highly significantp < 0.01
Specifically, the linear effect of X 1 , X 2 and X 4 illustrated highly significant ( p < 0.01) and remarkably significant ( p < 0.001) positive effects on FRAP, respectively, while their square terms (X 1 2 , X 2 2 and X 4 2 ) exhibited highly significant ( p < 0.01), significant ( p < 0.05) and remarkably significant ( p < 0.001) negative ones, respectively.