Avocado (Persea americana) is a tree belonging to the Lauraceae family, known for its nutritious and edible fruit.
Classification
Kingdom : Plantae (Plants)
Subkingdom : Tracheobionta (Vascular plants)
Superdivision : Spermatophyta (Seed plants)
Division : Magnoliophyta (Flowering plants)
Class : Magnoliopsida (Dicotyledons)
Subclass : Magnoliidae
Order : Laurales
Family : Lauraceae
Genus : Persea
Species : americana
3 horticultural races of P. americana
The races are recognized as
- Americana – West Indian
- Drymifolia – Mexican
- Guatemalenis – Guatemalan
Place of Origin
The eastern and central highlands of Mexico, along with the neighboring highland regions of Guatemala extending to the Pacific coast.
Climatic requirement
Avocado is a tropical to subtropical fruit, with subtropical cultivars suitable for cultivation at higher elevations ranging from 800 to 1000 meters.
Temperature
The optimum growing temperatures for avocado are 25–30°C during the day and 15–20°C at night. However, extreme temperatures can negatively impact reproductive processes. Daytime temperatures below 17°C or above 33°C, as well as nighttime temperatures below 12°C or above 28°C, can hinder pollen-tube formation and embryo development. For optimal floral development, pollen-tube growth, and embryo formation, the most suitable temperatures are around 20°C during the day and 15–20°C at night.
West Indian races
The west Indian races crop is best suited to tropical regions and thrives in humid, warm climates with monsoon rains. It is highly susceptible to frost, with a minimum tolerable temperature of approximately 1.5°C, below which its growth and survival are compromised.
Mexican races
The Mexican races are semi-tropical plant that demonstrates resilience to temperatures as low as -4 to -5°C without causing damage to its foliage; however, its flowers are vulnerable and can be damaged when the temperature drops below -4°C.
Guatemalan races
This subtropical plant is well-adapted to cool tropical climates but is less tolerant to low temperatures compared to the Mexican race. Its foliage can withstand temperatures as low as -2°C, though its flowers are damaged when the temperature drops below this threshold. Mexican Guatemalan hybrids, such as Fuerte, exhibit a higher capacity for cold tolerance.
Day length
Day length has little significance for the avocado.
Rainfall
Avocado is highly sensitive to water stress, requiring adequate and consistent moisture for healthy growth. It thrives best in regions with moderate annual rainfall ranging from 1200 to 1800 mm. However, in areas with excellent drainage, the crop can tolerate higher rainfall levels, up to 3000 mm, without suffering from waterlogging. Proper distribution of rainfall throughout the growing season is essential to ensure optimal growth and development.
Different cultivars of the crop exhibit unique adaptations to climatic conditions. West Indian cultivars are better suited to regions with high summer rainfall, making them ideal for wetter climates. In contrast, Mexican races are more resilient to water stress and perform well in areas with lower relative humidity (RH). Maintaining an RH of above 50% during flowering and early fruit set is particularly important to support pollen-tube growth and ensure successful fruit development.
Soil requirements
Avocado can be grown in a wide range of soil types, ideally with a pH between 5 and 7, but it is highly susceptible to root rot, making waterlogging critical. Plants may experience dieback if the water table is less than 1 meter, and they have low tolerance to salinity. However, West Indian races show greater salt tolerance compared to Mexican and Guatemalan races.
Botany
Plant type
The avocado is an evergreen tree with a variable shape, ranging from upright to widely spreading, and it can grow to a height of 15 to 18 meters. However, its height can be managed through regular pruning.
Leaves
The avocado tree features dark green leaves that are elliptic, ovate, or lanceolate in shape. These leaves vary in size, typically measuring 10 to 13 cm in width and 20 to 25 cm in length. The leaf margins are smooth and entire, contributing to the tree’s lush and attractive appearance.
Inflorescence
The avocado tree produces branched panicles that can be either axillary or terminal, exhibiting both determinate and indeterminate growth patterns. Indeterminate panicles terminate in a shoot bud, allowing continued growth and development, while determinate panicles conclude in a flower or inflorescence, marking the end of their growth. This branching structure supports the tree’s reproductive processes and contributes to its overall growth habit.
Flower
The flowers are small and pale green or yellowish green, classified as perfect flowers. They feature nine functional stamens, each comprising four pollen chambers. These stamens are arranged with six in an outer circle and three in an inner circle. Non-functional stamens, known as staminoids, are also present. A single pistil, containing one carpel and one ovule, completes the flower’s reproductive structure.
Pollination and Fruit Set
Dichogamy in avocado flowers limits self-pollination and promotes cross-pollination between two complementary groups. Environmental factors such as sun exposure, wind, and temperature fluctuations can influence the timing of floral anthesis, the duration of the open period, and the interval between successive openings. These variations may increase the likelihood of overlap between female and male phases on a single tree, enhancing the chances of self-pollination. Under moderate daytime temperatures of around 25°C and cool nighttime temperatures of approximately 20°C, sufficient overlap between flower types can occur, allowing up to 93% self-pollination. Pollination is facilitated by both insects and wind, which play a crucial role in the reproductive success of avocado trees.
Fruit
Avocado is a one-seeded berry.
Fruit development and maturation
Avocado exhibits typical sigmoid growth, with maturation times varying by cultivar. West Indian and Mexican cultivars typically mature within 150 to 240 days, while Guatemalan cultivars require over 250 days to reach full maturity.
Maturity indices
The skin color of mature avocado fruit varies by cultivar. In green cultivars, such as Fuerte, the skin changes from green to yellow-green as it ripens. In purple cultivars, like Hass, reddish streaks appear during early ripening, transitioning to a reddish, light, or dark purple hue at full maturity. A ripe fruit produces a hollow sound when tapped with fingers. The flesh is greenish-yellow to bright yellow with a creamy texture. The dry matter content ranges from 17% to 25%, increasing to as much as 30% in fully mature fruit.
Harvesting, storage, and ripening
Avocado fruit is harvested manually by cutting or snapping the stem at the base of the fruit. It is typically picked at the ripe stage to ensure optimal quality and flavor.
For storage, temperature requirements vary by cultivar: 5°C for Mexican, 8°C for Guatemalan, and 12.5°C for West Indian varieties. Maintaining a relative humidity of 80-90% can extend the storage period to 3 to 4 weeks.
To ripen avocados, they can be kept at 25°C for 3 to 6 days or treated with 10 to 100 ppm ethylene at 17-20°C for 12 to 72 hours. After such treatment, the fruit will ripen within 3 to 6 days.
