Xylem Anatomy and Hydraulic Traits of Two Co-Occurring Riparian Desert Plants

Ayup IAWAet al. –Journal Hydraulic 36 (1),traits 2015: of desert 69–83 plants 69

XYLEM ANATOMY AND HYDRAULIC TRAITS OF TWO CO-OCCURRING RIPARIAN DESERT PLANTS

Mubarek Ayup1,2,*, Ya-Ning Chen1,2, Maina John Nyongesah3, Yuan-Ming Zhang1, Vishnu Dayal Rajput1,2 and Cheng-Gang Zhu1,2 1Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China 2State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China 3School of Biological Sciences, Jaramogi Oginga Odinga University of Science and Technology, 210-40601 Bondo, Kenya *Corresponding author; e-mail: [email protected]

ABSTRACT

Populus euphratica Oliv. and Tamarix ramosissima Ledeb. are the dominant riparian plants in desert ecosystems in China, where they play a significant role in maintaining ecological balance. To obtain a better insight into the ecological adaptations of xylem structure and hydraulic traits in desert phreatophytes to extremely drought-stressed environments, we investigated various quantitative features of the vessels and intervessel pits, as well as the xylem hydraulic efficiency (Ks(max)) and native embolism rate (PLC, %), in the woody shoots and lateral roots (all c. 2–4.5 mm in diameter) of P. euphratica and T. ramosissima from natural populations in the Heihe River Basin, northwestern China. The relationships between xylem anatomy and hydraulic traits are also discussed. There were significant anatomical differences between lateral root and woody shoot xylem within individual species. For lateral roots , arithmetic, hydraulic and maximum vessel diameter (D, Dh, Dmax), average vessel area (Va), intervessel wall thickness (Tvw), intervessel pit membrane and pit aperture areas (APM, APA), and intervessel pit membrane and pit aperture diameters (DPM, DPA), were larger than in woody shoots (P < 0.05).The mean Ks(max) values in lateral roots were 6–11 times greater than in woody shoots for P. euphratica and T. ramosissima, respectively (P < 0.01). Woody shoots of T. ramosissima had higher native PLC values (68%) than P. euphratica (39%).The different vessel grouping patterns in the two species seemed to be related to their different native embolism level. It is possible that the lateral roots of these two riparian desert plants could be more resistant to embolism than the woody shoots, and that cavitation resistance in the root xylem of T. ramosissima is higher than that of P. euphratica. Keywords: Populus euphratica Oliv., Tamarix ramosissima Ledeb., xylem anatomy, hydraulic conductivity, native embolism, intervessel pits.

