Optimizing ear-plant height ratio to improve kernel number and lodging resistance in maize (Zea mays L.)

Highlights

• Plant growth retardants (PGR) regulated root and shoot structure in maize.

• Early PGR application raised lodging resistance but reduced yield in non-lodging year.

• Delayed PGR application promoted kernel/ear by optimizing assimilate allocation to ear.

• Ear-plant height ratio 36.6–39.4% balanced high yield and lodging resistance in maize.

Abstract

Exogenous plant growth retardants (PGRs) increase yield by reducing lodging occurrence but lower yield if lodging is absent in maize (Zea mays L.). It is hypothesized that PGRs can increase maize yield as well as improve lodging resistance by optimizing application timing. A two-year field experiment was conducted in 2018–2019 using plant growth regulator “Jindele” with active ingredients of ethephon and cycocel (EC; w/w=3:1), lodging susceptible (LS) and resistant (LR) maize hybrids, two plant densities of 60 000 and 90 000 plants ha⁻¹, and four EC application treatments. EC was sprayed at the 8-leaf stage T(8), 15-leaf stage T(15), and both stages T(8 +15), respectively, and no EC application was regarded as control CK. On average, T(8) and T(8 +15) significantly reduced lodging percent compared with T(15) and control due to the optimized morphology and strength of stem and root. Both T(15) and T(8 +15) resulted in a higher kernel number per ear (KN ear⁻¹) than T(8) and CK. In non-lodging conditions, T(15) yielded significantly better than CK, especially for LS hybrid in high plant density of 90 000 plants ha⁻¹ (13,417.4 vs 11,219.2 kg ha⁻¹), mainly as a result of increased kernel number. The increases in yield and kernel number were higher in LS hybrid than in LR hybrid. Both KN ear⁻¹ and KN m⁻² were significantly negatively correlated with internode length. The reduced internode length facilitated assimilate partitioning from leaves to the ear in T(8 +15) and T(15), thus increasing kernel number and weight. Ear-plant height ratio was considered an important parameter to evaluate lodging resistance and yield. The optimal ear-plant height ratios were estimated to be 36.6% for KN ear⁻¹, 39.4% for KN m⁻², and 38.4% for yield. The optimal range of ear-plant height ratio (36.60%39.43%) should be taken into account when breeding or selecting lodging-resistant and high-yielding hybrids as well as developing crop management to cope with lodging in practice.

Introduction

As one of the most important staple crops, maize (Zea mays L.) urgently needs to contribute more grain yields in the coming future due to its high yield potential in the increasing frequencies of weather extreme events (FAO, 2009, Bailey-Serres et al., 2019). It is expected that about 50% of yield increase is attributable to changes in crop management and 50% to genetic improvements (Duvick, 2005). Out of these strategies, the interaction of high plant density and selection of dense planting tolerant cultivar is more feasible and low-input for yield increase in practice (Tollenaar et al., 2006, Valadabadi and Farahani, 2010, Xue et al., 2016). Yield however attains a plateau level and then decreases when plant density exceeds the optimum level due to lodging (Rossini et al., 2011, Liu et al., 2012) and kernel abortion (Andrade et al., 1999, Subedi and Ma, 2005, Liu et al., 2015). Crop management that can reduce lodging occurrence and increase kernel formation becomes more important for improving maize yield at high plant density (Grassini et al., 2011, Bailey-Serres et al., 2019).

Plant growth retardants (PGRs) that are able to avoid/reduce lodging occurrence of cereal and grain crops are widely employed by farmers at a global scale (Cox and Andrade, 1988, Gaska and Oplinger, 1988, Norberg et al., 1988). Ethephon is an effective PGR to reduce lodging risk (Gaska and Oplinger, 1988, Konsler and Grabau, 1990, Shekoofa and Emam, 2008). Application of ethephon can reduce the height of center of gravity (Konsler and Grabau, 1990, Andria et al., 1997, Shekoofa and Emam, 2008), increase mechanical strength of basal stem (Langan and Oplinger, 1987, Gaska and Oplinger, 1988, Xu et al., 2017), and increase anchorage strength of brace roots in maize (Langan and Oplinger, 1987, Khosravi and Anderson, 1991). Cycocel (CCC) is a gibberellin acid (GA) inhibitor, which can inhibit the elongation of crop cells to reduce crop growth and thus increase lodging resistance (Dey et al., 2013, Wang et al., 2016, Tan, 2021). The mixture of ethephon and CCC (ethephon-cycocel; EC) is proved to be more effective in increasing lodging resistance (Li et al., 2007, Xue et al., 2008, Lu et al., 2015, Huang et al., 2017), and is commercially employed in PGR products such as “Jindele” in China.

