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# Plant injury resulting from high light intensity is due not to the

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Plant injury resulting from high light intensity is due not to the  [#permalink]

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23 Nov 2019, 04:52
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New Project RC Butler 2019 - Practice 2 RC Passages Everyday
Passage # 469, Date: 23-Nov-2019
This post is a part of New Project RC Butler 2019. Click here for Details

Plant injury resulting from high light intensity is due not to the light per se but to an excess of light energy over that utilized by photosynthesis. When light reaching the leaves is not used for photosynthesis, the excess energy triggers production of free radicals that can damage cells (oxidative damage). This often occurs when light intensity is high but photosynthesis is inhibited due to stress from temperature extremes, drought, or excessive soil water. When light intensity is at a low level where photosynthesis and respiration reach equilibrium and the net carbon gain is zero, no plant growth will occur. This light level is the light compensation point (LCP). Leaves exposed to light levels below the LCP for an extended period of time will eventually senesce. Both LSP and LCP vary among turfgrass species and with temperature and CO2 concentration.

Under high irradiance, warm-season grasses maintain a higher rate of photosynthesis than cool-season grasses. However, cool-season grasses have a lower LCP and exhibit higher photosynthetic rates under low light levels compared to warm-season grasses. Photosynthetic rates of both warm-season and cool season grasses exhibit a diurnal pattern on clear, sunny days, increasing from sunrise, reaching a maximum around noon, and then decreasing to the lowest levels by sunset.

Photosynthesis is affected by light duration because it occurs only during daylight. Increasing light duration may not increase the rate of carbon fixation, but the total amount of carbon fixed by photosynthesis will increase due to increased light exposure. Sunlight has all the colors of visible light and is composed of different wavelengths. Not all wavelengths are equally effective in driving photosynthesis, however. Most photosynthetic activity is stimulated by blue and red wavelengths — chlorophylls absorb blue and red light and carotenoids absorb blue light. Green light is reflected, thus giving plants their green color. Green yellow and far red are transmitted through the leaf.

This passage is excerpted from Applied Turfgrass Science and Physiology, by Jack Fry and Bingru Huang (Wiley Publishing, 2004).

Spoiler: :: OA
A

1. The authors of the passage are primarily concerned with

(A) discussing the impacts of light energy and photosynthesis on warm-season and cool-season grasses.
(B) arguing in favor of warm-season grasses, which are less prone to oxidative damage than cool-season grasses.
(C) exploring the important role of photosynthesis in sustaining turfgrass production.
(D) comparing different kinds of turfgrasses according to their responses to various levels of light energy.
(E) clarifying the scientific details of recent research into the photosynthesis of turfgrass.

Spoiler: :: OA
D

2. In the context of the passage, which of the following is the best definition for the word senesce (used in the next-to-last sentence of the first paragraph)?

(A) Grow faster
(B) Grow slower
(C) Turn darker green
(D) Die back
(E) Evolve

Spoiler: :: OA
B

3. According to the passage, which of the following is an important difference between warm-season and cool-season grasses?

(A) Cool-season grasses can better withstand higher light intensities such as those found nearer the equator, while warm-season grasses are better suited to northern climates.
(B) Warm-season grasses can handle the higher light levels of summer, while cool-season grasses can grow during the lower light conditions of winter.
(C) Most of the photosynthesis in warmseason grasses takes place during the day, while cool-season grasses usually photosynthesize at night.
(D) Warm season grasses use only the blue and red spectrums of light for photosynthesis, while reflecting harmful green light.
(E) Excess light reaching cool-season grasses can be responsible for damage to the plant’s cells, while warm-season grasses are unharmed.

Spoiler: :: OA
E

4. Which of the following can be inferred from the discussion on oxidative damage (in the first paragraph)?

(A) Oxidative damage most frequently occurs about one hour after sunrise and one hour before sunset.
(B) Homeowners should water their lawns as often as possible because damage to grass is caused by drought and not simply by light intensity.
(C) Oxidative damage to grass occurs when light reaching the leaves is not used for photosynthesis and, therefore, forms carbon fixation.
(D) Damage to grass occurs because of the high intensity of light and homeowners can do nothing to preserve their lawns.
(E) Both overwatering and underwatering a lawn can inhibit photosynthesis and damage grass.

Spoiler: :: OA
A

5. According to the passage, each of the following is true of turfgrass photosynthesis EXCEPT

(A) Its rate depends only on the amount of light energy.
(B) It occurs only during daylight.
(C) It is stimulated by blue and red wavelengths of light.
(D) It is inhibited by temperature extremes.
(E) It peaks around noon on clear, sunny days.

Source: GMAT For Dummies
Difficulty Level: 650

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Re: Plant injury resulting from high light intensity is due not to the  [#permalink]

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23 Nov 2019, 21:33
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can someone explain why question No. 4 answer is not C. It clearly stated directly in the passage oxidative damage occur when light is not reaching the photosynthesis.
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Re: Plant injury resulting from high light intensity is due not to the  [#permalink]

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23 Nov 2019, 23:54
sourabhbabel94 wrote:
can someone explain why question No. 4 answer is not C. It clearly stated directly in the passage oxidative damage occur when light is not reaching the photosynthesis.

Official Explanation

4. Which of the following can be inferred from the discussion on oxidative damage (in the first paragraph)?

Explanation

The question asks you to make an inference regarding the discussion of oxidative damage. In the first paragraph, you find out that oxidative damage occurs not only because of high light intensity but also because more light energy arrives than photosynthesis can use. Anything that hinders photosynthesis can contribute to oxidative damage. Choose an answer that you can logically deduce from the information in the passage without making wild assumptions.

Choice A is incorrect because light intensity is greatest at noon, not at sunrise and sunset. And because overwatering can impede photosynthesis and damage grass, B is out.

Eliminate C because oxidative damage results from the formation of free radicals, not carbon fixation.

Choice D is wrong because light intensity alone doesn’t damage grass; temperature change, drought, and excessive water can also inhibit photosynthesis. Because the passage lists these preventable impediments to photosynthesis, you can logically reason that homeowners can do something to prevent oxidative damage to their lawns (like watering them), which makes E the best answer.

Hope it helps
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Re: Plant injury resulting from high light intensity is due not to the  [#permalink]

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25 Nov 2019, 10:11
+1 Kudos to posts containing answer explanation of all questions
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Re: Plant injury resulting from high light intensity is due not to the  [#permalink]

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25 Nov 2019, 12:27
can anyone explain 5th ques ?
why is it A when in 2nd para it's given that photosynthesis rate is affected by light levels?
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Re: Plant injury resulting from high light intensity is due not to the  [#permalink]

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28 Nov 2019, 23:28
Naman29 wrote:
can anyone explain 5th ques ?
why is it A when in 2nd para it's given that photosynthesis rate is affected by light levels?

Official Explanation

5. According to the passage, each of the following is true of turfgrass photosynthesis EXCEPT

Difficulty Level: Hard

Explanation

To answer this exception question, simply return to the text and eliminate the answer choices that fit the description. In this case, eliminate the choices that are mentioned in the passage as being true of photosynthesis. You can locate the information in B and C in the third paragraph, so eliminate them. Choice D comes from the first paragraph, and E from the second paragraph, so eliminate those choices as well. You’re left with A, which isn’t found in the passage and actually contradicts information in the passage.

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Re: Plant injury resulting from high light intensity is due not to the   [#permalink] 28 Nov 2019, 23:28