The fine particle film of calcined and purified kaolin (KF) application is a technological product-oriented to sustainable use of water resources in various agricultural crops (Boari et al., 2014; Brito et al., 2018; Faghih et al., 2019). The use of KF is certified by the Organic Materials Review Institute (OMRI, 2021) as an authorized substance in organic food production, in the context of rising health and environmental concerns, especially for organically growing agricultural and horticultural crops (Sharma et al., 2015; Mphande et al., 2020). Adding to improved crop water management, KF can be applied to reduce the impact of heat and excessive light (Steiman et al., 2007; Santos et al., 2021) and also help in pest (Amalin et al., 2015) and pathogen control (Glenn et al., 2001; Tubajika et al., 2007). Fine kaolin films reduce negative UV impacts by increasing light reflection in apple, Malus domestica (Glenn et al., 2002); reduce leaf temperature by 1.1°C in tomato, Solanum lycopersicum (Boari et al., 2014); reduce water stress in pepper, Capsicum annuum (Creamer et al., 2005); increase water use efficiency (WUE) by 26% in tomato (AbdAllah, 2019), and stomatal conductance (g_s) in well-watered grapevines, Vitis vinifera (Glenn et al., 2010); increase leaf photosynthesis (A) in apple trees (Glenn et al., 2001); increase the sucrose biosynthesis in grapevines (Conde et al., 2018); increase height and diameter growth in young eucalyptus hybrid plants, Eucalyptus grandis × Eucalyptus urophylla (Santos et al., 2021). In tomato cultivation, the KF application results in a 23% increase of a marketable yield (average of 3 years) with significant net profit gain for the tomato producer of 600 € ha^–1 on non-saline soils, and 900 € ha^–1 in saline soils (Boari et al., 2014).

Coffee is one of the most consumed world beverages, and its production is made possible by the work of approximately 100 million coffee growers worldwide (Davis et al., 2019). Among 124 species of wild coffees, the global coffee trade relies on two species, namely Arabica (Coffea arabica) and Robusta coffee (Coffea canephora). The world’s two largest coffee producers, Brazil, and Vietnam are geographically distant from the African centers of coffee origin (Anthony et al., 2011). The environmental pressure of monoculture, the predominant Brazilian coffee-growing system (DaMatta et al., 2019) is far from deep forest environmental conditions in the centers of coffee origin, which causes their classification into shade-tolerant species (Ayalew, 2018). The C. arabica is an understory tree and an endemic species originated at the Ethiopian rainforests at altitudes above 1,500 m characterized with an average annual temperature of 20°C; C. canephora originated at West and Central Africa of altitudes from 0 to 1,200 m, characterized with average annual temperature between 24 and 26°C, where it evolved as a medium-sized tree (Charrier and Berthaud, 1985).

The expectation of the life of a coffee tree in their natural habitat is up to 100 years, while in plantations it is about 30–40 years (Gokavi et al., 2019). In plantations, tree production and yield cycles are regulated by diverse systems of training and renovations (Taugourdeau et al., 2014; Rakocevic et al., 2021a). In Brazil, about 300,000 ha are planted every year for orchard renovation or for the new area’s plantation, which corresponds to 13% of the areas under coffee crops (CONAB, 2021).

The coffee seedlings are produced in nurseries, normally covered with plastic mesh, which can block about 50 to 75% of photosynthetically active radiation (PAR), attaining up to 600–700 μmol m^–2 s^–1 at midday of one tropical sunny day (Matiello et al., 2010). Under actual air CO₂ concentration, this PAR range is considered as a light saturation point for C. arabica (Rodrigues et al., 2016; Rakocevic et al., 2021b). High light intensities provoke photoinhibition in both C. arabica and C. canephora, decreasing maximum photochemical efficiency (F_v/F_m) because of an increased initial and a quenched maximum fluorescence (DaMatta and Maestri, 1997; Martins et al., 2014). In coffee, generally limited and low A is explained by stomatal factor limitations, followed by the mesophyll and biochemical constraints (Martins et al., 2014). Moreover, coffee plants showed significant sensibility to UV radiation, mainly C. canephora, which displayed reduced root and total biomass, number of leaves and leaf area, increased leaf elongation rate under ambient compared with reduced UV radiation (Bernado et al., 2021).

The optimal development of Coffea seedlings in the nursery is dependent on water supply, with an average of 4.5 mm day^–1, summing about 600 mm (Vallone et al., 2010). When the seedlings formed in the nursery reach 4 to 6 pairs of leaves, they are subjected to a gradual increase of light and eventually gradual water reduction for about 30 days in a process of acclimatization. Afterward, seedlings are planted to the field, which is recommended to occur in the rainy spring in not irrigated fields (Mesquita et al., 2016). Until they are 1 year old, the young plants in the field need to be irrigated (Gervásio and Lima, 1998), otherwise without the irrigation, the mortality of transplanted coffee seedlings attains up to 73% after 6–8 months (Oliveira et al., 2015).

