Response of Leaf Optical Properties, Temperature, and Physiology to Variable Kaolin Application Rate

Kaolin alters grapevine leaf optical properties commensurate with application rate

The propensity of kaolin to reflect light is the primary interest of previous studies that have characterized its effects on leaf optical properties. Reflectance, transmittance, and absorptance for untreated leaves in this study (Table 2 and Figure 2) are largely similar to those previously reported, though the fractions of reflected and transmitted light were slightly lower than in other reports (Cabello-Pasini and Macías-Carranza 2011, Lobos et al. 2015, Tosin et al. 2019). Kaolin application can increase the fraction of reflected light by up to ~25%, particularly at important blue and red wavelengths (Dinis et al. 2022). In this study, the maximum observed increase in leaf reflectance was just over 10% (Table 3), similar to the response observed by Tosin et al. (2019). The difference in the effect is likely due to the density of kaolin application or deposition, which is not clearly reported by Dinis et al. (2022). A consistent effect of kaolin on reflectance across wavelengths in the visible spectrum (Figure 2) is related to the white color of the substance, whereby light across wavelengths is evenly reflected. This is particularly noticeable for the local reflectance maximum at green wavelengths, which is much less pronounced for kaolin-treated leaves compared to untreated leaves (Figure 2).

In addition to reflectance, this study presents the effects of kaolin on leaf transmittance and absorptance, which are seldom reported elsewhere for grapevines (Shellie and King 2013b, Lobos et al. 2015, Tosin et al. 2019). The effect of kaolin on the fraction of transmitted light was statistically significant, but marginal in proportion to total light (Table 2). The remaining and largest fraction of light is accounted for by absorptance. Reduced absorptance is context-dependent; kaolin-treated leaves may be protected from photooxidative damage under saturating light conditions but may experience reduced photosynthesis under light-limited conditions (Dinis et al. 2022). Thus, kaolin is most often applied under high light and temperature conditions to maximize both photooxidative protection and photosynthesis. Absorptance spectra responded somewhat differently to kaolin application than reflectance spectra insofar as the response of absorptance varied by wavelength and kaolin application rate (Figure 2). Absorptance values for the lower rates of kaolin were not significantly different from the control at all or some of the red wavelengths important for photosynthesis, which means that the use efficiency of photosynthetically active radiation (PAR) may not be significantly different from the control (Liu and van Iersel 2021).

Rosati et al. (2007) measured leaf reflectance, transmittance, and absorptance with kaolin application in almond and walnut and observed that, despite increases in reflectance and decreases in absorptance, kaolin can increase the use efficiency of absorbed light. As such, a decrease in absorptance may not necessarily mean a decrease in A_n because grapevine leaves do not use all of incident PAR under sunny conditions for photosynthesis (Keller 2020). The use efficiency of absorbed light and the implications for leaf photooxidative stress were not evaluated in this study.

Unsurprisingly, the positive relationship between kaolin rate and reflectance and the negative relationships between kaolin rate and transmittance or absorptance are strongly linear. The fractions of reflected, transmitted, and absorbed light across the visible spectrum (400 to 700 nm) presented in Table 2 further characterize these relationships. Reflectance increased proportionately with kaolin rate, whereas the reduction in transmittance was similar among all kaolin rates compared to the untreated control. Absorptance exhibited a stepwise decrease commensurate with the increase in kaolin rate, although the maximum rate (56 kg/ha) was the only treatment for which absorptance was significantly different from the others.

It remains unclear whether the effect of kaolin on leaf optical properties is less sensitive to environmental conditions than the leaf-level physiological responses measured in this study, because optical properties were measured only in 2024. Still, the effect of kaolin on leaf optical properties was significant in 2024 despite no significant differences in Ψ_leaf or g_s (Tables 2 and 4). Leaf optical properties are subject to variation in environmental conditions and plant development, so it is unclear whether similar results would have been observed in the other two years of the study (Cabello-Pasini and Macías-Carranza 2011). Overall, increasing kaolin rate predictably altered reflectance and absorptance despite subtle spectral differences among the treatments (Figure 2). Additional study is required to understand how the effects of varied application rate on leaf optical properties alter photosynthesis and photochemical efficiency.

Kaolin application can improve grapevine water stress response under warm conditions

The effect of kaolin on physiological responses related to water relations and gas exchange is widely documented in grapevine and other crop plants. It is commonly reported that kaolin application increases A_n, transpiration (E), and g_s, especially under high temperature conditions that would otherwise reduce these variables (Frioni et al. 2019a, 2019b, Dinis et al. 2022). Indeed, kaolin is thought to prevent high temperature-induced inhibition of these processes rather than functionally promoting them. Kaolin application has also been observed to have no effect (Cataldo et al. 2022) or even reduce A_n, E, and g_s under variable environmental conditions (Shellie and King 2013b, Lobos et al. 2015, Brillante et al. 2016).

