Baldwin & Schula, 1983) In a typical experiment scientists placed 15 poplar tree

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Baldwin & Schula, 1983) In a typical experiment scientists placed 15 poplar tree seedlings into gas-tight Plexiglas containers in a growth chamber. These were called the "true control group. In another similar chamber together they placed two more groups of 15, one of which was called the "experimental" gr control," they were exposed to a common airflow. Two leaves on the experim leaves in half. This was intended to mimic the action of caterpillars chewing on leaves group and the other "communication nental plants were damaged by tearing two The recarchersfound that the damaged plans rapidly incresed produsction of defensive chemicale in thcir lcaves (phenol and tannins), whidh are known to discourage herbivores. But surprisingly so did the ncighboring plants in the same chamber that were not themselves damaged. This anti-pest resistance happened within 52 hours. This did not occur in the "true controls" in another isolated chamber Questions 1. The scientists speculated that ethylene pas was the distress signal released from the damaged leaves and that chis discouraged further herbivory in the plants nearby. How might this hypothesis be tested 2. What would be the possible selective advantage for a plant to releasing distress chemicals alering ncighboring plants or is this just a byproduct of the injury process 3. How do you hink plants detect airborme chemiale Interestingy, Goldenrod plans can detect fruit By odors and build chemical defenses to discourage them from laying their eggs in the plant stem 4. Why don't plants have full strength defensive chemicals always ready rather than only building them when danger is at hand? "Revolt of the Fungus People" by Clyde Freeman Herreid Pags 5

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Answers

1. If ethylene gas is the diffusible signal that is released from the affected plants, it needs to be communicated to the surrounding plants by air. Hence to support this hypotheses, the airflow between the experimental group and the communication control needs to be aborted. If the diffusible ethylene gas is the distress signal, then the communication control group will not produce defense chemicals like the experimental group. Rather they will behave like the true controls in the isolated chamber.

2. The volatile organic compounds VOCs released by the plant are usually secondary metabolites which have a primary function at the plant-pest/herbivore interface. They act as signals for plant-plant communication. When a plant is attacked by a herbivore or pest, it identifies the injury and emits distress signal like ethylene, which is a diffusible signal. Therefore other plants in its vicinity receives an alert for the inevitable pest or herbivore attack, and primes itself by producing tannins or other defense chemicals in their leaves in large quantities such that they are enough to sicken or kill the herbivore. So these distress signals alarm the other plants in the vicinity, but the plant releasing it particularly surrenders itself to the pest/herbivore attack, giving no selective advantage to the plant. Rather it is a byproduct of the injury process.

3. The air-borne messenger molecules easily diffuse in through the stomata, pores on the leaf surface. Then the distress signal, for example ethylene, being gaseous diffuse across the semi-permeable membrane and are perceived by receptors inside the cell, localized in the endoplasmic reticulum membrane.

4. Defensive chemicals like phenol or tannin act as herbivore deterrent due to their acid taste and property of precipitating proteins. But keeping the plants primed with full strength defensive chemicals in absence of danger may be disadvantageous to the plant concerned as well as its neighboring plants. Firstly, the plants will not invest energy in producing such compounds when it does not sense any attack. In the growth-defense trade-off in plants, the system in absence of any danger like pest or herbivore attack will not trigger any defense signalling pathway, but rather reorient its resources towards growth, development or reproduction. Secondly, phenolic compounds reduce soil nutrient availability by stimulating soil respiration and the immobilization of nitrogen in the microbial biomass. Tannins bind to proteins like microbial enzymes or enzyme substrates and nitrogen-rich organic matter from soil, thereby lowers gross soil-nitrogen mineralization and nitrification rates. Many phenolic-rich plants may therefore negatively modify neighboring plant growth and nutrient accessibility by limiting nitrogen supply.

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