Stem and Root Wood Anatomy of the Shrub Phlomis Fruticosa (Labiatae)
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IAWA Journal, Vol. 25 (1), 2004: 71–77 STEM AND ROOT WOOD ANATOMY OF THE SHRUB PHLOMIS FRUTICOSA (LABIATAE) G.K. Psaras & I. Sofroniou Section of Plant Biology, Department of Biology, University of Patras, Patras 265 00, Greece [E-mail: [email protected]] SUMMARY Wood from the stems and roots of the malacophyllous Mediterranean drought semi-deciduous species Phlomis fruticosa was studied. The stem is diffuse-porous with narrow vessels. Though narrow vessels im- pose high conducting resistance, they are less vulnerable to cavitations, thus providing safety during summer drought and winter freezing. The geographical distribution of the plant (up to 1400 m altitude) may be relevant to the occurrence of narrow stem vessels, which provide high resistance to cavitations from low temperatures during winter. Vessel grouping in the stem adds to the safety against cavitations. Root vessels are mostly solitary and have almost double the diameter of stem ves- sels. Diffuse-porosity, the presence of narrow vessels in stems and the values of vulnerability and mesomorphy indices, are adaptations to the two stresses imposed by the Mediterranean climate, i.e. summer drought and winter cold. Key words: Phlomis fruticosa, ecological wood anatomy, Mediterranean plant life. INTRODUCTION Mediterranean plant life is affected by two seasonally separated climatic stresses: summer drought and winter cold stress (Mitrakos 1980). Perennial native plants of Mediterranean-climate areas may be classified as evergreen sclerophyll and drought deciduous shrubs or sub-shrubs. Evergreen sclerophylls possess hard and leathery leaves, with efficient stomatal control of excessive water loss or with a fine balance between water loss and uptake (Salleo & Nardini 2000). Drought deciduous plants have leaves that undergo desiccation and senescence during the period of dryness (Orshan 1963). A third group, malacophyllous species (from Greek, malakos = soft and phyllon = leaf), i.e. species having soft leaves, have adopted a semi-deciduous model. To avoid excessive water loss, they shed some of their leaves during the dry period. These plants often possess winter and summer leaves, which have distinct morphological features and thus are seasonally dimorphic (Orshan et al. 1989). In Greece the latter sub-shrubs constitute the so-called “phryganic” formations (Margaris 1976), which are synonymous with coastal sage of California and garigue formations of France. Downloaded from Brill.com10/02/2021 03:18:24PM via free access 72 IAWA Journal, Vol. 25 (1), 2004 Psaras & Sofroniou — Anatomy of Phlomis fruticosa wood 73 Phrygana, i.e. the components of phryganic formations, have developed certain strategies to survive and develop under the Mediterranean climate. The malacophyllous species Phlomis fruticosa L. (Labiatae) is a native sub-shrub of the east Mediterranean area (Greuter et al. 1986) dominating in the phryganic formations in Greece (Margaris 1976). Extensive work carried out under field conditions revealed that this species has in- teresting functional/ecophysiological characteristics. Maximum cambial activity of the plant takes place during winter (January-February; Psaras & Konsolaki 1986). Accord- ing to Grammatikopoulos et al. (1995) Phlomis fruticosa uses a “whole repertoire” of adaptations to tolerate the adverse conditions of the Mediterranean summer. The plant has two kinds of leaves. Summer leaves are small-sized with limited growth during the dry period (Kyparissis & Manetas 1993a). They resume growth and activity after autumn rains (Kyparissis & Manetas 1993b). They are relatively long-lived, about 13 months, and they avoid photoinhibitory damage through decreased chlorophyll con- tent (Kyparissis et al. 1995). Leaf tissues maintain their turgor during the dry period through elastic and osmotic adjustments (Grammatikopoulos 1999). Winter leaves, on the other hand, expand during early winter and are shed during summer. Although the relative water content of winter leaves during low temperature periods is kept high, photosynthetic rate remains low (Grammatikopoulos et al. 1995). This is attributed to a water stress imposed by increased water resistance to water flow from the soil to the leaves. The plant is also able to accumulate osmotically active compounds like pro- line (Rhizopoulou et al. 1990), as a response to drought and cold stress. The relative conducting area (Huber value), i.e. the conducting area in a cross section of a branch or a leaf petiole divided by the mass of the tissue supplied with xylem sap, was found to be lower in P. fruticosa compared to other co-occurring phryganic species (c. 1 vs 3 mm2 g-1; Psaras 2000), indicating limited xylem tissue. Phlomis fruticosa is a species for which we have information on its ecophysiology, but little is known of the anatomical features of its stem and root. Thus, this paper describes the wood anatomical features of the stem and the root of P. fruticosa, and