In the present study, the improvement—or maintenance—of g_s with kaolin application was most pronounced in 2022, which had the most extreme environmental conditions with respect to average summer temperature, summer GDD, summer ET_o, and greatest number of extremely hot (>40°C) days (Tables 1 and 4). This study provides some evidence that the relative effect of kaolin on the relationship between T_leaf and g_s is conserved across years and variable weather conditions (Figure 3). The effect on the relationship between absolute T_leaf and g_s, however, may scale with ambient weather conditions in a way that is difficult to characterize here. Some reports assert that the effectiveness of kaolin at reducing T_leaf and increasing g_s is more pronounced under warmer conditions (Shellie and King 2013b, Dinis et al. 2022). In other studies, ambient temperatures were mild enough (i.e., <35°C) that kaolin had less effect (Lobos et al. 2015, Cataldo et al. 2022). The effectiveness of kaolin to reduce T_leaf and improve A_n is likely temperature-dependent, though the exact relationship is unclear.

The response of Ψ to kaolin application is less variable than g_s, and there is often no difference between treated and untreated leaves (Lobos et al. 2015, Dinis et al. 2022). T_leaf has a less direct effect on Ψ than, for example, g_s, and thus one would expect a smaller effect of kaolin application on Ψ. Yet, grapevines can regulate Ψ through stomatal closure under the same stressful conditions in which kaolin may be more effective. Cataldo et al. (2022) observed that under extreme heat and drought conditions that severely depressed g_s, kaolin increased both predawn water potential and Ψ_leaf relative to the untreated control. Though Ψ appears to be more resistant to kaolin application by the magnitude of the response observed in this study (Table 4), this relationship may be altered under high temperature conditions.

The relationship between water status (e.g., Ψ) and g_s can describe the water stress response of a plant, or how resistant a plant is to reductions in productivity under increasing water deficits. In particular, the relationship between Ψ_leaf and g_s is used in grapevine studies to characterize the leaf-level response to increasing water deficit (Levin et al. 2019). Brillante et al. (2016) reported no difference in the relationship between stem water potential (Ψ_stem) and g_s among kaolin-treated and untreated leaves, even across a wide range of Ψ_stem values. The present study observed a difference between treated and untreated leaves, though kaolin’s ability to alter this water stress response was limited at low Ψ_leaf (<−1.4 MPa) (Figure 4). Cataldo et al. (2022) observed that kaolin can maintain A_n at high temperatures and low Ψ; g_s however, was significantly higher for kaolin-treated leaves only at moderate Ψ_leaf values between −1.5 and −1.2 MPa.

The effect of kaolin on the water stress response (i.e., g_s) may actually be nonlinear, increasing from high to moderate Ψ and decreasing as Ψ reaches severely water-stressed values. At the inflection point, the vine-level effect of increasing water deficit likely outweighs the positive impact of kaolin on leaf-level physiology. Under high water deficit, g_s is more strongly governed by Ψ and decoupled from temperature, the primary target of kaolin clay application (Gambetta et al. 2020). The linearity of the relationship observed in this study in 2022 may be attributable to both the narrow range of Ψ values and other environmental factors that influence g_s (Figure 4). A strong linear relationship was not observed in 2023 or 2024 and may be due, in part, to higher average Ψ_leaf across the season (Table 4 and Supplemental Figure 1). In conjunction with the observations of Cataldo et al. (2022) described above, this study provides discontinuous evidence that the effect of kaolin is maximized at moderate Ψ values (e.g., −1.5 to −1.2 MPa). Ultimately, kaolin can provide some relief to mild water stress, but supplemental irrigation is more effective at relieving severe water stress when it is available (Frioni et al. 2019b).

Water stress responses are also influenced by genotype and can be assessed along a spectrum of isohydric to anisohydric responses, or at what water status threshold (e.g., Ψ) grapevines increase stomatal closure to regulate Ψ (Levin et al. 2019, Gambetta et al. 2020). Despite grapevines having tighter stomatal control than some other species, different varieties exhibit wide variation from iso- to anisohydry and some studies have noted conflicting responses (Chaves et al. 2010, Levin et al. 2019, Serrano et al. 2024). Variable responses of g_s to Ψ by genotype may partially explain why the highest average g_s values were observed in 2022 despite the lowest average Ψ_leaf among all three years (Table 4).

