Light provides energy for the generation of plant assimilatory power and acts as a signal for photomorphogenesis (Kumar et al., 2016). Weak-light is a type of abiotic stress that seriously affects plant growth, development and production efficiency. Maize is a typical C₄ crop, and sufficient light is necessary to ensure its high and stable yield. However, over the past 50 years, global solar radiation has declined at an average rate of 1.4%–2.7% per decade (Stanhill and Cohen, 2001; Ramanathan and Feng, 2009), and the effective sunlight duration declined by 1.28% per decade in China (Che et al., 2005). Studies showed that maize yield reduced by 6%-7% when every 1 MJ/m² decrease in solar radiation (Chen et al., 2020). In southern China, the plum rain season from June and July overlaps with the grain-filling stage of spring maize. However, the grain-filling stage is the key period affecting the yield and quality of waxy maize (Lu et al., 2014). Weak-light stress during grain-filling stage led to yield reduction by reducing grain number and weight (Yang et al., 2016; Shi et al., 2018; Wen et al., 2019), which posed a serious threat to the production safety of maize. Fresh waxy maize is the special maize with the largest planting area in China. The grain starch is almost 100% composed of amylopectin, and the characteristics of high viscosity, low regeneration and easy digestion, made waxy maize the best edible maize. The development of fresh waxy maize plays an important role in promoting the adjustment of planting structure and increasing farmers’ income in China. Therefore, it is of great significance to study how to alleviate the effect of weak-light stress on fresh waxy maize. Weak-light throughout the growth period led to slower growth and development of maize plants, sterility of tassel and ear, lower pollen viability and filament differentiation, and lower DM accumulation, which ultimately resulting in a reduced yield (Cui et al., 2015). Weak-light stress at different periods had different influences on maize, and more yield reduction occurred under weak-light at reproductive growth stage than vegetative growth stage (Yang et al., 2019). Weak-light stress occurred in the early stages has less effect on crop production (Deng et al., 2009), possibly because crop could be able to compensate for stress damage in middle and late stages (Kobata et al., 2000). However, grain number, weight and yield were significantly decreased under weak-light during the grain-filling stage (Deng et al., 2009). Maize grain yield primarily come from direct accumulation of photosynthates at post-silking and remobilization of the non-structural carbohydrate reserved from vegetative organs at pre-silking, and the direct accumulation of photosynthates at post-silking is essential for grain development (Tollenaar and Daynard, 1982; Farooq et al., 2011).

Weak-light stress at post-silking stage decreased chlorophyll content, damaged the ultrastructure of mesophyll cells, decreased photosynthetic capacity (Ren et al., 2016), and reduced DM accumulation, eventually leading to the loss of grain yield (Wang et al., 2020). Previous studies found that weak-light stress after silking significantly reduced the number and size of maize endosperm cells and reduced the enrichment of endosperm cells, which also led to smaller endosperm metastatic cells and the number of mitochondria, which eventually led to lower yield (Jia et al., 2011). Other studies have demonstrated that the IAA, ZR and GA contents in maize grain reduced and the ABA content increased under weak-light stress at post-silking stage, which inhibited grain growth and development (Gao et al., 2018). Studies on other crops also showed that weak-light stress significantly reduced grain yield of wheat (Li et al., 2010) and rice (Wei et al., 2018). N metabolism is an important process for the energy metabolism that determines crop yield and quality. Crop photosynthetic capacity is closely related to leaf N content (Sharwood et al., 2014). RuBPCase accounted for 50 ± 5% of the leaf soluble proteins in C₃ crop, and it accounted for 10%-25% in C₄ crop (Schmitt and Edwards, 1981). NR, GS, and GOGAT are the important enzymes involved in the assimilation of intracellular ammonium into organic compounds. Weak-light stress reduced NR, GS, and GOGAT activities in maize leaves, interfered with N metabolism, caused reduction of DM and N accumulation, and ultimately led loss of grain yield (Wang et al., 2020; Sun et al., 2023).

