Photosynthesis Two-Way ANOVA Example (from lecture #38 1997-98)

The Table below shows diffusive resistance for leaves under two different atmospheres (Ordinary and CO₂) and three different light levels (dark, low, high).

Here is the result of a two-way ANOVA with interaction:

   SV                SS  DF         MS      F       P
 
Atmosphere       5.3333   1     5.3333    3.2   0.124
Light Level      0.6666   2     0.3333    0.2   0.824
Atmos x Light   32.6666   2    16.3333    9.8   0.013
Error           10.0000   6     1.6666
 
Total           48.6666  11

Hence we conclude that there is evidence (P = 0.013) that light level affects diffusive resistance, and that the effect of light level is different under the different atmospheres. Because the interaction is significant, the main effects are not interpretable and there is no need to test them.

To help interpret these results graphically, average over the replicate pairs of leaves so that there is just one mean for each combination of atmosphere and light level, then plot the table of means by rows and by columns. The graphs show the interaction dramatically. The resistance is slightly lower in CO₂ than in an ordinary atmosphere in a dark or low-light environment, but much higher in CO₂ than in an ordinary atmosphere when there is a high level of light.