Trees (1992) 6:216-224

9 Springer-Verlag 1992

Xylem structure and water transport in a twiner, a scrambler, and a shrub of Lonicera (Caprifoliaceae) Shau-Ting Chiu and Frank W. Ewers Department of Botany and Plant Pathology, Michigan State University, East Lansing, MI 48823-1312, USA Received March 17/Accepted May 4, 1992

Summary. Wood structure and function was investigated in different growth forms of temperate honeysuckles (Lonicera spp.). All three species had many narrow vessels and relatively few wide ones, with the measured Kh (flow rate/pressure gradient) approximately 24-55% of the theoretical Kh predicted by Poiseuille's law. Only the twiner, Lonicera japonica, had some vessels greater than 50 ~tm in diameter. The twiner also had the narrowest stem xylem diameters, suggesting the greater maximum vessel diameter hydraulically compensated for narrow stems. Conversely, the free-standing shrub, L. maackii, had the greatest annual increments of xylem but the least percent conductive xylem implying that a great portion of the wood was involved with mechanical support. The scrambler, L, sempervirens had low maximum vessel diameter, high Huber values (= xylem area/leaf area), and low specific conductivities (= measured Kh/xylem area), much like the shrub. The greatest vessel frequency occurred in the scrambler (901 vessels, mm-2), the highest thus far recorded in vines. The lowest Huber value and highest specific conductivity occurred in the twiner, suggesting little self-support but relatively efficient water conduction. LSC (= measured Kh/leaf area) and maximum vessel diameter of Lonicera vines were near the low end of the range for vines in general. Key words: Xylem - Water transport - Lonicera - Hydraulic conductivity - Vessel diameter

tion of descriptive, ecological and physiological features of wood structure has been "slow and difficult" (Carlquist 1988). Xylem functions include transport of water and minerals, mechanical support of the plant body, and storage of water and nutrients (Ewers and Cruiziat 1991; Ewers et al. 1991). Since in vines, the plant gets mechanical support from an external source, it is not surprising that vines differ from free standing growth forms in the mechanical properties of the stems (Gartner et al. 1990; Gartner 1991 a, c; Ewers and Fisher 1991). There are few studies of xylem structure and function that compare vines to closely related free-standing growth forms. Ewers et al. (1988, 1990, 1991) compared tropical tree, shrub and liana (= woody vine) species of Bauhinia. Gartner (1991 b) compared lianas to shrubs within the western North American species, Toxicodendron diversilobum (T. & G.) Greene. The present study investigated wood structure and function in different species of temperate honeysuckles (Lonicera spp.). In recent years attention has focused on quantifying and modelling the movement of water through plants (Waisel et al. 1972; Pickard 1981; Fiscus 1983; Gibson et al. 1985; Radcliffe et al. 1986; Schulte and Gibson 1988; Tyree 1988). For modelling water flow in tracheary elements, xylem transport efficiency is often expressed as the measured hydraulic conductance per pressure gradient (measured Kh; Tyree and Ewers 1991). Theoretical hydraulic conductance (theoretical Kh), which relates xylem anatomy to the ideal efficiency of water transport by the HagenPoiseuille equation, modified for elliptical cross area (Calkin et al. 1986), is calculated by the following formula: a2i.b2i .( 2ai'bi

Introduction

thoeretical Kh = 7~L, 1--]-~

Differences among taxa in features of xylem structure are often assumed to be of adaptive significance in plants (Carlquist and Hoekman 1985). However, the interpreta-

where ai and bi are the major and minor axes of elliptical cross area of the capillary in m; i = 1, 2 . . . . . n; and rl is the viscosity of the fluid (10 -9 MPa. s for water at 20 ~C). To determine how effective a stem is at supplying its leaves with water, one needs to consider not only measured Kh and theoretical Kh, but also the leaf area that the stem segment supplies. Leaf specific conductivity (LSC), mea-

Correspondence to: E Ewers

~. a2i+b2i j in m 4 .MPA 1. s-l (1)

