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Water and Mineral Nutrition in Plants

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Introduction

Water and Mineral Nutrition in Plants We have just concluded a series of lectures looking at the structure of the "higher" plant body and some of its features. At this time we can look at how the plant functions (physiology) and how plant growth and development is regulated. In other words, how does a plant do the things it needs to do, such as: Obtain nutrients for growth and survival, both from the "soil" and from the atmosphere Maintain water balance and transport water throughout the plant Transport nutrients and solutes to its cells and tissues Regulate growth and developmental activities You'll note the plants have many of the same problems that any animal does - its just that plants tend to solve their problems in different ways... Let's look now at some of these problems and how plants grow and survive in an often "hostile" environment. Obtaining Nutrients Plants are autotrophs. They obtain raw materials from their environment. Plants need about 18 elements, mostly mineral ions, along with oxygen, water, and carbon dioxide. We will discuss the specific mineral needs in the laboratory exercise on mineral nutrition. Plants then process these substances into their needed organic molecules for plant structure and function. Plants produce few waste products because they have no need to extract nutrients from pre-formed organic materials (like we do), and their fuel needed to do cell work is provided by photosynthesis How does the average plant obtain these raw materials? Needed gases are obtained by diffusion from the atmosphere, and will be discussed a bit more later. Water and most minerals must be absorbed as water-soluble ions from soil(never dirt) via the roots. Soil serves as the: * Reservoir for many mineral ions * Storage for some mineral ions The origins of any soil is the parent rock, of whatever type, which is "weathered" to tiny particles, often called clay, by mechanical and chemical processes (mostly involving water). ...read more.

Middle

to prevent water loss, or, if the surface is cork, the walls contain impermeable suberin. Plant cells have vacuoles to accumulate a volume of water, and cell walls to help maintain turgor. (This works better at preventing excess water than it does at preventing dehydration. Plants achieve "permanent wilt" when plasmolysis (loss of turgor) can not be reversed.) Many cells and tissues need not be maintained because they're dead (saves energy) Water Movement in Plants How does water enter and move through the plant, especially when plants have no pumps, and how does a plant move water upward against the forces of gravity as much as 300 feet. We know that water moves from the soil's environment by diffusion into the root through root hairs. It travels through the cortex of the root, mostly between cells, and from root stele upward through the xylem tissue, and out the leaves' stomata, with a few stops along the way for photosynthesis, turgor maintenance, and other water requirements. How does this happen? Let's look at the end point first, since this also functions in the overall movement of water throughout the plant. Water diffuses out of the plant via transpiration through the stomata. Transpiration is the term used to describe the evaporation of water from the plant. Transpiration also plays a role in the movement of water throughout the plant as we shall discuss. Transpiration loss is significant. In corn fields, as much as 90% of the water absorbed by the roots is lost by transpiration. How Water moves - The Tension-Cohesion Theory Much water is lost via transpiration. This creates a negative water potential in cells which exerts a "pull" on H2O in cell walls which connects to H2O in xylem creating a tension in the xylem. Since water molecules tend to cohere (stick to each other), this tension is transmitted to the xylem in roots making the root water potential negative, too. ...read more.

Conclusion

For all plants, hot summer weather increases the amount of water that evaporates from the plant. Plants lessen the amount of water that evaporates by keeping their stomates closed during hot, dry weather. Unfortunately, this means that once the CO2 in their leaves reaches a low level, they must stop doing photosynthesis. Even if there is a tiny bit of CO2 left, the enzymes used to grab it and put it into the Calvin cycle just don't have enough CO2 to use. Typically the grass in our yards just turns brown and goes dormant. Some plants like crabgrass, corn, and sugar cane have a special modification to conserve water. These plants capture CO2 in a different way: they do an extra step first, before doing the Calvin cycle. These plants have a special enzyme that can work better, even at very low CO2 levels, to grab CO2 and turn it first into oxaloacetate, which contains four carbons. Thus, these plants are called C4 plants. The CO2 is then released from the oxaloacetate and put into the Calvin cycle. This is why crabgrass can stay green and keep growing when all the rest of your grass is dried up and brown. There is yet another strategy to cope with very hot, dry, desert weather and conserve water. Some plants (for example, cacti and pineapple) that live in extremely hot, dry areas like deserts, can only safely open their stomates at night when the weather is cool. Thus, there is no chance for them to get the CO2 needed for the dark reaction during the daytime. At night when they can open their stomates and take in CO2, these plants incorporate the CO2 into various organic compounds to store it. In the daytime, when the light reaction is occurring and ATP is available (but the stomates must remain closed), they take the CO2 from these organic compounds and put it into the Calvin cycle. These plants are called CAM plants, which stands for crassulacean acid metabolism after the plant family, Crassulaceae (which includes the garden plant Sedum) where this process was first discovered. ...read more.

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