Atmospheric CO2 has been increasing rapidly since the late 1800s, largely due to human activities. Recall that CO2 enters leaves through stomata and can then be used for photosynthesis. However, transpiration occurs as a result of water evaporating through stomata. How have plants responded to elevated CO2 levels? The amount of water that evaporates from stomata over a period of time is referred to as stomatal conductance, which is determined largely by the number of stomata in a given area of leaf surface. Researchers obtained specimens from preserved collections and measured stomatal conductance in leaves from oak trees and pine trees that grew at various times under different CO2 levels. The data are shown in the following graph. In general, is the maximum stomatal conductance rate in plants more or less than it was a century ago?

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Welcome back. Let's look at our next question, which of the following statements about drought is false. So remembering that we're looking for a false statement. Let's through, through our answer choices. Choice A says a period of dry weather longer than normal is called drought. That is true. That's the definition of drought. So it's not our answer since we're looking for a false statement. Choice B says deforestation is a major factor in increasing drought worldwide. Well, this isn't autumn, uh isn't right off the bat an obvious answer. Um We understand that deforestation causes the runoff of water um from an area. So it makes the area drier, but drought is caused by um a lack of rainfall, dry weather. How can deforestation affect the amount of rainfall? Well, the answer is that forests store moisture in quite a number of different ways. Um When rainwater falls, that water is absorbed by the soil, that soil absorbs more water when it has tree roots, um penetrating it, allowing more water to come down. If you imagine soil without plants, it became very compact and hard and the water is more likely to run off rather than soak through. So the water is absorbed into the soil. Water is absorbed by tree roots and water droplets themselves are retained or I should say stopped. Might be a better word by leaves. Imagine a forest after a rain. And you imagine some of those raindrops are sort of sitting out on the leaves. So water droplets stay on the leaves of trees, um especially in a thick forest where you don't have a lot of sun, drying everything out. Uh The shade from the tree canopy allows the soil to remain moisture. And then most forests have a carpet of leaves on the ground and the leaves on the ground also hold on to moisture and help the soil to remain moist. So, in many different ways, forests hold on to moisture from rain. Well, when this moisture is released from the forest into the air as water vapor that aids in cloud formation, which will then result in more rain. So the stored moisture in the forest actually affects the amount of rain. So, deforestation is a major factor in increasing drought worldwide. So that's a true statement and therefore not. Our answer says the growth and development of plants are reduced as a result of drought. Um This is true. I mean, it seems obvious to us if they're getting less water, they grow less. What's the mechanism of that? Um In very dry environments, the plants close their stomata to reduce transpiration, to reduce the water lost as evaporation from their leaves. But when the stomata are closed, the stomata also how carbon dioxide enters the plant to allow photosynthesis. So when the plants are closing their stomata, in response to drought, less photosynthesis taking place, producing less food for the plant and its growth is slowed. So that's also true, the growth and development of plants are reduced as a result of drought. So finally, we're down to choice D which says plants respond to drought by increasing the rate of transpiration. And that is going to be a false statement because as we discussed, when we were talking about choice, C plants respond to drought by decreasing the rate of transpiration, closing their stomata to reduce the amount of water lost through the stomata and the leaves. So since that's a false statement, that is our answer. So which statement about drought is false. Choice. D plants respond to drought by increasing the rate of transpiration. See you in the next video.

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Key Concepts

Photosynthesis

Transpiration

Drought Resilience