Given the ongoing increases in global temperature, the frequency and intensity of extreme climate weather have increased, which cause marked damage to crops and food system infrastructure, with the potential to destabilize food systems and threaten local to global food security, especially the developing countries (Lesk et al., 2016). Changes in temporal and spatial patterns of precipitation directly influence the crop water cycle, and consequently, drought and waterlogging (W-B) stresses are increasingly becoming important factors that constrain crop yield (Tao et al., 2003a,b). Globally, around 12% of the world’s arable lands were affected by extreme rainfall events-induced W-B leading to approximately 20% of crop yield reduction (Setter and Waters, 2003). A continuous increase in population coupled with the increasing frequency and intensity of extreme rainfall events results in serious food security problems. Therefore, there is an urgent need for mitigation strategies to reduce the reduction of yield induced by W-B.

Maize is most vulnerable to climate variability and changes among the staple crops as the maize yields are significantly correlated with the seasonal precipitation in the North China Plain (Tao et al., 2008). Therefore, developing effective strategies to cope with the climatic risk in maize production is of great significance to ensuring food security. Previous studies suggested that W-B impedes maize roots’ growth and decreases the photosynthetic capability of the shoot and plant productivity. However, plant productivity is also determined by carbohydrate allocation, other than photosynthetic capacity (Moore et al., 2021). The ear is the main harvesting organ of maize, thus the growth and development of ears largely reflect the plant productivity of maize. The formation of maize ears is a dynamic development process of sequential differentiation. The maize ear is composed of spikelet pairs arranged in rings sequentially initiated at the ear apex. The ear growth cone can continuously differentiate into spikelet pairs until the sexual organ formation period (Bonnett, 1940; Kiesselbach, 1949). However, under adversity conditions, the insufficient supply of nutrients may reduce the differentiation time and the differentiation rate of spikelets, decreasing the total number of florets. Therefore, the ability of carbohydrate translocation to ears is a key for plant productivity.

Carbohydrate translocation from photosynthetic source tissues to non-photosynthetic sink tissues is in the form of sucrose. The entry of carbon from sucrose into cellular metabolism such as the synthesis of starch, protein, and cellulose in plants can potentially be catalyzed by either sucrose synthase or invertase. Sucrose translocation and sink strength are mediated by the sucrose-cleaving enzymes (sucrose invertase and sucrose synthase). The high activity of sucrose invertase is pivotal to ensure sufficient reducing sugar for spike differentiation and development in summer maize, thus ensuring sufficient florets to develop into grains (Hu et al., 2021a). It has been confirmed that increasing the sucrose invertase is an effective strategy to improve the source and sink relationship, thus increasing the crop yield in the context of adverse conditions (Julius et al., 2017; Xu et al., 2020).

Many investigations have recently been conducted to provide insights into sugar signaling and its interplay with hormones in the fine-tuning of plant growth, development, and survival (Sakr et al., 2018). Abscisic acid (ABA) regulates many plant adaptive responses to environmental constraints (Rohde et al., 2000; Finkelstein et al., 2002; Vishwakarma et al., 2017). Additionally, many mutations affecting sugar sensing and signaling are allelic to genes encoding components of the ABA synthesis or ABA transduction pathways suggesting that ABA and sugars can cross-influence each other (Laby et al., 2000; Rook et al., 2001; Brocard-Gifford et al., 2004; Akihiro et al., 2005). Moderate drought can increase the ABA content of stems, thus promoting the remobilization of carbohydrates into ears, increasing the grain-filling rate (Abid et al., 2017). GAs can mediate the photosynthetic (Tuna et al., 2008; Jiang et al., 2012) and sugar metabolic activities (Miyamoto et al., 1993, 2000; Chen et al., 1994; Mehouachi et al., 1996; Ranwala and Miller, 2008; Choubane et al., 2012; Liu et al., 2015; Machado et al., 2017), thus directly regulating the plant carbon status. Additionally, cytokinins and sugars acted both agonistically and antagonistically on gene expression, cross-influencing their metabolism and transport (Barbier et al., 2015; Wang et al., 2016), which may impinge on signaling pathways. Glucose has been confirmed to strongly affect genes involved in cytokinin metabolism and signaling (Kushwah and Laxmi, 2014). Besides, extensive crosstalk between sugars and other hormones such as brassinosteroids, ethylene, strigolactones, and auxin also function in regulating plant growth and development (Sakr et al., 2018).

