Ambient air temperatures and solar radiation affect OJIP fluorescence transients of coffee plants in an agroforestry system

Abstract

Physiological response of coffee plants to their environment was observed through leaf photosynthetic measurement. The leaf photosynthetic efficiency of coffee plants grown in agroforestry was identified at different elevations. Each elevation has variations in ambient air temperatures. OJIP transient fluorometry recorded that the photosynthetic efficiency responds differently at different elevations. The plants grown at 1200 MASL with higher ambient air temperatures experienced a rise in the OJIP steps of fluorescence transients, indicating the closure of some of the PSII reaction centers. However, the coffee plants grown at 1300 MASL, with lower ambient air temperatures and higher exposure to solar radiation, had better electron transport, as indicated by lower OJIP fluorescence transients, when compared to coffee plants grown at 1200 MASL. Regarding the OJIP fluorescence traits, it was found that the plants promoted better photosynthetic efficiency in the area with higher solar radiation and lower ambient air temperatures.

Abbreviations

Cyt b6f

cytochrome

PC

plastocyanin

PQ

plastoquinone

PSI

photosystem I

RC

reaction center

1 INTRODUCTION

Generally, coffee plants are grown at different elevations with different ambient air temperatures. Thus, solar radiation and ambient air temperature variations strongly influence coffee plants’ physiological performance, growth, and productivity (Estrada & Vaast, 2007).

Differences in elevation levels, topography, and vegetation generally affect micro-climate conditions, such as solar radiation and air temperatures. Different slopes share different air temperature lapse rates, as a variation in the reduction of air temperatures occurs every 100 m of increased elevation (Xiong et al., 2018). In most cases, changes in micro-climate conditions affect the increase in the leaf-to-fruit ratio, the net photosynthetic rate, and the fruit-ripening period (Paudel et al., 2021; Vaast et al., 2006). Plant growth and development are evaluated according to their photosynthetic performance, which is limited by environmental factors such as solar radiation and air temperatures (Azhar et al., 2018), which cause a decrease in yield (Thwe & Kasemsap, 2014).

In photosynthesis, light is absorbed by the antenna molecules, and energy is absorbed as excitation energy. The absorbed energy is trapped in a reaction center and used in photochemistry, while some energy is dissipated into the atmosphere as heat or fluorescence (Strasser et al., 2004). Chlorophyll fluorescence is a method used to monitor plant performance through the alteration process of photosynthesis and is done by measuring the radiation emitted from a single leaf (Krause & Weis, 1991).

Information about the photosynthetic efficiency of coffee plants in the field with different elevations is still limited. In this study, ambient air temperatures of a particular coffee growing area were observed with a particular focus on the efficiency of sunlight energy absorption by plants. The researchers hypothesize that differences in elevations where coffee plantations are located influence the photosynthetic efficiency of coffee in using light energy, which may affect the yield.

2 MATERIALS AND METHODS

This field survey was carried out at an organic smallholder coffee plantation on the slopes of Mt. Puntang in the Pangalengan Regency of West Java, Indonesia, from February to April 2022. The soil pH (H₂O) ranged from 5.5 to 6.0, with the percentages of sand, clay, and dust at 19%, 54%, and 27%, respectively. Seven-year-old coffee plants from the Arabica geisha variety were observed in this survey.

OJIP fluorescent transients were measured using a portable fluorometer (Pocket PEA; Hansatech Instruments Ltd., Norfolk, England). Measurement was conducted on one leaf per plant on 80 plants at each elevation level: 1200, 1300, and 1400 MASL. Leaves sampled for the measurement are leaves that are fully developed and come from the topmost branch of the plants. Before measurements were taken, the middle portion of the leaf surfaces (adaxial) was covered for at least 20 min using leaf clips provided by Hansatech Instruments for dark adaptation. The saturation pulse reaching the leaf was 3000 μmol photon m⁻² s⁻¹ to generate maximum fluorescence (F_M) for all leaf samples. Measurements were conducted between 09:00 and 12:00.

Ambient air temperatures and solar radiation at the coffee plantation were recorded using a weather-measuring device that was compiled using micro-electronic technology and the Internet of Things (IoT) data-based communications. The ESP32 (Espressife, China) integrated system was the microcontroller and handled environmental (weather) sensors that already had wifi communication devices.

Core Idea

• Different ambient air temperatures affect the chlorophyll fluorescence transient of coffee plants.
• Different vegetation densities affected the solar radiation exposure to the coffee plants’ leaf canopy.
• The area with lower ambient air temperatures and higher solar radiation promotes higher efficiency in the leaf photosynthetic performance.

FIGURE 1

Solar radiation (a), ambient air temperature (b), and air pressure (c) at a coffee plantation using the agroforestry system. Data were recorded in elevation levels at 1200 (blue), 1300 (orange), and 1400 MASL (gray)

The data of each parameter are presented as mean ± SE and reported with a statistical significance determined by the Tukey Contrasts Significant Different Test, using the R software (SoftVersion R 3.3.3 for Windows). Each elevation area had 80 plant replicates.

3 RESULTS AND DISCUSSION

The coffee plants showed better photosynthetic performance in the area at 1300 MASL, which was exposed to the highest solar radiation and lower ambient air temperatures. The highest solar radiation in midday time was recorded at 250 Wm⁻², followed by 150 Wm⁻² and 70 Wm⁻² at 1300, 1200, and 1400 MASL, respectively (Figure 1a). The difference in vegetation cover causes solar radiation differences at each elevation level. Although tree vegetation potentially influences solar radiation in the planting area (Rushayati et al., 2018), which may affect the ambient air temperature, different elevation levels dominantly affect the ambient air temperatures, in this case, in the coffee planting area. During the day, the highest air temperature was dominated by the area with the lowest elevation level at 1200 MASL, followed by 1300 and 1400 MASL, at 33, 30, and 27 °C, respectively. The lowest air temperature trend was recorded at 1300, followed by 1200 MASL, at 16 and 18 °C, respectively (Figure 1b).