In the post-planting period, under the combination of elevated air temperatures and elevated solar irradiance, especially its UV bands (Ulm and Jenkins, 2015; Bernado et al., 2021), one physiological disorder called leaf sunburn can occur in coffee seedlings (Santos et al., 2016). Sunburn is expressed by symptoms of chlorosis and necrosis in a great number of species (Racskó et al., 2010). Among deleterious effects of sunburn, the reduction of leaf gas exchange, reduction in plant height, leaf area, shortened internode, and branch length, and sunburn browning of fruits were reported in apple (Racskó et al., 2010) and coffee trees (Santos et al., 2016). Recently, KF application is shown to be the best method to prevent sunburn in fruits of pomegranate, Punica granatum (Yazici and Kaynak, 2006; Sharma et al., 2018).

Despite the great importance of the coffee crops in Brazil, and the continuous necessity of new sustainable strategies for agricultural production, natural resource conservation, and to guarantee the maximum setting of seedlings in the transition from the nursery to the field, no research was performed to test the use of KF on this crucial stage of the seedling’s establishment in the full sunlight. It was hypothesized that KF leaf protection from high light intensities and temperature stress can avoid the photoinhibition, improving leaf gas exchanges and WUE in young coffee plants, having more pronounced impacts on C. canephora than in C. arabica. Thus, the aim of this work was to assess the effects of KF technology on chlorophyll a fluorescence and net photosynthesis rate of young coffee plants in two seasons, autumn and summer, in their transition from nursery to full sun under the well-watered conditions, contributing to the conservation of local water resources, environmental sustainability, and increasing plant abilities to cope with abiotic stresses.

We first showed that during the transference of young Coffea plants from nursery shade to full sunlight, KF application decreased the stressed conditions of the new environment characterized by high light and elevated temperatures. Both species decreased T_leaf, which impacted a general increase of leaf gas exchange parameters (g_s, E, A) and I_g. In both Coffea sp., KF application reduced F₀, indicating reduced physical dissociation of the PSII reaction centers from the light-harvesting system (Sundby et al., 1986), which was supported with increased PI. Interestingly, only C. canephora leaves protected with KF achieved higher WUE compared with not-protected ones, which was one specific species response.

In cascade of plant responses during the transition from nursery to the sunlight, the KF application in young Coffea plants turned the leaf surfaces white (increasing PAR reflection), which firstly reduced T_leaf at the hottest daylight period up to 6.7°C during autumn, and up to 5.6°C during summer. Considering the effects of KF in other species, the reduction of T_leaf by 3°C is observed in grapefruit, Citrus paradisi (Jifon and Syvertsen, 2003), or by 2.5°C in rose, Rosa sp. (Sotelo-Cuitiva et al., 2011). The reduction in T_leaf is explained by the ability of KF to create a modified leaf/plant microclimate by the reflective nature of kaolin particles (Glenn et al., 2002; Steiman et al., 2007). The KF white color and formulation increase albedo on the fruit or leaf surfaces (Shellie and King, 2013), increasing radiation reflection on the canopy, impacting on T_leaf reduction (Campostrini et al., 2010), as was observed in both Coffee species, with high efficiency in midday.

The reduction in T_leaf of the KF-protected plants in Coffea sp. occurred in parallel with the increase in g_s, E, and I_g, and a reduction in CWSI. The microclimate created by the application of KF reduced the possible negative environmental effects of high PAR and high T_air, minimizing the partial or total closure of the coffee stomata, as happened without KF technology application (Martins et al., 2014; DaMatta et al., 2019). Without KF, C. canephora kept the leaves cooler at midday than C. arabica, which was likely related primarily to the higher E values linked to higher g_s, knowing that the increased transpiration rate results in increased latent heat loss and reduced leaf temperature (Ainsworth and Rogers, 2007; Jones, 2018). Additionally, overall, non-KF treated leaves from C. canephora have showed higher stomatal density than C. arabica (Rodrigues et al., 2018; Bernado et al., 2021), helping to understand the species-specific responses, i.e., could allow C. canephora leaves to respond more rapidly to changing environmental cues. C. arabica also showed increased g_s and E values when treated with KF, but the difference in g_s and E between the two coffee species was reduced with KF spraying, resulting from the reduction in C. canephora efforts to acclimatize on high light and temperature.

The A was fluctuated in two seasons, in well-watered Coffee plantlets, with generally higher assimilation in autumn than in summer as previously observed in Rodrigues et al. (2016). The season in adult Coffee plants grown in field conditions without irrigation can produce the opposite effect, showing higher assimilation in rainy summer than in dry autumn (Rakocevic et al., 2021b). Generally, A was higher in C. canephora than in C. arabica in plants protected with KF than in not protected, which was associated with increases in g_s. In fact, kaolin reduces abscisic acid accumulation in grapevine leaves, helping in the faster recovery of leaf gas exchanges under high light and temperature (Dinis et al., 2018), which was probably the mechanism of biochemical action in young coffee plants.