In 2023, when average g_s values were lowest, vines experienced moderate levels of water stress early in the season, followed by rewatering (Supplemental Figure 1); in 2022, vines were more slowly acclimated to water stress conditions. A study evaluating water stress recovery in potted vines reported that Syrah vines—the same variety used in this study—subjected to high water stress recovered only 66% of g_s capacity after rewatering (Dayer et al. 2017). It is therefore plausible that early water stress contributed to low average g_s values in 2023 compared to the other two years. The dynamic response of g_s to the timing, intensity, and genotypic influence of water stress cannot be fully explained in the present study and is further complicated by the variable stress responses exhibited by Syrah (Gambetta et al. 2020).

Kaolin has variable effects on fruit composition and may delay ripening in cool years

Measured improvements in A_n with kaolin application are presumed to improve photosynthate (i.e., sugar) production, but the consequences for fruit sugar accumulation are less direct. Kaolin application has seldom been reported to increase TSS (Shellie and King 2013a). Vine productivity and sugar accumulation are complicated by increases in fruit size promoted by kaolin application, which is consistently reported for other fruit (Spiers et al. 2008, Glenn 2012). For grapevines, larger berries have a higher photosynthate demand and thus may have lower concentration (i.e., TSS) despite a similar or higher sugar content (i.e., g/berry) (de Rességuier et al. 2024). This was observed in the present study insofar as berry mass trended higher and TSS trended lower with kaolin application and rate, but sugar content (g/berry) was largely the same (Tables 5 and 6). Increased fruit size in kaolin-treated vines with little-to-no change in TSS has previously been reported (Shellie 2015, Frioni et al. 2019a). Thus, kaolin can improve whole vine sugar production such that a greater mass of sugar is produced and translocated to a larger yield of fruit in kaolin-treated vines. This is arguably of less enological significance because berry sugar concentration (TSS) is the primary berry sugar trait evaluated for fruit and wine quality.

The inverse has also proved to be true: sugar accumulation in kaolin-treated vines with reduced A_n may lag behind untreated vines during ripening (Cataldo et al. 2022) and at maturity (Brillante et al. 2016, Dinis et al. 2020). In the present study, the greatest reduction in TSS for kaolin-treated vines, though not statistically significant, was observed in the final and coolest year (Table 6). It is important for practitioners to consider that kaolin application may delay sugar accumulation, particularly when temperatures are mild or cool.

The effect of kaolin on acid dynamics in ripening berries is likely a function of both delayed ripening and a cooling effect on clusters, which can slow degradation of malic acid. In this study, pH trended lower and TA higher with kaolin application (Table 6), in agreement with other reports (Frioni et al. 2019a, Dinis et al. 2020). Even more studies report an effect only on pH (Brillante et al. 2016), no effect (Shellie and King 2013a, Teker 2023), or even the opposite effect observed in this study (Lobos et al. 2015). The effect of kaolin on malic acid was significant only in 2022, which was the year with the greatest summer GDD and number of days above 40°C. The effect of kaolin and cluster cooling on malic acid is not consistent among other published reports, so any effect of kaolin application on fruit ripening rate is likely contributing to differences in malic acid.

Among the most reported effects of kaolin application on fruit composition are changes to phenolic compounds. The results of this study are quite variable and not statistically significant but do follow some of the trends observed in other reports (Table 7). Kaolin application consistently increases berry anthocyanin concentration (Song et al. 2012, Shellie and King 2013a, Lobos et al. 2015, Brillante et al. 2016, Dinis et al. 2016, Frioni et al. 2019a, 2019b, Valentini et al. 2021). The effect of kaolin is likely twofold; first, that reduced berry temperature optimizes stimulation of anthocyanin synthesis, and second, that reduced berry temperature slows the degradation of these temperature-sensitive compounds (Dinis et al. 2016, 2022, Keller 2020). Dinis et al. (2016) provided direct gene expression evidence for stimulation of anthocyanin synthesis while also suggesting that differences in anthocyanins between treated and untreated fruit at successive harvests are related to anthocyanin degradation in untreated fruit.

It is unclear why kaolin application did not influence anthocyanin concentration in this study, but variable varietal response may have contributed. Luzio et al. (2021) reported differences in the response of anthocyanins to kaolin application in seven varieties, which did not include the variety used in this study, Syrah. Unfortunately, there appear to be no reports of Syrah anthocyanins and kaolin application. The resistance of Syrah in this study to anthocyanin alterations is likely complex but may be influenced, among other attributes, by the loose cluster morphology (i.e., reduced berry temperature) and unique anthocyanin profile (e.g., higher proportion of more stable acylated anthocyanins) of Syrah (Smart and Sinclair 1976, Ortega-Regules et al. 2006).