217

s u r e d Kh d i v i d e d b y s u p p l i e d l e a f a r e a ( Z i m m e r m a n n 1978), has been used to determine "hydraulic dominance" o f t h e m a i n s t e m o v e r l a t e r a l b r a n c h e s ( T y r e e e t al. 1 9 8 3 ; E w e r s a n d Z i m m e r m a n n 1 9 8 4 a , b; E w e r s e t al. 1 9 8 9 ) a n d to c o m p a r e t w o t e m p e r a t e t r e e s t o a t r o p i c a l t r e e ( T y r e e e t al. 1 9 9 1 ) . H o w e v e r , it h a s s e l d o m b e e n u s e d t o c o m p a r e d i f f e r e n t g r o w t h f o r m s ( G a r t n e r 1991 b). S p e c i f i c c o n d u c t i v i t y ( = m e a s u r e d K h / x y l e m a r e a ) is a l s o u s e f u l i n c o m p a r i n g d i f f e r e n t t a x a b e c a u s e it r e l a t e s w a t e r t r a n s p o r t e f f i c i e n c y to t h e x y l e m a r e a . B e c a u s e x y l e m o f a s h r u b c o n tributes more in mechanical self-support than that of a liana (Gartner 1991c), one might expect that a shrub would d e v o t e a g r e a t e r p e r c e n t a g e o f its x y l e m t i s s u e t o m e c h a n i cal support and thus have lower specific conductivity than a l i a n a . H u b e r v a l u e ( = x y l e m a r e a / l e a f a r e a ) is r e l a t e d more directly to mechanical properties than hydraulic properties of stems. However, producing more wood per l e a f a r e a is o n e w a y o f e n h a n c i n g t r a n s p o r t ( E w e r s a n d Z i m m e r m a n n 1 9 8 4 a , b). F o r a g i v e n s t e m s e g m e n t , t h e r e l a t i o n s h i p b e t w e e n t h e s e v a l u e s is as f o l l o w s ( E w e r s a n d Z i m m e r m a n n 1 9 8 4 a, b): LSC = Huber value - Specific conductivity

(2)

W e e x p e c t e d H u b e r v a l u e s to b e g r e a t e s t a n d s p e c i f i c c o n ductivity to be lowest in the free-standing shrub.

Materials and methods Plant materials. We examined cultivated specimens of the twiner Lonicera japonica Thunb., the scrambler L. sempervirens L., and the freestanding shrub L. maackii (Rupr.) Maxim. A twiner is a vine whose shoots spirally twine around a support. A scrambler is a vine that is initially self-supporting but eventually falls over to be supported by an external host plant or object. The scrambler, L. sempervirens, has straight stems that do not twine but remain erect until they become too long and heavy to support their own weight. By the end of their first year, the stems normally fall upon the soil or adjacent support. Plants of L. sempervirens (up to 4 years old) were grown outdoors in pots at Michigan State University (MSU) and in the ground at Flushing, Michigan. Two outdoor individuals died in 1990. Because L. sempervirens is not hardy enough to survive some winters in Michigan, the potted plants were sheltered in the greenhouse for the winter. Plants of L. japonica and L. maackii (both up to 7 years old) were grown outdoors in the ground on the MSU campus. Specimens were collected between May and September during 1988 to 1991. Stem segments of mostly 15 cm length but 10 cm for densely branched specimens were labelled, a sketch of the branch architecture was then made, and leaves distal to labelled segments were collected for measurement of the leaf area. Fifty-five segments from 8 plants of L. japonica, 60 segments from 4 plants ofL. sempervirens, and 51 segments from 5 plants ofL. maackii were used for all the measurements described below, except for vessel frequency which is the number of vessels per transverse xylem area (Wheeler et al. 1989), mean and me~lian vessel diameter and theoretical Kh for which 20 (L. japonica), 17 (L. sempervirens), and 19 (L. maackii) stem segments were used. For wood macerations, 2 stem segments were sampled per species, MeasuredKh. Stem segments were collected early in the morning and cut under water to prevent the introduction of-air embolism. The cut surfaces were shaved smooth with a fresh razor blade, fitted with vinyl tubing at the proximal end, and the stems with tubing were submerged in water and evacuated at 0.07 MPa for 15 min to remove superficial air bubbles. A 150 mmol .m -3 acetic acid (pH = 5) solution (Calkin et al. 1986), filtered through 0.2 gm Gelman membrane filter (Sperry et al. 1987), was perfused through the segments under a gravity gradient with a maximum

pressure head of 4.1 kPa. Volumetric flow rate was determined with a pipette and stopwatch. Measured Kh was calculated as the measured volumetric flow rate (m3.s -1) divided by the pressure gradient (MPa. m -l; Tyree and Ewers 1991).

