Tropicals & Tender Perennials: Colocasia esculenta ID, 0 by ExoticRainforest

ExoticRainforest wrote:
This is from an article primarily written by aroid expert Julius Boos on aroid pollination for my website to which I added additional information contributed by a number of aroid botanists.

The inflorescences of both Colocasia and Alocasia are made up of imperfect unisexual flowers. Philodendron, Caladium, Amorphophallus and Xanthosoma species are also representatives of this group.  All produce inflorescences in which the spathe consists of two sections, both normally wrapped around the spadix with a constriction separating the two sections. The upper portion of the spathe is called the limb or blade while the lower is a convolute tube.  On the spadix the imperfect male and female flowers occur in separate regions or zones. Normally the zone of female flowers occurs at the very bottom of the spadix within the lowest portion of the spathe. The sterile flowers produce a pheromone (odor) which attracts insect pollinators in exactly the same fashion as bisexual species which have the female and male flowers growing together and known as perfect flowers. Normally, above these sterile flowers and within the upper spathes blade or limb, occurs the zone of fertile male flowers. The female floral chamber of some species is well hidden and often constricted making it difficult to see the female flowers.

The floral chamber of both Colocasia and Alocasia species can be tightly constricted just beneath the spathe. The spathe of these species is divided into a convoluted thicker lower region which hides the female flowers. The spathe is observed to have a constriction with the female zone being roundish to globe shaped (globose to ovoid). When ready to be pollinated the restriction loosens to provide the pollinating insects access to the flower pistils. Once pollinated the zone remains on the peduncle and opens to become the fruit producing region. Once fruit begins to develop the inflorescence is then known as an infructescence.
 
In some genera the uppermost section of the spadix also consists of a zone of sterile male flowers. In this group, female anthesis occurs just at or even before the spathe begins to show signs of opening. In Caladium species and perhaps Xanthosoma species it has been seen the peak period of female anthesis and receptivity actually occurs the day before signs of opening can be observed.
 
The beetles or insects that serve as pollinators in these genera are likely attracted by subtle odors propelled by natural heat produced within the aroid that is produced by the as yet unopened blooms. With their hairy bodies and legs covered with pollen from a previously visited bloom already at male anthesis these beetles actually force their bodies through the slightly relaxed spathe opening and down into the floral chamber containing the receptive female flowers. Pollination in nature is done by a species of insect and I just sent a note to botanist Pete Boyce in Malaysia asking if he knows which insect pollinates Colocasia esculenta.  They only stay inside the inflorescence long enough to pollinate the spadix, eat some of the pollen, breed and gather more pollen. The insects use the warmth of the spathe as a place to stay warm during the night as well as source of food since they eat the pollen and some of the flowers as well as mate within the spathe due to increased metabolism as it begins to close.
 
The inflorescence continues to open during female anthesis for only one day then on the second day the female flowers are no longer receptive and the upper male flowers open while the spathe is fully open and produce large amounts of pollen to be picked up on the bodies and legs of the departing beetles. Within unisexual genera male anthesis is usually complete in 24 to 36 hours, sometimes less. Male anthesis rarely lasts more than a single night. This serves to assure the newly collected pollen is transported to a newly opening inflorescence while it is fresh and when the second inflorescence is first approaching female anthesis.

French aroid pollination researcher Marc Gibernau (GHEE-ber-no) added this explanation, "one reason for the beetles to leave the inflorescence which is a great place to stay is that the spathes close and force the beetles up along the spadix. Once above the male zone, they will eat some of the pollen. If they don't go up, they can finish by crashing the spathe against the spadix, I observed it once in French Guiana. So the plants "decide" when the pollinators arrive and depart." Of additional interest is the fact Marc now is close to scientifically proving the spadix is capable of attracting the pollinator beetles by producing a "glow" visible only to the insect as a result of infrared heat detected by the beetle's antennae rather than visually.

The majority of aroids require a very specific insect species to do the work of pollination that insect is not present it is unlikely the plant will be naturally pollinated unless the species is capable of self pollination.
 
The insects are generally drawn to the inflorescence in the late day or at dusk and are attracted by a combination of pheromones (scent) and a source of food and shelter which is composed at least in part of an oil produced on the staminate flowers containing lipids along with the enclosure of the spathe. Shelter may play a part since the male often brings along his mate in to breed at the same time. Some species have sweet smelling pheromones while others show no noticeable aroma. This aroma is produced by the sterile male flowers on the inflorescence which are attempting to entice the pollinator and to the male of that insect species the scent may be similar to the same pheromone that attracts him to a mate when she is ready to be impregnated. This point is not factually certain. 

The insects arrive during the night due to thermogenesis. Quite simply, the spadix can warm enough to be noticeable to the touch and for the insects that may be tired from traveling long distances to perform their required tasks this additional source of heat in the rain forest creates a micro-climate and may actually increase their metabolism and encourage them to explore all portions of the spathe and spadix. A micro-climatic zone of warmth is now being generated within the spathe that offers both comfort and protection along with food. This feature alone may increase the chance of self pollination within the specimen, but another may inhibit the same.
 
The thermogenesis produced by the inflorescence during anthesis is simply a natural heat produced by many living beings and appears to stimulate the beetles into a period of copulation. Of major interest, even though the effects of thermogenesis have been observed for over 200 years, not until relatively recently did anyone know the cause. So what is the chemical cause? Salicylic acid, the same compound used to manufacture aspirin! The salicylic acid begins not only the heating process but also the production of the pheromones (scent). This unique process is not limited to Araceae (aroids) but is also found in other plant genera. Of interest, salicylic acid may also help to prevent self pollination which is an interesting contradiction in and of itself. The thermogenesis (heat birth or heat production) caused by the salicylic acid appears to be one of the stimulators to cause the beetles to be active and as a result to both feed and copulate.

It is known the rate of thermogenesis (heat rise) is sometimes dramatic, however thermogenesis does not produce a consistent temperature since the highest temperatures appear to last only 20 to 40 minutes. In fact, it may be the visit of the beetles that contributes to the effect botanists know as thermogenesis. The presence of beetles appears to increase the temperature produced by the event and the temperature increase appears to increase the amount of pheromone (perfume) being exuded by the tiny flowers.

Researchers have noted the highest temperatures appear to occur during the period when the highest number of beetles or insects are present, however the exact role of thermogenesis is still not well understood. The pheromones (scent or perfume) are not always detectable to the human nose. Some species have noticeably sweet scents in the early evening while some exude no noticeable smell on the first day of sexual anthesis. On other species the pheromone is noticeable only during specific hours of the day, normally in the evening. A more recent train of thought includes the likelihood infrared heat acts as an increaser to the production and distribution of the pheromones.  Marc suspects the beetles are also attracted to the spadix in the darkness of the forest due to the infrared heat produced during sexual anthesis. individually there is a significant increase in temperature above the ambient temperature of the rain forest at night once the inflorescence reaches anthesis. The average Philodendron temperature increase is approximately 12 degrees Celsius (a 21.6 degree Fahrenheit increase) above ambient but a few plant species can increase in temperature by as much as 20 degrees C (36 degrees F) above ambient. The heat can be so intense it can sometimes be felt on the palm of an opened hand held in front of the spadix increases in temperature 14 degrees C or 25.2 degrees F above the surrounding rain forest ambient temperature.

The photo included is of the "Black Magic" form of Colocasia esculenta. Not all forms of this variable species produce identical inflorescences.


This message was edited Apr 30, 2010 1:08 PM

This message was edited Apr 30, 2010 6:24 PM