Interpretation of lodging areas in rice fields using aerial photography
Rice field trials in the experiment were conducted in central Taiwan for 2 consecutive years (Fig. 1A), and an unmanned aerial vehicle was used to take multispectral images of the rice variety TNG71. First, a digital surface model (DSM) was used to express the spatial distribution of actual topographic features to the height difference of rice plants from the ground to be estimated, and orthomosaic images displayed the distribution of plants on the ground. The two images were then superimposed to evaluate the TNG71 lodging percentage under different types of water management (CP and AWD) and under different fertilizer levels (four nitrogen fertilizer levels). The results revealed that, in 2019, under CP management, both the fertilization rate and lodging area of each plot increased under different fertilization levels, and under AWD management, a lodging percentage of more than 40% occurred at the fertilization rate of 200 kg/ha (N4) (Fig. 1B). In 2020, the N3 and N4 nitrogen level areas exhibited more than 20% lodging percentages under AWD management (Fig. 1C). Our results revealed that the application of excessive nitrogen fertilizer might increase the occurrence of rice lodging.
Incidence of rice lodging in relation to field moisture, fertilizer management, and meteorological factors
Rice growth is affected by different field management methods and external environmental factors, which can lead to the occurrence of rice lodging. To understand the relationship between rice lodging and different types of water and fertilizer management, the rice lodging percentage, water management, and fertilizer application were analyzed for variance. The results demonstrated that water management and fertilizer application had a significant positive effect on the lodging percentage in 2019, but no significant positive effect was identified in 2020 (Table 1).
In 2019, under CP management, more than 135 kg/ha of fertilizer was applied, whereas under AWD management, more than 165 kg/ha of fertilizer was applied, and a lodging percentage of more than 15% occurred in this area (Fig. 2A). In the first phase of 2020, under AWD management, the lodging percentage exhibited an upward trend when the fertilizer application reached more than 120 kg/ha (Fig. 2B). To explore whether meteorological factors affected the lodging percentage of rice, the data from the weather station were used to analyze the changes in temperature, rainfall, and wind speed over the 2-year study period. The results revealed that the mean daily temperature in the maximum tillering stage of the rice (47 days after transplantation; DAT47) was 29.1 °C in 2019. However, the mean daily temperature in the same growth stage in 2020 was 16 °C, which was 13.1 °C lower than that in 2019 (Fig. 2C). The accumulated precipitation during the rice-growing period in the first phase of 2019 was 1168 mm, which was higher than the accumulated precipitation in the first phase of 2020 (450.5 mm). The accumulated precipitation in 2019 in the 10 days before harvesting was 410 mm, which was 9 times higher than that in 2020. We speculated that this was caused by continuous rainfall during the rice harvest period, and the lodging results were more severe in 2019 than in 2020 (Fig. 2D). According to the meteorological data in 2019, the average wind speed in the 10 days before harvest was 2.01 m/s, and the instantaneous maximum wind speed during the same period was 19.9 m/s. The average wind speed in the 10 days before harvest in 2020 was 1.71 m/s, and the instantaneous maximum wind speed during the same period was 16.4 m/s (Fig. 2E and F). Our results indicated that the higher lodging percentage of rice in 2019 was probably caused by the strong winds and heavy rains during the rice harvest period.
Correlation between plant height and lodging risk in rice
To evaluate the correlation between plant height and lodging risk during rice development, we further observed plant height growth during the whole growth period of TNG71. Our results revealed a significant correlation between rice plant height and nitrogen fertilizer levels in 2019 and 2020, and the interaction effect on plant height was significant for water management and nitrogen fertilizer levels in the middle tillering, maximum tillering, booting, heading, milk, dough, and mature stages of rice in 2019 (Table 2). The relationship between plant height and lodging and yield in different growth stages of rice was further analyzed. Plant height in the booting stage in 2019 was 72.2 ± 2.9 cm (Additional file 1: Table S1), and the Pearson correlation results revealed that plant height was positively correlated (r = 0.67) with lodging and negatively correlated (r =− 0.46) with yield (Fig. 3A). Plant height in the booting stage in 2020 was 62.5 ± 3.6 cm and had no significant positive correlation with lodging (r = 0.37) but was highly positively correlated with yield (r = 0.65) (Fig. 3B). Additional file 1: Table S1 presents the plant heights in different growth stages in 2019 and 2020. Additional file 1: Table S2 These results indicated that the difference in plant height in the booting stage between 2019 and 2020 was a key factor in subsequent rice lodging.
Comparing the continuous changes in the plant height of rice in 2019 and 2020, we noted that the growing degree of rice in the booting stage was 588.9 °C, corresponding to plant heights of 72.2 and 65.5 cm in 2019 and 2020, respectively. The difference in plant height may be related to temperature, rainfall, and other environmental factors. The high-temperature environment and abundant rainfall in 2019 enabled faster plant growth, producing more plant height than that in 2020 in the same growth stage (Fig. 4A) and leading to the occurrence of lodging in the later stage of growth. The difference in plant height between the lodging and nonlodging rice areas in 2019 was analyzed. The results demonstrated that if the rice plant height exceeds 70.7 ± 3.0 cm in the booting stage and sufficient nitrogen fertilizer continues to be applied, lodging risk increases in line with the predicted plant height growing curve. Compared with the plant height in 2019, the plant height in 2020 was only 65.6 cm in the booting stage, which may explain why lodging was less severe in 2020 (Fig. 4B and C).
Effect of lodging on rice yield and taste quality
The application of nitrogen fertilizer can increase the spikelet and panicle number and enhance rice yield (Kamiji et al. 2011; Xiong et al. 2013). In the present study, ANOVA was performed on our rice yield data, fertilizer content, water use efficiency, and lodging results to evaluate whether lodging affected rice yield. In 2019, rice lodging occurred more severely in the field with high levels of nitrogen, and the nitrogen fertilizer content was significantly correlated with the lodging results and final yield, suggesting that both lodging and yield were affected by the nitrogen fertilizer content. In 2020, lodging occurred less severely than in 2019. Nitrogen fertilizer did not exert a significant positive effect on lodging, but its relationship with panicle number, grain number, fertility, and yield were significant, revealing the positive effects of nitrogen fertilizer on rice yield (Table 3).
In addition to rice yield, taste quality is a key indicator of rice production. Rice is mainly composed of starch, protein, fat, and minerals. The taste quality of rice is affected by physical and chemical properties, including amylose and protein content, hardness, and cohesion (Aoki et al. 2017; Tran et al. 2005). ANOVA revealed that the application of nitrogen fertilizer had a significant positive effect on the lodging rate, taste, and protein and amylose content in 2019, indicating that lodging caused by a high nitrogen content might change the palatability and protein and amylose content of rice. The 2020 results indicated that water efficiency and nitrogen fertilizer application had a significant positive effect on the appearance, hardness, and taste quality of rice, indicating that these characteristics are also affected by fertilizer and water content (Table 4).
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