Zsolt Csintalan

Photosynthetic responses of a moss, Tortula ruralis, ssp. ruralis, and the lichens Cladonia convoluta and C. furcata to water deficit and short periods of desiccation, and their ecophysiological significance: a baseline study at present-day CO2 concentration

We report the changes in CO2 assimilation, potential photochemical activity (as measured by slow fluorescence), photosynthetic pigment concentrations, and dark respiration of two desiccation-tolerant (DT) lichens (Cladonia convoluta (Lam.) P. Cout. and C. furcata (Huds.) Schrad.), and a DT moss (Tortula ruralis (Hedw.) Gaertn. ssp. ruralis) during slow drying, and on rehydration following a 12 h period of desiccation. Initially there was a two to fourfold increase in net CO., assimilation due to reduction of CO2 -diffusion resistance by elimination of excess water. Optimum water content for photosynthesis was 100-150 % of dry mass (DM) in C. convoluta, c. 100 % DM in C. furcata, and 120-200 % DM in T. ruralis. The intensity of maximum and steady-state slow fluorescence showed little change above water contents of 56%, DM in the lichens and 73 % DM in T. ruralis (corresponding to c. 30-40 % cell relative water content), but fell sharply at lower water content. The variable duorophyll-fluorescence decrease ratio (Rfd) at 690 nm peaked at 56 % DM water content in the two lichens, and at 45% DM in T. ruralis. Photochemical activity ceased at the same point in the experiments as CO, assimilation; dark respiration ceased only when desiccation was complete. In all three species, the photosynthetic apparatus remained in a fully and quickly recoverable state. Chlorophyll and carotenoid concentrations remained substantially unaltered throughout. On rehydration, chlorophyll fluorescence parameters returned within 30 min to pre-desiccation levels, and photosynthesis recovered fully and rapidly (< 1 h). All three species attained a positive carbon balance within 20 min of re-moistening, in spite of high rates of dark respiration. The results confirm the significance of extracellularly-stored water to poikilohydric DT lichens and bryophytes. The measurements, in conjunction with published data on the full-turgor water content of similar mosses and lichens, show that the cell-physiological response of photosynthesis to water deficit is not greatly different from that of either normal or DT vascular plants. Small plant size and small cell volume in DT lichens and mosses, together with rapid recovery of photosynthesis after desiccation, allow the plants to utilize the small amounts of intermittently available water from brief showers or dew.

ECOPHYSIOLOGICAL RESPONSES OF HOMOIOCHLOROPHYLLOUS AND POIKILOCHLOROPHYLLOUS DESICCATION TOLERANT PLANTS : A COMPARISON AND AN ECOLOGICAL PERSPECTIVE

There is an apparently stark contrast in ecophysiological adaptation between the poikilochlorophyllous desiccation-tolerant (PDT) angiosperm Xerophyta scabrida and homoichlorophyllous desiccation-tolerant (HDT) lichens and bryophytes. We summarise measurements on Xerophyta and on the temperate dry-grassland lichen Cladonia convoluta and the moss Tortula ruralis through a cycle of desiccation and rehydration. Considered in a broad ecological and evolutionary context, desiccation tolerance in general can be seen as evading some of the usual problems of drought stress, and these plants as particular instances drawn from an essentially continuous spectrum of adaptive possibilities – related on the one hand to the physical scale of the plants, and on the other to the time-scale of wetting and drying episodes.

Reconstitution of chlorophylls and photosynthetic CO2 assimilation upon rehydration of the desiccated poikilochlorophyllous plant Xerophyta scabrida (Pax) Th. Dur. et Schinz