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INTRODUCTION

Xylem hydraulic conductivity and cavitation are important factors that influence plant productivity and survival (Lens et al. 2011; Scholz et al. 2013a). Numerous studies have been conducted to understand wood anatomical variations and hydraulic functions at the inter-specific, intra-specific and intra-plant levels (Carlquist 1980; Mayr & Cochard 2003; Sano 2005; Lens et al. 2011; Scholz et al. 2013a). Most of these studies have demonstrated that within a species, root xylem has wider conduits (Zimmermann 1983; McElrone et al. 2004; Psaras & Sofroniou 2004; Domec et al. 2009) and higher conducting efficiencies (Kavanaugh et al. 1999; Martínez-Vilalta 2002; McElrone et al. 2004; Pratt et al. 2007) than stem xylem. However, in some species, root conduit diameter has been shown to be comparable to (Zimmermann 1983) or even smaller than stem xylem conduit diameter (Machado et al. 2007). While the root system accounts for 50% or more of the total resistance to water flow along the soil-to-leaf continuum (Nobel & Cui 1992; Domecet al. 2006a), the pit membrane ultra-structure has a significant influence on the total hydraulic resistance in the plant. Since the hydraulic conductivity and cavitation resistance of xylem cannot be fully understood without considering the influence of intervessel pits (Prattet al. 2007; Lens et al. 2011), various studies have been undertaken to probe how variations in pit morphology between root and stem xylem influence xylem hydraulic functions (Alder et al. 1996; Sperry & Ikeda 1997; Domec et al. 2006b; Schulte 2012). Studies have revealed that roots tend to have larger interconduit pits (Alder et al. 1996; Sperry & Ikeda 1997; Domec et al. 2006b; Hacke & Jansen 2009), a more porous pit membrane (Alder et al. 1996) and are more susceptible to embolisms than stem xylem (Sperry & Saliendra 1994; Sperry & Ikeda 1997; Hacke et al. 2000; McElrone et al. 2004). However, most of these studies were conducted on conifers (Sperry & Ikeda 1997; Domec et al. 2006b; Hacke & Jansen 2009), and only a few focused on the structural and quantitative differences in intervessel pits between the roots and aboveground organs of angiosperms (Alder et al. 1996). Deciduous P. euphratica trees and T. ramosissima shrubs are the dominant indigenous plant species that are widely distributed in forelands of the Heihe River Basin in an arid region of China. They play a protective role in blocking winds and stabilizing sands, and their growth and survival exclusively depend on their capacity to extend their root systems towards the groundwater (Gries et al. 2003; Zhu et al. 2009). However, due to extensive use of the water and land resources in the upper and central parts of the Heihe River, from the 1960s onwards, water resources discharged to the lower reaches have significantly decreased and thereby led to degradation of the riparian vegetation (Guo et al. 2009). In order to restore the ecosystem of the Heihe River, the Chinese government initiated and implemented the ecological water conveyance project (EWCP) (Chen et al. 2006). This project led to favorable conditions for the groundwater and plant communities. However, change was recorded at a small scale (100–400 m away from the water channel) (Guo et al. 2009). In this study, we assessed the hydraulic traits of xylem in the two aforementioned species, by measuring the hydraulic xylem efficiency and native embolism rate, and

Downloaded from Brill.com09/25/2021 08:47:28AM via free access Ayup et al. – Hydraulic traits of desert plants 71 by analyzing the anatomy of xylem vessels and intervessel pits in both lateral roots and woody shoots (both 2–4.5 mm in diameter, including bark). Our objectives were to understand the variation in quantitative xylem vessel and intervessel pit features and hydraulic traits at the intra-plant level; and to discuss the possible relationships between xylem anatomy and hydraulic traits in these two species. This study should increase our understanding of the acclimatization strategy of riparian desert plant communities to severe drought stress.

MATERIALS AND METHODS

Study sites and plant materials Research was carried out in the lower reaches of the Heihe River Basin (42° 06' 012" N, 101° 03' 564" E) in Inner Mongolia, northwestern China. The Heihe River drains the second-largest inland arid catchment area in China, which is characterized by a fragile ecosystem with scarce water resources (Chen et al. 2006). Typical riparian desert forest grows along the river, where dominant species include Populus euphratica trees and Tamarix ramosissima shrubs (Xi et al. 2009). The region’s climate is extremely arid, with an average annual pan evaporation of 3755 mm. The mean annual precipitation is 42 mm where the minimum and maximum recorded precipitation is 7 and 103 mm, respectively. Average annual temperature ranges between 7.0 and 9.0 °C, with an absolute range from -36.4 to + 41.8 °C (Xi et al. 2009; Guo et al. 2009). We conducted xylem hydraulic measurements on lateral roots and woody shoots of 21–30 year old P. euphratica and older T. ramosissima during August, 2012. The tree age of P. euphratica was roughly estimated on the basis of trunk diameter (Lu 1978; Wang et al. 1996). Twelve P. euphratica trees (mean height of 9.3 ± 3.8 m, mean DBH of 21.5 ± 8.7 cm) and ten shrubs of T. ramosissima of comparable size (mean height of 2.8 ± 0.9 m, mean stem diameter of 10.0 ± 4.6 cm) were selected as target plants. Lateral root and woody shoot samples were collected from different individual trees. Three to four woody shoot samples (branches or twigs, 2–4.5 mm in diameter) were collected from the sun exposed upper canopy of each P. euphratica and T. ramosissima tree/shrub for 3 days in the morning between 05 : 30 am and 06 : 30 am. With the groundwater table being 1.5–3.5 m deep, lateral roots were excavated from the soil at a depth of 10 to 40 cm. From each target plant, hydraulic conductivity was measured in 5–8 lateral roots. The average gravimetric water content of the soil where these lateral roots were collected was 7.49 ± 3.18 (%) (mean ± SD). Detailed information about the sample numbers (N), hydraulic traits measurement sample length and diameter (including bark) is given in Table 1.

