BOSQUE 28(3): 220-225, 2007

ARTÍCULOS

Influence of radiation and drought on gas exchange of Austrocedrus chilensis seedlings

Influencia de la radiación y la sequía en el intercambio gaseoso de plantines de Austrocedrus chilensis

Javier Enrique Gyenge^a,b*, María Elena Fernández^a,b, Tomás Schlichter^a

*Corresponding Author: ^aINTA Estación Experimental Agropecuaria Bariloche, CC 277 (8400), San Carlos de Bariloche, Argentina, phone: 54-2944-422731, fax: 54-2944-424991, jgyenge@bariloche.inta.gov.ar

^bConsejo Nacional de Investigaciones Científicas y Técnicas, CONICET, Argentina.

SUMMARY

Austrocedrus chilensis is a conifer from Patagonia adapted to a wide moisture gradient. In xeric environments, its natural or artificial recruitment only occurs under nurse shrubs during normal rainfall years. In spite of its importance in ecological and economical terms, little information is available about the physiology of this species. Previous field measurements in experimental plantation revealed that seedlings without plant cover had lower survival, photo synthetic rates and predawn water potential than those planted under trees. So, the response of stomata to radiation and vapor pressure deficit, and the effect of drought stress on photosynthesis and transpiration of A. chilensis seedlings were studied under controlled conditions. Even when soil water was not limiting, all seedlings showed an early stomatal closure in response to moderate evaporative demand. Photo synthetic parameters and instantaneous water use efficiency suggested that microenvironments with intermediate shade are the best for A. chilensis C0₂ uptake. We suggest that the early stomatal closure could result in damage in leaves or the base of the stem by oveheating, explaining low survival when grown without shade. As a whole, our results may contribute to understanding the need for nurse plants in the establishment of the species.

Key words: photosynthesis, stomatal conductance, transpiration, instantaneous water use efficiency.

RESUMEN

Austrocedrus chilensis es una conifera endémica de la Patagonia adaptada a un amplio gradiente de condiciones de humedad. El reclutamiento en lugares xéricos sólo se produce bajo arbustos nodrizas en años de precipitación regular. A pesar de su importancia ecológica y económica, existe escasa información sobre la fisiología de esta especie. Plantaciones experimentales con esta especie revelaron que las plantas sin cobertura vegetal mostraron menores supervivencias, tasas fotosintéticas y potencial agua que los plantines bajo cobertura. En este trabajo se midió bajo condiciones controladas la respuesta estomática a la radiación y al déficit de presión de vapor, y el efecto de la sequía sobre la fotosíntesis y transpiración de plantines de A. chilensis. Los plantines cerraron sus estomas en respuesta a una demanda evaporativa moderada aun cuando el agua en el suelo no fue limitante. Los parámetros foto sintéticos demostraron que los ambientes semisombreados son los óptimos para plantines de A. chilensis. Se sugiere que el cierre estomático temprano podría implicar daños en las hojas o en la base del tallo por sobrecalentamiento. En conjunto, los datos contribuirían al entendimiento de la necesidad de una planta nodriza en el establecimiento de esta especie.

Palabras clave: fotosíntesis, conductancia estomática, transpiración, eficiencia instantánea en el uso del agua.

