Partial shading improves photosynthetic performance by increasing leaf greenness and delaying LS
It has previously been reported that plants exhibit different metabolic strategies in different light intensities (Keech et al., 2007). Maize leaves are highly sensitive to slight changes in light intensity, and any decrease in light intensity affects photosynthetic performance (Feng et al., 2019), which results in a change in leaf greenness, as shown in the current study. We attribute the delayed senescence in maize leaves as a contributor to increased plant photosynthetic capacity and carbon balance. The results indicate that partial shading delays the leaf senescence process in maize compared to NL, possibly via an enhanced photosynthetic process. However, the light interception by the whole plant is also important in the delayed leaf senescence process. For instance, our results of different partial shading degrees showed that 20% shading would delay leaf senescence, maintain leaf greenness and nitrogen content, prolong the senescence time of bottom leaves, and increase the leaf area (Supporting Information Figure S5, Table S2).
Improved photosynthetic performance is required for balanced growth in plants. In the present study, partial shade remarkably increased maize photosynthetic ability compared to full shade. Evidence has demonstrated that shade changes a plant’s micro-climate, which disrupts a plant’s photosynthetic performance, growth activity, and reduced yield (Gindaba & Midgley, 2005), while the right amount of shading improves plant growth and photosynthetic performance (Han et al., 2018). Consistent with the present study data, NL significantly reduced photosynthetic performance in maize plants, while partial shading improved the photosynthetic performance in maize plants. We observed that all the photosynthetic characteristics, Chl b, and nitrogen content were increased under partial shade (Fig.1, Supporting Information Figure S3). According to the results (Hachiya & Noguchi, 2011), the high transcription of nitrogen transporter geneZmNRT1.1a may maintain the nitrogen content and chlorophyll protein stability in PS leaves. Furthermore, the expression of chlorophyll degradation-related genes (ZmPaO ) increased at first ten days then stayed unchanged in PS, however in NL treatment, the expression level constantly increased during the whole experimental period, indicating leaves chlorophyll degradation in PS was reversed but continued to rise in NL (Fig. 7). Chlorophyll content and degradation process were positively correlated with aging senescence. We speculate that a possible mechanism is used by partial shade to enhance delay in leaf senescence in maize. We hypothesized that the improvement in the photosynthetic performance during the partial shade treatment might result from enhancement in plant hormonal balance under the partial treatment (Clarke et al., 2009; Wang et al., 2010). Others also reported that delay in leaf senescence could improve plant photosynthetic performance, which could, in turn, increase plant yield (Gregersen, 2011), which is consistent with our results (Table 3). Together, these results suggest that partial shade delays leaf senescence in maize by enhancing the photosynthetic rate and inhibiting chlorophyll degradation.
How Partial Shade Coordinates Phytohormones homeostasis to Regulate Leaf Senescence?
Phytohormones and their related genes regulate plant growth and development under various stresses (Bawa et al., 2020a; Bawa et al., 2020b; Tang et al., 2019). Indeed, all major plant hormones have been reported to play an important role in leaf senescence regulation (Schippers et al., 2007) by altering the time of age-related changes and interacting with the prevailing environmental conditions, thus influencing the speed of the overall senescence process (Jibran et al., 2013). Similarly, in the present study, we hypothesized that different light regimes might influence the hormonal regulations between the two sides of the maize plant, impacting leaf senescence. For instance, phytohormones like ethylene, abscisic acid (ABA), and salicylic acid (SA) accelerate the senescence process in contrast accumulation of auxin, gibberellin, and cytokinin delay senescence (Guo & GAN, 2012; Jibran et al., 2013). Previously, increased accumulation of some plant hormones such as IAA and GA3 under slight shading has been reported as growth promotion indicators (Han et al., 2018). Consistent with these findings, our results showed that the partial light conditions induced significant accumulation of IAA, ABA, GA3, and ethylene, specifically in PS leaves. The IAA content in PF leaves also increased; suggesting that some key factors might regulate phytohormone balance and limit leaf senescence in PS treatment (Fig. 9). PIF4 binds promoters of TAA1 , CYP79B2 , and YUCCA8 genes to promote auxin biosynthesis at high temperatures (Sun et al., 2012; Wang et al., 2017a). In the current study, may be a similar mechanism of ZmPIF4 controls the IAA level in maize leaves under PS treatment. PIF4 and PIF5 regulated GA biosynthesis via a circadian clock factor ELF3 as loss of ELF3 activity caused GA accumulation. Moreover, PIF4 is also involved in activating transcription factors EIN3 and ABI5 to regulate leaf senescence. In the present study, PIF5 and not PIF4 might play a key role in maintaining phytohormone balance because only PIF5 was strongly induced by the PS treatment (Fig. 6B). Another protein expression data of full shade leaves showed that continuous shading could significantly influence the expression of PHYA and EIN3 that leads to leaf senescence (Supporting Information Figure S6). However, the senescence inducing hormones (ABA and ethylene) decreased significantly during the succeeding days which could be a reason for the delayed leaf senescence under the partial light conditions.
