129 Annales Universitatis Paedagogicae Cracoviensis Studia Naturae, 5: 129–141, 2020, ISSN 2543-8832 DOI: 10.24917/25438832.5.9 Małgorzata Romańska Institute of Biology, Pedagogical University of Krakow, Podchorążych 2 St., 30-084 Kraków, Poland; *malgorzata_romanska@onet.pl Impact of water stress on physiological processes of moss Polytrichum piliferum Hedw. Introduction Plant responses to water de�cits depend not only on their age and development phase but also on the extent and rate of water stress. A  slow pace of water loss allows air conditioning and reduces damage caused by a water de�cit, while fast-pace water loss can block this process. �ough, a similar water de�cit can cause di�ering responses in sensitive plants compared to plants resistant to water de�ciency (Bray, 1997). In addition to water, oxygen availability is also an important element in plant bi- ochemical processes. Improper oxygen conditions of the soil adversely a�ect plants, causing root hypoxia and, consequently, inhibition of respiration. In hypoxic roots, growth is �rst restricted, followed by reduced water permeability and nutrient uptake. Maintenance of hypoxia and drought leads to irreversible physiological processes in both seed and spore plants, e.g. mosses (Kozlowski, 1984; Kozlowski, Pallardy, 1997; Rzepka et al., 2003; Rzepka, 2008). Mosses (Bryophyta) are a group of plants present in various relatively humid hab- itats around the world (Proctor, 2000, 2001). In general, their life requirements are small, which is why they o�en resettle in di�cult conditions, as pioneer plants. In some plant communities they cover huge areas, constituting the dominant �oristic group. �ey are commonly found on meadows, peat bogs, in brush, set-aside, rocks, stones, trees, in ditches, �res, debris and on walls. Mosses are characterised by high resistance to variability of habitat edaphic factors, temperature and humidity (Karcz- marz, 2000; Fojcik, 2011; Możdżeń, Skoczowski, 2016; Kula et al., 2018; Sołtys-Lelek et al., 2018; Możdżeń, 2019). �ey collect water thanks to the imbibition forces located in the plasma of cells, membranes and mucous substances that are secreted on the surface of the stalks or gametophyte leaves. �e role of rhizoids is limited to attaching the plant to the ground and is not important during intake of water. �erefore, for M ał go rz at a R om ań sk a 130 many species of mosses, rainwater and dew are a  direct source of moisture. Water conduction in mosses occurs due to the presence of layers of highly hygroscopic mu- cilaginous substances that draw in water and distribute over the entire surface of the turf. Water migration also takes place over the surface of leafy stalks, due to capillary forces present in the spaces between the leaves, or through primitive conductive tissue developed in the middle of the stem (Pressel et al., 2006). Bristly haircap moss Polytrichum piliferum Hedw. from the Polytrichaceae Schwägr. family belongs to cosmopolitan spore plants but is more commonly found in the North- ern Hemisphere. In Poland, it occurs both in the mountains and in the lowlands. It grows in sunny, sandy, dry and acidic places and creates brown-green, loose turf (Sza- fran, 1948). In its life cycle, like other mosses, the dominant generation is haploid game- tophyte (Fig. 1A–B). �e sporophyte has the form of a simple telome ending with a spo- rangial (Fig. 1C–D). �e underground part of the gametophores are made of rhizoids up to 5 cm long, and the above-ground part of the stalk has leaves that reach 4.5 to 6 mm in length and 0.5 mm in width. �ey are lanceolate, whole leaf edges, extended at the bot- tom in a single-layer, non-green sheath. �is moss has a double-layered leaf blade with a single-layer edge; a rib is present in the middle of the leaf (Fig. 1B). �e ventral side of the leaf has lamellas, whose apical cells are bottle-shaped and papillary (Wójciak, 2003). Fig. 1. Polytrichum piliferum Hedw.: A – gametophores, B – fragment of gametophyte leaf with rib, C – sporophytes, D – fragment of capsule with peristome (Source: https://atlas.roslin.pl/plant/9498) 131 Im pact of w ater stress on physiological processes of m oss Polytrichum piliferum H edw . Mosses are convenient organisms for studying the reaction of plants to water stress because they are relatively primitive for terrestrial plants. �ey do not have an epi- dermis, which is why they are more sensitive to moisture changes than most other plants (Harmens et al., 2011; Schröder et al., 2014). �erefore, they are o�en a research object for exploring plant hydration. �e aim of the study was to determine the e�ect of water stress on the activity of