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Photosynthesis: Research for Food, Fuel and Future—15th International Conference on Photosynthesis
Effect of Light on the Photosynthetic Activity during Desiccation of the Resurrection Plant Haberlea Rhodopensis Katya Georgieva*, Snejana Doncheva, Gergana Mihailova, Snejana Petkova Institute of Plant Physiology and Genetics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 21, 1113 Sofia, Bulgaria. * Corresponding author. Tel. No. +35929792611; Fax No. +35928739952; E-mail:
[email protected].
Abstract: The effect of light during desiccation of the resurrection plant Haberlea rhodopensis on the photosynthetic activity and some morphological parameters was evaluated using plants growing at low or high irradiance in natural habitat. Chlorophyll content was not only lower in sun plants compared to shade plants, but it declined to a higher extent when desiccation was carried out at high light irradiance. Regardless of lower chlorophyll content in sun plants their photosynthetic activity (PN) was about 30% higher compared to shade plants. However, during dehydration PN declined more rapidly in sun plants. The mean leaf thickness of fully hydrated leaves from sun plants was larger when compared with shade plants, which was due to higher thickness of the mesophyll. Following rehydration plants rapidly recovered and PN was higher by about 70% in sun than in shade plants. The results showed that the sun-exposed Haberlea plants exhibited good adaptation to desiccation under high irradiance. Keywords: Desiccation; Leaf thickness; Photosynthesis; Resurrection plant
Introduction Haberlea rhodopensis belongs to a small group of angiosperms, referred to as “resurrection plants” because they are capable of tolerating extremes of desiccation. It prefers shaded, northern, chiefly limestone slopes but can be found also on the sunexposed rocks. Our previous investigations have shown that detached Haberlea leaves as well as whole plants were able to survive desiccation in the dark or at low irradiance (about 30 μmol m–2 s–1) to water content below 10% with photosynthetic activity fully recovered after rehydration (Georgieva et al., 2005, 2007). However, it was found that these plants were very sensitive to photoinhibition (Georgieva and Maslenkova, 2006). Desiccation of shade-adapted plants at irradiance of 350 μmol m–2 s–1 PPFD induced irreversible changes in the photosynthetic apparatus, and leaves (except the youngest ones) did not recover after rehydration (Georgieva et al., 2008). The aim of the present study was to evaluate the effect of light during desiccation of Haberlea using plants growing
at low or high irradiance in natural habitat. Changes in the photosynthetic activity and some morphological parameters were studied at different degrees of desiccation as well as after rehydration of plants.
Materials and Methods Well-hydrated and naturally dried Haberlea rhodopensis plants growing on rocks below trees in deep shade (Bachkovo region) or sun exposed but briefly shaded by neighboring trees (Sitovo region) were studied. Adult rosettes of similar size and appearance were selected for the experiments. All measurements were conducted on fully expanded mature leaves from control (90% RWC), moderately (50% RWC), severely dehydrated plants (25% RWC) and dried leaves (8% RWC) as well as after 5 days of rehydration of dry plants. The RWC was determined gravimetrically by weighing Haberlea leaves before and after oven drying at 80 C to a constant mass and expressed as the percentage of water content in
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dehydrated tissue compared to water-saturated tissues, using the equation:
Chl (a+b) (mg g DW)
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Results and Discussion Chlorophyll content slightly decreased upon desiccation of shade Haberlea plants and it was 20% lower in dried leaves (Fig. 1A). Chl content was not only lower in sun plants compared to shade plants, but it declined to a higher extent when desiccation was carried out at high light irradiance. Following rehydration Chl content recovered reaching the control values. The content of carotenoids decreased during dehydration of both shade and sun-adapted plants but recovered after rehydration (Fig. 1B). Regardless of lower Chl content (13%, p < 0.05) in sunplants their photosynthetic activity (PN) was about 30% higher compared to shade plants (Fig. 2A). Similarly to the net photosynthetic rate, stomatal conductance (gs) was also higher in well-watered sunny leaves (Fig. 2B).
