes and style averaging 6.6 and two mm in length, respectively. The ovary reaches a length of 1.9 mm and has 46.2 ovules (Perea et al., 2017). A significantly greater number of flowers and pollinated flowers have been reported for CCN 51 than for Nacional cacaos that were used in 2010 in Ecuador (Montoya Bazán, 2010). The greater quantity of pollinated flowers can be related to pollen viability as recently found by García Talledo et al. (2019), who have reported that the viability of CCN 51 pollen lasts around 24 h, while in 10 cultivars of the Nacional type, viability is between 3 and 7 h (García Talledo et al., 2019). This gives an advantage for CCN 51 as a possible pollinator. The greater longevity of pollen can influence pollination and produce a high number of seeds compared to other clones. Additionally, CCN 51 has around 44,000 pollen grains per flower, above the average for Nacional cacaos (33,000 grains per flower) (García-Cruzatty et al., 2020) and a higher average number of pollen grains have been found to reach stigmas per flower per month (19.4) than other clones used at the Ecuadorian coast (Mena-Montoya et al., 2020). It has also been recorded that the highest number of grains arriving at the stigmas occurs in February (rainy season) and the lowest number between October and December, i.e., the last months of the dry period (Mena-Montoya et al., 2020). This suggests a reduction in pollen production and flowers due to the lower availability of water that leads to a lower probability of seed production. Also, a lower percentage of falls of small immature pods (cherelles wilt) has been reported in CCN 51 (Montoya Bazán, 2010), which corroborates its advantages for higher production.

CCN 51 has a low pod index (number of pods to get one kilogram of dried cacao- value (15.2)) as compared to other higher National-type cultivars (values between 17.8 and 22.8) (Jaimez et al., 2018). Similar values have been obtained in several locations in Ecuador and Colombia (Quintana Fuentes et al., 2015a; Perea et al., 2017; Jaimez et al., 2018). Its low value is related to an average of 71 seeds in 100 g and an average weight per seed that varies between 1.54 g in Ecuador (Boza et al., 2014) and 1.6 g in Colombia (Quintana Fuentes et al., 2015a). This seed weight is higher than those reported for Nacional-type cacao, suggesting that CCN 51 allocates a larger amount of photoassimilates to the development of seeds, more than for other parts of the pods. Other clones with a higher quantity of seeds in 100 g (between 94 and 85) present averages of weight per seed of 1.2 and 1.17 g (Boza et al., 2014).

Efforts to provide alternative cultivars to local farmers

One current challenge for CCN 51 utilization is to further optimize its agronomical management and to increase its use in breeding programs. The current evidence suggests that the osmotic adjustments observed in CCN 51 are a valuable trait conferring a greater tolerance to water deficit, which might represent an advantage for small farmers without an appropriated irrigation system. Nevertheless, additional research in this field is still required to further evaluate the effect of water supply on yield, as the current evidence suggests the possibility to achieve higher efficiency in the use of water in this clone. The review data suggest that during the end of the rainy season and the first three months of the dry season, pollination occurs preferentially, and consequently in the following dry months the development of pods reaches their highest production rate (October-December period). This occurs independent of irrigation, but with higher production in irrigated plants. The higher production also can be associated with a higher number of pollen grains that ensure better pollination success. Its high seed weight leads to a low value of the pod’s index. Unfortunately, all these physiological characteristics in response to environmental factors have not yet been widely used in breeding programs with goals to obtain cultivars with better adaptation to future climate change scenarios.

Undoubtedly, the development of novel cacao cultivars must be oriented to obtain clones not only with high productivity but also showing a low demand for fertilizers and displaying a high tolerance to diseases, thus reducing the use of fungicides and pesticides. For example, two clones have been recently released in Ecuador by INIAP (EETP 880 and EETP 801) using CCN 51 as parental. These clones have shown higher productions as compared to CCN 51 (Jaimez et al., 2018) and keep both the National-type organoleptic qualities (Loor-Solórzano et al., 2019) and high tolerance to frosty pod rot disease (Jaimez et al., 2020). It is noteworthy that the improvement of cacao is limited by the length of the breeding cycle of the crop that can take at least a decade and these efforts should start soon (Bekele & Phillips-Mora, 2019).

Efforts to develop an optimal agronomical management

The higher energy dissipation capacity of CCN 51 allows its cultivation under full sunlight exposure. CCN 51 has shown consistent and high yields when growing alone (i.e., monocultures under full sun exposure). Unfortunately, this has led to the displacement of other premium cultivars with excellent flavor potential and other interesting characteristics, thus reducing the diversity in the producers’ farms. For example, Melo & Hollander (2013) point out that between 2002 and 2009, 38% of the interviewed farmers had renewed their plantations using only CCN-51 and sowing it in full sun. On the other hand, the strong increase of this clone in cultivation bears risks of monoculture in cacao such as a higher incidence of pests and diseases (Andres et al., 2016). Diversification will reduce dependence on a single cultivar prone to the risk of being eliminated by the high incidence of insects and diseases (Niether et al., 2020). Therefore, avoiding monoculture protects against devastating losses and would also increase the likelihood of a recovery in less time. Probably each country should implement the possibility of cultivating CCN 51 combined with other cultivars that have been successfully used in each region. Unfortunately, the co-cultivation of CCN 51 and premium clones has resulted in the incorrect tendency to mix beans of this clone with premium cacao beans aimed to increase farm productivity. This has been shown to directly affect the sensorial quality of the final product (i.e., premium chocolate) with deleterious economic consequences. In this regard, as previously discussed, several methodologies have been developed to assure traceability based on the identification of selected molecular markers for each clone. Nevertheless, further research still must be done in this regard to discouraging these bad agronomical practices. Monocultures of CCN 51 (i.e., without shade trees or in the presence of temporary shade species) can lead to a faster depletion of the soil (Andres et al., 2016) and loss of pollinators, which significantly influences the production (Groeneveld et al., 2010). For this reason, despite its good production under full sunlight exposure, different production models (i.e., agroecological farming systems) that combine CCN 51, and shade trees still must be designed, implemented, and transferred to the local farmers.

An important advantage of the cultivation of CCN 51, is its lower accumulation of Cd as compared to other clones. This is certainly a field that still must be explored as Cd uptake is a current bottleneck to access the European premium markets.

Efforts to develop an optimal post-harvest management

Generally, CCN 51 is considered bulk cacao with negative impacts in the fine-flavor market. However, as mentioned before, several alternatives have been proposed to further engineer the post-harvesting process to demystify its utilization as a potential clone for controlled production of fine-flavor cacao either by standardized controlled fermentations, or by breeding, or both, to eventually enhance its sensory profile to unite the best of the two worlds. Research in this topic is still in its infancy and additional experiments will be required to engineer a process that redefines the flavor profile of CCN51, thus opening better opportunities for this clone in the complex cacao market.