The reported effects of kaolin on total phenolics and other phenolic compounds are less consistent, but an increase in concentration has been observed (Brillante et al. 2016, Dinis et al. 2016, Frioni et al. 2019b, Valentini et al. 2021). The present study does not clarify the complex influences of environmental conditions, abiotic stress, and kaolin on non-anthocyanin phenolic compounds. Lobos et al. (2015) observed similarly variable responses as reported here. In contrast to anthocyanins, reports of improved phenolic concentrations with kaolin application do not align neatly with significant improvements in leaf-level physiology (Brillante et al. 2016).

Practical considerations for optimizing kaolin application in vineyards

The present study contributes to the existing literature on vineyard kaolin applications by evaluating the effect of variable application rate on leaf optical properties, T_leaf, and leaf-level physiology. Application rate was evaluated in 2023 and 2024 by using 100, 50, 25, and 0% of the maximum label rate (56, 28, 14, and 0 kg/ha/application) for the predominant commercial kaolin product in the United States (Surround WP, NovaSource). The label for this product recommends a range of 14 to 56 kg/ha for insect pests, sunburn, and heat stress, but the results of this study indicate that the most consistent and greatest improvement for the measured responses was achieved at 56 kg/ha. This rate produced significantly lower T_leaf, higher Ψ_leaf, and higher g_s in 2022 and was the only rate to significantly reduce T_leaf in 2024 (Table 4). For other measurements of yield and fruit composition, the response, while not statistically significant, was usually scalar with the application rate (Tables 5 to 7). Relatively few studies report the rate (kg/ha or w/v and v/ha) of kaolin application and instead report only the concentration of the mix (w/v), which complicates comparisons of the application rate effect across studies. The studies reviewed herein that report application rate (kg/ha or w/v and v/ha) used a high application rate between 56 and 60 kg/ha (Shellie and King 2013a, 2013b, Shellie 2015, Ferrari et al. 2017).

Increases in global mean temperature will indubitably affect grape cultivation, but extreme heat events pose a more acute and unpredictable challenge (Gambetta 2016). A higher frequency of extreme heat events is forecasted for grapegrowing areas in the region of this study (McKinnon and Simpson 2022). The number of days over 40°C is used herein to approximate the frequency of extreme heat events that would suppress physiological function or even cause damage to vegetative and reproductive organs (Keller 2020). Consequently, the effect of kaolin on leaf-level physiology was strongest in 2022, which had the highest number of days over 40°C, highest average summer temperature, and highest summer GDD (Table 1). Because ambient temperatures of 30°C can produce leaf and fruit surface temperatures greater than 40°C, kaolin application may be useful when temperatures exceed 35°C for several days or for single 40°C days (Riley 2014, Frioni et al. 2019a). The weather conditions in 2023 and 2024 at the study site may not be the ideal conditions for judicious kaolin application, and ultimately, the vintage effect was more significant than kaolin application effect for most measured variables.

This study can also guide future work that interrogates the role of kaolin in irrigation scheduling. Measurements of Ψ are often used to initiate irrigation according to thresholds that are empirically correlated with desirable vegetative growth and fruit quality responses (van Leeuwen et al. 2009, Levin and Nackley 2021). Moderate water stress is associated with Ψ_stem values approaching −1.1 MPa and Ψ_leaf values between approximately −1.3 and −1.1 MPa (van Leeuwen et al. 2009, Deloire et al. 2020). In the present study, g_s was over 26% higher for kaolin-treated leaves above approximately −1.3 MPa, indicating an altered water stress response (Figure 4). Under moderate water stress, the Ψ_leaf irrigation threshold could theoretically be decreased by 0.1 to 0.2 MPa without a significant penalty to g_s.

In regions with a Mediterranean climate characterized by warm summers and wet winters, the altered water stress response may allow producers to save water by delaying irrigation initiation, albeit with important implications for fruit yield and quality (Kar et al. 2023). Munitz et al. (2020) imposed various Ψ_stem thresholds for irrigation onset and reduced water use by 9 to 22 mm (20 to 32%) by delaying onset by 0.2 MPa within the range of −1.2 to −0.8 MPa. Kaolin application could confer similar water savings when Ψ is consistently monitored. Such irrigation strategies are increasingly important for grapegrowers in the western U.S., where water availability is increasingly unpredictable and precarious (Bambach et al. 2022). The present study did not investigate the interaction of kaolin and irrigation scheduling, so future work is required to understand whether kaolin can be used to delay irrigation initiation and contribute to climate-resilient water management.