Protogyny

It’s that time of the year, the time when female flowers go stepping out ahead of the bachelors.

I am not talking about debutantes, of course. I’m talking about flowering of the Titan Arum, Amorphophallus titanum. The Huntington has a population of about 40 plants, most of which are mature enough to reproduce. The story of their ecology, growth,and maturation is available through The Huntington website, so I want to take the discussion in a different direction.

Since 1694 botanists have increasingly understood that flowering and cone-bearing plants produce seed through a sexual process, parallel to that of animals, fleshing out the tales of birds and bees with those of flowers and trees. In the case of Amorphophallus, we have to begin with the huge caveat: though people refer to this incredible production as a flower, botanically it’s an “inflorescence” – a flowering stem harboring hundreds of flowers (tiny, simple flowers). That’s the way the Aroids (the Arum family) roll. They pack hundreds of flowers in a tight column-like stem called a spadix.

Many Aroids, such as Anthurium, make flowers on the spadix that are “perfect” – each producing male pollen and female ovaries. Amorphophallus and many other aroids, however, make two kinds of flowers – female (ovary producing) flowers in one zone and male flowers (pollen producing) in another, just above. This “monoecious” habit (separate male and female flowers on the same plant) is reasonably common in the plant world. As with animals, there is pressure to limit in-breeding, which means self-pollination is not the mainline strategy for populations that retain high genetic diversity.

For plants that might self-pollinate, timing could be everything. Many plants have evolved simple strategies that limit self-pollination like the tried and true “Rhythm” method. In Amorphophallus, the female flowers are receptive one day, but the nearly adjacent male flowers open 24 hours later, spewing their sticky strings of pollen when most female flowers (below) are no longer receptive. Because the female flowers mature and are receptive for earlier, than the males, we call this protogyny.

But the consequences of this behavior are yet more complex. Individual plants have to orchestrate incredible timing, with receptivity tied to heat generation. Pollen-bearing flowers have their own warm period the following day. Any flies (the pollinators) that remained in the chamber surrounding the flowers until the following day will pick up pollen as they escape, ready to deliver that pollen to another plant that might be receptive. For this to be a successful strategy, therefore, other plants in the vicinity have to key into the same environmental cues so those plants are also flowering over just a very few days, or all bets are off. Thankfully, flower position and maturation periods are closely spaced, so there is always the chance that a few female flowers could still be receptive when the adjacent pollen-producing flowers open. A bit of insurance against the lack of a mate doesn’t hurt.

Of course, in cultivation (far from the native tropical lowland forest) things are artificial. Lacking a natural population, lacking the right pollinators, the best way to consummate this union ex situ (apart from the natural setting) is with human intervention. Someone has to be the matchmaker, and that person has to have a stock of viable pollen available. With our first flowering of this “stinky plant” many years ago, staff collected nearly-open male flowers and forced them to mature under a warm light – yielding a few grains of good pollen, enough to produce a seed crop. But the great thing is that Amorphophallus pollen can be collected in advance (even years earlier) and stored in the freezer.

But we are out of pollen this year because our stock was used last year. What’s a matchmaker to do? Minimally, this will be the year to collect good pollen for future use. So this plant, nicknamed ‘Scentennial’, will become the stud plant for future seed production.

Protogyny (females flower first) is one strategy. But there are others; in some plants the timing is reversed? Pollen bearing flowers mature first, before the pistils are receptive. It’s a similar strategy, but we give it a different name – Protandry.

Ingolf Lamprecht and Roger Seymour, 2010. “Thermologic investigations of three species of Amorphophallus” Journal of Thermal Analysis and Calorimetry 102(1):127-136. DOI: 10.1007/s10973-010-0891-9

https://www.researchgate.net/publication/243958289_Thermologic_investigations_of_three_species_of_Amorphophallus