The plant growth regulators are widely used in plant adaption to abiotic stresses in recent years. They can modulate the content and balance of endogenous hormones, regulate the synthesis and translocation of carbohydrates, and improve the source and sink relationship, thus playing a wide range of roles in plant growth and development. For example, the application of cytokinins can increase the florets of maize by counteracting the inhibition of ABA on sucrose translocation; application of SA can promote the translocation of photosynthesis assimilates toward ears, thus increasing the grain yield (Thompson, 2014; Kang et al., 2021). Our previous studies suggested that the application of 6-benzyladenine (6-BA) increased the yield of waterlogged summer maize from many aspects such as improving photosynthesis, balancing the metabolism of carbon and nitrogen, and delaying the leaf senescence (Ren et al., 2017). In addition, the 6-BA effects on endogenous hormones have been widely discussed. Moreover, the hormonal regulation of the source-sink relationship has been studied (Albacete et al., 2014). However, less is known regarding the effects of 6-BA on carbon translocation and spike differentiation of waterlogged summer maize that are directly related to the grain yield. Therefore, in this study, we focused on the effects of 6-BA on the ear differentiation processes, the changes of endogenous hormones, the carbon partitioning between source and sink tissues, and the cross-talk between hormones and sugars to find out the underlying mechanism of 6-BA, improving the grain yield of waterlogged summer maize.

Grain yield of maize was mainly determined by the kernels per ear and 1,000-kernel weight. Significantly, the kernels per ear were more vulnerable to climate variability and change (Gao et al., 1989). The decrease of kernels per ear contributed to about 70% of W-B-induced grain yield losses (Hu et al., 2021a). The kernels were developed from florets. Therefore, the florets’ differentiation processes were the key to the formation of final mature kernels per ear. The paired spikelets and florets differentiated before the T2 stage can grow well and were likely to develop into mature kernels. However, after entering the sexual organ formation period, especially in the meiotic period, the differentiation of spikelets and florets tends to be unbalanced and more florets tend to develop into degradation or deformed florets (Zheng et al., 1990). In this study, W-B significantly decreased the number of florets at the T1 stage. Although W-B had less effect on the number of florets that formed during the period of sexual organ formation, the development of apical florets and the growth of filaments were significantly affected by W-B, leading to an increased pollination time gap between the basal and apical kernels. As a result, the ability of florets differentiated at the T2 stage to develop into kernels was significantly decreased. Therefore, promoting the florets differentiation during the T1 stage and balancing the differentiation of spikelets and florets at the T2 stage were crucial to determining the final mature kernels per ear. Interestingly, we found that the application of 6-BA could effectively increase the number of florets at the T1 stage, promote the growth and development of apical spikelets and florets, and improve the growth and structure of filaments, thus increasing the locus of effective florets and grain yield of waterlogged summer maize.

The ear differentiation processes are extensively regulated by hormones. Studies have proved that the content of cytokinins in ears of summer maize was peaked at the spikelet differentiation stage suggesting the pivotal role of cytokinin in florets differentiation. In addition, the high ratio of JA/CTK, ABA/CTK contributed to the abortion of florets (Wang, 2019). Besides, SA has been reported to act as a positive role in florets differentiation. In this study, W-B significantly decreased the content of cytokinin and SA in the ears, but increased the content of JA, which contributed to the impediment of florets differentiation. Previous studies have confirmed that the application of 6-BA could effectively mitigate the W-B effects on hormones (Hu et al., 2021b). In consistent with our previous study, Gao et al. (2007) reported that the application of 6-BA could modulate the endogenous cytokinin and ABA contents by promoting and inhibiting their biosynthesis. Additionally, cytokinin treatment has also been reported to elevate SA production by directly inducing the expression of isochorismate synthase (Wildermuth et al., 2001; Gaille et al., 2002, 2003; Choi et al., 2010). Moreover, the interplay between hormones is extensively established (Takei et al., 2004; Tsai et al., 2012), suggesting that the application of 6-BA could affect the metabolism of endogenous hormones in a wide range. Furthermore, the application of 6-BA could regulate MAPK and phosphorus signals to do a long effect on the synthesis and signal transduction pathways of hormones (Hu et al., 2021b). In consistent with these studies, in this study, application of 6-BA effectively mitigated the decrease or increase of cytokinin, SA, and JA in ears at the T1 stage, which further proved that application of 6-BA could promote the differentiation of florets contributing to the improvement of grain growth by doing a long effect on the metabolism of hormones.