INTRODUCTION

Austrocedrus chilensis (D. Don) Pie. Ser. et Bizzarri is a native conifer from Northern Patagonia, Argentina (38° to 42° S). Considered one of the most drought tolerant tree species of the region, it ranges along a wide geographic area, from sites with 300 mm of mean annual precipitation to sites exceeding 2,500 mm (Dezzotti and Sancholuz 1991). Northern Patagonia has a typical Mediterranean climate, with fall-winter precipitation and summer droughts. Freezing temperatures limit growth in early spring and low soil moisture and high vapor pressure deficit (VPD) limit growth in late summer (Dezzotti and Sancholuz 1991). Generally this type of climate imposes hydrologic and temperature limitations to ecosystem gas exchange. Several studies have pointed out that at xeric sites (< 1,000 mm yr¹ of precipitation), natural regeneration of A. chilensis is highly restricted, both temporally and spatially. Mostly, seedlings of A. chilensis are restricted to the presence of shrubs, under which soil water in microsites is higher than in the open (Kitzberger 1995). However, it seems that excess radiation was a more limiting factor than soil water content for seedling survival of this species (Kitzberger et al. 2000, Letourneau et al. 2004). Precipitation and average temperature during summer explain recruitment patterns of A. chilensis (Gobbi and Schlichter 1998, Kitzberger et al. 2000). Soil temperature, at 1 and 5 cm of soil depth, were 100 and 70% higher in sunny places than under shrubs (Letournaeu et al. 2004). Those authors found an increase in the facilitative strength of shrubs when the atmospheric demand was more severe. Seedlings showed larger shoot-root ratio (S/R) and leaf area- sapwood area ratio (L/SA) in shaded than in un-shaded places (Letournaeu et al. 2004). These changes in allometric parameters increase the capacity to drive water to leaves, changing specific leaf area conductivity (Maherali etal. 1997). However, another important limiting factor in this pathway is the stomatal conductance (gs) which could be important in terms of survival. Kolb and Robberecht (1996) found a correlation between pine seedlings survival and gs. They suggested that a high transpiration indirectly cooled seedlings stems by means of increased rate of water flow through the stem xylem. Therefore seedlings in which transpiration was restricted by stomatal closure (because of low soil water availability and/or high VPD) would have a limited capacity for stem cooling and so would be more susceptible to heat stress. In addition, high radiation and temperature could damage photosynthetic apparatus when stomata are closed (Epron 1997). Gyenge et al. (2005) found that seedlings of A. chilensis had high total plant resistance, with more than 70% of their water hydraulic resistance placed at leaf level. These could imply a low capacity to transport water from the soil to the leaf. Also, seedlings of Quercus spp. subjected to soil and stem base heat showed rapid declines in rates of transpiration, photosyntesis and survival; demonstrating that heating caused shoot death even without exposition of the upper stem or foliage to heat (Huddle and Pallardy 1996). Based on the hydraulic resistance of seedlings, we hipothesized that A. chilensis close their stomata in response to VPD even when soil water content is relatively high. This might be the reason of the decrease in seedling survival in sunny places due to overheating or photo-destruction.

There exists considerable ecological information about A. chilensis (Dezzotti and Sancholuz 1991, Gobbi and Schlichter 1998, Kitzberger et al. 2000). However, little is known about physiological characteristics of this species that could help to understand its ecological patterns, particularly in relation to effects of soil water shortage and climatic conditions on leaf gas exchange. The present study is part of a larger study dealing with the feasibility of using A. chilensis in commercial forest plantations. A previous study established an experimental plantation of A. chilensis under shelter provided by a young (15 years old) ponderosa pine (Pinus ponderosa L.) plantation and in an open grassland area (Schlichter 2004). In the later situation, survival of seedlings was less than 40%, while survival of seedlings growing under pines was over 90%. In addition, field-measured photosynthesis and predawn water potential were lower in seedlings growing in the open compared to seedlings growing under pines (Schlichter 2004). Following those results, the objective of this study was to characterize, under more controlled conditions, the effect of drought and climatic conditions (mainly VPD and radiation level) on leaf gas exchange of A. chilensis seedlings. In particular, we studied (a) the relationship between stomatal conductance (gs) and climatic conditions (radiation and VPD) as well as with plant water status; (b) the limitation in carbon acquisition imposed by stomata; (c) the instantaneous water use efficiency and (d) the response of net photosynthesis to radiation level. All these variables were measured in seedlings grown under controlled environmental conditions in a glasshouse.

