Why Weeds Are Tough
    By Enrico Uva | June 23rd 2011 09:13 AM | 3 comments | Print | E-mail | Track Comments
    About Enrico

    I majored in chemistry, worked briefly in the food industry and at Fisheries and Oceans. I then obtained a degree in education. Since then I have...

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    While cycling to work this morning, I spotted a dandelion coexisting with a sedum in a rock garden. It wasn't a coincidence. Each has evolved one of two known biochemical strategies to deal with high temperatures and low humidity.  The dandelion's biochemistry is also one of the reasons why a lawn left to its own devices will soon cease to be a monoculture. 

    Unlike dandelions and sedum, the majority of land plants are like Kentucky bluegrass, which is the common grass found in lawns. They are C3 plants. During photosynthesis, the RuBisCO enzyme captures CO2 and bonds it to a five-carbon compound to produce a pair of three-carbon molecules. One remains within the cycle, and the other is used as a building block to produce sugars.

    Unfortunately, high temperatures make it easier for RuBisCo to catalyze the reaction between the five-carbon compound and oxygen, instead of CO2.  With one less carbon,in what's known as photorespiration, a three and a two-carbon compound (2PG) are produced . The latter is not totally wasted because a metabolized version leaves the chloroplast, eventually goes to mitochondria and works its way back into photosynthesis in the form of carbon dioxide. But the process wastes time and energy. Worse, when there is little moisture available and flabby guard cells cause a plant's pores to close, carbon dioxide does not enter, and the rate of wasteful photorespiration increases.

    So what is the strategy that makes weeds like dandelions so tough? They have two types of chloroplast-bearing cells: one type near the leaf surface where the oxygen-avoiding PEP enzyme produces a four-carbon compound, and a second type found deeper and below the surface where oxygen levels are low. There, CO2 is released from the 4-carbon compound, and it's taken care of by a now less distracted RuBisCo. Having more specialization requires a larger investment on the part of the C4 plant, but it leads to a bigger profit because it avoids the losses of photorespiration. Also, if dandelions can photosynthesize more efficiently, their pores don't have to be as fully open as often, and less water is lost to evaporation.

    Water is needed in photosynthesis to return electrons to chlorophyll molecules after they are excited. When losing electrons water splits into oxygen and into H+. The subsequent proton gradient across chloroplast membranes then provides the energy required for ATP synthesis.

    By the way, sedum's strategy known as CAM photosynthesis is similar to C4, but it fixes its carbon dioxide at night so that its pores can remain closed during the day, saving even more water.

    We cannot ignore the dandelion's varied means of reproduction in discussing its ubiquitous nature. It can regenerate itself from its roots should the rest of the plant be eaten or severed. Every so-called flower is actually a composite of many flowers, each of which can produce a seed, even in the absence of sex. But bluegrass also has this ability, so it's fair to say that the dandelion owes its success over grass more to C4 metabolism than to its reproductive versatility.


    but that doesn't explain why when you water a plant it dies but not watering a weed makes them grow more and faster

    There are many things I could say to respond to this. Though the key point is.. why do you think watering ANY plant means it will survive better than any other?

    Sure, we all know plants need water but this is such a vauge idea.. how much.. what is best? What plants do better with how much water? How do more "healthy" plants react to water compared to less "healthy"? A plant with only water can't survive so its not the only important thing.

    If you give a terminal cancer patient all the water in the world, he will still die.

    Turning to complete guessing. I doubt many grass we plant in our lawns were evolved to live in those conditions. The weeds however are more likely to be native. Even without help they might thrive... water them when they are already more suited to the conditions you are just increasing the advantage they have.

    Here's a photo I took today, revealing dandelions' and other C4 plants' clear advantage over C3 grass(yellow) in times of drought.