symbiosis – Wandering through Wadis

The distinctive yellow flower stalks and the pearl-like fruit of this desert shrub make it easy to recognize and remember. The pearl plant (Ochradenus baccatus) is quite common in the region and are spotted often on my wanders in the wadis near Dahab. In the springtime, they are buzzing with flying insects – bees, wasps, flies, hover flies, beetles, and more I’m sure. This shrub has been reported as one of the most important food sources for many animal species.

There’s one species in particular that the plant has a special relationship with – the Egyptian spiny mouse (Acomys cahirinus). The fruit – a fleshy, juicy berry – is attractive to desert animals. But when an enzyme in the flesh of the berries combines with what’s inside the seed, it creates a toxic “mustard oil bomb”, deterring most animals from munching on this fruit (and destroying the seeds in the process). The spiny mouse has adapted to this, however. They will collect fruits and bring them to a different, rocky area, one that is safer for them. There, the mice chew and eat the flesh, careful not to bite the seed which they then spit out, avoiding any nastiness and helping to disperse the plant’s seeds. One study suggests these safer places the mice choose are actually “the best places for young O. baccatus plants to germinate, grow and survive.” How’s that for some symbiotic behavior!

The Bedouin of Sinai have also found benefits of the pearl plant, using it in traditional medicine to cure joint pain. A bowl of water in which the leaves have been boiled is placed in a hole in the ground above which a makeshift tent is constructed. The patient then lies beneath its cover for 24 hours. Pearl plant is also used to cure aches and pains in a camel’s body except instead of boiling the leaves, the plant is placed on embers in a hole. In Saudi Arabia, the plant is used to lower blood cholesterol and to counteract malaria.

Recently, a friend and fellow plant-lover asked me if this species had separate male and female plants as she had noticed that some plants were full of berries, while others only had a few. In dioecious plants, only the plants that grow female flowers produce fruit. Date palms are a good example. It turns out, though, that the pearl plant is gynodioecious, meaning that some plants have only female flowers and some plants are bisexual, having both male and female flowers. This explains the phenomenon my friend noticed – two plants, side by side, both in full bloom but only one seeming to fruit fully.

I’ve always liked this plant’s Latin name, Ochradenus baccatus. Ochradenus comes from the Greek for “pale yellow” or “yellow ochre”, and baccatus means “adorned with berries”. It is also known as taily weed and shrubby or sweet mignonette in English and is called gurdhi by the Bedouin in South Sinai.

References:

Bailey, C., & Danin, A. (1981). Bedouin plant utilization in the Sinai and the Negev. Economic Botany, 35(2), 145–162.

K.C. Burns. Seed Dispersal: The Blind Bomb Maker. Current Biology, Volume 22 (Issue 13), 2012, Pages R535-R537. https://doi.org/10.1016/j.cub.2012.05.014.

If you follow the Wandering through Wadis Facebook page, you might recall the photographs of orchids that I shared a few weeks ago. I had no idea that there were orchids growing in the desert, but after my friend sent me photos of the blooms she had seen, I obviously had no choice but to go see them for myself.

I had identified the orchids in my friend’s photos as Scarce (or Eastern) Marsh Helliborine (Epipactis veratrifolia) but, admittedly, I knew nothing about orchids. So I’ve been reading up on them. And I’ve learned a lot of fascinating things about orchids in general, but also about these rare beauties that are native to Sinai. (They are not found in mainland Egypt.)

Most orchids (more than 99% of all species) are epiphytic and use their roots to attach themselves to and grow on trees. The Scarce Marsh Helleborine, however, is a terrestrial, or ground, orchid and grows its roots firmly in soil.

This helleborine is a perennial herb and grows, from a fleshy rhizome, to be between 25 – 150 cm tall. The leaves are ovate (egg-shaped) and pointed at both ends. They grow along the stem and can be 8 – 25 cm long. The inflorescence, or cluster of flowers, grows atop an erect stem. The flowers are fairly open and are green to yellowish-green in color with purplish or reddish radial stripes. The lip, or bottom middle petal, is tipped in white. The upper part of the stems, bracts, ovaries, and sepals are covered in short, fine hairs. In Dahab, the orchids were found growing among native grasses in a wet area.

Like all orchids, this helleborine is dependent on a mycorrhizal symbiosis, a mutually beneficial relationship between a plant and a fungus, to complete its life cycle. The plant’s fruit capsule is full of microscopic seeds (in some species, over a million), but these seeds all lack endosperm. Endosperm is the tissue usually found inside seeds that provides nutrition to the plant as it sprouts. Because an orchid’s seeds don’t have this inborn nutrition, they rely on fungi to provide them with the nutrients they need to germinate. The chance of germination is so small that only a minute fraction of the released seeds grow into adult plants.

But before a plant can even produce any of these seeds, it must first be pollinated. And to help ensure that, the Scarce Marsh Helleborine employs a trick, a special mimicry, to lure pollinating hoverflies to its flowers. The flowers emit three chemical substances that are usually released as alarm pheromones among aphids. Aphids are the preferred diet of hoverfly larvae. So female hoverflies smell these chemicals, interpret this to mean that aphids are nearby, and proceed to lay their eggs near the source of the scent – the flowers. The hoverflies are rewarded with a small sip of nectar, but their larvae are doomed to starve because, when they hatch, there will be no aphids around to consume. (This is a strange contradiction from an evolutionary perspective because since the larvae die, the number of potential pollinators decreases.) The orchids are mimicking the aphids, taking advantage of the female hoverflies and deceiving them into pollinating the flowers.