Physiological Basis of Heterosis for Nitrogen Use Efficiency of Maize

Abstract

Efficient use of nitrogen inputs for concurrent improvements in grain yield and nitrogen use efficiency (NUE) has been recognized as a viable strategy for sustainable agriculture development. Yet, there is little research on the possible physiological basis of maize hybrid heterosis for NUE and measurable traits that are corresponding to the NUE heterosis. A field study was conducted for two years to evaluate the heterosis for NUE and determine the relationship between NUE and its physiological components. Two commercial hybrids, 'Xianyu335' and 'Zhengdan958', and their parental inbred lines, were grown at 0 (0 N) and 150 kg N ha^-1 (150 N), in a randomized complete block design with four replications each year. Compared to their parental lines, both hybrids displayed a significant heterosis, up to 466%, for NUE. N internal efficiency (NIE) accounted for 52% of the variation in heterosis for NUE, while there was generally negligible heterosis for nitrogen recovery efficiency (NRE). Heterosis for NIE and thereby for NUE in maize was ascribed to (i) an earlier establishment of pre-anthesis source for N accumulation, which phenotypically exhibited as a faster leaf appearance rate with higher maximum LAI and photosynthetic nitrogen use efficiency; (ii) a larger amount of N being remobilized from the vegetative tissues, especially from leaves, during the grain filling. Phenotypically, there was notably a rapid reduction in post-anthesis specific weights of leaf and stalk, but with maintained functionally stay-green ear leaves; and (iii) a higher productive efficiency per unit grain N, which was characterized by a reduced grain N concentration and enhanced sink strength.

Figure 1

Coefficients of determination (r²) of mid-parental heterosis for NUE (MPH_NUE) (dependent variable) versus mid-parental heterosis for NRE (MPH_NRE) and mid-parental heterosis for NIE (MPH_NIE) in 2014 and 2015.

Figure 2

Comparison of grain yield increment per-unit N applied, N uptake increment per unit N applied and grain yield increment per-unit N uptake of hybrids and parental inbred lines of maize in 2014 and 2015. Means followed by different letters indicate significant differences according to an ANOVA-protected LSD_0.05 test.

Figure 3

Ratio of vegetative N uptake and reproductive N uptake to total N uptake of maize hybrids and parental inbred lines under different nitrogen rates. Means in the same growth stage within a year followed by different letters indicate significant differences according to an ANOVA-protected LSD_0.05 test.

Figure 4

Nitrogen remobilization ratio (NRR) of maize hybrids and their parental inbred lines under different nitrogen rates in 2014 (left) and 2015. Means in the same year followed by different letters indicate significant differences according to an ANOVA-protected LSD_0.05 test.

Figure 5

Relationship between nitrogen internal efficiency and shoot remobilized nitrogen (A) and remobilized leaf and stalk nitrogen (B) of hybrids and parental inbred lines.

Figure 6

Relationship of nitrogen internal efficiency with shoot %N and grain %N (A) at physiological maturity, and with specific leaf weight and specific stalk weight (B) at physiological maturity of hybrids (black color) and parent inbred lines (white color).