Resynthesis of the photosynthetic apparatus and resumption of CO2 assimilation upon rehydration is reported for the monocotyledonous and poikilochlorophyllous desiccation-tolerant (PDT) plant Xerophyta scabrida (Pax) Th. Dur. et Schinz (Velloziaceae). During desiccation there was a complete breakdown of chlorophylls whereas the total carotenoid content of air-dried leaves was reduced to about 22% of that of functional leaves. The prerequisites for the resynthesis of photosynthetic pigments and functional thylakoids were the reappearance of turgor and maximum leaf water content at 2 and 10 h after rehydration, respectively. The period of increased initial respiration after rewetting leaves (rehydration respiration) lasted up to 30 h and was thus 6 to 10 times longer than in homoiochlorophyllous desiccation-tolerant plants (HDTs) in which chlorophylls are retained during desiccation. Accumulation of chlorophylls a + b and total carotenoids (xanthophylls and βcarotene) started 10 h after rehydration. Normal levels of chlorophyll and carotenoids were obtained 72 h after rehydration. Values for the variable-fluorescence decrease ratio (Rfd690 values), an indicator of photochemical activity, showed that photochemical function started 10 h after rehydration, but normal values of 2.7 were reached only 72 h after rehydration. Net CO2 assimilation started 24 h after rewetting and normal rates were reached after 72 h, at the same time as normal values of stomatal conductance were obtained. The increasing rates of net CO2 assimilation were paralleled by decreasing values of the intercellular CO2 concentration. All photosynthetic parameters investigated showed values normal for functional chloroplasts by 72 h after the onset of rehydration. Fully regreened leaves of the presumed C3 plant X. scabrida exhibited a net CO2 assimilation rate which was in the same range as that of other C3 plants and higher than that of recovered HDT plants. The fundamental difference between air-dried PDT plants, such as X. scabrida, which have to resynthesize the photosynthetic pigment apparatus, and air-dried HDT plants, which only undergo a functional recovery, is discussed.

Resynthesis of Thylakoids and Functional Chloroplasts in the Desiccated Leaves of the Poikilochlorophyllous Plant Xerophyta scabrida upon Rehydration

Summary The ultrastructural changes in chloroplasts and other cell organelles in the desiccated, achlorophyllous leaves of Xerophyta scabrida, a poikilochlorophyllous desiccation tolerant (PDT) monocotyledonous plant, were examined during reconstitution of the photosynthetic apparatus after rehydration of airdried leaves. In the desiccoplasts (the former chloroplasts) of the air-dried leaves no thylakoids were present, only osmiophilic lipid material in the place of former grana and stroma thylakoids and groups of translucent plastoglobuli. Ten to 12 h after the start of the rehydration of air-dried leaves the resynthesis of chlorophylls and thylakoids began, fundamental structural changes occurred in desiccoplasts: the appearance of a small amount of starch, of primary thylakoids and of primary grana consisting of two appressed thylakoids, whereas the size of plastoglobuli descreased. At this stage the mitochondria appeared to be fully functional and to recover before the reconstitution of chloroplasts. Grana with 2–3 thylakoids were predominant 24 h after the start of rehydration; the degree of stacking and the ratio of appressed to non-appressed membranes increased. Translucent plastoglobuli were no longer seen, only much smaller osmiophilic plastoglobuli were visible. At 72 h after rehydration of air-dried leaves, grana of up to 7 thylakoids appeared, the degree of stacking increased further, starch granules became larger, as did the plastoglobuli, which also again turned translucent. The thylakoid system was then fully reconstituted and capable of ensuring the energy requirements of a normal rate of CO2 assimilation as well as synthesis and accumulation of excess lipids in the translucent plastoglobuli.

Regreening of desiccated leaves of the poikilochlorophyllous Xerophyta scabrida upon rehydration