Moreover, since florets differentiation was associated with active growth or the activation of biological processes, a possible link to the carbohydrate supply had been suggested (Kuiper, 1993). Carbohydrates were translocated from source (leaf) toward sink (ear) in the form of sucrose (Evert, 1982). The ability of an organ or a tissue to unload carbohydrates from the phloem defined its sink strength. Enzymes involved in the immediate sucrose metabolism are expected to be important both for phloem unloading and for the import of sucrose into sink organs (Ho et al., 1991). The sucrose must be cleaved into glucose and fructose to function in sink metabolism pathways. In this study, the number of florets at the T1 stage was positively correlated with the sucrose-cleaving enzymes of the ears. However, the activities of sucrose-cleaving enzymes in ears at the T1 stage were substantially lower in waterlogged summer maize, thus impeding the entry of carbon from sucrose into cellular metabolism. The cross-talk of sugar metabolism and hormones had been widely discussed. Cytokinin and SA have been confirmed to induce invertase activities, thus particularly participating in regulating sink strength, photosynthate partitioning, and phloem unloading (Ehness et al., 1997; LeClere et al., 2008). However, the high content of ABA and JA could inhibit the activities of sucrose invertase and transporters, thus impeding the long-distance transport of carbohydrates (Parish et al., 2012). In this study, the activity of sucrose invertase in the ears was positively correlated with the content of cytokinin but negatively correlated with the ratio of JA/CTK and ABA/CTK. These results underlie a close connection between hormones, the activities of sucrose-cleaving enzymes, and the floret differentiation processes. To some extent, it confirmed that the application of 6-BA improved the metabolism of endogenous hormones to regulate the activities of sucrose-cleaving enzymes, thus promoting the differentiation of florets at the T1 stage.

For increasing the effective florets differentiated during the T2 stage, it was critical to improve the plant nutrition status. The blocked nutrient supply to ears during this period affected the development of florets, especially the apical florets as the supply of assimilates to the apical kernels was inferior to the supply to the middle and basal kernels (Shen et al., 2018). Furthermore, the fertilization of the basal, oldest ovaries can stop the development of younger, apical ovaries and cause their abortion (Freier et al., 1984; Cárcova and Otegui, 2001). In this study, W-B significantly decreased the content of cytokinin and increased the content of JA and SA, thus decreasing the activities of sucrose invertase and synthase, leading to the lower proportion of ¹³C in spike nodes. Accordingly, the contents of sucrose, soluble sugar, and starch in the spike node were all decreased suggesting that the carbohydrates supporting the growth and development of the spike node were insufficient. Therefore, the number of large and small bundles, the area of large bundles, and sieve elements in spike nodes were decreased. As a result, the translocation ability of spike nodes was decreased. In addition, the activities of sucrose invertase and synthase in ears were also significantly decreased by W-B, which suggested that the capacity of ears to utilize sucrose was decreased. Thus the carbohydrate flux to ears was significantly impeded by W-B. Moreover, the competitive ability of apical florets was weaker than that of basal florets. Therefore, W-B inhibited the development of apical florets and reduced the growth rate of apical ovaries filaments more significantly than it impeded the growth rate of basal florets. As a result, the pollination time gap between basal and apical florets may be increased, leading to the abortion of apical kernels and the decrease in grain weight.

Significantly, the application of 6-BA effectively increased the content of cytokinin and decreased the JA content in the spike node and ears, thus increasing the activities of enzymes involved in the immediate sucrose metabolism. As a result, the structure of the spike node was improved, promoting the translocation of sucrose into the ears. Moreover, the ability of ears to unload and metabolize sucrose was increased. Therefore, the growth and development of spikelets and florets were more coordinated and synchronized. Consequently, the structure of filaments of apical florets was improved, the fertilization ability of apical florets was increased, and the fertilized florets, kernels per ear, and grain yield were increased.

The application of 6-BA increased the contents of cytokinin and SA and decreased the JA content during the crucial phase of floret differentiation, thus inducing the sucrose-cleaving enzymes and establishing the sink strength. Therefore, the number of florets was increased. Additionally, the application of 6-BA regulated the hormones during the phase of sexual organ formation, thus improving the carbon translocation ability of the spike node and increasing the utilization ability of the ears. Consequently, the spikelets and florets were developed coordinately and synchronously, thus improving the development of apical florets and the structure of filaments and increasing the fertilized florets, kernels per ear, and grain yield of summer maize (Figure 9).

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

JH: data curation, writing—original draft, visualization, and investigation. BR: writing—review and editing. YC, PL, and BZ: supervision. JZ: conceptualization, writing—review and editing, and funding acquisition. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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