Pitcher plants

PITCHER plants (Nepenthaceae) have been well covered in these pages.

Books have been written about them, the idea that they serve as receptacles for urine from rodents has been discussed and discarded, and hundreds if not thousands of pictures have been taken.

We know the plants have sides that create a slippery surface leading insects to tumble down into the liquid.

Insects are attracted by the sugary nectar that is produced by glands at the edge of the pitchers (peristome).

Ultraviolet and visible light waves of the colour of the pitcher plants also attract insects.

If insects slip in, they cannot escape as the slippery waxy zone is between the zone of attraction and the digestive zone.

However, aerial pitcher plants, which prey primarily on flying insects, often lack a waxy zone.

It has been suggested that the shape of the pitcher prevents the escape of the prey.

When they tumble in, they drown and are digested by the acidic detergent-like digestive fluids.

Our next problem is to find out what micro-organisms live in the liquid that drowns the insects.

The fluid is highly acidic and could create a micro-environment for certain bacteria.

It would, after all, be considered to be an extreme habitat.

Filter feeding mosquito larvae are also able to survive in this acidic environment.

The first studies began with Dr JS Hepburn in 1918. He performed experiments on unopened, developing, and opened pitchers that had not caught any prey and regular open insect catching plants.

All samples were incubated in sterile conditions at 37 degrees Centigrade.

He reported no bacterial growth for the unopened plants and growth for all the others.

He concluded that the liquid was sterile and did not have any contact with the outside world until the pitchers were partially or fully open, allowing bacteria to enter.

The results were retested in 1993, 2007, 2010, 2011 and 2012 and they concluded that indeed the unopened liquid part of the plant is sterile and holds no micro-organisms.

However, all of these experiments were conducted in a temperate zone under strict laboratory conditions and did not reflect the habitat of the tropical rainforest in the wild.

The one that did achieve growth in 1998 thought it was a mistake.

For this experiment, three separate pitcher plants were selected in Selangor. Of these three, five separate plants were haphazardly selected.

For the opened pitcher plants, the liquid was poured into a sterile tube.

For the unopened ones, a sterile small needle was inserted in the base of the plant and the liquid was extracted and placed into a sterile tube. All samples were transported on ice to the lab.

After DNA analysis, the researchers did find bacteria growing in the unopened plants – an astounding discovery.

So why did the rest of the researchers get it so wrong? Possibly specialised bacteria, found only in the tropics, made themselves available to the unopened plant.

A second reason could be the movement of plants  from the tropical to the temperate zone could have altered the flora and fauna of the unopened plant.

A third reason is the greenhouses in temperate zones are kept to minimise bacterial growth.

The next question is how does bacteria get into the sealed unopened pitcher? It is speculated by the authors that there are microscopic openings between the lid and the chamber during development.

It is also surmised that secreatory tissues (the tissues that secrete the fluid) that line the chamber could possibly play a role.

The authors concede that future studies are required to test the two hypotheses.

The authors further proposed that there are two types of bacteria. One type is a native type that colonises the plant while still in development.

The second type involves those that are carried on the bodies of the arthropods that land in the chamber and are digested.

The first type is possibly transferred from one developing plant to another.

This study has produced three major findings.

First , the pitcher plants do have microorganisms living in the unopened fluid of the plant.

Secondly, once the plant opens, the original bacteria is replaced by the micro-organisms introduced by the critters that fall in for digestion.

And finally, bacterial communities vary by the particular species of the plant.

Adapted from ‘Bacterial communities associated with the pitcher fluids of three Nepenthes (Nepenthaceae) pitcher plant species growing in the wild’ by Lee Yiung Chou, Charles M Clarke, and Gary A Dykes; Springer-Verlag Berlin Heidelberg 2014.

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