Aloe

CAM (Crassulacean Acid Metabolism) respiration is a physiological adaptation that saves water in arid environments. It works by temporally dividing the stages of photosynthesis and carbon metabolism.

Overnight (carbon accumulation phase).

The stomata (microscopic openings on the leaves) open, because there is more moisture and minimal water loss.
The plant absorbs CO₂ from the atmosphere.
The CO₂ is converted to malic acid and stored in the vacuoles of the cells of the aquifer parenchyma.
O2 is released into the atmosphere.

During the day (photosynthetic phase)

The stomata close to prevent water loss through evaporation.
Accumulated malic acid is decomposed, releasing CO₂ internally.
The internal CO₂ is used in photosynthesis (Calvin cycle), allowing the plant to fix carbon without opening the stomata.
Photolysis of water (cleavage of H₂O) produces oxygen (O₂).
Because the stomata are closed, O₂ accumulates temporarily in the internal spaces and can be used in cellular respiration or released slowly.

Recent taxonomic revisions

The use of molecular data (multilocus, nuclear and chloroplast sequencing, and more recently phylogenomic approaches) has revealed that “Aloe” in a broad sense does not constitute a natural group of related species, but gathers several lineages with an independent evolutionary history that have been elevated to the rank of genera distinct in morphology, habit and biogeography.

Aloidendron

Previously referred to as “tree aloes”; woody stems and dichotomous branching (e.g., “quiver tree”); large corymbs or raceme panicles; often iconic species in arid habitats.

Kumara

Only two species: K. plicatilis and K. haemanthifolia. Distinguished by the distichous fan of flattened leaves.

Aloiampelos

Sarmentose or climbing/arbustive plants with elongated stems and leaves distributed along internodes, suitable for rocky substrates and scrub.

Aloe

Prevalence of acaulescent or short-stemmed species; great variability in size and shape; simple or compound inflorescences; flowers often slightly curved. It is the richest grouping, counting more than 500 species.

Gonialoe

Includes former species of Aloe “partridge breast” ( A. variegata group); rosettes often distichous or in multiple rows with flattened leaves and geometric variegation.

Aristaloe

Genus monotypic(A. aristata), small rosette with aristate (filamentous) leaf apices and spotting; morphological transition to forms observed in other related genera(Gasteria, Haworthia, Tulista).

Evolution and radiation

Combined analysis of morphology and molecular data suggests that the arborescent forms(Aloidendron) are older than the other Aloe s.l.; diversification of evolutionary lines is related to climatic gradients and arid niches of South and East Africa, and to later colonizations of Madagascar and the Arabian Peninsula.

Key ecological adaptations

To adapt to the adverse conditions of the arid climates in which they are widespread, Aloes have developed specific adaptations

• Leaf succulence + thick cuticle: reduce transpiration and allow water storage.
• CAM metabolism: allows stomata to open at night.
• Rosettiform architecture/fan: channels rainwater and dew to the base of the plant.
• Different habitats (arboreal, climbing, compact) as adaptive strategies for light, pollinators and dispersal.

Reproductive ecology

Many orange-red tubular-flowered species depend on nectarivorous birds (nectarinias, in English “sunbirds,” and other opportunistic passerines) while others with shorter or clearer flowers are visited by bees; studies show that generalist birds can transfer more pollen than nectarinias(sunbirds) specializing in certain species(A. ferox); there are also species pollinated primarily by bees or with morphological specializations (narrow necks) to select more effective visitors.

Uses of Aloe

Aloe vera (syn. A. barbadensis Miller) is a popular species, but A. ferox, A. arborescens and others are also used to make gels, extracts and cosmetics; polysaccharides(acemannan and others) and anthraquinones (e.g. aloin in latex) are studied for their healing, anti-inflammatory, glycemic control support, and skin care effects, but there is evidence on risks that can be derived from improper oral use of latex, which can have laxative and other adverse health effects.

Conservation

Aloes are threatened in the wild by illegal harvesting of the rarest species, loss of habitat (due to agriculture and urbanization), climate change (e.g., regression of Aloidendron dichotomum populations in portions of the range toward the equator), limited protection of some micro-areas of high endemism, and international trade.

CITES Status

Almost all Aloe species are included in CITES: most in Annex B with the exception of a limited group (about twenty) in Annex A (more protected) while cultivated Aloe vera is excluded from the regime because it is not threatened in the wild. CITES continues to play a primary role in order to prevent the exploitation of natural populations. Responsible purchasing should give priority to plants obtained by artificial propagation in production nurseries.

Comparison of Aloe s.l. with related genera

• Gasteria: lingulate leaves often in two rows (distichous) especially when young, smooth or warty surface; “vest” flowers (gasteriform).
• Haworthia / Haworthiopsis / Tulista: small rosettes, often with translucent “windows” or white tubercles; revision segregated morphologically distinct groups to resolve polyphilia.
• Astroloba: spiral columnar rosettes of rough triangular leaves; flowers more angular, whitish-green.

Growing Aloe Plants

Cultivating Aloe plants varies depending on their specific needs.

General Tips

• Pots: Use pots with drainage holes and provide a generous layer of drainage material (e.g., expanded clay) at the bottom to prevent water stagnation.
• Fertilization: In spring and summer, use a balanced fertilizer for succulent plants every 4-6 weeks.
• Pests: Monitor for mealybugs on the leaves or aphids on the inflorescences.
• Winter cold: Although some species are frost-resistant, it’s always better to protect aloes in environments that don’t drop below 2°C (35.6°F), keeping them perfectly dry.

Rosette Aloe with Soft Leaves

• Exposure: They prefer partial shade. They tolerate hot climates well.
• Watering: Water when the soil is completely dry, about once every 7-10 days. Avoid water stagnation.
• Soil: They prefer well-draining soils, such as those specific for cacti with a generous addition of coarse drainage material.

Rosette Aloe with Very Tough Leaves

• Exposure: They love direct, warm light, requiring several hours of sun per day.
• Watering: Water moderately, reducing in winter. Avoid stagnation that can cause root rot.
• Soil: They prefer a mineral and sandy substrate for good drainage.

Tree-like Aloe

• Exposure: They prefer full sun or partial shade. They tolerate hot climates well.
• Watering: Water moderately, reducing in winter. Avoid stagnation that can cause root rot.
• Soil: Use a well-draining substrate, maintaining a neutral or slightly acidic pH.

Miniature Aloe with Soft Leaves

• Exposure: They require indirect sunlight or light shade. Perfect for bright interiors.
• Watering: Regular watering is recommended but only when the soil feels dry to the touch.
• Soil: Drainage is crucial. They grow well in cactus and succulent mixes.