Table 3. Effect of N on photosynthetic CO₂ assimilation (Ph, μmol m⁻² s⁻¹) in different corn cultivars.

Corn cultivars	Treatment	August 16th	September 6th	September 17th
Jingyu7	LN	20.88 ± 0.57cC	22.94 ± 0.64 cC	16.64 ± 0.54 cC
	MN	26.69 ± 0.45 bB	34.69 ± 0.98 bB	24.69 ± 0.62 bB
	HN	33.70 ± 0.88 aA	42.32 ± 1.06 aA	36.72 ± 1.43 aA
Nongda80	LN	18.26 ± 0.66 cC	26.26 ± 0.011 cC	19.24 ± 0.39 cC
	MN	25.56 ± 0.85 bB	33.43 ± 0.74 bB	27.12 ± 0.48 bB
	HN	31.67 ± 0.75 aA	41.43 ± 0.42 aA	34.45 ± 0.68 aA
Tangyu10	LN	17.94 ± 0.67 cC	21.64 ± 0.42 cC	14.46 ± 0.24 cC
	MN	24.59 ± 0.42 bB	32.55 ± 0.87 bB	22.76 ± 0.74 bB
	HN	29.59 ± 0.67 aA	40.21 ± 1.13 aA	32.43 ± 0.68 aA

Note: The statistical analyses are based on each cultivar. With respect to each cultivar, the values in the same column followed by different uppercase and lowercase letters are significantly different at the 0.01 and 0.05 probability levels, respectively. The corn cultivars were subjected to low (LN, 0 kg N ha⁻¹), medium (MN, 75 kg N ha⁻¹), and high (HN, 150 kg N ha⁻¹) levels of N application over the cropping season (2002 and 2003). The data are average values across 2 years with ± one standard error.