photosynthesis (i) and dark respiration (ii) in Bristly haircap moss P. piliferum. Material and methods Plant material �e turf of Polytrichum piliferum mosses collected in situ in spring 2010, from the natural forest habitat surrounding Rybna (50°03ʹ04ʺN 19°38ʹ50ʺE – Southern Poland) site was used for the research. Conditions Plant material was acclimated for 2 weeks in a  growth chamber. Conditions for mosses during the acclimatisation were a 12 h/12 h photoperiod, density of quan- tum stream in the PAR range of 70 μmol × m–2 × s–1 – with the �uorescent light source Fluora-Osram (Poland), a temperature of 15°C (±2°C) and a relative humid- ity that oscillated around 70–100%. Mosses were regularly watered with distilled water. A�er the acclimatisation period, similar morphological P. piliferum gameto- phores ± 1.5 cm long were selected for the study. Plants were rinsed with distilled water and dried with �lter paper. �en they were placed into holes of plexiglas plates in glass vessels with a volume of 1 dm3 with 10 ml of distilled water and stored in a growth chamber. Gas exchange An infrared gas analyser ADC–225 MK3 (UK) operating in a closed system was used to measure photosynthesis and respiration of gametophores. �e entire system con- sisted of an assimilation chamber with an air humidifying system and a water jacket, and its volume was 0.664 dm3. Measurements were carried out in air with an oxygen content of 21%. �e temperature inside the assimilation chamber was 25°C through- out the measurements. During photosynthesis measurements, the intensity of light reaching the gametophores was 100 μmol × m–2 × s–1. �e concentration of carbon dioxide was 300–400 μmol CO2 in 1 litre of air in a  closed system, and the relative humidity of the air was approximately 75%, which allowed air to pass through the scrubber with distilled water. M ał go rz at a R om ań sk a 132 Experimental groups �e control group for photosynthesis and dark respiration were P. piliferum game- tophores arranged on Plexiglass plates, immediately a�er acclimatisation. A�er the fresh mass determination and gas exchange measurements, the mosses were dried for 1 hour at room temperature, and then their mass, photosynthesis and dark respiration were measured. A�er this treatment, the gametophores were sprayed with distilled water and allowed to rehydrate in air for 15 h. �en their mass was determined, photo- synthesis and dark respiration were measured. A�er measurement, the gametophores were placed in a dryer (Wamed SUP-100, Poland) until completely dry to determine their dry mass (DM). �e second experiment consisted of periodic �ooding of gametophores with distilled water for 1 h, a�er which photosynthesis and respiration parameters were determined, and then the plant material was transferred to the atmosphere for 15 h. A�er this time, the gas exchange parameters were determined. A�er completing these measurements, the plant material was placed in the dryer to completely deter- mine the DM. Statistical analysis Statistical analysis was performed using one-way analysis of variance, ANOVA/ MANOVA, tests. �e signi�cance of di�erence between means (± SE), n = 5, was determined with Tukey test at p = 0.05. �e data was analysed with STATISTICA Data Analysis So�ware System (StatSo�, Inc., 2018, Version 13.1, www.statso�.com). Results Drying and rehydration In the control group, photosynthesis intensity for Polytrichym piliferum gametophores was 61.3 μmol CO2 gDM –1 × h–1. Drying the plant material for one hour reduced the intensity of photosynthesis to 0 μmol CO2 gDM –1 × h–1. Rehydrating the same material for 15 hours resulted in the restoration of photosynthetic activity to 32 μmol gDM –1 × h–1, which constituted 51% of the initial value (Fig. 2A; Tab. 1). �e dark respiration rate for the control plants was 35.5 μmol CO2 gDM –1 × h–1. A�er drying the mosses for 1 hour, respiration decreased to 34.0 μmol CO2 gDM –1 × h–1, com- pared to the control. A�er subjecting the same gametophores to 15 h of rehydration, the intensity of dark respiration increased signi�cantly to 45.6 μmol CO2 gDM –1 × h–1 and was 28.5% higher than the control value (Fig. 2A; Tab. 1). 