relates wood structure to known ecophysiological traits of the plant. MATERIALS AND METHODS Plant material was sampled from five perennial shrubs ofPhlomis fruticosa L. (Labia- tae), growing wild near the campus of the University of Patras (38° 15' N, 21° 44' E, 150 m above sea level) in southwestern Peloponnisos, Greece. The shrubs were 80–100 cm high; stem wood was taken about 20–30 cm above soil. Root samples, excavated with hand tools, were collected about 20 cm distal from the root-stem junction. Prepa- ration of sections and macerations for light microscopy were made using standard microtechnical methods. Sections were observed with a Zeiss Axioplan microscope. For scanning electron microscopy (SEM) small blocks or sections exposing transverse, radial or tangential surfaces were mounted on stubs, sputter-coated with gold and examined with a Jeol 6300 scanning electron microscope. For terminology and de- termination of the quantitative features of the wood, recommendations of the IAWA Committee (1989) were followed. Downloaded from Brill.com10/02/2021 03:18:24PM via free access 72 IAWA Journal, Vol. 25 (1), 2004 Psaras & Sofroniou — Anatomy of Phlomis fruticosa wood 73 Table 1. Quantitative anatomical features of Phlomis fruticosa wood. Stem Root Growth rings not observed not observed Vessels per mm2 93.7 ± 8.6 (80–105) 94.2 ± 7.1 (82–104) Tangential vessel diameter (μm) 27.01 ± 6.33 (15–40) 51.52 ± 11.39 (30–90) Vessel element length (V; μm) 233 ± 28 (175–310) 260 ± 31 (150–350) Number of vessels per group 5 (1–9) 2 (1–4) Intervessel pit size (μm) 4 5 Fibre length (F; μm) 590 ± 75 (390–700) 620 ± 81 (410–740) Vulnerability index (v) 0.29 0.55 Mesomorphy index (m) 67.1 143.5 Ray height (μm) 370 ± 30 (150–600) 400 ± 34 (150–680) Multiseriate ray width (μm) 35 ± 4 35 ± 5 Uniseriate ray width (μm) – 10 ± 2 Number of rays per mm 12 ± 2 18 ± 2 F/V ratio 2.5 2.4 Vessel element and fibre dimensions are means of at least 100 measurements; ± values rep- resent s.d.; range shown in parentheses. For all other quantitative data the recommendations of IAWA Committee (1989) were followed. WOOD DESCRIPTION Table 1 gives the quantitative anatomical features of Phlomis fruticosa stem and root wood. Stem wood (Fig. 1–7) — Diffuse-porous, vessels usually in tangential bands. Ves- sel elements with oblique simple perforation plates, non-vestured, crowded alternate intervessel pits. Fibres thin- to thick-walled, non-septate, with simple to minutely bordered pits. Axial parenchyma absent. Rays 1- to 3-seriate, homocellular with square cells and one marginal row of upright cells. Root wood (Fig. 8–12) — Diffuse-porous, vessels usually solitary, partly in radial multiples of 2. Vessel elements with oblique simple perforation plates, non-vestured, alternate intervessel pits. Ground tissue fibres thin- to thick-walled, non-septate. Axial parenchyma absent. Rays 1- to 3-seriate, homocellular with square cells and one mar- ginal row of upright cells. DISCUSSION The mean vessel diameter of Phlomis fruticosa stem wood (27 μm) is close to that of desert shrubs (29 μm) given by Carlquist (1977), but is much lower than that given by Fahn et al. (1986; 82 μm). However, vessel member length (233 μm) is higher than that of desert species (150 μm; Fahn et al. 1986) and closer to that of Mediterranean species (275 μm; Fahn et al. 1986). The same holds true for the fibres means. Simple Downloaded from Brill.com10/02/2021 03:18:24PM via free access 74 IAWA Journal, Vol. 25 (1), 2004 Psaras & Sofroniou — Anatomy of Phlomis fruticosa wood 75 Fig. 1–7. Stem wood. – 1: TS, vessels in tangential arrangement. – 2: LS, vessel member with sim- ple perforation plate. – 3: intervessel pits. – 4 & 5: macerated wood, vessel member and fibres, respectively. – 6: TLS, multiseriate ray. – 7: RLS, ray cells. — Scale bar in 1, 6, 7 = 100 µm, in 2–5 = 10 µm. perforation plates found in P. fruticosa vessel members are considered to be an advan- tage for xerophytic species (Carlquist 1975; Fahn et al. 1986). The vessel frequency of P. fruticosa (94/mm2) is also close to that recorded for desert shrubs (90/mm2; Fahn et al. 1986), and lower than Mediterranean evergreen sclerophylls (158/mm2; Fahn et al. 1986). Carlquist (1977) introduced two indices as ecological indicators for wood anatomy trends: vulnerability (v; the mean vessel diameter divided by vessel frequency) and mesomorphy (m; the vulnerability multiplied by mean vessel element length). Low Downloaded from Brill.com10/02/2021 03:18:24PM via free access 74 IAWA Journal, Vol. 25 (1), 2004 Psaras & Sofroniou — Anatomy of Phlomis fruticosa wood 75 Fig. 8–12. Root wood. – 8: TS, diffuse-porous wood. – 9: macerated wood, vessel member, fibres. – 10: LS, vessel member with simple perforation plate. – 11: intervessel pits. – 12: TLS, uniseriate ray. — Scale bar in 8 = 100 µm, in 9–12 = 10 µm. values of v and m (less than 1 and 50, respectively) indicate adaptation to xeric con- ditions, whilst high values of v and m (more than 1 and 800, respectively) indicate adaptation to mesic conditions.