Exogenous PGR has contrasting effects on yield in maize, depending on application timings (Xue et al., 2008, Li and Zhang, 2010). PGR application at the 5- to 7-leaf stage reduced kernel number per ear due to the negative impacts of EC on maize kernel formation (Xue et al., 2008, Li and Zhang, 2010). In contrast, kernel number per ear was increased when PGR was sprayed after the 12-leaf stage (Konsler and Grabau, 1990, Li et al., 2014, Huang et al., 2017). A co-regulation of ear growth and internode elongation was detected by Xu et al. (2004), in which uniconazole as a GA synthesis inhibitor suppressed internode elongation at the middle section of maize plants but promoted ear size at the silking stage. This co-regulation was also supported by a meta-analysis, revealing that late application of ethephon was able to increase kernel number per ear and thus increase maize yield (Zhao et al., 2021).

The formation of fertile florets is one of the key determinants of the final kernel number in maize (Gonzalez et al., 2019). PGRs applied during floret formation can directly inhibit initiations of spikelet and floret (Konsler and Grabau, 1990). In addition, kernel number is a function of crop growth rate (CGR), assimilate partitioning to the ear (PR) during the critical period (~15 d per-anthesis to 15 d post-anthesis), and the minimum assimilate flux (AG) for a kernel formation in maize (Fischer and Palmer, 1984, Otegui and Bonhomme, 1998, Andrade et al., 1999, Borrás and Vitantonio-Mazzini, 2018). CGR is associated with assimilate productivity (Vega et al., 2001a) and greatly depends on genotypes (Tokatlidis and Koutroubas, 2004) and light distribution in the canopy (Perez et al., 2019). Exogenous PGRs can minimize leaf size at the upper canopy layer and increase leaf angle at the middle layer, thus optimizing light distribution in maize canopy (Subedi and Ma, 2005, Huang et al., 2017, Perez et al., 2019, Huang et al., 2021). More importantly, PGRs application can reduce stem internode length (Zhang et al., 2020a, Zhang et al., 2020b), which is expected to minimize the competition between vegetative organ growth and ear development (Xu et al., 2004, Rebetzke et al., 2011, Slafer et al., 2015). In this way, more assimilates can be partitioned to the ear and increases kernel number (Konsler and Grabau, 1990, Zhao et al., 2021). Hence, stem internode lengths at different sections of maize plant affect both lodging resistance and kernel formation, but the relevant information is very limited. Besides, there should be optimal internode lengths at different plant sections when reducing lodging and increasing yield. However, it is complex and difficult to determine so many optimal values from the basal to the upper internodes. Instead, the ear-plant height ratio that is determined by stem internode length at different plant sections is speculated to have close relationships with kernel formation and lodging resistance in maize. Hypothetically, both kernel number and lodging resistance can be increased by optimizing optimal ear-plant height ratio.

The objectives of this study were to (I) detect the responses of lodging resistance and grain yield to the EC application time, (II) attempt to find out chemical strategies to raise yield for different maize hybrids in different lodging conditions, and (III) seek the optimal ear-plant height ratio for reducing lodging and increasing yield. To achieve these objectives, we conducted a two-year field experiment with two maize hybrids, two plant densities, and four application timings of PGR. The most popular PGR commercially named “Jindele” with main active ingredients of ethephon and cycocel (EC) in China was used. Results from this PGR are more effective and more useful in guiding agricultural production.