Table 1. Growth characteristics and nitrogen, carbon and nitrate concentration of red and green lettuce cultivated at different nitrogen concentration and photosynthetic photon flux density.

	Nitrogen	PPFD	Head mass	number of leaves	total N	NO3	total C	C/N ratio
Red Lettuce	0.75	678	18.6	10.3	9.7	0.09	396.9	41.0 a
	3	678	76.0	17.7	16.9	0.05	382.9	22.8 c
	12	678	204.4	23.0	32.9	5.02	388.1	11.8 d
	0.75	339	17.4	11.3	10.9	0.09	395.3	36.4 b
	3	339	70.5	16.7	18.2	0.08	379.9	20.9 c
	12	339	184.2	22.8	33.5	5.37	384.5	11.5 d
Main effects
0.75 mM N			18.0 c	10.8 c	10.3 c	0.09 b	396.1 a	38.7
3 N			73.3 b	17.2 b	17.6 b	0.06 b	381.4 c	21.8
12 N			192.9 a	22.9 a	33.2 a	5.19 a	386.3 b	11.7
678 μmol PPFD			99.7	17.0	19.8 B	1.72	389.3 A	25.2
339 μmol PPFD			90.7	17.5	20.9 A	1.85	386.5 B	22.9
Significances
N			*	*	*	*	*	*
PPFD			ns	ns	*	ns	*	*
N * PPFD			ns	ns	ns	ns	ns	*
Green Lettuce	0.75	678	28.7	13.7	9.5	0.04 c	395.4 a	41.6
	3	678	93.6	22.3	18.6	0.46 c	384.0 b	20.8
	12	678	292.3	27.3	37.7	8.58 b	367.1 c	9.8
	0.75	339	26.0	14.3	12.1	0.56 c	396.8 a	33.5
	3	339	99.6	21.7	21.3	0.22 c	368.1 c	17.3
	12	339	278.5	27.7	39.1	10.08 a	362.2 c	9.3
Main effects
0.75 mM N			27.4 b	14.0 c	10.8 c	0.30	396.1	37.5 a
3 mM N			96.6 b	22.0 b	19.9 b	0.34	376.1	19.1 b
12 mM N			285.4 a	27.5 a	38.4 a	9.33	364.6	9.5 c
678 μmol PPFD			138.2	21.1	21.9 B	3.02	382.2	24.1 A
339 μmol PPFD			134.7	21.2	24.1 A	3.62	375.7	20.0 B
Significances
N			*	*	*	*	*	*
PPFD			ns	ns	*	*	*	*
N * PPFD			ns	ns	ns	*	*	ns

Nitrogen (N) concentration in the nutrient solution is given in millimol per liter (mM). Photosynthetic photon flux density (PPFD) is given in μmol m⁻² s⁻¹. Head mass is given in gram fresh matter. Nitrogen, carbon (C) and nitrate (NO₃) concentration is given in milligram per gram dry matter. Data was evaluated via two-way ANOVA, factors: mM N and PPFD, α = 0,05, followed by Tukey HSD test (mean, n = 3). Identical letters indicate that values do not differ significantly. Asterisks indicate significantly influential factors.