Increased PPD coupled with MC application improves seed yield through the enhancement of the biological yield
Cotton lint yield is determined by either three yield components (boll density per unit land area, boll weight, and lint percentage) or dry matter accumulation and partitioning. In terms of the latter, the biological yield was increased, but the harvest index decreased as the PPD level increased (Table 2), which was supported by Dai et al. (2015) and Zhang et al.( 2016), but did not agree with Cordeiro et al. (2021) who reported that MC application improves cotton harvest index. The MC application decreased the biological yield but increased the harvest index across both years, thus did not alter seed cotton yield (Table 2). Among the six combinations of PPD of three levels and MC of two levels, MC1D3 had the greatest seed cotton yields, which was largely attributed to the highest biological yield (Table 2). Also, MC1D3 displayed a maximum in lint yiled resulted from a concurrent greatest boll sensity (Tang and Luo, 2023; Luo and Tang, 2003).
Increasing PPD coupled with MC application delays leaf senescence as indicated by higher SLW, Chl concentration, and Rubisco activity at the FB to FBO stage
The SLW was roughly decreased with increasing PPD levels before the FBO stage, but the trend was reversed at the FBO stage (Fig. 1a, 1b). At that time, MC1D3 exhibited the maximal SLW, being 0.3% higher in 2019, and 2.4% higher in 2020 than the second largest combination MC0D3. Pettigrew and Johnson (2005) reported an increased SLW by 4% due to MC application. Enhanced Chl concentration (either Chl a or Chl b) after cotton plants' exposure to MC application has been well recognized (Reddy et al., 1996; Tung et al., 2018a). The Chl a, Chl b, and Chl a + b concentrations pooled across all PPD levels were consistently higher during the PS to FBO stages in the MC application regime than in the MC free regime (Table 6, Fig. 2, 3). MC1D3 demonstrated the greatest Chl a, Chl b, and Chl a + b concentrations either at the FB stage in 2019 or at the FBO stage in 2020 (Fig. 2, 3). Chlorophyll concentration is an important indicator of leaf senescence (Kong et al., 2016; Chen et al., 2018). Rubisco is a crucial rate-limiting enzyme responsible for CO2 fixation in the Calvin cycle during photosynthesis. The Rubisco activity showed a rising trend through the PS to the FB and then declined sharply down to the FBO. MC1D3 exhibited the maximum Rubisco activity at the FB stage in 2019, and at the FB and FBO in 2020. The Rubisco activity was 2.6–53.2% higher at the FB stage in both years, and 2.4–52.7% higher at the FBO stage in 2020 than those in the other treatments (Fig. 6a, 7a). MC application-decreased Rubisco activity was observed at the early reproductive stage as indicated by lower activity at the PS stage in 2019 (P = 0.024 ) and the FF stage in 2020 ( P = 0.040). The result accords with the studies by Reddy et al. (1996) and Tung et al.(2019) where the activity of RuBP carboxylase is decreased in MC-treated plants. However, in the late reproductive stage such as the FBO, the MC application improved the Rubisco activity by 17.2% in 2019, and 28.1% in 2020. In addition, the positive correlations between the Rubisco activity and Chl a, Chl b, and Chl a + b were detected in both years (Fig. 8). Furthermore, delayed cotton leaf senescence due to increasing PPD has been observed (Dong et al., 2006; Dong et al., 2012; Luo et al., 2018). Taken together, MC1D3 was characterized by greater SLW, Chlorophyll concentration, and Rubisco activity at the late reproductive stage such as the FB and FBO stages, which should contribute to delayed leaf senescence together.
Increasing PPD coupled with MC application improves cotton leaf photosynthetic carbon production as indicated by enhanced starch and TNC concentrations at early reproductive growth and their utilization efficiency in terms of transformation rate
All carbohydrate components and TNC concentrations peaked at the FF stage and then declined down to the FBO stage (Fig. 4, 5). MC1D3 displayed higher starch and TNC concentrations at the FF stage, but no difference from or even lower than the other treatments at the FBO stage over both years. Therefore, the greatest transformation rates of starch and TNC were produced in MC1D3 (Table 11). The higher starch concentration at the FF stage in MC1D3 may be related to a higher Rubisco activity at that time (Fig. 6, 7). The higher Rubisco activity can be attributable to the higher chlorophyll (a, b, and a + b) concentration as evidenced by the positive correlation between them (Fig. 8), while the higher chlorophyll concentration is typically caused by MC application (Reddy et al., 1996; Tung et al., 2018a). As little information about the PPD effects on cotton leaf physio-biochemical characteristics is available, the following discussion focuses on the MC application effects on them. To date, there were a large number of controversial reports on the MC application effects on the carbohydrate content, Pn, and Rubisco activity in the cotton leaf and their interrelationships. Tung et al. (2019) reported that photosynthesis is decreased by 1–26% as a result of reduced Rubisco activity by 3–37% in MC-treated plants, and in turn leads to the accumulation of starch and sucrose and yield loss. MC application increases the accumulation of sucrose, hexose, and starch in the cotton leaf, and thus reduces photosynthesis (Tung et al., 2018a). Pn and Rubisco activity are decreased, but chlorophyll and starch contents are enhanced, and sucrose content is not changed in MC-treated cotton leaves(Reddy et al., 1996). MC application increases leaf CO2 exchange rate and starch content but does not affect photoassimilate export from leaves to young bolls (Zhao and Oosterhuis, 2000). Different from the above Pn measured on a single cotton leaf, canopy gross photosynthesis is increased within 48 h after MC application (Hodges et al., 1991). In addition, Hu et al. (2017) reported that elevated air temperature increases starch content, but decreases sucrose content, which is attributed to low Rubisco and FBPase activities in cotton leaves. Liu et al. (2013) observed that the contents of sucrose, starch, hexose, Rubisco and FBPase activities, and SLW are increased, but Pn and sucrose transformation rates are decreased by low-temperature treatment due to late planting. Taken together, there have existed complicated associations among photosynthesis, related enzyme activity, and carbohydrate content in cotton leaves that deserve more exploration.
MC1D3 is expected to enhance the canopy photosynthetic production on a population basis by increasing the population density as indicated by the first or second largest Leaf area index throught the whole reproductive growth phase (Tang and Luo, 2023). Mao et al. (2014) found that increasing plant density significantly enhances light use efficiency as mediated by improving light distribution in the cotton canopy. Moreover, the population photosynthetic production is potentially further elevated by the additional MC application, because the measure typically inhibits leaf expansion, and creates compact plant stature, thus allowing more light penetration to the low-middle canopy. Gonias et al. (2012) reported that radiation use efficiency is significantly enhanced by 33.2% in MC application treatment, which is probably due to changes in leaf photosynthetic capacity and light configuration throughout the cotton canopy.
Starch is a predominant form of carbon reserves in plants. High starch and TNC concentrations at the FF stage (early reproductive growth) in MC1D3 imply a great carbon supply potential that is required for heavy boll load, which accords with the previous observation of a higher boll density in MC1D3 relative to the other treatments in both years (Tang and Luo, 2023; Luo and Tang, 2023). The combination of the high PPD and MC application is prone to render boll setting more concentrated, increasing synchronous demand for photosynthate (Gwathmey and Clement, 2010; Chen et al., 2021). The high transformation rate of starch throughout the FF to FBO stage in MC1D3 probably means a rapid starch degradation into soluble sugars for translocating into developing bolls, which help to synchronize boll maturation.