Elucidating the physiological mechanism of 'stay green' in maize hybrids - crop growth processes and nitrogen economy

Abstract

The study reported in this thesis was established during 2008/09 and 2009/10 at Lincoln University, New Zealand, to investigate the physiological mechanisms of the 'stay-green' trait in maize hybrids. In 2008/09, the response of ‘P39K38’ (sgr 6), ‘P38V12’ (sgr 7), ‘P38F70’ (sgr 8), and ‘P38G43’ (sgr 9) to 0 or 270 kg nitrogen (N) ha-1 and nil or full irrigation was quantified. Only the response of ‘P39K38’ (sgr 6) and ‘P38G43’ (sgr 9)) to four rates of N (0; 0 (V6) and 50 (V12); 150 (V6) and 0 (V12) and 150 (V6) and 50 (V12) kg N ha-1) applied at six (V6) and 12 (V12) fully expanded leaves was examined in 2009/10, when crops were fully irrigated. Increased ‘stay-green’ rating either had no influence or led to a decline in dry matter accumulation and grain yield because of the reduction in total intercepted photosynthetically active radiation (iPAR). Total dry matter and grain yield were similar amongst the hybrids in 2008/09. However, ‘P39K38’ (sgr 6), yielded 2.0 and 1.8 t ha-1 more dry matter and grain yield, respectively, than ‘P38G43’ (sgr 9) in 2009/10. Total iPAR was 5% higher among the low ‘stay-green’ (sgr 6 and 7) hybrids because for ‘P39K38’ (sgr 6), leaves in positions 8-14 were 10% larger. Thus, the critical green area index (GAI) was reached 70-110 oC.d earlier in these hybrids than in the high ‘stay-green’ hybrids. Consequently, mean maximum GAI in the low ‘stay-green’ hybrids was 4-13% higher and better synchronised with the peak of solar radiation. GAI decline during grain filling was consistent and influenced by the nitrogen x hybrid interaction. The ‘stay-green’ trait was only exhibited in the presence of adequate N. When additional N was not provided, the ‘stay-green’ trait had no effect on leaf senescence. However, when N was available, ‘P38G43’ (sgr 9), senesced 1-2 fewer (P<0.046) leaves at physiological maturity compared with the other hybrids. The decline in GAI was also affected by the interaction of water and N, with the least (P<0.001) number of leaves senesced when both were provided, suggesting that both acted synergistically to delay leaf senescence. The stay green hybrids did not show any preferential sequestration of N, but its remobilisation within the crop differed during grain filling. For example, ‘P39K38’ (sgr 6) had 25 kg N ha-1 more in its kernels from the remobilization of stalk and leaf N than ‘P38G43’ (sgr 9) at physiological maturity. As a consequence of the smaller leaf size and slower decline in leaf N content during grain filling, the high ‘stay-green’ hybrids, maintained a high specific leaf N (SLN) concentration (g m-2) for longer than the other hybrids. The decline in SLN was consistent with the increase in the number of leaves senesced, suggesting that leaf senescence was associated with the decline in SLN. Total dry matter converged between the hybrids in 2008/09 because the radiation use efficiency (RUE) provided a compensatory influence. RUE was therefore 8% (2.68 vs. 2.48 g dry matter MJ-1 PAR absorbed) higher in the high ‘stay-green’ (sgr 8 and 9) hybrids compared with the low ‘stay-green’ (sgr 6 and 7) hybrids. Overall, the selection for high ‘stay-green’ rating may have led to a decline in the partitioning of N to the kernels and consequently contributed to lower grain N levels. Thus, the hybrid ‘stay-green’ trait may only provide an opportunity for a prolonged harvest window before crops are ensiled. Otherwise it had a negative effect on dry matter yield and grain protein content in these environmental conditions.