Environmental Influences on SPAD Values in Prunus mume Trees: A Comparative Study of Leaf Position and Photosynthetic Efficiency Across Different Light Conditions

1. Introduction

The appropriate environment for photosynthesis is essential for the plant's survival, and the light environment is especially important. For instance, shade-intolerant plant species require high levels of light and activate shade-avoidance responses that concentrate the resources for growth while reducing the energy of the defense system when the light is limited (Ballaré, 2014). Light availability in the shaded environment, including the understory, is essential to seeding survival and plant growth in forest ecosystems (Lin et al., 2014). Within a single plant, the light environment can vary depending on the position of each leaf or the surrounding environment. Zhen et al.(2022) reported that far-red photons could account for over 50% of the total incident photons between 400-750 nm in vegetation shade, and it could contribute 10-14% of leaf gross photosynthesis in a tree and an understory species in deep shade.

As highlighted, it is imperative to find ways to utilize light because of its importance to plants effectively. Before studying the light environment, it is important to consider that the light environment is constantly changing. The sun's position shifts over time, and weather changes affect the amount of daylight, influencing the light environment crucial for plant survival. Leaves in strong light environments develop a thicker palisade parenchyma layer to utilize high light intensities more effectively than those in shaded areas (Arrigoni-Blank et al., 2022; Yang et al., 2023). Additionally, leaves exposed to light tend to have higher nitrogen and chlorophyll content compared to those in shaded areas (Arrigoni-Blank et al., 2022). The nitrogen-to-phosphorus ratio is higher, and leaf mass per area is greater in high-light environments than in shaded places (Arrigoni-Blank et al., 2022; Yang et al., 2023).

The position of individual leaves significantly influences photosynthetic efficiency by determining their exposure to sunlight in terms of both duration and intensity. Leaves located higher within the canopy are more likely to receive direct sunlight for extended periods, enhancing their photosynthetic capacity. Leaf angles and density adjust to optimize light capture as the sun's position changes (de Casas et al., 2011). The leaf angle strategy also varies depending on the leaf's position, with more horizontal leaf angles observed in low-light environments to maximize light capture (Migliavacca et al., 2017). These traits are different from plant species to species. Each plant species adapts their leaf traits differently based on its light environment (de Casas et al., 2011; May and Oberbauer, 2021). These adaptations affect photosynthetic efficiency, with some species adapting to a wide range of environments and others to more specific conditions.

In this study, Prunus mume is used as plant material. The P. mume originated in China and is an important fruit crop in the subtropical region (Shi et al., 2020). P. mume is a culturally significant tree valued for its flowers; in East Asia, the fruits of this plant are extensively utilized in traditional foods and medicines and are recognized for their nutritional benefits, including vitamins, minerals, and antioxidants such as phenolic compounds (Ali et al., 2017). In Korea, the application of an open-field smart farm system to P. mume is being promoted (Lee et al., 2020), and for this purpose, research on the light environment of plum plants is needed. Aside from this, light environment research to increase the efficiency of fruit production is being conducted all over the world. The light intensity and exposure during fruit development significantly impact many traits of the fruit that are related to the final quality and price (Ismail et al., 2009). Also, the leaf condition directly related to photosynthetic capacity can impact the supply of carbohydrates and other nutrients to the quality of the developing fruits (Pino et al., 2023). Therefore, the light environment of the leaf is an important factor in fruit.

Soil plant analysis development (SPAD) measures leaves' greenness or relative chlorophyll content by measuring the leaf's transmittance in the red and near-infrared regions of the electromagnetic spectrum (Zhang et al., 2022). The SPAD meter provides a quick and non-destructive way to estimate chlorophyll content. It is widely used in agriculture to Estimate nitrogen status and fertilizer requirements (Vishwakarma et al., 2023). It is also used to monitor plant health and stress levels. Han et al.(2022a) report that the blueberry plant changes the SPAD values over time and differs from cultivars. Also, it was reported that the blueberry's planted place (pot vs ground) affects the SPAD value changes in a day (Han et al., 2022b).

In this study, we confirm the change in the SPAD value of P. mume in a planted place with a different light environment. One area is sunny, while the other is shaded. We also investigate the SPAD value differences of each leaf based on canopy height and time. This comprehensive analysis provides insights for optimizing the light environment to improve plant growth and fruit quality.

2. Materials and Methods

2.1. Character of the experiment area

The trees of P.mume located at Cheongju National University of Education in Sugokdong, Cheongju City, Korea, were measured to conduct this study. Fig. 1 represents Site A, the sunny area, and Site B, the shaded area. Table 1 represents the photosynthetic photon flux density (PPFD) value of sites A and B for three days, from 11:00 to 11:30 and from 16:00 to 16:30. The first two days were sunny, and the last day, 26 July, was rainy and cloudy during measurement (Table 1).

Fig. 1.

Experiment areas. Site A is a sunny area, and site B is a shaded area. The image is from Google Maps.

Table 1.

The day envelopment of each measuring day (2023 July 24 – 26)

3. Results and Discussion

The trees of P. mume planted at site A are in the sunny area in front of the building, and the direction of the building is on the north side of the trees. In contrast, the trees planted at site B are in the shaded area on the side of the building. The direction of the building is east of the P. mume trees; Because of the height of the building, the trees are shaded most of the day (Fig. 1).

