ФИЗИОЛОГИЯ РАСТЕНИЙ, 2011, том 58, № 1, с. 95-101
ЭКСПЕРИМЕНТАЛЬНЫЕ СТАТЬИ
УДК 581.1
SPECIES-SPECIFIC, SEASONAL, INTER-ANNUAL, AND HISTORICALLY-
ACCUMULATED CHANGES IN FOLIAR TERPENE EMISSION RATES IN Phillyrea latifolia AND Quercus ilex SUBMITTED TO RAIN EXCLUSION IN THE PRADES MOUNTAINS (Catalonia)
© 2011 J. Llusia, J. Penuelas, G. A. Alessio, R. Ogaya
Global Ecology-CEAB-CSIC-CREAF Unit-Center for Ecological Research and Forestry Applications, Autonomous University of Barselona, Bellaterra, Barcelona, Spain Received December 29, 2009
Mediterranean vegetation emits large amounts of terpenes. We aimed to study the effects of the decreases in soil water availability forecast for the next decades by global circulation models and ecophysiological models on the terpene emissions by two widely distributed Mediterranean woody species, Phillyrea latifolia L. and Quercus ilex L. We subjected holm oak forest plots to an experimental soil drought of ca. 20% decrease in soil moisture by partial rainfall exclusion and runoff exclusion. We measured the emission rates throughout the seasons for two years with contrasting precipitation and soil moisture (16.6% average in 2003 vs. 6.4% as average in 2005). Among the detected volatile terpenes, only a-pinene and limonene were present in detectable quantities in all of the studied periods. Total terpene emitted ranged from practically zero (spring 2003) to 3.6 and 58.3 p.g/(g dry wt h) (winter 2005 and summer 2003 for P. latifolia and Q. ilex, respectively). A clear seasonality was found in the emission rates (they were the highest in summer in both species) and also in the qualitative composition of the emission mix. Maximum emissions of a-pinene occurred in spring and maximum emissions of limonene in winter. Neither the inter-annual differences in water availability nor the rain exclusion treatment significantly affected the emissions in P. latifolia, but Q. ilex showed by 17% lower emissions during the drier second year of study, 2005, but more than two- and threefold increases with the drought treatment in summer 2003 and in summer 2005, respectively, showing historical accumulated effects. These results, which show increased monoterpene emission under the moderate drought produced by the treatment and decreased emission under the severe second year drought, and a much higher sensitivity to drought in Q. ilex than in P. latifolia, are useful in understanding the behavior of plant volatiles under Mediterranean conditions and in modeling future emission under changing climate conditions. They show that the usage of current models could lead to under- and overestimations of the emission under summer dry conditions, because most current algorithms are based on light and temperature only.
Key words: Phillyrea latifoli — Quercus ilex — monoterpenes — water stress — isoprenoid emission
INTRODUCTION
Mediterranean vegetation emits large amounts of isoprenoids, mainly a-pinene, limonene, and iso-prene [1—3]. Abiotic factors greatly influence the emission rates of isoprenoids [3—5]. Among them, changes in water supply may induce important changes in Mediterranean plants that often suffer of water scarcity, especially in the dry summer season. Global circulation, climatic, and ecophysiological models predict a further reduction in the water availability in the Mediterranean region [6, 7]. Soil drought is known to be involved in the short-term control of emission, increasing [8, 9] or decreasing [1, 10, 11], the emission rates being dependent on the water stress intensity. But
Corresponding author. J. Llusia. Center for Ecological Research and Forestry Applications, Autonomous University of Barselona, 08193 Bellaterra, Barcelona, Spain. Fax. 349-3581-4151; e-mail. j.llusia@creaf.uab.cat
less is known about what happens at the longer term seasonal and annual scale in different species. Moreover, most studies have been conducted under laboratory conditions on potted plants and at the leaflevel [1, 11, 12].
In this study we aimed to test the effect of the soil drought forecast for the Mediterranean basin in the coming decades by global circulation models and ecophysiological models [6, 7] on the emission rates of the widely distributed species P. latifolia and Q. ilex. We studied their responses to experimental rain and runoff exclusion (producing ca. 20% relative decrease in soil moisture) in a Mediterranean forest in southern Catalonia. We measured their emission throughout the seasons for two years with contrasting precipitation. We aimed thus to study how emission varied, depending on the species, the season, the year, and the drought conditions, including the historical ones.