Summary The poikilochlorophyllous and desiccation tolerance nature of the leaves of the monocotyledonous Xerophyta scabrida and the physiological recovery of the air-dried leaves upon rehydration after a long-term desiccation is described in this paper. Water uptake through the leaf surface upon rehydration was found to be of prime importance for recovery. Field observations in the natural habitat showed that at the onset of the rainy season water uptake through the leaf surface preceded the development of new adventitious roots. In dry leaves carotenoids were found as the only photosynthetic pigments. Upon rehydration de novo chlorophyll and carotenoid synthesis began just before the leaves reached their maximum water content and specific leaf area. The accumulation of chlorophylls and carotenoids was found to rise very fast and maximum content was reached about 72 hours after rehydration. The values of the chlorophyll fluorescence ratio, F690/F735, and of the fluorescence intensities at 690 nm and at 730 nm (both at maximum Fm and steady-state fluorescence Fs) continuously decreased from the begin of regreening until reaching maximum chlorophyll content after 3 days. The fluorescence ratio F690/F735 proved to be a very suitable non-destructive diagnostic method to monitor the chlorophyll content during regreening of the poikilochlorophyllous leaves of X. scabrida upon rehydration.

Optimizing Phytoremediation of Heavy Metal-Contaminated Soil by Exploiting Plants' Stress Adaptation

Soil phytoextraction is based on the ability of plants to extract contaminants from the soil. For less bioavailable metals, such as Pb, a chelator is added to the soil to mobilize the metal. The effect can be significant and in certain species, heavy metal accumulation can rapidly increase 10-fold. Accumulation of high levels of toxic metals may result in irreversible damage to the plant. Monitoring and controlling the phytotoxicity caused by EDTA-induced metal accumulation is crucial to optimize the remedial process, i.e. to achieve maximum uptake. We describe an EDTA-application procedure that minimizes phytotoxicity by increasing plant tolerance and allows phytoextraction of elevated levels of Pb and Cd. Brassica juncea is tested in soil with typical Pb and Cd concentrations of 500 mg kg−1 and 15 mg kg−1, respectively. Instead of a single dose treatment, the chelator is applied in multiple doses, that is, in several small increments, thus providing time for plants to initiate their adaptation mechanisms and raise their damage threshold. In situ monitoring of plant stress conditions by chlorophyll fluorescence recording allows for the identification of the saturating heavy metal accumulation process and of simultaneous plant deterioration.

Responses of CO2 assimilation, transpiration and water use efficiency to long-term elevated CO2 in perennial C3 xeric loess steppe species

Summary CO 2 assimilation (A), transpiration (E), water use efficiency (WUE), leaf-nitrogen and carbohydrate responses to 11 months elevated (700 μmol mol -1 ) CO 2 exposure in four perennial C 3 species ( Festuca rupicola, Dactylis glomerata, Filipendula vulgaris, Salvia nemorosa ) from a xeric temperate loess steppe are reported. The responses in the species varied greatly owing to their differing acclimation. The acclimation of photosynthesis was somewhat downward in F. rupicola , fully downward in D. glomerata , and upward in S. nemorosa and F. vulgaris . The reduction in the initial slope of the A/q response curve in F. rupicola and D. glomerata suggested a decrease in Rubisco capacity. Net CO 2 assimilation at 700 μmol mol -1 CO 2 c a in the high CO 2 F. rupicola was higher than in those grown at present (350 μmol mol -1 ) CO 2 ; there was no difference in D. glomerata . The initial slope of the A/c i curve indicated an increased Rubisco capacity in high CO 2 F. vulgaris and S. nemorosa . Their net CO 2 assimilation was higher in the plants grown in the high CO 2 treatment at c i s ovet 200 μmol mol -1 than that in the plants grown at present CO 2 . The A/c i response curves, which were saturated in all species grown at present CO 2 , did not reach saturation in the plants grown at elevated CO 2 , reflecting that the Pi limitation of CO 2 assimilation was alleviated in the plants grown at high CO 2 . Transpiration decreased with an increase in q in both the present and elevated CO 2 F. rupicola and D. glomerata . In F. vulgaris , an increase in q caused a reduction in transpiration in the plants grown at high CO 2 only. Transpiration rate in both the present and elevated CO 2 S. nemorosa was not affected by any change in c i . It is suggested then that long-term exposure to high CO 2 causes a similar acclimation of stomatal regulation and transpiration to that of photosynthesis. High CO 2 caused a significant decrease in protein-nitrogen content only in D. glomerata . Starch increased in F. rupicola and D. glomerata and soluble sugar content was higher in all species grown at high CO 2 than at ambient. Instantaneous WUE significantly increased in all species grown at elevated CO 2 .