Table 2 presents the PPFD measurements at sites A and B over three days, specifically during the peak sunlight hours of 11:00 to 11:30 at site B and the late afternoon period of 16:00 to 16:30, marking the end of significant photon availability from the sun.

Table 2.

PPFD value of each day during measuring SPAD value (from 11:00 to 11:30 and 16:00 to 16:30)

On July 24, 11:00, the PPFD values of min, max, and average were higher than those in site B. Also, from 16:00 to 16:30, the values in site A were higher than those in site B (Table 2). This trend was maintained the next day; All PPFD values were higher at site A than at B. However, on day 3, on July 26, this trend changed, caused by the cloudy and rainy weather. This weather can be confirmed in Table 1, which is the record of a rain shower. On that day, at 11:00, the maximum PPFD value was higher at site A, but the other values were higher at site B. Similarly, at 16:00, only average PPFD values were higher at site A, and the others were higher at site B (Table 2).

Table 3 illustrates the ANOVA results for SPAD values among trees at sites A and B. On 24 July, a significant difference in SPAD values was observed between all trees at both sites (p < 0.001). This significant variation persisted throughout the observation period (24 July to 26 July), consistently showing higher SPAD values at site A, a sunny area, compared to site B, a shaded area, despite the trees being of similar age (Table 3).

Table 3.

ANOVA result of SPAD value difference in a day between three P. mume trees

The variation in SPAD values over time was analyzed, with Table 4 detailing the differences between 11:00 and 16:00 measurements. On 24 July at Site A, SPAD values for Trees 2 and 3 showed no significant difference (p > 0.05), whereas Tree 1 exhibited a significant difference (p > 0.05). This pattern persisted on 25 July. On 26 July, Trees 1 and 3 showed no significant difference, contrasting with Tree 2, which did significantly differ. These discrepancies could potentially be attributed to daily environmental variations or differing conditions. Consequently, the three days of merged data reveal that time does not significantly impact SPAD values for any trees at Site A (p > 0.05). This tendency in SPAD values was consistent across Site B as well. The statistical analysis result of Site B is similar to that of trees in sunny areas; there were no significant differences (p > 0.05) in SPAD values over time (Table 4).

Table 4.

T-test result of SPAD value of P. mume at 11 AM and 4 PM for three days

Table 5 shows the difference in SPAD values between leaves positioned above and below 180 cm (Table 5). At Site A, significant differences (p < 0.001) in SPAD values were observed between higher and lower leaves for all trees, except for Tree 2 on 26 July. On that day in Tree 2, The SPAD value by the position of the leaves is not significantly different (p > 0.05). This anomaly may be attributed to daily environmental variations. Merged data from the three-day period across all trees at Site A confirmed significant differences in SPAD values based on leaf position (p < 0.001). A similar trend was noted at Site B, except for Tree 6. For Trees 4 and 5, SPAD values significantly differed by leaf position on all measurement days, a finding that was consistent in the aggregated data (p < 0.001). However, for Tree 6, no significant difference was found in SPAD values between the positions of the leaves (p > 0.05).

Table 5.

T-test result of SPAD value of P. mume at over 180 cm and under 180 cm height for three days

In summary, after 13 years of acclimatization to their environments, P. mume trees displayed distinct adaptive responses. Significantly, trees situated in Site A, a sunny area, consistently registered higher SPAD values than those in Site B, a shaded region. It was observed that SPAD values remained consistent throughout the day for each tree, regardless of the light conditions at their respective sites. This indicates that diurnal shifts in sunlight intensity, associated with the sun's movement, do not markedly affect SPAD values. However, it is important to note that the data for this study were collected over only three days. Despite the short observation period, the reliability of the findings is enhanced by the high number of data points (1800 single data points per tree) used in the analysis. This large sample size provides a reliable basis for the conclusions. A similar observation was reported in blueberry; However, in that plant, the SPAD value was differently affected by sun to blueberry cultivars (Han et al., 2022a). Moreover, our research decisively shows that leaf position within the canopy significantly influences SPAD values, with a notable statistical variance in SPAD readings based on the vertical position of leaves. This finding emphasizes the importance of leaf placement in optimizing photosynthetic efficiency. While these outcomes underscore the multifaceted factors impacting SPAD values in P. mume, the specific influence of leaf position is identified as a pivotal factor. Notably, the efficiency of different leaf positions did not present a uniform pattern, potentially due to varying daylight intensity or duration. An expanded sample size would be beneficial to obtain more definitive data. In addition, Wang et al.,(2021) reported that open-field smart farming has developed to a practical level, making it possible to control cultivation environment parameters such as light intensity. Combined with our findings, it could increase the efficiency of photosynthesis in orchards with open-field smart farming.

4. Conclusion

In this study, we found that the light environment and the height of the leaf position could affect photosynthetic efficiency. Furthermore, the impact of the environment varies, as each plant and each leaf adjusts with different reactions. In the past, due to a lack of technology, optimizing every leaf's photosynthetic potential was difficult. However, with the advent of smart farming technology that senses and analyzes complex factors, it is now possible to optimize the photosynthetic potential of most leaves, even in open-field conditions. This study suggests that even under field environment conditions, open-field smart farming technology can control and optimize the photosynthetic potential of plants. Therefore, further research should consider larger sample sizes and a detailed examination of environmental variables to ascertain the precise elements affecting leaf position efficiency. Identifying these factors could enable the integration of this knowledge into smart farming practices, potentially enhancing crop yield and system efficiency.

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