Weak-light stress posed a serious threat to maize production safety. However, there is few effective and reasonable protective measures to abate the influence. In recent years, a wide range of plant growth regulators, such as 6-benzylaminopurine (6-BA), gibberellic acid (GA₃), auxin (IAA) and cytokinin (CTK) have been widely used to reduce the damage of various abiotic stresses in crop production. The application of IAA and CTK increased grain yield, 1000-grain weight and filled-grain percentage of rice under salt stress (Javid et al., 2011). The application of GA₃ improved maize growth under salt stress (Rauf et al., 2022). It was well established and known that 6-BA could promote plant cell division, inhibit and scavenge free radicals, delay leaf senescence, increase DM and N accumulation (Roitsch and Ehneß, 2000). Exogenous application of 6-BA effectively alleviated the adverse effects of waterlogging on maize by increasing the leaf area index and chlorophyll content, reducing the MDA content, maintaining the stability of chloroplast ultrastructure (Ren et al., 2017), and reducing the ABA content (Hu et al., 2022). Exogenous 6-BA application during the fertile florets abortion stage of wheat increased the number of florets and number of grains by primarily suppressing the number of degenerated and aborted florets, which result in a further increase in grain yield (Li et al., 2019). Yuan et al. (2014) reported that soaking maize seeds by 6-BA could alleviate the physiological damage under drought stress. At present, most studies on 6-BA regulation of maize growth characteristics focused on waterlogging, drought and other stresses, and few studies reported on regulating growth of fresh waxy maize under weak-light stress. The main component of phytase Q9 is fulvic acid (fulvic acid content≥200 g/L) (Huang et al., 2020). Fulvic acid was reported to have significant effects in stress resistance. Exogenous fulvic acid application substantially reduced the damage of drought stress on maize by sustaining the chlorophyll contents and gas exchange possibly by enhanced SOD, POD and CAT activities and proline levels (Anjum et al., 2011). Huang et al. (2020) found that exogenous spraying phytase Q9 improved the leaf area index, SPAD value and net photosynthetic rate under weak-light conditions at the whole growth period (Huang et al., 2020). However, whether phytase Q9 could alleviate the effect of weak-light stress on fresh waxy maize and its regulation mechanism need further study. This study aimed to investigate the effects of exogenous spraying 6-BA and phytase Q9 on the yield and photosynthetic characteristics of fresh waxy maize under weak-light stress, and provide theoretical basis and technical support for stress-resistant cultivation of fresh waxy maize under climate change.

Solar radiation is the energy source for the accumulation of photosynthates (Yang et al., 2021a). Photosynthate accumulation especially at post-silking is a key determinant of maize yield (Barnabás et al., 2008; Liu et al., 2020). The leaf N content is closely associated with plant photosynthetic capacity. A high N content in leaves enhanced photosynthesis and delayed leaf senescence (Sinclair et al., 2000). In this study, the results showed that weak-light stress decreased fresh ear and grain yield, DM and N accumulation in fresh waxy maize, which was consistent with the previous results (Yang et al., 2021a; Yang et al., 2021b). Exogenous hormone played an important role in the response to various abiotic stresses. Previous studies have showed that spraying phytase Q9 improved yield and DM accumulation on summer maize (Huang et al., 2020), and this is consistent with the results in this study on fresh waxy maize. Furthermore, we also found that spraying 6-BA and phytase Q9 both increased the yield, DM and N accumulation, and the increase was greater in 6-BA.

