Hi Heidy,

It would be helpful to have the whole paper by Vazquez-Luna to answer this with some more confidence but here is my guess:

Imaging you are performing seed germination in Petri dishes with wet filter paper for 7 days.

Tt: is your control sample, i.e., no toxic substance on it. The value you have to place on the equation is the number of germinated seeds after the 7 days.

Tp: is your treatment sample with a specific concentration of the toxic substance (HTP is probably an abbreviation of the substance they used in this specific paper). Again, the value you have to place on the equation is the number of germinated seeds after the 7 days.

R1: simply refers to the Tt and Tp values obtained in the first repetition of the experiment. Then you would do the same for your second, third... repetition (R2, R3...)

I hope that helpful.

Javier