METHODS

Three-years old seedlings of A. chilensis were grown in the tree nursery of the Forest Genetic Group of EEA Bariloche INTA (National Institute of Agricultural Technology, Argentina) with the same procedure as the one used for seedlings planted in the field (Schlichter 2004). Seeds were collected from trees (cones of 399 trees from 16 populations, Pastorino 2000) growing around Bariloche city. One year old seedlings were grown in 0.7 1 (6 x 25 cm) pots in a soil similar to that of the site where seeds were collected (sandy to loam-sandy texture). Neither fertilization nor artificial light was supplied. At the beginning of the experiment, seedlings were in average 3.3 mm in diameter at the root collar and less than 15 cm in height. Plants used in both experiments were randomly selected from more than 1,000 seedlings of the tree nursery.

Experiment 1: response of stomata to climatic conditions. Air temperature, air relative humidity, photosynthetic photon flux density (PPFD, μmol m^-2 s^-1), stomatal conductance (gs, mol m^-2 s^-1), transpiration (E, mol m^-2 s^-1) and net photosynthesis (A, μmol m^-2 s^-1) of well watered and water stressed seedlings were simultaneously measured. Measurements were carried out for three days during the growing season, with a LICOR 6200 (Li-Cor, Lincoln, Nebraska, USA). The experimental treatments consisted of different levels of water stress. Forty seedlings were randomly distributed in the mentioned two treatments: well watered (20 seedlings) and water-stressed plants (20 seedlings). In the well watered treatment, seedlings were irrigated to field capacity every two days. Seedlings in the drought-stressed treatment were watered to field capacity only one time at the beginning of the experiment. In each day of measurement, drought-stressed seedlings were randomly selected from the initial pool of seedlings. At each sampling point, measurements were taken every one or two hours starting in pre-dawn and finishing in the evening. Different plants were used each day. Only 12 plants could be measured during each day of sampling to have minimum variation in climatic conditions. Eight plants were measured during the first day of measurement (Feb 29) (well watered, original physiological state of seedlings); 12 plants were measured in March 2 (six well watered and six water stressed plants); and 10 plants in March 13 (six well watered plants and four water stressed plants). Due to the small size of plants and their limited number of branches, water status of all plants measured with the LICOR 6200 equipment was monitored with a pressure chamber (PMS Instrument, Corvallis, Oregon, USA) in pre-dawn (ψpd) the day after the measurements. To assess the area and the specific leaf area (SLA, m² kg-¹), branches sampled with the LICOR 6200 were cut after measurements, measured using a digital scanner, oven dried (70°C) and weighed.

Photosynthesis at PPFD > 500 μmol m^-2 s¹ (at light saturation) and leaf transpiration of well watered and water stressed plants were related with gs measured on March 2 and 13. Instantaneous water use efficiency was calculated as the ratio between carbon gain in photosynthesis and water loss in transpiration (A/E). Vapor pressure deficit (VPD, kPa) was assessed from temperature and relative humidity of the air assuming that leaves had the same temperature as the air (Ewers and Oren 2000). The adjusted models that described the relationships between variables in each day were compared using F tests with an a = 0.05 (Neter and Wasserman 1974).

Experiment 2: relationship between net photosynthesis and PPFD. In order to determine photosynthetic performance of A. chilensis seedlings under non limiting water availability, curves relating net photosynthesis and PPFD of eight well watered plants were drawn. Instantaneous radiation levels were obtained with shade cloth of different densities, from 2,000 to 0 μmol m~² s^-1 similarly to what was done by Fernández et al. (2006). Carbon dioxide exchange, PPFD, gs, relative humidity and temperature, were measured with a LICOR 6200 (Li-Cor, Lincoln, Nebraska, USA). After that, sampled branches were cut, oven-dried and weighed.

The model fitted to the data was a rectangular hyperbola commonly used to describe the response of photosynthesis to radiation (Landsberg 1977):

where A= C0₂ assimilation (net photosynthetic rate); A_max= maximum assimilation rate; QY= apparent quantum yield (slope of the first part of the curve); CP= light compensation point, and PPFD= photosynthetic photon flux density (independent variable).