Xylem hydraulic trait measurements The maximum hydraulic specific conductivity (or xylem hydraulic efficiency) (Ks(max), kg/m/s/MPa) and native embolism rate (percentage loss of hydraulic conductivity, PLC, %) of the xylem were measured on lateral root and woody shoot (branch/

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Table 1. Mean native embolism rate and hydraulic efficiency of different organs ofPopulus euphratica and Tamarix ramosissima. P-values are for the ANOVA between roots and woody shoots within the same species; different letters indicate significant differences between roots and woody shoots within the same plant (P < 0.05). Values are means±SD of sample diameter and length; values are means±SE of PLC and KS(MAX) values.

Number Diameter Segment K Organs PLC (%) S(MAX) P (N) (mm) length (cm) (Kg-1s-1m-1MPa) KS (MAX) Populus euphratica woody shoots 48 4.17 (0.7) 9.5 (2.8) 38.9 (2.8) 3.1 (0.2) a 2.73E-13 lateral roots 55 3.3 (1.1) 11.7 (2.0) – 20.5 (2.7) b

Tamarix ramosissima woody shoots 38 3.7 (0.6) 7.4 (2.1) 68.1 (3.7) 2.4 (0.2) a 8.82E-15 lateral roots 57 3.8 (0.7) 8.7 (3.3) – 26.5 (2.9) b twig) segments (7–12 cm long and 2–4.5 mm in diameter) from P. euphratica and T. ramosissima, using a XYLEM ®xylem embolism meter (Bronkhorst, Montigny- les-Cormeilles, France). The hydraulic conductivity and PLC values were measured with a prototype of the new Xylem apparatus (INRA license, http://www.instuctec.fr). The longest vessel length for each species was determined before excising the woody shoot segment for assessment of hydraulic traits. The maximum vessel length was determined by an air-perfusion technique (Zimmermann & Jeje 1981; Choat et al. 2003). For each species, the longest available un-branched stem (2–5 mm in diameter) was selected for the measurement of the longest vessels. A total of 29 branches were collected from 21–30 year old trees (Lu 1978; Wang et al. 1996) (n = 5), 11–20 year old trees (n = 9) and two year old (n =15) saplings of P. euphratica. A total of 14 twigs were collected from older tamarix shrubs (stem diameter of 10 ± 4.6 cm) and main shoots from two year old saplings of T. ramosissima for measurements of maximum vessel length. The distal ends of the woody shoot sections were placed under water while air was supplied at the proximal end at a pressure of 0.1 MPa. The woody shoot was cut again a few cm from the submerged end until bubbles were detected in the water (Zimmer- mann & Jeje 1981; Choat et al. 2003). The remaining woody shoot segment length was considered as the maximum vessel length for this sample. The 2–3 year old branches of P. euphratica were collected at a height between the top of the tree and the middle crown (collected at approximate heights between 7~11 m). As to T. ramosissima, one year old twigs were collected at height 2~3 m. Woody shoot samples were excised in air at the junction with older branches in the morning (at 05 : 30–06 : 30 am), and immediately put under distilled water, then at least 5 cm from the cut end, they were cut off under water with a razor hand pruner to minimize the air entering from the open vessels during the initial woody shoot cutting. The leafy end of the woody shoot was wrapped with black plastic bags and the cut end submerged in water during transportation to the laboratory. It took 30 to 40 min