Different air pressure due to altitude variance primarily affects the heat transfer rate (Saidi & Abardeh, 2010) due to heat conduction and air convection (Wicky & Hauck, 2020); this can be seen in Figure 1c. Typically, ambient air temperature decreases with the increase in altitude. In some plants, this condition reduces heat stress and, in some cases, increases the leaf photosynthesis, fruit formation, and fruit maturation period (Vaast et al., 2006; Worku et al., 2018).

The variations in ambient air temperatures in the coffee habitat were reflected in the OJIP fluorescence transients (Table 1). At 1300 MASL, the coffee plants showed a better photosynthetic efficiency, while at 1200 the photosynthetic efficiency was low (Figure 2 and Table 2). The rise of the O–J part of the fluorescence at an elevation of 1200 MASL indicates the closure of some PSII reaction centers, which results in a reduction of electron transport on the acceptor side of PSII (Kalaji et al., 2016; Strasser et al., 2005) due to the reduction of the primary plastoquinone electron acceptor of PSII (Q_A) in the photosystem II (Strasser et al., 2004). The rise of the J–I part of the fluorescence at 1200 MASL corresponds to the inhibition of the secondary electron acceptor (Q_B), as well as plastoquinone (PQ), cytochrome (Cyt b6f), and plastocyanin (PC) (Guha et al., 2013). The increase in the I–P part is typically related to the reduction of electron transporters (ferredoxin, intermediary acceptors, and NADP) of photosystem I (PSI) as the acceptor.

TABLE 1. OJIP steps in chlorophyll fluorescence transients of coffee plants leave at different elevation levels

Elevation (MASL)	Fo	FJ	FI	FM	Area
1200	7514.1^b	9427^b	21,008.3^b	36,900.2^b	934,973.6^b
1300	7028.6^a	8552.8^a	16,334.7^a	27,646.2^a	678,020.4^a
1400	6855.4^a	8299.9^a	17,152.4^a	29,224.6^a	867,324^b

• Note. The different letters indicate significant differences at the 0.01 level, according to the Tukey Contrasts Significant Difference. Data correspond to the mean value (n = 80) of the total sampling data in Figure 3.

FIGURE 2

OJIP fluorescence transients of coffee leaves at different elevation levels

FIGURE 3

OJIP traits, specific energy flux per reaction center, and the performance index of all coffee plant samples at different elevation levels

TABLE 2. Specific energy flux per reaction center and the performance index of coffee plants grown at different elevation levels

Elevation (MASL)	ABS/RC	TRo/RC	ETo/RC	DIo/RC	PIABS
1200	4128.96^a	263.76^a	0.45^a	0.20^a	5213.40^b
1300	10751.38^b	1769.64^b	0.52^b	0.35^c	3521.13^a
1400	5192.15^a	131.50^a	0.45^a	0.25^b	4495.78^b

• Note. The different letters indicate significant differences at the 0.01 level, according to the Tukey Contrasts Significant Difference. Data correspond to the mean value (n = 80) of the total sampling data in Figure 3.

Higher exposure to solar radiation at 1300 MASL also affects the coffee plants reducing the area, indicating a blockage in the electron transfer from the reaction center (RC) to the quinone pool (Kumar et al., 2020). The area is relative to the pool size of the electron acceptors Q_A on the reducing side of PSII. At 1300 MASL, there was a significant increase in specific energy flux per reaction center (ABS/RC, TR_O/RC, ET_O/RC, and DI_O/RC), describing chlorophyll fluorescence and its function (Yang et al., 2016). The higher values of ABS/RC at 1300 MASL indicate a significant increase in the adequate antenna size, while an enormous value of TR_O/RC indicates a transformation of the RCs, due to the inhibition of electron transport from Q_A to Q_B (Yusuf et al., 2010) and inactivation (Liang et al., 2019) or impairment (Braga et al., 2020) of the oxygen-evolving complex. Higher ET_O/RC values and DI_O/RC at 1300 MASL indicate that excess light energy was dissipated as an adaptive mechanism to minimize photodamage to the photosynthetic apparatus (Akhter et al., 2021). These changes in specific energy flux per reaction center result in a significant decrease in the performance index on an absorption basis (PI_ABS).

The chlorophyll fluorescence measurement on coffee plant leaves at three elevation levels suggests that the photosynthetic performance of the coffee plants is more efficient at 1300 MASL with sufficient solar radiation and lower air temperatures.

AUTHOR CONTRIBUTIONS

Conceptualization, data curation, formal analysis, funding acquisition, methodology, resources, writing—original draft, and writing—review and editing: Aidil Azhar. Project administration and resources: Eka Merdekawati. Formal analysis, investigation, and methodology: Aris Pramudia. Resources and software: Andi Nur Cahyo. Supervision. Lili Dahliani: Funding acquisition and project administration: Hiroshi Ehara.

ACKNOWLEDGMENT

This study was supported by e-RISPRO LPDP Grant numbers PJR-80/LPDP/2021 and 0770/D6KU.04.00/2021, to all of whom we express our gratitude.

CONFLICT OF INTEREST

The authors declare no conflict of interest.