133 Flooding and reoxygenation Before �ooding P. piliferum gametophores, photosynthesis intensity was 78 μmol CO2 gDM –1 × h–1 (control). Hourly (1 h) �ooding of plant material reduced photosynthesis to 61.8 μmol CO2 gDM –1 × h–1, i.e. 21% relative to the control. In addition, subjecting the plant material to 15 hours of reoxygenation in the air resulted in an increase in photosynthesis intensity to 66.3 μmol CO2 gDM –1 × h–1, i.e. its intensity was 15% lower than the initial value (Fig. 2A; Tab. 1). �e intensity of dark respiration in the control sample was 41 μmol CO2 gDM –1 × h–1. A�er 1 h of �ooding the plants, respiration decreased to 37.2 μmol CO2 gDM –1 × h–1 (by 9% compared to the control). A�er subjecting the same gametophores to air reox- ygenation for 15 hours, the intensity of this process increased to 38.0 μmol CO2 gDM –1 × h–1 (by 7% of the initial value) (Fig. 2B; Tab. 1). Fig. 2. Changes in photosynthesis intensity (P) in Polytrichum piliferum Hedw. gametophores: A: P1 – a�er 1 h drying at room temperature, P2 – a�er 1 h drying and 15 h rehydration; B: P3 – a�er 1 hour of �ooding, P4 – a�er 1 hour of �ooding and 15 hours of reoxygenation in the air; mean values (± SE) marked with di�erent letters di�er signi�cantly according to Tukey test p = 0.05 Im pact of w ater stress on physiological processes of m oss Polytrichum piliferum H edw . M ał go rz at a R om ań sk a 134 Tab. 1. Changes in photosynthesis and respiration processes in Polytrichum piliferum Hedw. gameto- phores during drought and water stress; values expressed in % in relation to the control Percentage of control Mosses treatment A�er 1 hour of drying A�er 1 hour of drying and 15 h rehydration A�er 1 h �ooding A�er 1 hour of �ooding and 15 h reoxygenation P 0.00 51.02 79.12 85.44 R 96.01 128.50 91.14 93.25 P – photosynthesis, R – respiration; control – measurement taken at the beginning of the experiment, P1, R1 – a�er 1 h drying at room temperature, P2, R2 – a�er 1 h drying and 15 h rehydration, P3, R3 – a�er 1 hour of �ooding, P4, R4 – a�er 1 hour of �ooding and 15 hours of reoxygenation in the air Fig. 3. Changes in dark respiration intensity (R) in Polytrichum piliferum Hedw. gametophores: A: R1 – a�er 1 h drying at room temperature, R2 – a�er 1 h drying and 15 h rehydration; B: R3 – a�er 1 hour of �ooding, R4 – a�er 1 hour of �ooding and 15 hours of reoxygenation in the air; mean values (± SE) marked with di�erent letters di�er signi�cantly according to Tukey test p = 0.05 135 Discussion As a solvent, water is an irreplaceable environment for biochemical reactions. It is a sub- strate for many reactions and a factor that keeps the protoplast in proper physical con- dition. With only a 10% water de�cit, the photosynthetic process was already decreasing due to the reduction of turgor and the automatic closing of stomata. Water is also asso- ciated with the transport of organic substances. It is an important temperature regula- tor in plant tissues, and in the process of photosynthesis it is a source of hydrogen for assimilation force. Dehydration of the green crumb inhibits chlorophyll synthesis and accelerates its degradation. However, plasma hydration has a signi�cant impact on the entire course of respiration (Martim et al., 2009; McElrone et al., 2013; Silva et al., 2014). Most mosses are very sensitive to changes in water conditions (Rzepka et al., 2003, 2005), dependent on surface water and capillary forces action, which are o�en sup- ported by setting gametophore leaves (Wójciak, 2003). Mosses belong to poikilohydric plants, i.e. they are characterised by variable hydration. In drought conditions, these plants can dry out completely, but their structure is not completely destroyed (Lou, 2006; Ruibal et al., 2012; Zhou et al., 2011). Increasing the availability of water, e.g. as a  result of atmospheric precipitation, causes rapid hydration of their tissues and activates metabolic processes that gradually dried up during drying. Poikilohydric plants are hydrolabile, which means that their water balance can be negative for a long period. �is is the result of the low sensitivity of the stomata to dehydration, as only very low environmental humidity causes their gradual closing and limiting of transpi- ration. �e protoplasm of these plants is resistant to signi�cant and rapid �uctuations in water potential (Rock et al., 2009). Experiments carried out on Polytrichum piliferum gametophores have shown that mosses adapt relatively quickly to changing water conditions (Fig. 2–3; Tab. 1). �e rate of photosynthesis of gametophores dried for 1 h was 0 μmol CO 2 gDM –1 × h–1, and a�er 15 h of rehydration it constituted 51% of the control sample. �e dark respira- tion rate measured in P. piliferum, a�er 1 h of drying, decreased by 4% of the original value. A�er 15 h of rehydration, this process increased by 28.5% relative to the control sample. �is clearly illustrates a lower sensitivity of the dark respiration process than photosynthesis to a periodic lack of water (Rzepka, 1990). �e process of drying and then rehydrating mosses can be repeated several times, without lethal changes to the body (Proctor et