Downloaded from Brill.com09/25/2021 08:47:28AM via free access Ayup et al. – Hydraulic traits of desert plants 73 to transfer the samples to the laboratory. In the laboratory, lateral root and woody shoot samples 2–4.5 mm in diameter were selected and 7–12 cm long segments were re-cut under distilled water with a sharp razor blade for hydraulic measurements (the meas-urement length of 7–12 cm was selected because of plugging and leaching of perfusion solution from the petiole-stem /leaf-stem internodes during hydraulic measurements). The basal cut end was then immediately attached to the hydraulic apparatus. The hydraulic pressure head was adjusted to avoid refilling of embolized vessels. For measurement of hydraulic conductivity, we chose 1–2 kPa, and 2–3 kPa for roots and woody shoots respectively of both species. Initial conductivity (Kinit: kg/s/MPa) of the segments was measured by gravity perfusion with natural mineral water containing several ions (1.35 ± 0.56 mg/l K+, 5.7 ± 0.79 mg/l Na+, 7.24 ± 0.48 mg/l Ca2+, and 1.15 ± 0.22 mg/l Mg2+) (mean ± SD). A reference perfusion fluid was selected with the aim of accurately simulating the native xylem sap ion concentrations, with respect to fluids containing only one or a few cations (Van Ieperen & Van Gelder 2006; Nardini et al. 2007, 2010). The solutions were filtered through a 0.1 µm filter and perfused at low pressure (1 to 3 kPa). Afterwards, the sample was flushed at high pressure (at 150 to 180 kPa) with the same solution as that used for Kinit measurement. The maximal conductivity (Kmax, kg/MPa) was then measured, and the specific conductivity was calculated as Ks = k (kg/s/MPa) × segment length (m) / segment surface area (m2). Finally, the percentage loss of hydraulic conductivity was computed as PLC = 100 (1-Ks (init) /Ks(max)).

Anatomical measurements Vessel and pit anatomy of the lateral root and woody shoot xylem were characterized for each species; the same plant materials were used for both hydraulic measurements and anatomical study. Shoot segments were fixed in FAA. Using a sliding microtome, semi-thin transverse sections (8–10 µm) were cut, double-stained with 1% safranin and 1% fast green, and observed under a light microscope (Olympus, BX51, Japan) equipped with a digital camera (Olympus DP 70, Japan). Intervessel pits were studied with scanning electron microscopy (SEM). Samples were wrapped using a wet towel covered with black plastic and then transported to the Xinjiang Institute of Ecology and Geography. Six samples (three lateral root and three woody shoot samples) per species were prepared for SEM according to Jansen et al. (2009). Small woody shoot and root segments from fresh samples were cut into segments 5–10 mm in length, dehydrated with an alcohol series (50, 70, 90, 100% etha- nol) between 10 and 30 minutes per step, and air-dried for 12 hrs at room temperature. Sample segments were split longitudinally with a razor blade and fixed to aluminum stubs with electron-conductive carbon cement (Neubauer Chemikalien, Munster, Ger- many). All samples were observed with a Zeiss Supar-55VP emission scanning electron microscope (Carl Zeiss SMT, AG Co., Germany) at an accelerating voltage of 2 KV. A list, including definitions and acronyms, of vessel and pit-level anatomical features assessed in this study is provided in Table 2. Most features were assessed following the method of Scholz et al. (2013b). The analyses of the microscopic images were per-

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Table 2. List of vessel and pit level characters measured with their acronyms, definitions and units (Scholz et al. 2013b).

