Journal of Experimental Botany, Vol. 48, No. 315, pp. 1835-1841, October 1997
Journal of Experimental Botany
Effects of drought on photosynthesis and on the thermotolerance of photosystem II in seedlings of cedar [Cedrus atlantica and C. libani) Daniel Epron1 Laboratoire de Sciences Vegetales, Institut des Sciences et des Techniques de I'Environnement, Universite de Franche-Comte, Pole Universitaire, BP 427, F-25211 Montbeliard cedex France Received 31 January 1997; Accepted 2 June 1997
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This wide geographic distribution probably contains a high genetic variability. The success of artificial regeneraThe rate of net CO2 assimilation (A) in cedar needles tions will depend on our capability to provide users with declined during drought progression, while the well-adapted genotypes showing a great potential for both quantum yield of electron transport measured under growth and stress resistance. This requires a better undermoderate light and high CO2 (AF/Fm) remained almost standing of the ecophysiological behaviour of cedars and constant. A slight but significant decrease in AF/Fm the elaboration of efficient, physiologically-based, screenwas observed in severely draughted plants. This suging tools. gested that the decline in A during drought progression The Mediterranean-type climate is characterized by was mostly due to stomatal closure, and that the rather long drought periods in summer during which the photosynthetic apparatus of cedar was only impaired temperature may reach potentially-damaging thresholds by severe water deficits. A Turkish provenance of C. for physiological processes. It has been demonstrated that libani displayed a significantly higher thermotolerance Cedrus species are rather tolerant to water deficit accordthan a French provenance of C. atlantica at whatever ing to Levitt's classification (Levitt, 1972; Ludlow, 1989), levels of drought. Nevertheless, there was a strong even if periods of severe drought may limit seedling interaction between drought and heat stress in both establishment and growth (Aussenac and Granier, 1978; cases. The temperature at which FJFm was lowered Aussenac and Finkelstein, 1983; Grieu et ai, 1988a; to 15% of its original value was increased by more Guehl et ai, 1991). The photosynthetic apparatus is than 3-4 °C in draughted plants. This drought-induced known to be very resistant to water deficits. However, shift in the PSII thermotolerance was still evident when most of studied species were herbaceous plants or broadheat treatments were done under high light rather than leaved trees (Kaiser, 1987; Comic and Briantais, 1991; in darkness. Epron and Dreyer, 1993). It remains unclear whether coniferous species have similarly high resistance to water Key words: Cedrus, chlorophyll fluorescence, drought, high deficits. In contrast, the photosynthetic apparatus, and temperature, photosynthesis. particularly photosystem II (PSII), is known to be rather sensitive to heat stress. Dissociation of the light-harvesting antennae from the PSII core and denaturation of the Introduction PSII reaction centre have been postulated in heatdamaged leaves (Berry and Bjorkman, 1980; Bilger et al., Cedars are increasingly used for afforestation of marginal 1987; Yordanov, 1992). It may be the consequence of an lands in some Mediterranean areas because of rural excessivefluidityof membrane lipids and/or of alterations changes. Cedars (Cedrus atlantica Manetti, C. libani [G Don] Loudon, C. brevifolia [Hook F] Henry, and C. of protein-lipid interactions in the thylakoid membrane. deodora Loudon) are present in most of the mountainous Various environmental factors may alter the thermostability of PSII in a given species. It has been shown that countries of the Mediterranean basin (Ducrey, 1994). Abstract
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Materials and methods Plant material
The cedar species studied were Cedrus atlantica (seed origin: Menerbes, Luberon, France) and Cedrus libani (seed origin: Arslankoy, Taurus, Turkey). Seedlings were grown from seeds in a forest nursery for one year in small containers (450cm 3 ). Before the beginning of the second growth season (in March), seedlings were planted out in 3 dm 3 pots filled with a 1 :1 (v/v) mixture of a commercial organic soil and sand, and raised outdoor for 6 months. The plants were watered with tap water two or three times a week depending on weather conditions. Two grams of a complete fertilizer specially designed for conifers (N,P,K; 5,4,8 by vol.; Profertyl, Herouville St Clair, France) were added twice during the raising period. In August, plants were transferred in a growth cabinet (28/20°C, 40/70% relative humidity, 180 ^mol m~ 2 s~' at the top of the plant for 12 h). Half of the plants were subjected to drought by withholding water supply for 3 weeks.
Gas exchange
The rate of net CO 2 assimilation (A) was recorded with an open gas exchange system (Li 6400, Li-Cor, USA) located in the growth cabinet. A small shoot was inserted in the ventilated chamber and CO 2 exchange was monitored until a stable value was recorded. Average air temperature, leaf-to-air vapour mole fraction difference, ambient CO 2 mole fraction, and photon flux density were, respectively. 28 °C. 8mmolmol~', 355 /itnol mol" 1 . and 150/j.molm" 2 s" 1 . At the end of the experiment, the needles of the shoot were harvested and ovendried for 48 h at 70 °C A was expressed on a per unit dry weight. Chlorophyll fluorescence
Chlorophyll a fluorescence was measured at room temperature with a modulated fluorometer (mini PAM, Walz, Germany) on 8-10 needles affixed to transparent tape and enclosed into an aluminium chamber. The chamber was flushed with water saturated-air containing 1% CO 2 The air stream entered the chamber at a flow rate of 2 dm 3 min" 1 . The chamber temperature was maintained at 30 °C. The Fo value was recorded after 12 h in darkness. Superimposing a saturating flash of white light (1 s; 4500 ixmo\ m" 2 s" 1 ) yielded the maximal fluorescence level ( F J . The FJFm ratio (FJFm=\ -FJFJ was used as an estimate of the maximal quantum yield of PSII photochemistry in dark-adapted needles (Butler, 1978; Bjorkman and Demmig. 1987). Then, the needles were illuminated by a red LED array (LH36U, Hansatech, England) at the photon flux density of 220 ^mol m~ 2 s" 1 . PFD was measured with a quantum sensor (LI-190B, Li-Cor, USA). A saturating flash of white light yielded the maximal fluorescence level (F'm) was imposed every 5 min until a constant AFjFm ratio, calculated as 1 —F/F'm, was reached. It took about 40 min. The AFjFm ratio was used as an estimate of the quantum yield of PSII electron transport in the light (Genty el ai, 1989). It has regularly been checked that AF/Fm values remained constant after having increased the CO 2 concentration from 1 to 2% to be sure that CO 2 did not limit the rate of PSII electron transport. Fv/Fm was also recorded 20 min after an exposure to a high light (1 lOOftmol m~ 2 s~' provided by a red LED array) in CO2-free air for 60 min. PSII thermotolerance
So/7 and plant water status
The pots were weighed regularly to calculate the fresh weight of the soil and then the relative water content of the soil
(RSWC): RS WC=(SFfV-SD W)I(SFW,-SD W) where SFW was the fresh weight of the soil. SDW the dry weight of the soil oven-dried for 72 h at 105°C and SFW, the initial weight of the soil. It is assumed that the soil was at its water holding capacity at the beginning of the experiment. Predawn leaf water potential of small shoots (
