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± 1.85 27.1 5a ± 1.10
30.2 20.2
*Different letters show significant variance at the confidence level of p < 0.05.
The abaxial epidermal cells of the cauline leaf are smaller vs. the rosette leaf, but this variance is not significant. The number of abaxial epidermal cells and stomata per 1 mm2 is larger in the cauline leaf (367.36±6.69 cells and 158.72±6.19 stomata) vs. the rosette leaf (343.04±5.85 cells and 134.40±5.77 stomata) while the stomatal index of the rosette (27.97%) and cauline (30.02%) leaves is practically the same and has no significant variance. The stomata are oblong-rounded, 25.03-38.05 pm long and 21.20-
189 Leaf blade anatomy of the rare Siberian flora species
28.66 pm wide, chaotic and surrounded with epidermal cells (Table 1, Figure 1). The abaxial epidermis thickness of the rosette and cauline leaves has no significant variance.
The adaxial side of the leaf comprises glandular trichomes surrounded with pronounced rosettes of cells in the base part (Figure 2). The number of trichomes in the rosette and cauline leaves has no significant variance and amounts to 0-32 pcs per 1 mm2.
Fig. 2. Mertensia sibiricaglandular trichomes in juvenile (A) and old leaves (B)
The rosette leaf is thicker (1360.7-1629.8 pm) within the midrib vs. the cauline leaf (839.49-1006.29 pm), which is due to its more developed vascular system: the vascular bundle and is parts are 2-2.5 times larger vs. the rosette leaf. However, the vascular tissue area ratio (xylem/phloem) does not have significant variance (Table 2).
Table 2. Mertensia sibirica rosette and cauline leaf anatomy characteristics.
Rosette leaf
Cauline leaf
Characteristic
M ± m CV, %
Leaf thickness (midrib), pm Leaf thickness (blade), pm Mesophyll thickness, pm Palisade mesophyll thickness, pm Spongy mesophyll thickness, pm Palisade mesophyll / spongy mesophyll ratio Cell length of the upper mesophyll layer, pm Cell width of the upper mesophyll layer, pm Bundle cross section area, pm2 Xylem cross section area, pm2 Phloem cross section area, pm2 Xylem/phloem area ratio
1 526.65a ± 14.51 914.73B ± 10.86
4.8 5.9
373.10a ± 8.31 369.67a ± 7.43
11.1 10.1
290.60a ± 6.55 289.56a ± 3.84
11.3 6.6
74.88a ± 2.06 89.59B ± 2.60
13.8 14.5
214.86a ± 6.69 204.21a ± 5.47
15.6 13.4
0.36A ± 0.01 0.45B ± 0.02
19.4 23.7
80.80a ± 3.06 90.66B ± 3.03
18.9 16.7
29.53a ± 2.46 28.51a ± 1.78
41.6 31.2
207411 .68a ± 3651.90 82777.69B ± 1746.60
8.8 10.6
611 77.74a ± 1555.00 31229.01B ± 684.42
12.7 11.0
4911 0.47a ± 1508.62 24744.63B ± 796.88
15.4 16.1
1.26a ± 0.04 1.29a ± 0.05
* Different letters show significant variance at the confidence level of p < 0.05.
The leaf mesophyll is 242.90-369.90 pm thick, dorsiventral, consists of a single layer of palisade and multiple layers of spongy parenchyma (Figure 3). The thickness of the rosette and cauline leaf mesophyll, spongy mesophyll, blades has no significant variance, while the palisade tissue is thicker in the cauline leaf (Table 2) and the palisade/spongy mesophyll ratio is also higher for the cauline leaf (0.45 vs. 0.36). The palisade coefficient is low, amounts to 25.8% and 30.9% for the rosette and cauline leaves, respectively.
Leaf section by veins
Leaf section (blade) Fig. 3. Mertensia sibirica rosette and cauline leaf blade anatomy respectively.