Acronym Definition Unit Vessel features D Arithmetic vessel diameter = the simple average of the equivalent circle diameter, D = P/π μm 4 1/4 DH Hydraulic weighted diameter, DH = (ΣD / N) D: equivalent circle diameter, N: total conduit numbers μm

DMAX Maximum vessel diameter μm 2 -2 VD Vessel density = number of vessels per mm mm TVW Inter-vessel wall thickness measured as the double inter-vessel wall in the middle of adjacent vessels μm

LVW Inter-vessel wall length=length of vessel wall in contact with other vessels as based on transverse sections μm

FC Inter-vessel contact fraction = portion of vessel wall in contact with other vessels as based on transverse sections –

FVM Vessel multiple fraction = ratio of grouped vessels to total number of vessels – 2 VA Average vessel area μm VG Vessel grouping index = ratio of total number of vessels to total number of vessel groupings (including solitary and grouped vessels) –

VS Solitary vessel index = ratio of solitary vessels to total vessel groupings including solitary and grouped vessels) –

Pit features 2 APA Inner pit aperture surface area μm APM Inter-vessel pit surface area = area occupied by pit border or the inter-vessel pit membrane μm2 DPA Diameter of inner pit aperture as measured at the widest part of the openings μm DPM Horizontal pit membrane diameter at its widest point μm FPF Fraction of inter-vessel wall area occupied by inter-vessel pits – Apa w/s Ratio of the diameter of the inner pit aperture as measured the widest and shortest axis –

Fp Mean fraction of the vessel area occupied by inter-vessel pit, Fc × FPF – formed by the open source software ImageJ. The measurements of vessel characteristics (Table 3) were conducted on one cross section each of 5–8 different lateral roots and woody shoots from each species. Only the last two growth rings in branch samples of P. euphratica were analyzed in the case of three-year-old branches. Statistical analyses were performed using SPSS v. 12.0. Comparisons of hydraulic and anatomical parameters within individuals were made with a one-way ANOVA.

RESULTS

Intra-plant variation in vessel and intervessel pit characteristics There was considerable anatomical variation in xylem at the intra-plant level, especially in vessel diameter, vessel density, and quantitative intervessel pit characteristics (Table 3 & 4).

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PA C w/s 0.11 a 0.11 0.07 b (no unit) (0.007) (0.005) (0.001) (0.003) 0.025 a 0.022 a Inner A Inner 1.9 (0.6) a 2.4 (0.6) b 4.6 (1.5) b 3.03 (0.7) a F V M

0.4 a (0.02) (0.07) (0.02) (0.04) 0.87 a 0.78 a 0.64 b (no unit)

P 0.02 0.01 0.01 0.04 (no unit) F G (0.1) (0.3) (0.1) 2.5 a 1.8 b (0.03) 1.15 a 1.32 a (no unit)

F V S PF F 0.3 a 0.6 b (0.03) (0.07) (0.01) (0.04) 0.84 a 0.83 a (no unit) (no unit) 0.45 (0.1) a 0.48 (0.1) a 0.34 (0.1) a 0.39 (0.1) a V VW T (0.3) (0.8) (0.3) (0.1) 4.2 a 6.1 a 6.7 b ( µ m) 10.6 b

( µ m) VW 28 a 49 b (0.8) (2.2) (5.1) (1.1) ( µ m) Inner D 1.5 (0.2) a 2.3 (0.6) a 1.9 (0.3) b 3.7 (0.7) b 21.1 a 21.7 a ) 2 L D (9.5) (4.7) (6.8) (30.1) 354.7 a 81.68 b 89.52 b

119.13 a 119.13 (n/mm P M ( µ m) ) 8.1 (0.9) a V 2 3.6 (0.5) b 8.9 (1.4) b 2.95 (0.3) a A ( µ m (28.7) (55.3) (55.3) (483.9) 901.8 a 741.5 a 4454.4 b 2075.2 b D V PA ) 2 M A X (0.9) (4.3) (1.9) (2.8) ( µ m) 48.8 a 39.9 a 91.5 b 66.8 b ( µ m Inner A Inner 2.5 (1.3) a 3.3 (1.4) b 0.44 (0.1) a 0.64 (0.1) b D H D (0.4) (3.5) (1.1) (0.9) ( µ m) 40.2 a 34.2 a 77.8 b 62.5 b )

