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).