About Opuntia engelmannii var. lindheimeri (Engelm.) B.D.Parfitt & Pinkava
Opuntia engelmannii var. lindheimeri is a variety of prickly pear cactus that typically grows as a shrubby plant, forming dense clumps up to 3.5 metres (11 ft) high, and usually has no visible trunk. Its flattened stem segments, called pads or cladodes, are most often green (rarely blue-green), shaped obovate to round, and measure around 15–30 cm long and 12–20 cm wide. In the Sonoran Desert, terminal pads of this cactus are oriented mostly east-west, an arrangement that maximizes solar radiation absorption during summer rains. Cladodes form the main stem of the plant and act as its primary photosynthetic structure. O. engelmannii most commonly grows at high altitude desert, where lower atmospheric pressure increases water stress and can make gas exchange challenging. This has led the species to rely on CAM photosynthesis, which enables more efficient gas exchange and reduces water loss during the process. Its glochids (small, barbed bristles) are yellow when young and turn brown as they age. The larger cactus spines are non-vascularized, modified leaves that grow from areoles, and usually occur in groups. Spine characteristics are extremely variable: there can be anywhere from 1 to 8 spines per areole, and spines are often absent from lower areoles. Spines are yellow to white, slightly flattened, and 1–6 cm long; they harden from the tip downward and develop a waxy cuticle. Spines do not perform photosynthesis, and their primary function is defense. However, spines also support asexual reproduction: they can catch on animal fur, and when the animal struggles to free itself, it may carry away a detached cladode that can later fall off and grow into a new individual plant. All members of the Opuntia genus grow both tap roots and adventitious roots, and some tap roots act as water storage structures. Deeper roots anchor the plant and absorb water from deeper groundwater stores, while shallow roots spread over a wide area to absorb rainfall quickly. A notable trait of Opuntia species is that both root types can develop from areoles on detached cladodes. This is particularly useful for propagation, as separated cladodes can grow new roots and new shoots from their areoles to form an independent new plant. Opuntia species are commonly colonized by arbuscular mycorrhizal fungi, which colonize the plants more heavily in summer than in winter, and when water is abundant. These mycorrhizal fungi help reduce drought stress for the cactus. The flowers of O. engelmannii are yellow, occasionally reddish, and measure 6–9 cm in both diameter and length. The flowers are hermaphroditic with inferior ovaries, and their stamens are thigmotropically sensitive. Flowering occurs in April and May, with individual blooms lasting 1–2 days each. Single-day blooms open around 11 AM and stay open for 6 hours. Two-day blooms open around 10 AM and remain open until 4:30 PM on the first day, then open again from 8:30 AM to 2:00 PM on the second day, resulting in an average total open time of 12 hours per bloom. Flowers produce nectar to attract pollinators, and O. engelmannii is known to produce more nectar than other species in the Opuntia genus. Opuntia species most often undergo cross-pollination, but are generally also self-compatible. Cross-breeding between different species within the Opuntia genus has also been observed. Known pollinators include solitary bees and sap beetles. Most pollinators of O. engelmannii are polylectic, meaning they pollinate a range of unrelated plant species, but several specialized pollinators have adapted to only feed on Opuntia. These specialists include members of the families Anthophorini (previously classified as the family Anthophoridae), Andrenidae, and Megachilidae, which are mostly solitary bees. Large and medium-sized bees are considered the most effective for pollination between separate plants. Beetles and smaller bees usually act as "pollen thieves", as they often collect pollen without successfully pollinating the flowers. The fleshy fruits of O. engelmannii are purple, measure 3.5 to 9 cm long and 2–4 cm wide, and are spineless and hairless. They are typically ovate-elongate or barrel-shaped, with a sweet flavor that is consumed by many animals, including humans. Opuntia seeds are ovoid or lens-shaped, averaging 0.45 cm long, 0.35 cm wide, and 0.16 cm thick. The seed funiculus is thick and white, and the embryos are typically curved. Fruits of O. engelmannii usually contain an average of 144 to 172 seeds each. Seeds can remain viable in soil for at least 19 months before germination. Within its native North American range, O. engelmannii seeds are most often dispersed by coyotes. O. engelmannii is native to subtropical regions of North America. It prefers hot, dry conditions, but is more tolerant of frost than other Opuntia species. Its native range extends from California to Louisiana in the United States, and from the Mexican states of Sonora and Chihuahua to the Tamaulipan matorral in northern and central Tamaulipas. While it is occasionally found in the Mojave Desert, it is mostly replaced there by Opuntia basilaris, which does not require summer rain. The species has become naturalized in southern and eastern Africa, including Loisaba in Kenya, where it is classified as invasive. It was also recorded in Spain in the early 2000s, where it remains invasive. The fruits of O. engelmannii were a dependable summer food source for Native American tribes. The Tohono O'odham people of the Sonoran Desert specifically classified the fruits by their color, ripening time, and storage quality. Today, O. engelmannii is cultivated as an ornamental plant for drought-tolerant gardens, container plantings, and natural landscaping projects. It has also been used as a living hedge and as cattle fodder. These uses, combined with its ability to survive drought, led to its intentional import to multiple locations across Africa.