Photoreceptors recognize the light conditions in plants, and as a component of light signaling, they are associated with leaf senescence (Kim et al., 2017). In this perspective, identifying some major transcription factors (TFs) that play a crucial role in leaf senescence through TFs-mediated senescence regulations has enhanced the importance of TFs-mediated senescence regulation. Therefore, we investigated the expression of several transcription factors that are involved in light signaling and phytohormone responses like ZmPHYA1b (Zea maize Phytchrome A1 b),ZmPIF4.1 (Zea maize Phytochrome Interacting factor 4.1),ZmPIF5.1 , ZmEIN3.1 (Ethylene Insensitive 3),ZmABI5.1 (ABA Insensitive 5) and SAG (Senescence associated genes) to verify their contribution in the regulation of leaf senescence under the different light regimes in maize plants. Several reports have suggested the involvement of PIFs in dark-induced and developmental senescence as they transduce light information to primary senescence regulators (Kim et al., 2017). PIF4/PIF5 acts in the signaling pathways of two senescence-promoting hormones, ethylene and abscisic acid, by directly activating the expression of EIN3, ABI5, and EEL (Sakuraba et al., 2014). Similarly, in our study, during the first ten days under partial light conditions, the PS leaves of maize plants showed significant upregulation of ZmPHYA1b , ZmPIF4.1 , and ZmPIF5.1, ZmABI5.1, and ZmEIN3.1 genes (Fig. 6A). Here, PIF4 protein was not expressed in NL and PL treatments. However, in FS, the expression of PIF4 was significantly induced by early full shading. The same results were found in PHYA, PIF5, EIN3, indicated heavy shade would certainly lead to rapid leaf senescence (Fig. 6B). Contrarily, their expression was suppressed in the PF leaves compared to NF, NS, and PS. It shows that the leaves under PF conditions compensated the leaves on the other side of the maize plant to counter the influence of shading which could be a reason for delayed leaf senescence. In agreement with our results, different studies reported the activation of senescence-related genes inA. thaliana under partial shading (Brouwer et al., 2014). However, our research found that heterogenous shading under PL conditions induced a regulatory mechanism between the two side leaves. The PF leaves balanced the initial increase of senescence regulators in PS leaves.
Several senescence-associated genes (SAGs) have been identified in Arabidopsis, wheat, rice, and maize that have been linked to the complicated and highly regulated process of leaf senescence (Li et al., 2014; Wu et al., 2016). Therefore, to further verify the information revealed through phytohormone signaling regulation and expression of relevant transcription factors, we quantified maize SAGs. Notably, the reduced expression of ZmSAG2and ZmSAG12 in PS and significant reduction after an initial increase in the first ten days under partial light conditions suggested a synergy between the two side leaves. This observation verified the delayed leaf senescence under partial light conditions in maize (Fig.7). Furthermore, in agreement with our hypothesis, seed dry matter accumulation increased from day 5 up to day 25 under NL. In contrast, partial shading increased dry seed matter accumulation from day 5 up to day 35 (Fig. 8), which could be due to increased leaf greenness under partial light conditions. Collectively, it could be displayed from the hormonal signaling and expression pattern of senescence-related TFs that the decrease in light intensity improved the growth response and greenness of maize leaves due to the compensatory interaction between the two lateral sides of the maize plant under heterogeneous light conditions.