Anatomical study. Following the measurements of Kn, a filtered 0.5% safranin solution was perfused through the stem segments for 5 h to demarcate the conducting xylem (Ewers and Zimmermann 1984a, b). The middle of each stem segment was transversely sectioned with a sliding microtome. The 20 gm sections were dehydrated through an ethanol-xylene series and mounted in permount. Kodachrome slides of the sections were made with a Nikon-SMZ1 photomicroscope and the images projected on a large sheet of paper for measuring the vessel diameters (Ewers and Fisher 1989), and vessel frequency which is the number of vessels per transverse xylem area (Wheeler et al. 1989). In the young stems, every vessel seen in a transverse section was measured. In stems older than 2 years, all the vessels in randomly selected sectors were measured. Each sector had vascular rays for marginal boundaries and the pith and the vascular cambium as its inner and outer boundary. Measurements from the sectors were extrapolated to the total xylem area for calculation of theoretical Kn from Eq. 1. Wood macerations were necessary to learn cellular characteristics to distinguish narrow vessels from fibers or tracheids, since in transverse sectional view narrow vessels appeared similar to tracheids. A 5 mm long segment adjacent to the sectioned region was used for macerations. All tissues outside the cambium were removed and the wood containing pith was cut in longitudinal slivers. The material was treated with Jeffrey's solution (10% chromic acid: 10% nitirc acid = 1 : 1 v: v) for 36 h in a 60 ~C oven until it was soft to the touch (Johansen 1940). The tissue was washed in water, stained with 0.5% aqueous safranin, dehydrated in an ethanol-xylene series, and mounted in permount. Cell wall thickness and lumen diameter of vessel elements, tracheids and fibers, which were determined at the middle of the long axis of each cell, were measured directly with an ocular micrometer under a Zeiss compound microscope. For each segment, cell populations were randomly sampled from the macerated materials. Xylem area and leaf surface area. Xylem transverse areas were determined from the 20 g m sections. The conducting xylem area was identified based on the presence of safranin dye in the vessel walls. The conducting and nonconducting transverse areas of xylem were separately traced with a camera hicida attached to a Nicon SMZ-10 microscope. The area of paper cutouts was measured with a Li-Cor 3000 portable area meter. Stem xylem diameter was determined as the mean of 4 diameters measured from the traced drawings. One-sided leaf surface areas of all the leaves distal to the stem segments were directly measured with the area meter. Huber value. After measurements of leaf area, leaves were dried in a 100 ~C oven for at least 4 days until their dry weight remained constant. Leaf weight was determined to the nearest 0.01 g with a balance. Both (a) leaf area based and (b) leaf weight based Huber values were calculated as the xylem transverse area of the stem segment divided by (a) the leaf area or (b) the oven-dried weight of leaves distal to the stem segment. Moisture content of wood. Stem segments from 3 to 6 cm long abutted to those used for measured K~ were collected. The longer segments were needed for narrower stems. Immediately after the bark was removed, the fresh weight was determined to the nearest 0.01 g with a balance and fresh volume was measured to the nearest 0.05 ml by displacement of water upon submerging the specimen into a slender graduated cylinder. The moisture content of fresh wood was calculated by the following equation (Stewart 1967; Pallardy et al. 1991): 1 Ws = ~ - 0.65

fresh weight of wood Where D~ = fresh volume of wood

(3)

Statistical analysis. Differences among taxa and differences between current year (1-year-old) and older (older than 1 year) stems were evaluated from the SAS GLM procedure for analysis of variance with un-

218 Table 1. Range in cell lumen diameters (~m) o f vessels, tracheids and fibers of the twiner (L. japonica), scrambler (L. sempervirens) and shrub