2 P M A µ m D ( µ m) (2.2 (0.6) (1.2) (0.5) ( 32.7 a 29.7 a 65.8 b 45.5 b 5.7 (1.3) a 7.7 (2.1) b 43.1 (15.2) a 55.9 (16.1) b

Species / Organ Species / Organ Lateral roots Tamarix ramosissima Tamarix shoots Woody Populus euphratica shoots Woody Lateral roots 2 for explanation of acronyms. Table Table 3. Quantitative vessel features of woody shoots and lateral roots (2–Table 4.5 mm in diameter). (n): sample numbers; letters different indicate significant between differences roots and woody shoots within the same species (P Values are < 0.05). means ± of acronyms. 2 for explanation Table SE. See Populus euphratica shoots (n = 7) Woody Lateral roots (n = 6) 4. Quantitative intervessel pit features of woody shoots and lateral roots (2–4.5Table mm in diameter). Different letters indicate significant differences between roots and woody shoots within the same species (P Values < 0.01). are means ± SE. See

Tamarix ramosissima Tamarix shoots (n = 5) Woody

Lateral roots (n = 8)

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The variation of vessel size and vessel distribution patterns in woody shoots and lateral roots are presented in Figure 1a, b, c, d. The mean, maximum and hydraulic vessel diameter (D, Dmax, Dh, respectively), and average vessel area (Va) were larger in lateral roots than in woody shoots for both P. euphratica and T. ramosissima (P < 0.05) (Table 3). Lateral root intervessel walls (Tvw) (Fig. 1f, h) were thicker than in woody shoots (Fig. 1e, g) in both species (P < 0.05). The vessel grouping index (VG) varied significantly only within P. euphratica, with mean values ranging from 2.53 for branches to 1.80 for lateral roots (Table 3; Fig. 1a, b). Variation in the solitary vessel index (VS) was comparable to that of VG (P < 0.05). The intervessel wall length (LVW) was also greater in roots than in woody shoots in P. euphratica (P < 0.01) while T. ramosissima (a species with mainly solitary vessels) did not exhibit such differences (Table 3; Fig. 1c, d). The average maximum vessel length of P. euphratica branches was 8.7 ± 2.6 cm (total branch length of 24 ± 2.9 cm) (Mean ± SD) (n = 5), and 33 ± 11cm (total twig length of 37 ± 12 cm) (n = 6) for T. ramosissima. The pit membrane diameter (Dpm) of intervessel pits was significantly larger in lateral roots than in woody shoots, for both species (P < 0.01; Table 4). Compared with woody shoots, lateral roots tended to have higher values for pit membrane surface area (Apm). Variation in inner pit aperture diameter (Dpa) and pit aperture surface area (Apa) exhibited patterns that were similar to those for Dpm and Apm ( P < 0.01) (Table 4). Our observations of SEM images indicated that there were no obvious qualitative differences in the intervessel pit membrane structures between lateral roots and woody shoots. In both species, the intact pit membrane was observed as a uniform deposition of microfibrils across the surface, with no visible pores (Fig. 2a, b). A dense layer of randomly oriented microfibrils was observed after removing the incrustations (Fig. 2c, d).

Intra-plant variation in hydraulic traits The mean values of Ks(max) for xylem in the lateral roots were 6.7-fold and 11-fold greater than in the woody shoots for P. euphratica and T. ramosissima, respectively (P < 0.01) (Table 1). According to our observation, the native embolism rates (PLC) of T. ramosissima shoots were higher (> 60%) than those in P. euphratica branches (39%) (Table 1).