shade sun
A
5 4 3 2 1 0
Car (x+c) (mg g DW)
B
-1
1.5 1.0 0.5 0.0
90
50
8
R
RWC (%)
Fig. 1 Changes in chlorophyll (A) and carotenoid content (B) during dehydration and after 5 days of rehydration (R) of shade and sun Haberlea rhodopensis.
shade sun
A PN (mol m-2 s-1)
The net photosynthetic rate (PN) was measured at 500 µmol m–2 s–1 PPFD using a portable photosynthesis system LCpro+ (ADC BioScientific Ltd., Hertfordshire, UK). CO2 assimilation (µmol CO2 m–2 s–1) and stomatal conductance (mmol m–2 s–1 PPFD) were calculated according to von Caemmerer and Farquhar (1981). Chlorophyll (Chl) a, chlorophyll b and total carotenoids were extracted from leaf disks with 80% acetone. The pigment content was determined spectrophotometrically according to Lichtenthaler (1987) and the data were calculated on a dry weight basis (80 C for 48 h). Control and water stress treatments were statistically compared. Comparison of means from six measurements from different plants was done by the Student’s t – test. For ligh microscopy samples of the middle portion of fully hydrated and dehydrated leaves were fixed, dehydrated and embedded in Durcupan (Doncheva et al., 2009). Semithin sections (1 to 2 m) were stained with fuchsine and methylene blue and examined under a light microscope Nikon Eclipse 50 (Tokyo, Japan) eqquiped with a video camera. Thickness of the leaf and mesophyll were measured in the 5 representative semithin cross sections. Thickness data were done using the ImageJ software (http://rsb.info.nih.gov/ij/)
8 6 4 2 0 B
gs (mmol m-2 s-1)
RWC (%) = (mfresh–mdry) × 100/(msaturated–mdry)
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0.20 0.15 0.10 0.05 0.00
90
50
25
R
RWC(%)
Fig. 2 Changes in CO2 assimilation (PN) and stomatal conductance (gs) during dehydration and after 5 days of rehydration (R) of shade and sun Haberlea rhodopensis plants. Mean of six measurements from different plants with standard error.
It has been shown that under high light conditions there are increases in the amounts of photosystems, electron transport, ATP synthase complexes, and enzymes of the Calvin–Benson cycle (Walters, 2005). Conversely, under low light there is an increase in the relative amounts of light-harvesting complexes and in the stacking of thylakoid membranes to form grana. It
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Leaf thickness (m)
is believed that these changes are of adaptive significance: an increase in photosynthetic capacity reduces susceptibility to photodamage, while changes in photosystem stoichiometry serve to optimize light utilization. As a result of dehydration PN declined more rapidly in sun plants and the values obtained were close to those in shade plants at 50% RWC (Fig. 2A). Moreover, when RWC dropped to 25%, CO2 assimilation in sun plants was inhibited to a higher extent that in shade plants. The results showed that stomatal conductance measured in moderately desiccated plants (50% RWC) was reduced by 20% and 40% in shade and sun plants, respectively. On the other hand, gs was strongly inhibited in severely desiccated leaves (25% RWC) especially in sun plants. Stomatal control determines the rate of CO2 assimilation and slows and minimizes the development of stress to the system over a range of RWC before metabolism is disrupted (Lawlor and Cornic, 2002). It has been shown that droughtinduced decrease of leaf net CO2 uptake observed in Ramonda mykoni (Schwab et al., 1989) and Haberlea rhodopensis (Peeva and Cornic, 2008) when RWC decline to 40% was strictly diffusion limited both by stomatal closure and a decrease in mesophyll conductance. The diffusion pathway through intercellular airspaces can be influenced by leaf thickness, cell shape, and packing relative to the position of stomata (Evans et al., 2009). The mean leaf thickness of fully hydrated leaves from sun plants was larger when compared with shade plants (Fig. 3A), which was due to higher thickness of the mesophyll layer (Fig. 3B). The higher photosynthetic activity of well-watered sun plants could be due to the higher thickness of the mesophyll layer. C3 leaves should have sufficient mesophyll surfaces occupied by chloroplasts to secure the area for CO2 dissolution and transport because the affinity of Rubisco for CO2 is low. To increase the mesophyll surface area, the leaf can either be thicker or have smaller cells (Terashima et al., 2006). Both leaf and mesophyll thickness decreased during desiccation (Fig. 3). Following rehydration plants rapidly recovered and PN was higher by about 70% in sun than in shade plants (Fig. 2A). The stomatal conductance also showed better recovery after rehydration of sun plants. The results showed that the sun-exposed Haberlea plants exhibited good adaptation to desiccation under high irradiance.
