The Leaf Trichome, Venation, and Mesophyll Structural Traits Play Important Roles in the Physiological Responses of Oak Seedlings to Water-Deficit Stress

Abstract

In this study, we investigated the effects of water-deficit stress on the leaf anatomical traits, physiological traits, and stem starch content in Quercus acutissima Carruth and Quercus serrata Murray by subjecting their seedlings to well-watered (WW) and water-deficit stress (WS) treatments. The water stress-induced changes in trichome density, trichome-to-stomata ratio, mesophyll thickness, vein density, vein distance, vein loopiness, vessel diameter, transpiration (E), stomatal conductance (g_s), water use efficiency (WUE), and starch content were analyzed between two time points. While trichome density did not vary between treatments in Q. acutissima, it dramatically increased in Q. serrata (62.63-98.96 trichomes mm^-2) at the final week. The WS-treated seedlings had a thicker palisade mesophyll (162.85-169.56 µm) than the WW-treated samples (118.56-132.25 µm) in both species. The vein density and loopiness increased significantly in the WS-treated Q. serrata seedlings. Small-sized vessels (10-50 µm) were more frequent in the WS than the WW in Q. serrata. The E, g_s, WUE, and starch content declined significantly in the WS-treated seedlings compared with WW-treated samples in both species. Further, principal component analysis revealed significant relationships between anatomical and physiological traits, particularly in the WS-treated seedlings of Q. serrata. The coordinated changes in leaf anatomical traits, physiological traits, and stem starch content indicate an important role in the survival of Q. acutissima and Q. serrata seedlings in water-deficit stress environments, although Q. serrata may show higher survivability under prolonged water stress than Q. acutissima.

Keywords: drought stress; drought tolerance; oak species; starch reserves; stomatal conductance; water use efficiency.

Figure 1

Density of glandular and non-glandular trichomes of Quercus acutissima and Q. serrata in the well-watered (WW) and water-deficit stress (WS) treatments. Comparison means derived using Tukey’s test (p < 0.05) are shown for the significant interaction between water stress and time. Different lowercase letters indicate significant differences (n = 10). Still-green leaves were the ones harvested for leaf anatomical traits measurement.

Figure 2

Trichome-to-stomata ratio (T/S) of Quercus acutissima and Q. serrata in well-watered (WW) and water-deficit stress (WS) treatments. Vertical bars indicate the SE (n = 10). Comparison means derived using Tukey’s test (p < 0.05) are shown for the significant interaction between water stress and time. Different lowercase letters indicate significant differences.

Figure 3

Glandular (g) and non-glandular trichomes (ng) of Quercus acutissima and Q. serrata in (a,b) well-watered (WW) and (c,d) water-deficit stress (WS) treatments at the final week. The bar represents 59. 5 µm.

Figure 4

(a) Thickness of palisade mesophyll and (b) spongy mesophyll tissues of Quercus acutissima and Q. serrata in well-watered (WW) and water-deficit stress (WS) treatments. The vertical bars indicate the SE (n = 10). Different lowercase letters indicate significant differences between treatments at α = 0.05. Still-green leaves were the ones harvested for leaf anatomical traits measurement.

Figure 5

Changes in leaf anatomical structure of Quercus acutissima and Q. serrata showing the palisade mesophyll (pm) and spongy mesophyll (sm) tissues in well-watered (WW) and water-deficit stress (WS) treatments at the final week.

Figure 6

Frequency distribution of leaf midrib xylem vessel diameter classes of (a) Quercus acutissima and (b) Q. serrata in well-watered (WW) and water-deficit stress (WS) treatments after eight weeks. Vertical bars indicate the SE (n = 10). Still-green leaves were the ones harvested for leaf anatomical traits measurement.

Figure 7

(a,b) Transpiration (E), (c,d) stomatal conductance (g_s), and (e,f) water use efficiency (WUE) of Quercus acutissima and Q. serrata in well-watered (WW) and water-deficit stress (WS) treatments. Vertical bars indicate the SE (n = 10). Comparison means derived using Tukey’s test (p < 0.05) are shown for the significant interaction between water stress and time. Different lowercase letters indicate significant differences. Gas exchange measurements were performed in still-green leaves.

Figure 8

(a) Loading plot of the relationship between anatomical and physiological traits and (b) loading plot of individuals of well-watered (WW) and water-stressed seedlings of Quercus acutissima and Q. serrata. Abbreviations: WWQA—well-watered seedlings of Q. acutissima (samples 1–10); WSQA—water-stressed seedlings of Q. acutissima (samples 11–20); WWQS—well-watered seedlings of Q. serrata (samples 21–30); WSQS—well-stressed seedlings of Q. serrata (samples 31–40); gs—stomatal conductance; E—transpiration; vdis—vein distance; vden—vein density; vloop—vein loopiness; WUE—water use efficiency; pm—palisade mesophyll; sm—spongy mesophyll; ts—trichome-to-stomata ratio; td—trichome density; and xvd—xylem vessel diameter.

Figure 9

Starch content of (a) Quercus acutissima and (b) Q. serrata in well-watered (WW) and water-stressed (WS) treatments. Vertical bars indicate the SE (n = 8). Comparison means derived using Tukey’s test (p < 0.05) are shown for the significant interaction between water stress and time. Different lowercase letters indicate significant differences.

Figure 10

Average weekly substrate dryness index (SDI) of all pots of Quercus acutissima and Quercus serrata from day 0 (no treatment) to day 49 of treatment imposition.

Figure 11

The schematic diagram for the measurement of the stomatal density (black arrow), trichome density (red arrow), mesophyll traits showing (a) palisade tissue and (b) spongy tissue, venation traits showing (c) loopiness, i.e., number of closed loops in the FOV divided by the FOV area, (d) vein density, i.e., the total length of veins in the FOV divided by the FOV area, (e) vein distance, i.e., the mean diameter of the largest circular masks that can fit in a closed loop, and xylem vessels (green arrow) [53] measured in the leaves of Quercus acutissima and Q. serrata.