Bioscience Methods 2025, Vol.16, No.6, 270-279 http://bioscipublisher.com/index.php/bm 2 74 Positive correlations can also be found between physiological indicators such as photosynthetic rate, chlorophyll concentration, and flag leaf fluorescence and grain yield. So these traits are not only useful in breeding but also have great reference value for field evaluation. Finally, only those varieties that can balance high yield and high NUE are truly worthy of long-term use in sustainable production as "star varieties". 5 Mechanisms of Fertilizer Regime Effects on Rice Performance 5.1 Regulation of growth and development by nitrogen supply intensity Whether rice grows well or not, nitrogen supply is an unavoidable factor. Generally speaking, when nitrogen is provided sufficiently, the plants naturally grow taller, have more tillers, larger leaves, and the biomass increases accordingly, and the yield is also guaranteed. But then again, more nitrogen fertilizer is not necessarily better. Excessive application may instead cause problems, such as slow aging of leaves and inadequate filling of panicles, mostly due to hormonal imbalance or decreased activity of certain key enzymes (Chen et al., 2022). When there is less nitrogen, it will definitely affect the growth, the chlorophyll content will also be low, and the yield is likely to drop. But not all varieties are so brittle. Some "fertilizer-saving" ones can withstand low nitrogen and have a relatively strong growth recovery ability (Qi et al., 2025). In addition, not only the amount should be well controlled, but also the application time is crucial. During critical periods such as panicle differentiation, if nitrogen supply is insufficient, it is easy to miss the yield window (Zhang et al., 2025). 5.2 Impact of nitrogen levels on enzyme activity and chlorophyll content Once the nitrogen supply changes, the alterations in some enzymes and chlorophyll within plants can be immediately observed. The guys that dominate nitrogen metabolism, such as nitrate reductase (NR) and glutamine synthase (GS), as well as GOGAT, are all very sensitive. When there is sufficient nitrogen, the enzyme activity becomes active, the contents of chlorophyll a and b will also increase, and the efficiency of photosynthesis will rise accordingly (Ali et al., 2021; Ullah et al., 2021; Liao et al., 2022). However, don't be too happy too soon. Excessive use of nitrogen not only fails to infinitely increase the photosynthetic rate, but sometimes even causes problems in reverse, such as increased photosuppression or oxidative stress. At this time, plants have to activate the antioxidant defense line, and enzyme activity and energy allocation are all messed up (Kiran et al., 2016; Cisse et al., 2020; Wang et al., 2022). The situation of low nitrogen is also rather complex. Although the chlorophyll and enzyme activities of some varieties decrease, certain antioxidant systems or starch metabolic pathways in the body are more active instead, indicating that they have their own countermeasures. 5.3 Interaction between fertilization methods and soil nitrogen dynamics How to fertilize also directly affects the movement of nitrogen in the soil. There are significant differences among various methods. In some places, attempts have been made to combine the "old-fashioned methods" such as biochar, manure and straw with modern synthetic fertilizers. The results are quite good. Not only has the organic carbon in the soil increased, but also the availability of nitrogen and the activity of microorganisms have improved. The root system is well-developed, the enzyme activity is more active, and the nitrogen mineralization is faster. From another perspective, applying fertilizer deeper or to the side can also increase the total nitrogen content in the deep soil and stimulate the activity of beneficial microorganisms. However, if the traditional surface application method is used, it may be more conducive to the accumulation of surface carbon (Xie et al., 2025). Some farmers also use nitrification inhibitors or slow-release fertilizers. Long-term use can make the soil condition more stable and the crop yield does not decrease (Tang et al., 2024). Therefore, fertilization is not merely about "quantity" to win. It depends on what the crops need and what the soil lacks. Tailoring to the specific needs is the key. 6 Case Studies: Regional Field Trials 6.1 Fertilizer-variety matching studies in the middle and lower Yangtze River Basin In the middle and lower reaches of the Yangtze River, it's not the case that the less nitrogen fertilizer there is, the lower the yield will be. The situation is actually not that simple. Many field trials have found that nitrogen-saving varieties (NSV) did not have a decrease in yield after reducing nitrogen fertilizer application by 40%-60% (from the original 300 kg N/ha to 120-180 kg N/ha), and even broke records, while those common varieties could not
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