Current year stems

(L. maackii) Growth forms

Twiner Scrambler Shrub

Older stems

twiner Vessels n = 170

Tracheids n = 70

Fibers n = 220

7.6-103.0 1 0 . 1 - 52.9 7 . 6 - 58.0

3 . 8 - 17.6 7 . 6 - 15.1 5.0-13.9

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scrambler Table 2, Range in cell wall thickness (~tm) of vessels, tracheids and fibers of the twiner (L japonica), scrambler (L. sempervirens) and shrub (L. maackii) Growth forms

Vessels n = 170

Tracheids n = 70

Fibers n = 240

Twiner Scrambler Shrub

0 . 6 - 2.5 0.6 - 2.5 0 . 6 - 2.5

1.3- 5.0 2.5 - 3.8 1.9-4.4

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Analysis of variance indicated that there were statistically significant differences between current year stems and older stems in most examined features for all three species. Therefore, the transectional and physiological data below are presented separately for current year and older stems.

Maceradon A comparison of cell lumen diameter (Table 1) and wall thickness (Table 2) from the macerated xylem indicated that vessels had generally wider cell lumens and thinner walls than the tracheids and fibers. However, the very narrow vessels were similar to the tracheids except for the presence of perforations. To exclude the tracheids and fibers in vessel measurements from the wood transection, below, we included only cells (vessels) that had walls thinner than 2.5 ~tm and lumens more than 18 ~tm in twiners or more than 15 ~tm in scramblers and shrubs.

Transections Vessel diameter frequency distributions are shown for one representative current-year and one 4-year-old stem of the twiner, scrambler and shrub (Fig. 1). Except in the older stems of twiners, the frequency distribution of the vessel diameters tended to the Poisson distribution (Steel and Torrie 1980) with a high percentage of the narrow vessels (Fig. 1). The pattern for the older stems of twiners was a positively skewed normal distribution with many more narrow than wide vessels, and a pronounced tail to the wide

Fig. 1. Percentage of total theoretical Kb (open circles) for each class of vessel diameter and the frequency distributions of vessel diameter (histograms) in current year and 4-year-old stems of a twiner Lonicerajaponica, a scrambler L. sempervirens, and a shrub L. maackii. Each graph is from one representative stem. Arrows indicate the largest vessel diameter in each sample

vessel diameter (Fig. 1). For all the distribution patterns, a great portion of total theoretical Kh resulted from the wider vessels. While only 1.1% (twiners), 15.3% (scramblers), 12.2% (shrubs) of the total number of vessels in the current year stems and 0.6% (twiners), 4.7% (scramblers), 8.9% (shrubs) in the older stems were grouped in the highest 2 classes of vessel diameter in each sample, these classes contributed 30.5% (twiners), 62.7% (scramblers), 50.6% (shrubs) of the total theoretical Kh of current year stems and 27.7% (twiners), 25.5% (scramblers), 46.9% (shrubs) of that of older stems. Conversely, the narrow vessels were frequent but contributed little to the theoretical Kh (Fig. 1). When stained vessels and tracheids below the 18 ~tm (twiner) and 15 ~tm (scrambler and shrub) threshold were included, they increased theoretical Kh by less than 1% (data not shown). Qualitative features of wood anatomy can be seen in Figs. 2 - 4 . Based on the examination of vessel diameter in 56 stem segments, the twiner had the greatest maximum vessel diameter (Fig. 5) but also many small vessels (Figs. 1, 2a, b). As stems aged, the outer growth rings of xylem contained the widest vessels (Fig. 2b). The means of maximum vessel diameter for twiners, 65.8 ~tm in current year stems and 94.7 ~tm in older stems, were significantly greater than those of the other growth forms (Fig. 5).

219

Fig. 2 - 4 .

Transverse sections of the current year stems (a) and the older stems (b) of the twiner Lonicerajaponica (2), the scrambler L. sempervirens (3) and the shrub L. maackii (4). Bar = 100 ~tm

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The shrub had greater stem xylem diameters than the twiner and the scrambler species. Likewise, the xylem diameter increment and total xylem area increment per year were greatest in shrubs (Table 3). The ratio of conductive xylem area to total xylem area (percent conductive xylem area) was lowest in the shrubs (Table 3). The greatest vessel frequency (vessels.mm-2) occurred in scramblers, especially in current year stems, and the least occurred in twiners, especially in older stems (Table 3).