Photosynthetic capacity is the major determinant of crop productivity. Our study showed that weak-light stress decreased the Pn of ear leaves, which was consistent with the previous results on normal maize (Sinclair et al., 2000; Sharwood et al., 2014). Weak-light stress reduced leaf area index (LAI) and chlorophyll content and damaged the mesophyll cell ultrastructure, which led to the decrease of photosynthetic capacity, and thus resulted in significant yield loss (Ren et al., 2016). Previous study found that LAI, SPAD value and Pn of summer maize significantly increased under spraying phytase Q9 (Huang et al., 2020). In this study, we found that spraying 6-BA and phytase Q9 increased Pn significantly under weak-light stress, and the increase was higher in 6-BA. RuBPCase and PEPCase are key enzymes involved in carbon metabolism, and NR, GS, GOGAT are key enzymes involved in N metabolism. The activities of RuBPCase, PEPCase, NR, GS and GOGAT were decreased under weak-light on summer maize (Sharwood et al., 2014; Wang et al., 2020). This is generally consistent with the results of this experiment in fresh waxy maize. Studies reported that weak-light stress reduced the activities of photosynthesis-related enzymes in maize leaves (Sharwood et al., 2014), which reduced the net photosynthetic rate and led to the decrease of photosynthetic production capacity (Zhong et al., 2014). The decrease in grain weight and volume under weak-light may be due to reduced photosynthetic rate and decreased assimilate production capacity resulting from the inhibition of leaf N metabolism (Sharwood et al., 2014). In this study, spraying 6-BA and phytase Q9 increased the activities of PEPCase, RuBPCase, NR, GS and GOGAT under weak-light stress. This promoted the improvement of photosynthetic capacity, which in turn increased DM and N accumulation. Our experimental results also indicated that the spraying effect on carbon and N metabolism of 6-BA was better.

SOD, CAT and POD are key enzymes involved in antioxidant systems, and MDA is one of the products of membrane lipid peroxidation, its content can be used as one of the indicators to investigate the severity of cell stress. Previous study found that weak-light stress reduced the activities of SOD, CAT and POD, and increased the MDA content in ear leaves of summer maize (Huang et al., 2020). In this study, we also found that weak-light stress decreased the activities of SOD, CAT and POD, and increased the content of MDA, which accelerated the oxidative damage of leaf in fresh waxy maize. Under weak-light stress, plasmolysis occurred in mesophyll cells and the endomembrane system was destroyed, which resulted in the dissolution of cell membrane, karyotheca, mitochondria, and the mesophyll cell ultrastructure was damaged, which led to the decrease of photosynthetic capacity, and thus resulted in yield loss (Ren et al., 2016). Spraying phytase Q9 could increased the activities of SOD, CAT and POD (Huang et al., 2020). Our results also indicated that spraying 6-BA and phytase Q9 increased the activities of SOD, CAT and POD, and reduced the MDA content, which delayed the leaf senescence. We also found that the spraying effect on antioxidant systems of 6-BA was better than phytase Q9. Spraying exogenous hormones significantly alleviated the decline in anti-aging ability and photosynthetic productivity of fresh waxy maize under weak-light stress, thereby promoting dry matter accumulation and increasing the fresh ear and grain yield. Further research is needed on the hormone metabolism and molecular physiological mechanisms of exogenous regulators applied to improve the yield of fresh waxy maize under light stress.

The results showed that weak-light stress during the grain-filling stage severely limited photosynthetic production capacity by reducing the activity of enzymes involved in carbon and N metabolism, accelerating leaf senescence, and thereby reducing the accumulation of photosynthetic products, resulting in reduced yield in fresh waxy maize. The effect of weak-light stress was greater on JKN2000 than SYN5. Spraying exogenous phytase Q9 and 6-BA alleviated the decrease in carbon and nitrogen metabolism enzymes and antioxidant enzyme activities under weak-light stress, improved photosynthetic capacity, delayed leaf senescence, and thus increased dry matter and nitrogen accumulation, increasing yield. The improvement effect of 6-BA was more significant on JKN2000. These results indicated that in the actual production of fresh waxy maize, 6-BA can be widely used as an effective regulator to alleviate weak-light stress.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

GL, WL, and DL designed the experiments. GL,WL, and YL did the field management. GL, WL, and WPL performed data analysis. GL, WL, and DL contributed to the preparation of the manuscript. All authors contributed to the article and approved the submitted version