Thermoluminescence studies on the function of photosystem II in the desiccation tolerant lichen Cladonia convoluta

The effect of desiccation and rehydration on the function of Photosystem II has been studied in the desiccation tolerant lichen Cladonia convoluta by thermoluminescence. We have shown that in functional fully hydrated thalli thermoluminescence signals can be observed from the recombination of the S2(3)QB− (B band), S2QA− (Q band), Tyr-D+QA− (C band) and Tyr-Z+(His+)QA− (A band) charge stabilization states. These thermoluminescence signals are completely absent in desiccated thalli, but rapidly reappear on rehydration. Flash-induced oscillation in the amplitude of the thermoluminescence band from the S2(3)QB− recombination shows the usual pattern with maxima after 2 and 6 flashes when rehydration takes place in light. However, after rehydration in complete darkness, there is no thermoluminescence emission after the 1 st flash, and the maxima of the subsequent oscillation are shifted to the 3rd and 7th flashes. It is concluded that desiccation of Cladonia convoluta converts PS II into a nonfunctional state. This state is characterized by the lack of stable charge separation and recombination, as well as by a one-electron reduction of the water-oxidizing complex. Restoration of PS II function during rehydration can proceed both in the light and in darkness. After rehydration in the dark, the first charge separation act is utilized in restoring the usual oxidation state of the water-oxidizing comples.

Ecophysiological consequences of contrasting microenvironments on the desiccation tolerant moss Tortula ruralis

Tortula ruralis is a homoiochlorophyllous-desiccation-tolerant (HDT) moss that retains all pigments when dehydrated and rapidly recovers physiological function upon rehydration. This moss forms extensive cover in exposed and shaded areas in the sandy semi-arid grasslands of Central Europe. We hypothesized that contrasting drying regimes between these microhabitats would affect plant N status, constraints to gas exchange and growth, as well as result in altered pigment concentrations and ratios, and photochemical light-response dynamics. Furthermore, we believed T. ruraliss HDT habit would limit its ability to acclimate to altered light environment. We found that sun plant T. ruralis had lower plant mass, as well as lower tissue N, C, total photosynthetic pigment concentrations and carbon isotope discrimination (Δ) values compared to shade plant counterparts. Carotenoid/chlorophyll ratios in sun plants were typical of high light-adapted tissue, but chlorophyll a/chlorophyll b ratios were lower, more characteristic of low light-adapted tissue. This unique combination of pigment responses was accompanied by sustained lower levels of optimal quantum efficiency of PSII (Fv/Fm) in sun plant T. ruralis, even during favorable diurnal conditions, and reduced engagement of energy-dependent thermal dissipation (NPQ). Reciprocal transplants of sun and shade plants showed that T. ruralis is capable of short-term adjustment to altered light level, as evidenced by increases in Fv/Fm, NPQ, and light-adapted PSII yield (φPSII) in transplanted sun plants, and concurrent decreases in sun-transplanted shade plants. However, the performance of transplanted sun plants remained consistently below that of undisturbed shade plants. These findings show that microenvironmental variation results in different patterns of resource acquisition in this HDT moss, and that growth in the open imparts greater desiccation tolerance, and the development of a greater standing engagement of slowly reversing photoprotective mechanisms. In contrast, prolonged activity and greater resource acquisition in shaded populations may allow T. ruralis to rapidly adjust to changes following disturbance to the plant canopy, fostering the persistence of T. ruralis in these semi-arid grasslands.

Responses of nine bryophyte and one lichen species from different microhabitats to elevated UV-B radiation

Chlorophyll fluorescence parameters (Fv/Fm, RFd) of nine bryophyte and one lichen species were investigated after prolonged exposure to elevated UV-B radiation. The majority of the investigated bryophytes showed a prompt or inducible tolerance to increase UV-B irradiation. Among the investigated species high degree of UV-tolerance coincides with strong desiccation tolerance.