al., 2007). One of the properties of mosses that allows them to sur- vive di�cult periods of drought is the ability to preserve undamaged ribosomes. �is provides the opportunity to quickly regain the ability to synthesise proteins when the cells are rehydrated. It is an ecological adaptive feature not found in other plants be- cause irreversible destruction of the protein synthesis apparatus occurs during drying (Krupa, 1974; Zeng et al., 2002). Another feature of mosses is their relatively high level Im pact of w ater stress on physiological processes of m oss Polytrichum piliferum H edw . M ał go rz at a R om ań sk a 136 of sucrose (~10% of dry mass), which remains constant during dehydration and rehy- dration (Bewley et al., 1978; Alpert, Oliver, 2002). During rehydration, the moss cells quickly return to homeostasis and become convex, and the cell organelles gradually return to their normal shape and function (Pressel et al., 2006; Proctor et al., 2007). Dehydration causes general condensation of cell content, fragmentation of the central vacuole system, increased viscosity of the cytoplasm, condensation of chro- matin and dense packing of ribosomes (Pressel et al., 2006; Proctor et al., 2007). �e plasma membranes of dried cells, e.g. Syntrichia ruralis (Hedw.) F. Weber & D. Mohr (= Tortula ruralis), look like typical lipid bilayer membranes, with scattered endothe- lial particles. Nuclei, chloroplasts and mitochondria lose their elongated shape and become round or ovate, probably due to the loss of the depolymerised microtubule cy- toskeleton (Platt et al., 1993; Ligrone, Duckett, 1996, 1998; Hoekstra et al., 2001). �e e�ects of water de�cit are revealed in almost every cellular process. One such response may be the appearance of oxidative stress. Reactive oxygen species are formed, which can trigger reactions leading to damage to cellular structures (Rzepka, 2008). �is stress leads, e.g. for lipid peroxidation, to changes in thylakoid structure, inhibition of photosystem activity of PSI and PSII and electron transport chain and inhibition of photosynthesis due to the inactivation of thiol groups of the Rubisco enzyme. Reactive oxygen forms are removed using oxidants, e.g. catalase, peroxidase and superoxide dismutase (Hoekstra et al., 2001; Franca et al., 2007). Under conditions of water de�cit, the processes of incorporating carbon are clearly modi�ed, RuBP carboxylase activity is reduced and the incorporation of carbon into organic compounds is decreased. In- hibition of photosynthesis activity is also caused by partially closed stomata. �e dark respiration process is less sensitive to periodic lack of water than photosynthesis and continues even when the potential values are low. Experiments carried out on vari- ous species of bryophytes capable of withstanding periods of drought prove that the respiration rate, as these organisms are dried, clearly decreases (Krupa, 1974; Rzepka, Krupa, 1996; Rzepka et al., 2001). In the experiment conducted with P. piliferum, �ooding with water followed by reoxygenation in the air caused changes in the photosynthesis and the dark respi- ration processes of gametophores. A�er 1 hour of �ooding, photosynthesis activity decreased by 21% and dark respiration by 9%, compared to the control. A�er 15 h of reoxygenation in the air, the reduction of photosynthesis had reached approximately 15% and respiration 7%, compared to these processes in the control (Tab. 1). Hypoxia can induce various plant responses (Rzepka, 2008; Rzepka et al., 2003, 2005). Many changes that adapt plants to oxygen de�ciency are preceded by the activation or re- pression of speci�c genes (McDaniel et al., 2007; Trouiller et al., 2007). According to recent hypotheses, morphological adjustment consists of the development of aeren- chyma, whose large intercellular spaces cause the movement of oxygen from shoots to 137 underground organs and these �ooded organs (Kopcewicz, Lewak, 2005). Osakabe et al. (2014) wrote that water stress is an important factor limiting plant growth and pro- ductivity. Longer exposure to the stress factor causes weakness and loss of vigour of the body’s cells, which in turn increases susceptibility to diseases caused by pests and pathogens. �is leads to disturbances in the functioning of the plant and a decrease in its biological value. �e plant cell regains full physiological activity a�er being saturated with water. �e speed of recovery is generally inversely proportional to the rate and intensity of stress, but most mosses regain their normal morphology in a  few minutes and rebuild their architecture in 1–2 days (Proctor, 2001; Pressel et al., 2006; Proctor et