RESULTS

Response of stomata to climatic conditions. Stomatal conductance increased linearly with PPFD only at values of PPFD < 50 μmol m^-2 s^-1 (figure 1), after which it remained nearly constant.

Maximum stomatal conductance (at saturation PPFD) of all plants was exponentially related with ψ_pd (figure 2). This pattern was observed during the day in which VPD was always lower than 2 kPa. Stomata reduced their conductance abruptly when ψ_pd was lower than -0.5 MPa. After that value, the rate of gs reduction was lower and nearly constant, reaching a minimum gs of about 0.04 mol m~² s^-1. A similar pattern was found in A/E in relation to ψ_pd (data not shown), indicating a great decrease in A/E when ψ_pd was lower than -1 MPa.

Daily patterns of gs versus VPD measured in well watered and water stressed plants during three days (February 29, and March 2 and 13) are shown in figure 3. To avoid the influence of light level on gs, only gs values when PPFD> 500 μmol m^-2 s^-1 were plotted. No differences in radiation were measured among the three days, reaching maximum values of 1650 μmol m^-2 s^-1. Morning values of gs measured in well watered plants (maximum values) were around 0.11 mol m^-2 s^-1. Stomatal conductance was relatively stable during February 29 and March 2 (figure 3) in correspondence to nearly constant values of VPD, which never exceeded 2.5 kPa. During the third day (March 13, figure 3), VPD reached higher values with a maximum of 3.5 kPa at midday, while gs decreased in relation to the increment of VPD. The lower values of gs were about 0.05 mol m^-2 s^-1 in the afternoon. On the other hand, gs of water stressed plants responded to VPD with a pattern similar to that for well watered plants, but the magnitude of the response depended on ψpd. Severely stressed plants (ψpd < -2.0 MPa) had their stomata almost closed during the whole day, and for this reason did not respond to VPD.

The reduction in gs due to water stress implied a decrease in carbon exchange. A linear relationship was observed between photosynthesis at PPFD > 500 μmol m^-2 s^-1 and gs in well watered and water stressed plants (figure 4). There was a positive net C0₂ assimilation even when stomata were almost closed (approximately 0.03 mol m^-2 s^-1). Photosynthesis and transpiration had different values in days with different VPD (P<0.001, test F, figure 4). For these analyses, we chose values obtained from well watered plants during March-2 (low VPD) and March-13 (high VPD). Generally, at similar values of gs, photosynthesis was lower and transpiration higher in days with high VPD. In agreement with these observations, A/E was higher in the day with lower VPD (figures 4). However, at very low gs (i.e. when the plant is subjected to intensive drought conditions, 0.03 mol m^-2 s^-1), A/E is independent from evaporative demand.

Relationship between net photosynthesis and PPFD. Maximum assimilation values were 5.07+0.43 (+SE) μ.mol C0₂ m^-2 s-¹ and CP was 10.46+15.19 μmol photon m^-2 s^-1. Quantum yield was 0.023+0.007 mol C mol photon¹. Specific leaf area was 9.6+0.3 m² kg-¹ (n=20).

