In order to identify the optimum period of spring water-restrictive irrigation for winter wheat (Triticum aestivum L.) in the Haihe Plain, China and elucidate its effects on flag leaf senescence and yield formation, field experiments were conducted at the Xinji Experimental Station of Hebei Agricultural University from 2016 to 2019 by using different irrigation regimes in spring, including the conventional regime involving two irrigation periods (control (CK), the 3-leaf unfolding stage and the anthesis stage) and a series of single, restrictive irrigation regimes (SRI) comprising irrigation at the 3-leaf unfolding stage (3LI), 4LI, 5LI, and 6LI.  There are five major findings: (1) The senescence (determined by the green leaf area, GLA) in the 4LI treatment occurred moderately earlier than that in CK, showed no significant difference with that in 5LI and 6LI, and occurred significantly later than that in 3LI.  (2) Compared with other SRI treatments, the GLA value and photosynthetic rate in 4LI were 14.82 and 20.1% higher, respectively.  Microstructural analysis of flag leaf also revealed that the mesophyll cells and chloroplasts were irregularly arranged under drought stress in 3LI and 6LI; however, drought stress had minimal negative effects on the microstructure in 4LI and 5LI.  (3) Postponed irrigation in spring could significantly increase superoxide dismutase (SOD) and catalase (CAT) activities in the early stage of grain filling; however, these activities would subsequently decrease.  Among the four SRI treatments, the overall enzyme activities were the highest in 4LI, and the combined malondialdehyde (MDA) content in flag leaves in 4LI and 5LI was 14.5% lower on average than that in 3LI and 6LI.  (4) The soluble sugar (SS) and proline (Pro) contents in 4LI were the highest among the four SRI treatments; however, they were lower than those in CK.  The abscisic acid (ABA) hormone content in 4LI and 5LI was lower than that in 3LI and 6LI, respectively, suggesting a smaller drought stress effect in 4LI and 5LI.  (5) In two growing seasons, there was a larger number of spikes per unit area in 4LI (i.e., 13.4% higher than that in 5LI and 6LI) and the 1 000-grain weight in 4LI was the highest among the four SRI treatments (i.e., 6.0% higher than that in the other three SRI treatments).  Therefore, a single restrictive irrigation regime at the 4-leaf unfolding stage is recommended to be effective in slowing down the senescence process of flag leaves and achieving high yield.

 

Over-use of N fertilizer in crop production has resulted in a series of environmental problems in the North China Plain (NCP).  Thus, improvement of nitrogen use efficiency (NUE) in summer maize has become an effective strategy for promoting sustainable agriculture in this region.  Using twenty maize cultivars, plant dry matter production, N absorption and accumulation, yield formation, and NUE in summer maize were investigated under three N levels in two growing seasons.  Based on their yield and yield components, these maize cultivars were categorized into four groups including efficient-efficient (EE) cultivars, high-nitrogen efficient (HNE) cultivars, low-nitrogen efficient (LNE) cultivars and nonefficient-nonefficient (NN) cultivars.  In both two seasons, the EE cultivars improved grain yield together with increased plant biomass, and enhanced accumulative amounts as well as higher average grain yields than the other cultivar groups under deficient-N conditions.  Significant correlations were observed between yield and kernel numbers (KN), dry matter (DM) amount and N accumulation at both post-silking and maturity stages.  DM and N accumulation at late growth stage (i.e., from silking to maturity) contributed largely to the enhanced yield capacity and improved NUE under N-deficient conditions.  Compared with the NN cultivars, the EE cultivars also showed increased N assimilation amount (NAA) and N remobilization content (NRC), and elevated N remobilization efficiency (NRE), NUE and nitrogen partial factor productivity (PFPN).  Our investigation has revealed N-associated physiological processes and may provide guidance for cultivation and breeding of high yield and NUE summer maize under limited N conditions in the NCP.