Aquatic Plant Science

The science of submerged aquatic plants in the aquarium hobby


    What are the signs of toxicity?

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    Posts : 10
    Join date : 2016-07-20

    What are the signs of toxicity?

    Post by Admin on Fri Jul 22, 2016 4:06 pm

    These are my observations but are in no way definitive.

    1. Reduced photosynthesis

    I used to wonder why plants reduced pearling right after I dosed iron, manganese, or CSM.  It never made any sense to me but I've been able to repeat it many times.  Only after a large water change is performed do the plants return to pearling at the previous rate.  Now I know why.

    The first sign of toxicity is reduced photosynthesis. This is supported by many experiments in the literature.  Excess of certain nutrients impair the physiological processes preventing carbon dioxide assimilation and oxygen from being produced. We would observe that plants don't pearl as much and CO2 levels are higher.  Only when the concentrations of the excess nutrients are reduced to more optimum levels will the physiological processes start to function normally.

    I'm not the only one to make this observation.  Tom Barr also noticed this, too:
    "I do a large water change in the morning right after the lights come on, then late in the day, there is mad pearling and obviously better growth than any other day of the week.
    I've measured plant growth to confirm this also."
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    It's not the water change that causes the increased photosynthesis and growth. It's the reduction of toxic concentrations of nutrients.

    Under the conditions of high light, 100+PAR, and high CO2, 40+mg/L, plants should pearl a lot more and growth should also be very fast.  However, in my tanks with high light and low CO2 (<10mg/L) plants still pearl and grow very fast, faster than should be expected at such low CO2 levels.  The difference is that the nutrients are closer to within the tolerance levels that plants require and can function normally even at low CO2 levels.


    2. Physical signs and symptoms

    The next stage of toxicity results in physical symptoms.  This may include chlorosis, necrosis, crimpling, twisting, and deformation of growth.  It may result in loss of color or even the enhancement of color if chlorophyll is reduced or damaged.  The reduction in the number of new side shoots is another sign as well as complete stunting of growth. A lot of these symptoms will look like deficiencies.  Overall plant aesthetics will look ugly.  Algae may start to take over and smother plants.


    3. Death

    When the conditions are so bad that plants can no longer tolerate them, they die.  Some plants are much more sensitive and will die sooner than the more tolerant ones.

    I've killed off a few plants already:
    Rotala wallichii, Rotala "Vietnam", Rotala macrandra, Pogostemon erectus, Hygrophila pinnatifida and other Hygros, and some others.  Too bad because these were some of my favorite plants.


    So these are the three stages of toxicity. Tell me what you think or if you can add more.

    Lawrence So

    Posts : 21
    Join date : 2016-07-20
    Location : San Francisco, CA

    Re: What are the signs of toxicity?

    Post by Lawrence So on Fri Jul 22, 2016 4:16 pm

    If you've never grown plants in optimal conditions, how would you even know that plants have reduced the rate of photosynthesis? You wouldn't. You would think it's normal for the plant even as the light and CO2 are both high.

    The same applies to the physical signs. The only way you'd know there's an issue is if you had something to compare to.

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