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Wood moisture content

L.j. L.s. L.m. L.j. L.s. L.m. Current yeor stems Older stems Fig. 5. Comparison of vessel diameter among different growth forms: a twiner Lonicera japonica, a scrambler L. sempervirens and a shrub L. maackii. Bars indicate • standard error. Asterisks indicate the statistically significant difference between species. ~ median; ~ mean; I I maximum

In the current year stems, the shrub had the highest moisture content of fresh wood and the twiner had the lowest (Table 3). However, the decline of moisture content of fresh wood from current year to older stems was greatest in the shrubs and least in the twiners (Table 3). For the older stems, there was no statistically significant difference in moisture content among the different species.

Measured and theoretical Kh Stems of the scrambler contained relatively narrow vessels (Figs. 1, 3 a, b). In this species, for each growth ring the widest vessels occurred at about the 5th to 7th cell layer outside the previous year's latewood (Fig. 3 b). Unlike the twiner, when the stems became older, the maximum vessel diameter did not increase by much (Figs. 1, 3b, 5). In shrubs (Fig. 4 a, b), in going from earlywood to latewood, vessel diameter consistently decreased and the number of tracheids and fibers consistently increased. As with the scrambler, when the stems became older, the diameter of maximum vessels increased little in the older growth rings (Figs. 4b, 5). The anatomical characters of the scrambler were generally similar to those of the shrub. However, in the scrambler, there was a wider distribution area of moderate vessel diameters throughout the growth ring (Figs. 3, 4), a greater number of very small vessels (Fig. 1), a smaller xylem area and a higher percentage of stem conductive xylem area (Table 3). For all growth forms, the median vessel diameter was usually lower than the mean vessel diameter (Fig. 5). The difference between median and mean vessel diameter was greatest in the twiners (Fig. 5).

The relationship between measured Kh and theoretical Kh was linear and highly correlated in all 3 species. The r values of the regression lines of measured Kh over theoretical Kh ranged from 0.72 to 0.98 (Fig. 6). The mean ratios of measured Kh over theoretical Kh were 34.2% (twiner), 45.3% (scrambler), 54.6% (shrub) in the current year stems (Fig. 6a) and 25.2%, 35.0%, 24.3% in the older stems (Fig. 6b). These percentages were not statistically different among species.

Huber value, specific conductivity, LSC The leaf area based Huber value was greatest in the current year stems of scramblers and least in the older stems of twiners (Fig. 7). In current year stems, scramblers significantly differed from twiners and shrubs in this parameter, but twiners and shrubs were not distinguishable from each other. For older stems, twiners were distinguishable from scramblers and shrubs but scramblers and shrubs were not statistically different from each other. In addition, leaf weight based Huber value was much higher in current year

Table 3. Quantitative features (mean • standard error) of xylem in the twiner (L. japonica), scrambler (L sempervirens) and shrub (L. maackii) A g e of stems Growth forms

Leaf weight based Huber value (mm2. g 1)

Moisture content of fresh wood (%)

Increment of xylem diameter (ram. year -1)

Current year stems: Twiner Scrambler Shrub

1.54 _ 0.27 2.64 • 0.20 1.55 •

36.6 + 2.0 48.0 • 5.6 56.0+4.7

1.465 • 0.131 1.728 • 0.162 3.033•

Older stems: Twiner Scrambler Shrub

1.75 • 1.77 _+O. 16 1.51 •

3 4 . 2 • 3.4 38.3 • 4.0 3 3 . 4 + 1.4

0.905 • 0.095 1.090 • 0.091 2.410•

Increment of total xylem area (ram2 9year -1)

Percent conductive xylem area

Vessel frequency (per m m 2)

1.93 • 0.40 1.57 • 0.32 3.49•

87.9 • 3.6 81.5 • 8.0 65.3 •

774 • 100 901 • 47 787 • 96

3.22 • 2.16 • 0.40 13.08•

65.4 _+5.7 82.2 • 3.8 59.0+5.1

561 • 830 • 62 738•

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