al., 2007). �e experiments carried out here showed that subjecting P. piliferum gametophores to water stress causes an imbalance of physiological processes such as photosynthesis and dark respiration. It can be argued that the plant’s ability to maintain homeostasis in the cir- cumstances of the stressor or change homeostasis through adaptation determines the resistance of plants to stress and allows them to survive or overcome prolonged stress. Conclusion Mosses are poikilohydric organisms and resistant to various environmental stress fac- tors. Tolerance to drying or �ooding is a  relatively common phenomenon in moss plants. It helps them survive adverse environmental conditions. (i) Drought stress caused complete inhibition of photosynthetic activity of Polytrichum piliferum game- tophores. However, rehydration for 15 h resulted in the restoration of the photosyn- thetic activity of the plants. �is is in contrast to hypoxia stress, which did not signif- icantly a�ect photosynthesis or drying. (ii) Dark respiration in the tested mosses was clearly less sensitive to drought stress or hypoxia, compared to photosynthesis. �ere were slight di�erences in the activity of dark respiration, compared to the controls. Perhaps, the ability to change homeostasis by adapting, surviving or overcoming ad- verse living conditions plays an important role. Global climate change causes a  rise in temperature, carbon dioxide and variable distribution of atmospheric precipita- tion. It is these factors that increase the demand for the monitoring and testing, in- cluding photosynthesis and dark respiration, of mosses and other plant species in the eco-physiology �eld. Acknowledgements I would like to thank Prof. Jan Krupa for valuable advice and help in editing the paper. I also thank PhD Grzegorz Rut and PhD Katarzyna Możdżeń for help in carrying out the experiments. Con�ict of interest �e author declares no con�ict of interest related to this article. 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Combined e�ects of nitrogen deposition and water stress on growth and physiological responses of two annual desert plants in northwestern China. Environmental and Experimental Botany, 74, 1–8. DOI: 10.1016/j.envexpbot.2010.12.005 Abstract Mosses are convenient organisms for studying the reaction to water stress because they do not have an epidermis, which makes them more sensitive to changes in humidity than most other plants. �e aim of the study was to determine the e�ect of water stress on the course of physiological processes of mosses using Polytrichum piliferum Hedw. �e present study showed that the action of the abiotic stressor, which is water, adversely a�ects the photosynthesis and dark respiration processes by reducing their intensity. However, it is worth noting that the respiration process is less dependent on tissue hydration than the photosynthesis, which is clearly demonstrated by the study results. �e bryophytes’ resistance to stress factors is responsible for the plant’s ability to maintain homeostasis under stress conditions. �e ability to change homeostasis by adapting, surviving or overcoming adverse living conditions also plays an important role. Key words: dark respiration, hypoxia, photosynthesis, Polytrichum piliferum, rehydration Received: [2020.03.31] Accepted: [2020.06.16] 141 Wpływ stresu wodnego na przebieg procesów fizjologicznych u mchów Streszczenie Mchy są organizmami dogodnymi do badania reakcji na stres wodny, ponieważ nie posiadają epidermy, przez co odznaczają się większą wrażliwością na zmiany wilgotności niż większość innych roślin. Celem pracy było określenie wpływu stresu wodnego na przebieg procesów �zjologicznych mchów na przykładzie Polytrichum piliferum Hedw. Przeprowadzone badania pokazały, że działanie abiotycznego stresora, jakim jest woda, wpływa niekorzystnie na przebieg procesów fotosyntezy i oddychania, poprzez zmniejszenie ich natężenia. Jednak warto zaznaczyć, że proces oddychania jest w mniejszym stopniu uzależniony od uwod- nienia tkanek niż proces fotosyntezy, co wyraźnie widać w przeprowadzonych tu badaniach. Za odporność mszaków na czynniki stresowe odpowiada zdolność rośliny do utrzymania homeostazy w czasie działania stresora. Również ważną rolę odgrywa zdolność zmiany homeostazy przez adaptację, przetrwanie albo po- konanie niekorzystnych warunków życiowych. Słowa kluczowe: oddychanie ciemniowe, hipoksja, fotosynteza, Polytrichum piliferum, rehydratacja Information on the authors Małgorzata Romańska She is interested in mosses physiology in di�erent stress factors. Im pact of w ater stress on physiological processes of m oss Polytrichum piliferum H edw .