DISCUSSION

All plants closed their stomata when VPD increased above 2.5 kPa, even under non limiting soil water availability. This threshold VPD value is not a high value for late spring or summer in Patagonia¹. In addition, stomata sharply closed when *P_pd decreased from -0.3 to -0.8 MPa, i.e. under relatively high pre-dawn water potential values. Schlichter (2004) reported that seedlings of A. chilensis growing in the open grassland had *P_pd lower than -1 MPa 30 days earlier than seedlings growing under pines reached these values. In addition, plants in the open had minimum W_pd values of-3.5 MPa, while seedlings growing under pine shelter only reached minimum values of -2.0 MPa. The relationship between gs and pre-dawn water potential found in this study was similar to that described for Pinus taeda L., a subtropical species, which grows in humid environments and closes its stomata when the ψsoil is near -0.6 MPa (Wakamiya-Noborio et al. 1999). However, the low but positive rates of photosynthesis at low stomatal conductances are similar to data reported for several Cedrus and Quercus species (Epron 1997, Fotelli et al. 2000), two Mediterranean species able to survive during periodic droughts. Hydraulic resistance measured in whole plants and their compounds showed that the main resistance to water flow in A. chilensis is located at the leaf level (Gyenge et al. 2005). Based on these results, it appears that the strategy of A. chilensis seedlings is to sustain favorable water conditions during a drought, by maintaining relatively high water potential by means of an early stomata closure (desiccation avoidance response). This same strategy was found in Q. ilex L., which closes its stomata early during the season compared to other Quercus species that overcome drought by means of desiccation tolerance (Fotelli et al. 2000). However, in spite that the effect of temperature on photosynthesis was not measured in our study, several published studies point out that this conservative strategy may be detrimental when transpiration has to act as a heat transfer mechanism (for more information about this mechanism, see Kolb and Robberecht 1996). Other authors have also shown the negative effects of high temperatures applied on the base of the stem or the leaves on plant transpiration and photosynthesis (Huddle and Pallardy 1996, Hamerlynck et al. 2000). This could be taken as one of the causes for the high mortality of A. chilensis seedlings under full light conditions even when soil water content was high or similar to that present under shade conditions (Kitzberger et al. 2000, Letournaeu et al. 2004).

Compared with other species, A. chilensis shares some physiological characteristics with shade tolerant species. Stomata of A chilensis were fully open at values of PPFD around 50 μmol m^-2 s^-1, whereas in "heliophilous" species, maximum stomata aperture is reached at values of PPFD of 280-400 μmol m^-2 s¹ (Leverenz 1995, Le Roux et al. 1999). In addition, light compensation point was comparatively low (10 compared to 16, 23 and 19 μmol m^-2 s^-1 for P. ponderosa, Pseudotsuga menziesii (Mirb.) Franco, and Tsuga heterophylla (Raf.) Sarg., respectively; Bond et al. 1999). Also, Amax was comparable to that of T. heterophylla, a shade tolerant species (Bond et al. 1999). However, low QY values of A. chilensis (0.023 mol C mol photon¹ against 0.052, 0.046 and 0.039 mol C mol photon^-1 for P. ponderosa, P. menziesii and T. heterophylla, respectively; Bond et al. 1999) may be detrimental in shaded microenvironments. Based on those results and A/E measurements, microenvironments with intermediate shade may be the best for A. chilensis seedling growth.

In summary, the results may indicate that drought avoidance behavior through early stomatal closure could be the main mechanism used by this species to survive under soil water deficits coupled with high evaporative demand. However, studies in other species with the same strategy indicate that the risk of photoinhibition or cell damage due to over-heating is increased under this situation. For this reason, shelter imposed by shrubs (natural conditions) or other trees (artificial plantations) may be a possible way to avoid these processes derived from excessive radiation. Further research is needed in order to evaluate this hypothesis. In addition, photosynthetic performance was comparable to that of shade tolerant species regarding some parameters (light compensation point, stomatal conductance at low PPFD levels). However, at the same time, quantum yield indicated a low C fixation capacity at low light levels. As a whole, the results further our understanding on the need of nurse plants for successful recruitment of A. chilensis, and at the same time, they help explaining its very low growth rates under highly shaded conditions (Kitzberger et al. 2000).

ACKNOWLEDGMENTS

This research was partially funded by INTA (National Institute for Agricultural Technology) and SAGPyA through the project PIA 16/98. ME Fernández was supported by a fellowship for graduate students of CONICET. We also thank the support of the Genetics Group of INTA EEA Bariloche, which provided the plants for the experiments. We gratefully acknowledge Guillermina Dalla Salda for her valuable comments on this manuscript.

NOTES

1 Bustos C. INTA EEA Bariloche, unpublished data.

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Recibido: 01.02.07, Aceptado: 23.08.07