Effects of leaf-trichome resistance on gas exchanges
Our simulation analysis based on field-obtained leaf traits at each site shows that leaf temperature was consistently higher than the ambient air temperature in daytime across all elevational sites (Figure 4a,c,d). While daytime leaf temperature can be higher than air temperature for a number of reasons other than leaf trichomes (i.e., leaf size, transpiration, PPFD), leaf-trichome resistance itself actually increased leaf temperature under the daytime conditions especially in leaves with thicker leaf trichomes (Figure 4g). In daytime, the effect of leaf‐trichome resistance was consistently much greater on sensible‐heat flux than H2O flux across the elevational gradient (e.g., 7.6 % for H2O‐flux resistance and 45.1% for the sensible‐heat‐flux resistance at the 2400 m site on average; Figure 5a-c), which strongly influences the gas-exchange rates through the “indirect” effect (Figure 1). For carbon budgets, leaf‐trichome resistance slightly increased the daily photosynthetic rates at the 1800 and 2400 m sites (+0.1% and +0.8%, respectively) but decreased it in the other sites (−1.1‐−0.4%; Figure 4e,h, and 5d,g). For water budgets, leaf‐trichome resistance slightly decreased the daily transpiration rates only at the 100 m site (−0.2%) but increased it in the other sites (+0.2‐+5.2; Figure 4f,i, and 5e,g). Leaf‐trichome resistance consistently decreased the daily water‐use efficiency in all study sites due to either increased water loss (the 2400 m site) or suppressed photosynthesis (the 150 m site) or both (the 700, 1280, and 1800 m sites) (Figure 5). Across various trichome thickness (from 0 to 1 mm at 0.05 mm intervals), the effects of leaf‐trichome resistance on H2O and sensible‐heat fluxes monotonically increased (Figure 6a,b) and the differences in leaf temperature between with and without leaf-trichome resistance became monotonically wider at all sites (Figure 6c,d). On the other hand, with increasing trichome thickness, the effect of leaf‐trichome resistance on the daily photosynthetic rate showed an upward‐convex curve at the 2400 m site (Figure 6e), and the daily photosynthetic rates can be enhanced only at the 2400 m site when trichome thickness was up to 0.8 mm which is similar to the largest trichome thickness observed in this study (0.776 mm). At the other sites, the daily photosynthetic rates were reduced by leaf-trichome resistance in any trichome thickness (Figure 6e). The daily transpiration rates showed an upward‐convex curve across trichome thickness at all sites except for the 100 m site where the daily transpiration rates were consistently reduced by leaf-trichome resistance (Figure 6f). The daily transpiration rates can be enhanced by leaf-trichome resistance in any trichome thickness from 0 to 1 mm at the 1280, 1800, and 2400 m sites (Figure 6f). The daily water-use efficiency was consistently suppressed by leaf-trichome resistance with any trichome thickness across all sites (Figure 6g).
In the second analysis, we conducted the sensitivity analysis on the effects of leaf trichomes on the photosynthetic rate (A ), the transpiration rate (E ), the water-use efficiency (WUE), leaf temperature (Tl ) with varying environmental factors (air temperature, relative humidity, PPFD, wind speed, and atmosphere pressure) and other leaf traits (characteristic leaf dimension, stomatal coefficient, and Vcmax ) (Figure S5-8). The effects of leaf-trichome resistance on the gas-exchange rates strongly depend on air temperature (Ta ), relative humidity (h ), PPFD (Q ), characteristic leaf dimension (d ), and stomatal coefficient (a ) as well as on trichome thickness (δt ) but not on wind speed, atmosphere pressure, and Vcmax (Figure S5-8); thus, we henceforth focus on the six factors: Ta , h , Q ,d , a , and δt . In order to visualize these complex interactions, we employed two kinds of heatmaps to present how the gas-exchange rates depend on those environmental factors and leaf traits, and also to present how the effects of leaf-trichome resistance on the gas-exchange rates depend on those environmental factors and leaf traits (Figure 7). First, we draw a heatmap of the photosynthetic rates across various characteristic leaf dimension and trichome thickness at a given combination of other variables (air temperature, relative humidity, PPFD, and stomatal coefficient). Figure 7a is such example, the photosynthetic rates are expressed across varying characteristic leaf dimension and trichome thickness at a condition of Ta = 15 °C, h = 0.8, Q= 2400 µmol m-2 s-1, a = 2.5. Similar heatmaps are made for all combinations of air temperature (Ta = 5, 10, 15, 20, 25, 30, 35°C) and PPFD (Q = 300, 600, 1200, 2400 µmol m-2s-1) at fixed stomatal coefficient (a = 2.5; isohydric leaves. relevant for M. polymorpha ) and relative humidity (Figure 7c). In order to visualize the effect of leaf-trichome resistance on the photosynthetic rates, we draw another heatmap which shows absolute differences in the photosynthetic rates between leaves with trichomes (δt = 0.05-1.00 mm in Figure 7a) and leaves without trichomes (δt = 0 mm in Figure 7a) (Figure 7b). Figure 7b is such example where orange and navy colors mean that the leaf-trichome resistance increases and decreases the photosynthetic rates respectively. Similar heatmaps are made for all combinations of air temperature and PPFD (Figure 7d). In Figure 7a-b, points and bars represent the means and standard deviations of field-observed trichome thickness and characteristic leaf dimension at each site. Furthermore, these analyses are also made for the transpiration rate and water-use efficiency at three values of stomatal coefficient (a = 2.5, 5, 10) (Figure 8). Sensitivity analyses in relation to other relative humidity (h = 0.2, 0.5) are available in Figure S9-10.
At low stomatal coefficient (a = 2.5; Figure 8a,d,g; isohydric leaves) that may represent the stomatal behaviors of M. polymorpha (Table 3), leaf-trichome resistance tends to decease the photosynthetic rate (A ) when air temperature is more than 15 °C, but it increases A at low air temperature (Figure 8a). In relation to characteristic leaf dimension, leaf-trichome resistance tends to increase A in smaller characteristic leaf dimension (Figure 8a). For water budgets, the leaf-trichome resistance tends to decrease the transpiration rate (E ) at middle air temperature (15-30 °C) though it tends to increase E at high PPFD and low air temperature (Figure 8d). The leaf-trichome resistance tends to increaseE in leaves with smaller characteristic leaf dimension (Figure 8d). Overall, the leaf-trichome resistance tends to increase the water-use efficiency (WUE) under low light and low air temperature but tends to decrease it under high light and high air temperature (Figure 8g). The leaf-trichome resistance in leaves with smaller characteristic leaf dimension tends to decrease WUE (Figure 8g). The magnitudes of effects of leaf-trichome resistance on A , E , and WUE tend to increase with increasing trichome thickness (Figure 8). Among different humidity conditions, humidity, the effects of leaf-trichome resistance on A, E, and WUE does not differ so much (Figure 8 and S9-10).
In order to explore the roles of leaf trichomes in wider context (i.e., considering other plant species), we also conducted similar sensitivity analyses with higher stomatal coefficients (a = 5, 10; Figure 8; more anisohydric leaves). With higher stomatal coefficient, the leaf-trichome resistance tends to increase A , decrease E , and increase WUE compared to lower stomatal coefficient (Figure 8). However, the values of WUE themselves are much higher in leaves with lower stomatal coefficient than in leaves with higher stomatal coefficient (Figure 8).