Help need to decide what stat test to use.

For my dissertation I am investigating the effect of zinc nano particles in plant growth. I want to test for significance in the change in plant growth. For example the average growth of my control plant was 41mm. Then from here the growth of my plants under different concentrations of zinc nano particles were:

10mg/L = 39mm
50mg/L = 32mm
100mg/L = 10mm
500mg/L = 5mm
1000mg/L = 0mm (Complete growth inhibition)
2000mg/L = 0mm

So in my discussion I want to state "Zinc nano particles did/ didn't have a significant effect on plant growth".

I've been scratching my head over this for a while so any advice would be greatly appreciated!
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TS Contributor
So you have n=7 individual plants? You could correlate
zinc versus growth across these seven subjects (Spearman's
rank correlation rho). I am not sure how to deal with the
one case way beyond complete inhibition, though. Maybe
you include it anyway.

With kind regards

If I understand your post properly, you want to calculate the effect of zinc over the growth. Apparently, it has dosis-depend effect with complete inhibition with concentrations of Zinc higher than 1000 mg/dl.
In my opinion, you could create a linear regression where the effect is "y" and "x" the concentration. In this case, Spearman is right too.