Table 5.

Multiple linear regressions (stepwise) explaining biomass variation from stable isotopes (Δ¹³C_s and Δ¹⁸O_s); Δ¹⁸O_s and Δ¹³C_s variations from gas exchange parameters (A_sat, E, g_s, C_i/C_a, T_cum) plus SLN, SN, and LN; and C_i/C_a and A_sat variations (g_s, SLN, SN, and LN) derived from water and genotype means within each N treatment.

Initial variable, first variable entering the model; initial r² and mean square error (MSE), adjusted coefficient of determination (r²) and MSE after including the first variable in the model; final r² and MSE, adjusted r², and MSE obtained with the final stepwise model.

N treatment	Trait	Initial variable	Initial r2	Initial MSE	Final stepwise model	Final r2	Final MSE
Low N	AB	Δ18Os	0.59**	7.36	AB= –15.1×Δ18Os–8.5×Δ13Cs+716.8	0.76***	5.70
	Δ18Os	Tcum	0.67**	0.33	Δ18Os= –0.19×Tcum+36.3	0.67**	0.33
	Δ13Cs	SLN	0.37*	0.45	Δ13Cs= –1.8×SLN+5.6×gs+20.0	0.68**	0.32
	Ci/Ca	–	–	–	–	–	–
	Asat	LN	0.72***	1.32	Asat=3.3×LN+17.4×gs–1.4	0.85***	0.96
High N	AB	Δ18Os	0.48**	8.4	AB= –11.6×Δ18Os–5.8×Δ13Cs+538.5	0.84***	6.64
	Δ18Os	Tcum	0.61**	0.58	Δ18Os= –0.4×Tcum+36.8	0.61**	0.58
	Δ13Cs	LN	0.59**	0.65	Δ13Cs= –1.7×LN+10.7×gs+23.1	0.87***	0.36
	Ci/Ca	gs	0.59**	0.03	Ci/Ca=0.68×gs–0.06×LN+0.74	0.74***	0.03
	Asat	gs	0.43**	1.39	Asat=31.5×gs+2.9×LN–4.5	0.74***	0.93

AB, aboveground biomass; Δ¹³C_s, shoot carbon isotope discrimination; Δ¹⁸O_s, ¹⁸O enrichment in shoots; E, transpiration rate; g_s, stomatal conductance; C_i and C_a, intercellular and ambient CO₂ concentrations; SLN, specific leaf nitrogen; T_cum, cumulative transpiration.

*P <0.05; **P <0.01; ***P <0.001.