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Please Help! Question with plant use of CO2 and Statistics..

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Please Help! Question with plant use of CO2 and Statistics..

Postby flux101 » Sat Apr 07, 2007 9:03 pm

Hello,
I have graphed Respiration rate against Specific Leaf Area (area to mass of leaf) and found no correlation on a linear regression (line of best fit).
I have also graphed Photosynthetic rate against Specific Leaf Area and found a strong correlation on a linear regression (line of best fit).
Can anyone provide me with an answer to the following questions?

1. Why is there no relationship bt. specific leaf area and respiration rate?
2. Why is there a relationship bt. specific leaf area and photosynthetic rate?
3. Why isn't or is the line of best fit the best way to analyse the trends in the data?

If you can help answer these questions, particularily the 1st and the 3rd, it would be hugely appreciated.

Many Thanks,
Fluxone
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Postby blcr11 » Fri Apr 13, 2007 1:58 pm

This is way out of my area, but since no one else has I suppose I can take a stab at it.

Respiration should be a dark process not requiring light. The surface area of the leaf should be largely irrelevant to the rate of respiration. The photosynthetic rate, however, should be proportional to the surface area--a wide leaf with little thickness (large specific area) should have more of its light capturing machinery exposed to light compared to either a narrow leaf or a thick leaf, assuming the density of the light harvesting assemblies are comparable.

Not sure how to answer 3. Even if there is no trend, calculating a mean and standard deviation is the same as fitting a general model of the form, y=mx+b, with m=0. If there is no obvious trend, I guess there is no point in trying to fit a line through the data, other than to prove statistically that the slope cannot be distinguished from zero. If the trend is not linear--say it's quadratic, or multivariate--a simple univariate linear model isn't adequate; a univariate linear model may still be statistically significant, but the residuals should tell you there is something more to consider.
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Thanks!

Postby flux101 » Sat Apr 14, 2007 9:09 pm

Hey thanks,
I ended up concluding that most respiration is occuring at shoot and root apical meristems, or other photosynthate "sinks", whereas the leaves are "sources". This would explain the lack of respiration in the leaves (or lack of relationship to leaf area), as sugars are not really getting broken down in the mature leaves. As for the photosynthesis, I concluded on something like what you said too.
Also, I figured a similar thing to what you wrote for the 3rd question.
Thanks for helping.
Cheers
C.
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