沙漠研究 19-1, 133 - 136 (2009 )Journal of Arid Land Studies“Desert Technology IX ”
Refereed Paper

Potential of Photosynthetic Characteristics in Polymorphic Leavesof Populus euphratica Olivier
Yukako MONDA1, Naoko MIKI1, Ken YOSHIKAWA1
Abstract: A distinct leaf shape polymorphism occurs within single plants of Populus
euphratica Olivier. Gas exchange was investigated in broad-ovate (BOL) and lanceolate (LL)
leaves, located respectively in the upper crown and lower crown of mature P. euphratica
growing in its native habitat. This study was designed to clarify the differences ofphotosynthetic characteristics in wet conditions between those of BOL and LL. BOL showed
high Pnmax; LL showed low light compensation. Results show that, during wet periods, BOL
performs very high photosynthesis production with much water loss. LL has a low
photosynthetic rate, but LL can perform effective photosynthesis production in a darkcondition.
Keywords: Photosynthetic characteristic, Polymorphism, Populus euphratica Olivier

1. Introduction
Being sessile and long-lived, perennial plants might experience widely various environmental
changes during their lives. Riparian forests in oases are unstable water resource environments. Oases
can reduce negative effects of environmental conditions on plants through evaporative cooling: theso-called oasis effect (Oke, 1987). However, such water-resource-based oasis ecosystems are unreliablebecause of anthropogenic effects and environmental impact. Riparian forests in oases will experience
radical changes in conditions: from wet to hyper-arid. Therefore, specific strategies are necessary tomaintain high levels of growth and carbon gain. Populus euphratica Olivier is the most common species
in oases (Wickens, 1998), distributed from Morocco in the west to the Ordos Plateau in the east (Kurschner,2004). Mature P. euphratica have broad-ovate leaves (BOL) at the upper crown, and lower in crown withsprout of lanceolate leaves (LL) (Fig. 1). Juveniles have only the LL type.
In a preceding study, the photosynthesis of BOL and LL with a crown on mature P. euphratica wasmeasured in the actual state (Wang et al., 1997; Zhang et al., 2003). For average daily photosynthetic
characteristics, the water use efficiency (WUE) of BOL was higher than that of LL as a result of the higher
transpiration rate (E) of LL (Wang et al., 1997). Regarding the net photosynthetic rate (Pn) of BOL,
severe photosynthetic depression was observedat midday. In contrast, no midday depression
was observed in LL (Zhang et al., 2003).
Those results, especially that for midday,
indicates the photosynthesis characteristic of awater stress condition.
The objective of this study is to clarify thedifferences of photosynthetic characteristics inwet conditions between the BOL and LL in acrown on mature P. euphratica by measuring
photosynthesis under conditions that let a branch Fig. 1. Morphology of broad-ovate leaves (left, BOL)
and lanceolate leaves (right, LL) of Populus
absorb water.
euphratica.

2. Materials and Methods
This study was conducted in a natural P. euphratica forest established along the Heihe River in theEjina Oasis, Inner Mongolia, China (41˚57'–59'N, 101˚04'–10'E) (Fig. 2). This region is in a hyper-arid
climatic zone. It has low precipitation and humidity, and wide temperature variation: annual precipitation
is 34 mm; relative humidity ranges from 15 to 53%; maximum and minimum air temperatures were 40 ˚Cand -26 ˚C, respectively, in 2004. Therefore, Ejina Oasis ecosystems rely heavily on water resources
１Graduate School of Environmental Science, Okayama Univ., 1-1-1 Tsushima-naka, Okayama 1008530, Japan13414(a) Pn ma x
a10Pn max (μmol m-2-1s )12t = 3.73 ※86b420LL