Discussion
As for dermal tissue parameters, medium variance is typical of the thickness of the abaxial (CV=20.2-30.2%) and adaxial (CV=28.9-23.9%) epidermis and the cell size of the adaxial (CV=20.2-22.3%) and especially abaxial (CV=38.5-39.9%) epidermis. The stomatal complex and mesophyll parameters under study are primarily characterized by low variance. Coefficients of variation for the cells of the upper mesophyll layer (CV=31.2-41.6%) and the number of stomata on the lower epidermis of the rosette leaf (CV=21.5%) demonstrate medium and high variance. A very high coefficient of variation (116.2-174.0%) is registered for the adaxial epidermis parameter characterizing the density of trichomes per 1 mm2. Glandular trichomes having a similar structure are also relevant to some genera of the Boraginaceae family, according to F. Selvi and M. Bigazzi (Selvi & Bigazzi, 2001). Significant variance was established for 13 (56.5%) of the 23 studied parameters of rosette and cauline leaf blades belonging to this species.
Thus, the cauline leaf significantly differs from the rosette leaf in finer cells of its adaxial and abaxial epidermis (and, consequently, their larger number per 1 mm2), while the adaxial epidermal cells are thicker, and in a larger number of stomata in the abaxial epidermis. The palisade mesophyll in the cauline leaf is more developed vs. the rosette leaf, while the cells are longer and the palisade/spongy mesophyll ratio is higher. This variance is due to the fact that cauline leaves develop in better light environments vs. the rosette leaves and therefore have more pronounced signs of being heliophytic. The rosette leaves have a more developed system of vascular tissues vs. cauline ones, as they play the main role in providing plants with water and nutrients. However, the cauline and rosette leaf blades have no variance in terms of the xylem/phloem ratio and mesophyll thickness.
The contribution of the cauline leaf palisade mesophyll to the photosynthetic potential of M. sibirica is higher vs. that of the rosette leaf (the ratio between palisade and spongy mesophyll is 0.45 vs. 0.36, respectively), which characterizes the cauline leaf as more heliophytic.
M. sibirica differs from M. maritima in thinner hypostomatous rather than amphistomous leaves being 369.67-373.10 pm thick vs. 471.34-550.46 pm for the second species, a smaller number of stomata per unit of area (134.4-158.72 pcs per 1 mm2 vs. 1.5-3.25 pcs/cm2 or 150-325 pcs/mm2), which is probably reasoned by M. maritima adaptation to open, well-lighted areas with saline soils (sand beaches and pebblestones of sea littorals) and winds.
The established thickness of the M. sibirica leaf blade (373.10±8.31 plm) is comparable with data provided by M.R. Slaton et al (2001) for the North American species Mertensia viridis(A. Nels.) A. Nels. (366±13 pm).
Thus, the M. sibirica leaf blade structure has hygromesophytic and scioheliophytic signs: quite fleshy hypostomatous leaves with a thin cuticle, large-cell stomata and epidermis, dorsiventral mesophyll, increased contribution of spongy parenchyma to the photosynthetic potential of this species, large amounts of water-storage tissues, predominant rising current development.
Leaf blade anatomy of the rare Siberian flora species
191
Apparently, the large thin-walled mesophyll cells have a water storage function. The poor development of trichomes found only on the adaxial epidermis, the large-cell mesophyll also reflect species preference for high air and substrate humidity.
Conclusion
The studies of the M. sibirica leaf blade anatomy allow classifying this species as hygromesophytes. We have determined 13 signs, according to which the rosette leaves significantly differ from the cauline leaves. We have also established the presence of glandular trichomes on the adaxial leaf epidermis.
Acknowledgements
The study was performed within the framework of a state assignment of the Ministry of Science and Higher Education of the Russian Federation (project No. 0721-2020-0019).
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Citation:
Belaeva, T.N., Butenkova, A.N. (2020). Leaf blade anatomy of the rare Siberian flora species Mertensia sibirica (L.) G. Don fil. Boraginaceae).
Ukrainian Journal of Ecology, 10(b), 186-191. I ("OE^^MI This work Is licensed under a Creative Commons Attribution 4.0. License
