Reading Passage 2
You should spend about 20 minutes on Questions 14-27, which are based on Reading Passage 2 below.
Mention the words “chemical warfare” or “deployed armies” in any conversation, and your interlocutor might immediately assume you’re talking about wars between humans. In reality, however, there are other kinds of wars out there where these techniques are employed far more frequently and in a far more intricate manner: those waged in the plant kingdom.
We might not normally think of plants this way, but much like humans and animals, they too have to fight for survival on a daily basis. Nutrients, light and water are the three things any plant needs in order to grow; unfortunately, none of these is ample in supply, which means that the competition between plants can grow fierce. Some plants and trees are at an architectural advantage: taller trees have greater access to natural light, while plants with deeper roots have the ability to absorb more water and nutrients. Others, though, manage to defend their territory through “allelopathy”, or chemical warfare.
So how does this chemical warfare work exactly? As Dr Robin Andrews explains, plants convert the nutrients they absorb from the ground to energy with the aid of a type of organic compound known as metabolites. These metabolites can be divided into two categories: primary and secondary. Primary metabolites are what allows a plant to live, playing a direct role in its growth and development, and are thus present in every plant. Secondary metabolites, on the other hand, can vary from plant to plant and often play the role of a defence mechanism against neighbouring competitors.
Out of these secondary metabolites, there are two that are incredibly interesting: DIBOA and DIMBOA. These two cyclic hydroxamic acids were at the forefront of a study conducted by Sascha Venturelli and colleagues in 2015, which found that once they are released into the soil by the plants that produce them, they degenerate into toxic substances that have the power to inhibit growth in nearby plants once they soak them up. As Dr Claude Becker notes, “the phenomenon itself has been known for years”, but we now finally understand the “molecular mechanism” behind it – and its supreme intricacy would put to shame any chemical bombs created by humans.
But plants do not just fight wars against other plants; chemical warfare also comes into play in their defence against herbivores. As Brent Mortensen of Iowa State University describes, plants “actively resist” attacks made by herbivores through qualitative and quantitative chemical defences. What’s the difference? Qualitative defences can be lethal even in small doses, and are often employed to protect “young” or “tender leaves or seeds”. They can also be recycled when no longer necessary. Quantitative defences, in contrast, are only effective “in larger doses”, but unlike qualitative defences, can protect the plant against all herbivores. Quantitative defences are also not as immediately lethal, as they usually lead to indigestion, pain, irritation of the mouth and throat, and inflammation or swelling in the skin.
And what about the “deployed armies” I mentioned before? Well, chemical attacks are not the only way plants elect to defend themselves against herbivores. Some plants, such as the African acacia, also recruit armies to assist them in their war. As Angela White of the University of Sheffield explains, the acacia tree has “hollowed-out structures” which invite ant colonies to build a home in them by providing not just shelter, but also food in the form of a special nectar. In return, ants protect them against herbivores – and this includes not just the small ones like bugs, but also the ones as big as giraffes.
At this point, of course, you might be wondering what all this has to do with you. The territorial nature of plants might be fascinating in its own right, but what is its application in real life? Well, Dr Venturelli of the 2015 study mentioned before has an answer for you: apparently, certain allelochemicals – the aforementioned chemical compounds that are responsible for stunting growth in plants – have been found to have an effect on human cancer cells, too. According to Michael Bitzer and Ulrich Lauer of the same study, “clinical trials at the University Clinics Tubingen currently assess the efficacy of these plant toxins in cancer patients”. This means that comprehending the way plants defend themselves against the enemies in their environment might not just be of interest to plant biologists alone, but to medical researchers as well.
Complete the sentences below.
Choose NO MORE THAN THREE WORDS from Reading Passage 2 for each answer.
14 Plants are very similar to ________ as they also struggle to stay alive every day.
15 The height of a tree or plant can affect how much ________ it receives.
16 Chemical warfare in plants also goes by the name of ________.
17 Water and nutrients are both taken from the soil, and the latter is later turned into ________.
18 Secondary metabolites are an ________ that functions as a defence mechanism for plants.
19 DIBOA and DIMBOA are two types of secondary metabolites that can ________ once absorbed by a plant.
20 The 2015 study by Sascha Venturelli and colleagues examined the ________ of chemical warfare in plants.
Plant Defences Against Herbivores
– can kill a herbivore in 21 _______
– can be recycled when no longer necessary
– only works in larger doses
– effective against 22 _______
– causes a variety of symptoms, none 23 _______
– uses the help of ant colonies that reside in its 24 _______
– ants can protect it against herbivores of all sizes, even 25 _______
Do the following statements agree with the information given in Reading Passage 2?
TRUE if the statement is true according to the passage
FALSE if the statement is false according to the passage
NOT GIVEN if the information is not given in the passage
26 Allelochemicals are secondary metabolites.
27 Plant biologists and medical researchers are currently cooperating to assess the efficacy of plant toxins in preventing the growth of cancer cells.