China

supplied by the Heihe River.
In Ejina Oasis, P. euphratica grows in apatchy forest stand with diverse structure (Mondaet al., 2008). Three trees of P. euphratica wereselected, which were 4.0 ± 0.3 m height, 3.1 ± 0.4
cm DBH on average and free of disease and pestdamage. Furthermore, BOL appeared in theupper part of crown, and LL appeared in the lower
part of crown. The vertical distance of BOL andLL was about 2 m.
To obtain a leaf sample that is not in a water
stress condition, 2–3 year branches were taken at
8:00; the base of branch was cut off in the water
immediately after sampling. The Pn and E ofupper fully extended leaves were measured using aportable photosynthesis system (Li-6400; Li-Cor
Inc., Lincoln, NE, USA) during June 3–4 in 2005.
The CO2 concentration and vapor pressure deficit(VPD) in the cuvette were held respectively to 360
ppm and less than 1.5 kPa during measurements.
Then WUE was calculated from the Pn/E ratio.
To construct light–photosynthesis curves, Pnvalues at different levels of photosynthetic photon
flux (PPFD) were measured repeatedly for 3–4 setsof BOL and LL. The Pn was stabilized at anirradiance of 1300 μmol m-2 s-1; then gas exchange
was measured at 11 stepwise irradiance intervals tozero. To obtain the maximal net photosynthetic
rate (Pnmax) and light compensation, we calculated
gas exchange characteristics (Kaleida Graph
Version 4.0; Synergy Software Technologies Inc.)

Light compensation point(μmol m -2 s-1 )Fig. 2. Study area.

60(b) Ligh t compensation
a50t = 2.89 ※40b30
20100
LL16BOL(c ) WUE

ns141210WUE

Fig. 2 Study area

70BOL86420LLBOLFig. 3. Photosynthetic characteristics of BOL and LLat different locations in the crown (a)Maximal net photosynthetic rate (Pnmax), (b)Light compensation, (c) water use efficiency(WUE) in P. euphratica. Vertical bars
represent SD (n = 4–5). Different letters denotesignificant difference using Student’s t-test. (※p<0.05, ns = not significant)135

by non-rectangular hyperbola equation (Johnson and Thornley, 1984) using the observed photosynthesis–PPFD relation.
Differences in photosynthesis characteristics between BOL and LL were analyzed using Student’st-test.
3. Results
The Pnmax of BOL was significantly higher than LL (t (5) =3.73, p<0.05) (Fig. 3a). The lightcompensation of LL was significantly lower than BOL (t (5) =2.89, p<0.05) (Fig. 3b). Those of BOL andLL showed no significant differences in WUE (Fig. 3c).
4. Discussion
Results show a difference in the potential of photosynthetic characteristics between BOL and LL.
Our result suggests that, in the wet condition, BOL can achieve higher Pnmax than LL (Fig. 3a). No
significant difference was found between WUE of BOL and LL (Fig. 3c). Therefore, the achievement ofhigh Pnmax of BOL is accompanied by the loss of much water.
In the water stress condition, no significant difference was found for average daily Pn between BOLand LL (Wang et al., 1997) by stomatal closure in midday (Zhang et al., 2003). As with other Populus
species composing riparian forests (Rood et al., 2000, 2003; Horton et al., 2001), P. euphratica adapts
itself to changing severe drought conditions through the sacrifice of branches of the upper part of the crownthrough dieback (Hukin et al., 2005; Monda et al., 2008). To depress evaporation loss during severe
water shortage periods, the dieback phenomenon is observed among desiccation-avoidant trees (Sperry andPockman, 1993; Horton et al., 2001; Rood et al., 2000, 2003; Cooper et al., 2003).
Therefore, mature P. euphratica adapt to environmental changes: during the wet condition, BOL
performs very high photosynthesis production with much water loss; during periods of dry conditions,
BOL adapts to the available water resources by controlling the loss of the water through stomatal control
and crown dieback.
Another interesting point revealed by our study was that light compensation of LL was significantly
lower than that of BOL (Fig. 3b), suggesting a shade-leaf type which fitted shaded conditions.
Consequently, LL has a low photosynthetic rate, but LL can perform effective photosynthesis production indark conditions. This photosynthetic characteristic will be advantageous for leaves of the lower part ofcrown, which easily receives self-cover shade and cover shade from other individuals. All development
stages in P. euphratica have LL in all or part of the crown. Additional studies are necessary to clarify LL
traits for adaptation to dry conditions.
Acknowledgments
This study was partly funded by a contribution from the Oasis Project (Historical evolution ofadaptability in an oasis region to water resource changes), promoted by the Research Institute for
Humanity and Nature (RIHN), Japan.
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