DISCUSSION Variation in xylem anatomy and hydraulic traits within individual trees Our assays indicated that lateral root vessel dimensions (D, Dmax, Dh and Va) of both Populus euphratica and Tamarix ramosissima were greater than in woody shoots. This general trend is in agreement with many other studies (Zimmermann 1983; McElrone et al. 2004; Psaras & Sofroniou 2004; Domec et al. 2009). However, some vessel characteristics, particularly vessel grouping features (VG, VS, FVM) were species- specific where differences were only observed within P. euphratica trees that had higher VG in woody shoots than in roots. With respect to T. ramosissima, both lateral roots and twigs had a lower VG .

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a e

b f

c g

d h

Figure 1. Cross-sectional LM micrographs of woody shoots and lateral roots of Populus euphratica and Tamarix ramosissima. – a, e: P. euphratica, woody shoots; b, f: lateral roots. – c, g: T. ramosissima, woody shoots; d, h: lateral roots. – a–d: Vessel size and vessel distribution pattern; scale bars = 100 μm. – e–h: Variation of intervessel wall thickness (TVW); scale bars = 10 μm.

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a c

b d

Figure 2. SEM micrographs of intervessel pit membranes in woody shoots for Populus euphratica (a, c) and Tamarix ramosissima (b, d), showing intact pit membranes with incrustations (a, b) and intervessel pit membranes after removal of pit incrustations (c, d). — Scale bars: a, c = 2 μm; b = 200 nm; d = 1 μm.

Significant variations in interconduit pit characteristics have been observed when comparing roots and aboveground organs within plants of the same species (Alder et al. 1996; Sperry & Ikeda 1997; Domec et al. 2006b; Hacke & Jansen 2009). Consistent with previous studies, our results showed that intervessel pit dimensions (Apm, Apa, Dpm, Dpa ) in both P. euphratica and T. ramosissima were significantly larger in the roots than in the woody shoots. It has been reported previously that roots of Acer grandidentatum have more porous pit membranes than stems (Alder et al. 1996). However, another study revealed that the pit membrane structures in the roots of Douglas-fir (Pseudotsuga menziesii) are similar to those in the stems, based on SEM observations (Sperry & Ikeda 1997). In the present study, we did not observe structural differences in intervessel pit membranes between these two organs using SEM imaging. In both organs, intact pit membranes appeared to be non-porous, and only randomly orientated microfibrils were occasionally observed after removal of pit incrustations. Variation in pit membrane porosity in angiosperms has been investigated in several recent studies (Sano 2005; Jansen et al. 2009; Lens et al. 2011); however, pores in homogeneous pit membranes are not always observable through electronic microscopy (Wheeler 1981; Choat et al. 2003). Electronic microscopy itself may introduce some artifacts into the pit membrane’s structure (Choat et al. 2008). Methods used in the

Downloaded from Brill.com09/25/2021 08:47:28AM via free access Ayup et al. – Hydraulic traits of desert plants 79 preparation of SEM samples may also strongly impact pit membrane structures, as proven by Jansen et al. (2008). A previous study reported a high native embolism rate in the stems of T. ramosissima where the mean rate in different seasons ranged from 47 to 87% (Pockman & Sperry 2000). Our previous study also reported that the mean PLC values in the lateral roots, main stems and twigs of two-year-old T. ramosissima seedlings (under optimal soil moisture levels) fluctuated within the range of 50 to 70% (Ayup et al. 2012). We further attempted to find out how these plants manage to grow well by having higher embolism levels in their xylem, especially T. ramosissima. It is possible that there may be some other water loss compensating mechanism, such as an “ionic effect”, which is the embolism-induced loss of hydraulic conductance alleviated by increased hydraulic conductance of still functioning vessels through a synchronous increase in the ionic concentration of the xylem sap (Nardini et al. 2011). These could have prevented the occurrence of catastrophic cavitations in xylem which leads to dieback of plants.