Photosynthetic carbon assimilation increased in C. canephora and C. arabica when protected with KF, but in C. arabica the increases in E and g_s were proportionally greater than the increases in A, and therefore, the WUE and iWUE were reduced compared with GL plants. In summer, C. canephora protected with KF increased A more than E, resulting in elevated WUE when compared with GL plants, or to C. arabica protected with KF. A similar response is observed in grapevine, where KF application reduces canopy temperature and the thermal stress, impacting on increased WUE and productivity (Glenn et al., 2010). On the other hand, C. arabica decreased WUE and iWUE in both seasons due to relatively higher increases in g_s and E than in A, when compared with C. canephora. Kaolin applied at high doses acts as an antitranspirant, impacting the direction of leaf A and E reductions in some stages of grapevines (Frioni et al., 2019, 2020). Two coffee species differ in anatomical leaf characteristics: C. arabica is characterized by a greater thickness of the abaxial epidermis and the spongy parenchyma, and by the lower thickness of the palisade parenchyma and reduced stomatal density than C. canephora (Bernado et al., 2021). Considering those anatomical species specificities and their differential responses in leaf gas exchanges with KF application, the question is: Could kaolin spraying dose be different between C. arabica and C. canephora species to provoke a positive response in water savings? In future research, reduced doses could be tested in C. arabica to promote WUE elevation, and water savings. The elevated WUE in C. canephora in the summer period can lead to water savings. In fact, the KF application in other species, such as in strawberry (Fragaria ananassa) seedlings during transplanting, allowed savings between 20 and 40% of the water volume without affecting plant growth and green intensity (Santos et al., 2012). The protective effect of KF places this technology as a sustainable development tool to mitigate the effects of ongoing global warming and allows water economy (Roy et al., 2018).

When leaves are submitted to heat stress, the increase of chlorophyll fluorescence (F₀ parameter) is observed (Smillie and Nott, 1979). KF influenced the chlorophyll fluorescence emission and minimized damage to the photochemical apparatus before the appearance of visual symptoms in these two coffee species, C. canephora and C arabica (data not shown). Coffea sp. plants protected with KF had lower F₀, higher F_v/F_m, and higher PI than those not protected. This effect presumably reflects the physical dissociation of the PS II reaction centers from the light-harvesting system (Sundby et al., 1986). F_v/F_m values less than 0.75 indicate a photo-inhibitory effect of the PSII-associated photosynthetic apparatus (Bolhar-Nordenkampf et al., 1989), which occurred in coffee exposed to full sunlight, regardless of KF treatment. The PI values of coffee seedlings not protected with KF indicated that the activity of PS I and PS II was compromised during the transition of the seedlings from a nursery shade to full sunlight. In C. canephora protected with KF, PI values were upto 25-fold higher than those not protected with KF on DFS 2 during the summer. Results about coffee seedlings not protected with KF may suggest some destabilization of membranes and proteins, production of reactive oxygen species, and cell death, as observed in apples (Gindaba and Wand, 2007).

In conclusion, the application of KF on coffee leaves would reduce T_leaf under high PAR and high T_air during the sensitive agronomic management of young Coffea plants, confirming the initial hypothesis. KF impacted on F₀, F_v/F_m, and PI modifications in Coffea sp., minimizing possible damages of the photochemical apparatus, preventing the stomatal closure, and permitting higher net CO₂ assimilation. For the C. arabica, it seems that autumn can be considered as the best season for planting, although KF application improved the plant acclimatization to elevated light and temperatures at midday. On the other hand, C. canephora showed greater plasticity than C. arabica related to the planting season. Observing the species-specific responses in water management efficiency with KF applications, C. canephora showed higher WUE and iWUE than C. arabica, indicating water savings in C. canephora cultivations, from the practical point of view. The second practical point of view could be related to diminished costs and risks, where the dilution of 1 kg of KF (the commercial price of Surround WP is about 4–5 USD) in 20 L of water (5% w/v) can protect 1,550 m² of nursery bed or around 300,000 to 450,000 young coffee plantlets. In the field, 1 kg of KF in 5% w/v protects 1 hectare, i.e., 3,000–5,000, of newly planted coffee plants. Thus, the processed-kaolin particle film technology is important in the transition of seedlings from the nursery to the field planting condition, given that the young plant price is 0.12–0.20 USD, whereas the KF cost per plant is less than 0.002 USD. The use of KF can be used as a management strategy to protect leaves from the two coffee species against excess solar radiation, elevated temperatures, and excess water spend, especially in summer and in C. canephora.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

DA: investigation, data curation, and writing original draft. NR: resources and conceptualization. GA: software and data curation. WB: investigation. WR: methodology. EC: resources, conceptualization, definition, and validation. MR: conceptualization, validation, reviewing, and editing. All authors read and approved the final manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.