MATERIALS AND METHODS
The study site and species description. The study was carried out in a holm oak forest in the Prades Mountains of southern Catalonia (41°13' N, 0°55' E), on a south-facing slope (25% slope) at 930 m a.s.l. The soil is a stony xerochrept on bedrock of metamorphic sandstone, and its depth ranges between 35 and 90 cm. Summer drought is pronounced and usually lasts for 3 months. This holm oak forest is very dense (1.66 trees/m2). Trees are about 4—8 m high and have a mean stem diameter (measured at 50 cm height) of 5 cm. The forest is dominated by Quercus ilex L., Phillyrea latifolia L., and Arbutus unedo L. with abundant presence of other evergreen species well adapted to drought conditions (Erica arborea L., Juniperus ox-ycedrus L., and Cistus albidus L.), and occasional individuals ofdeciduous species, such as Sorbus torminalis (L.) Crantz and Acer monspessulanum L.
Experimental design. Four 15 x 10 m plots were marked out at the same altitude and aspect along the slope. Half the plots received the dry treatment and the other half were used as control plots. The dry treatment consisted of partial rain exclusion, achieved by suspending PVC strips at a height of 0.5—0.8 m above the soil. The strips covered approximately 30% of the total plot surface. In addition, a 0.8-m-deep ditch was excavated along the entire top edge of the treatment plots to intercept runoff water supply. This drought treatment was started in March 1999. Temperature and precipitation were monitored every half-hour by an automatic meteorological station installed between the plots in a forest gap. Soil moisture was measured every two weeks throughout the experiment by time domain reflectometry (Tektronix 1502C, "Beaver-ton", United States) with three stainless steel cylindrical rods, 0.25 m long, permanently fully driven into the soil at randomly selected points, 4 beneath the strips and 4 clear of the strips, in each plot. We also determined the soil moisture at permanent wilting point measuring soil desorption curves with a WP4 Dew Point Hygrometer ("Decagon Devices", Pullman, United States).
Gas exchange and terpene sampling and analysis. In
every season of the two years, 2003 and 2005, little stems of P. latifolia and Q. ilex with 1—4 current-year leaves were measured on sunny middays in each of the four plots. Measurements of CO2 and H2O exchange, and terpene sampling were conducted simultaneously. A calibrated IRGAporometer (LCA-4, "ADC", Hod-deson, Hertfordshire, United Kingdom) was used for determination of CO2 and H2O exchange. Air coming out of the cuvette flowed through a T system to a glass tube (8 cm long and 0.3 cm internal diameter). The tube was filled manually with terpene adsorbents Car-bopack B, Carboxen 1003, and Carbopack Y ("Supel-co", Bellefonte, United States) separated by plugs of
quartz wool. Samples were taken using a Qmax air sampling pump ("Supelco"). The hydrophobic properties of the tubes minimized sample displacement by water. In these tubes terpenes did not subjected to chemical transformations, as checked with standards (a-pinene, p-pinene, camphene, myrcene, p-cymene, li-monene, sabinene, camphor, and dodecane). Prior to usage, these tubes were conditioned for 10 min at 350°C with a stream of purified helium. The sampling time was 5 min, and the flow varied between 470 and 500 ml/min, depending on the packing of the adsorbent and quartz wool. A calibrated air sampling pump was used to trap isoprenoids. The trapping and desorption efficiency of liquid and volatilized standards, such as a-pinene, p-pinene, or limonene was practically 100%. In order to eliminate the problem of memory effect of previous samples, blanks of 5-min air sampling without plants were taken immediately before and after each measurement. The glass tubes were stored in a portable fridge at 4°C and taken to the laboratory. There, they were stored at —28°C until analysis (within 24—48 h). There were no observable changes in terpene concentrations after storage of the tubes, as was checked by analyzing replicate samples immediately after sampling and also after 48-h storage. Emission rate calculations were made on mass balance basis and subtracting the control samples values (without plants) from the values for samples from plants.
The analyses of volatile organic compounds were performed using a GC-MS system (Hewlett Packard HP59822B, Palo Alto, United States). Tubes with trapped emitted monoterpenes were inserted in a TDU (Thermal Desorption Unit, Model 890/891; "Supelco") and desorbed at 250°C for 3 min and passed into a 30 m x 0.25 mm x 0.25 mm film thickness capillary column (Supelco HP-5, Crosslinked 5% Me Silicone, "Supelco"). After sample injection, the initial temperature (40°C) was increased at 30°C/min up to 60°C and thereafter at 10°C/min up to 150°C, maintained for 3 min, and thereafter at 70°C/min up to 250°C, which temperature was maintained for another 5 min. Helium flow was 1 ml/min. The identification of monoterpenes was conducted by GC-MS and comparison with standards from "Fluka" (Switzerland), literature spectra, and GCD Chemstation G1074A HP. Internal standard dodecane, which did not mask any terpene, together with frequent calibration with common terpene a-pinene, A3-carene, p-
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