Possible correlations between xylem anatomy and hydraulic characteristics Our results are consistent with those of similar studies (Kavanaugh et al. 1999; Martínez-Vilalta 2002; McElrone et al. 2004; Pratt et al. 2007), where the lateral roots of both species exhibited larger vessel diameters and areas, with greater hydraulic efficiency than the woody shoot xylem. In angiosperms, thinner pit membranes are usually more porous than thicker pit membranes, which is generally correlated with Tvw and thicker Tvw is then associated with greater resistance to embolism (Cochard et al. 2008; Jansen et al. 2009). Besides, pit aperture shape is also correlated with embolism resistance, with cavitation-resistant species exhibiting narrow and elliptical pit apertures (Lens et al. 2011; Scholz et al. 2013a). Results from this study show that lateral roots of both species had thicker Tvw than woody shoot samples, and T. ramosissima had thicker Tvw than P. euphratica (P < 0.05). A similar trend was found in inner pit aperture shape (Pa w/s), narrower pit apertures in lateral roots than woody shoots and narrower pit apertures in T. ramosissima compared to P. euphratica. Considering these two cavitation resistant related parameters, it is possible that the lateral roots of these two desert riparian plants are more resistant to embolism than woody shoots. Moreover, T. ramosissima is more cavitation resistant that P. euphratica. Some previous studies have also suggested that the roots of P. euphratica are more cavitation-resistant than the stems and midribs of leaves (Hukin et al. 2005). In the Sonoran Desert, stems of T. ramosissima were observed to be more cavitation-resistant than those of surrounding native riparian plants (Pockman & Sperry 2000). Several studies have contributed to our understanding of the functional role of vessel groupings across angiosperms (Carlquist 1984, 2009; Loepfe et al. 2007; Lens et al. 2011; Jansen et al. 2011; Scholz et al. 2013a) with two different interpretations of such roles. The first postulation indicated that drought-adapted species have higher vessel groupings (with non-conductive ground tissue) than their relatives in more mesic habitats. The larger vessel groups may provide redundant pathways when some vessels become embolised in dry environments (Carlquist 1984, 2009). Contradictory

Downloaded from Brill.com09/25/2021 08:47:28AM via free access 80 IAWA Journal 36 (1), 2015 to Carlquist’s hypothesis, the theoretical hydraulic model revealed that high vessel connectivity decreases resistance to embolism by increasing the risk of embolism spreading via air-seeding (Loepfe et al. 2007; Martínez-Vilalta et al. 2012). Our study appears to be consistent with Carlquist’s hypothesis, so that P. euphratica exhibited a lower embolism level with higher grouping of vessels in their branches, and vice versa in T. ramosissima. Lateral roots in both species revealed more solitary vessels and a relatively higher native embolism level (data not shown) compared to their woody shoots. However, it must be noted that Carlquist’s hypothesis is based on the correlation between vessel grouping and soil moisture availability, with species growing in xeric habitats developing a greater degree of vessel grouping than their relatives in more mesic sites (Carlquist 1984, 2009; Scholz et al. 2013a). The two species assessed in the current study grow under identical environmental conditions, but revealed differences in their pattern of vessel grouping with different native embolism rates.

CONCLUSION This study has revealed clear anatomical differences between the root and woody shoot xylem in both P. euphratica and T. ramosissima (two co-occurring desert phreatophytes). The values of all vessel dimensions (D, Dh, Dmax, Va) and quantitative intervessel pit parameters (Apm, Apa, Dpm, Dpa, Apa w/s) assessed in this study, as well as those for the hydraulic efficiency were higher in lateral roots than woody shoots in both species. Tamarix ramosissima shoots had relatively higher native embolism rates than Populus euphratica branches growing in the same habitat. It is however necessary to conduct further research on the structure and composition of pit membranes through different microscopy techniques. In addition, tests of ionic effects are necessary to better understand the acclimatization strategy of riparian desert plants to extreme drought conditions, from the perspective of xylem structure and function.

ACKNOWLEDGEMENTS

This study was funded by the National Natural Science Foundation of China (91025025 and U1303102) and supported by the open fund project of the State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, CAS.

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Accepted: 18 September 2014

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