A
shade sun
500 400 300 200 100 0
Mesophyll thickness (m)
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600
B
500 400 300 200 100 0
90
50
8
R
RWC(%) Fig. 3 Changes in leaf thickness (A) and mesophyll thickness (B) during dehydration as well as after 5 days of rehydration (R) of shade and sun Haberlea rhodopensis plants.
Acknowledgement This work is supported by National Science Fund of Bulgaria under research project DO02-208/2008.
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the Changes in Photosynthetic Activity of the Homoiochlorophyllous Desiccation-Tolerant Haberlea Rhodopensis and Desiccation-Sensitive Spinach Leaves during Desiccation and Rehydration. Photosynth. Res. 85: 191-203 Georgieva K, Szigeti Z, Sarvari E, Gaspar L, Maslenkova L, Peva V, Peli E, Tuba Z (2007) Photosynthetic Activity of Homoiochlorophyllous Desiccation Tolerant Plant Haberlea Rhodopensis during Desiccation and Rehydration. Planta 225: 955-996 Lawlor DW, Cornic G (2002) Photosynthetic Carbon Assimilation and Associated Metabolism in Relation to Water Deficits in Higher Plants. Plant Cell Environ. 25: 275-294 Lichtenthaler KH (1987) Chlorophylls and Carotenoids: Pigments of Photosynthetic Biomembranes. Methods Enzymol. 148: 350-382 Peeva V, Cornic G (2009) Leaf Photosynthesis of
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Haberlea Rhodopensis before and during Drought. Environ. Exp. Bot. 65: 310-318 Schwab KB, Schreiber U, Heber U (1989) Response of Photosynthesis and Respiration of Resurrection Plants to Desiccation and Rehydration. Planta 177: 217-227 Terashima I, Hanba YT, Tazoe Y, Vyas P, Yano S (2006) Irradiance and Phenotype: Comparative Eco-Development of Sun and Shade Leaves in Relation to Photosynthetic CO2 Diffusion. J. Exp. Bot. 57: 343-354 von Caemmerer S, Farquhar GD (1981) Some Relationships between the Biochemistry of Photosynthesis and the Gas Exchange of Leaves. Planta 153: 376-387 Walters RG (2005) Towards an Understanding of Photosynthetic Acclimation. J. Exp. Bot. 56: 435447
Tingyun Kuang Congming Lu Lixin Zhang
Photosynthesis Research for Food, Fuel and Future 15th International Conference on Photosynthesis
With 508 figures
Editors: Prof. Tingyun Kuang key Laboratory of Photobiology Institute of Botany Chinese Academy of Sciences Beijing, China Email:
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Prof. Congming Lu Head, Photosynthesis Research Center Institute of Botany Chinese Academy of Sciences Beijing, China Email:
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Prof. Lixin Zhang key Laboratory of Photobiology Institute of Botany Chinese Academy of Sciences Beijing, China Email:
[email protected]
ISBN 978-7-308-09694-2 Zhejiang University Press, Hangzhou ISBN 978-3-642-32033-0 Springer Heidelberg New York Dordrecht London
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