Changes of fluorescence and xanthophyll pigments during dehydration in the resurrection plantSelaginella lepidophylla in low and medium light intensities.

The changes in photosynthetic efficiency and photosynthetic pigments during dehydration of the resurrection plantSelaginella lepidophylla (from the Chiuhahuan desert, S.W. Texas, USA) were examined under different light conditions. Changes in the photosynthetic efficiency were deduced from chlorophyll a fluorescence measurements (Fo, Fm, and Fv) and pigment changes were measured by HPLC analysis. A small decrease in Fv/Fm was seen in hydrated stems in high light (650 µmol photons·m-2·s-1) but not in low light (50 µmol photons·m-2·s-1). However, a pronounced decline in Fv/Fm was observed during dehydration in both light treatments, after one to two hours of dehydration. A rise in Fo was observed only after six to ten hours of dehydration. Concomitant with the decrease in photosynthetic efficiency during dehydration a rise in the xanthophyll zeaxanthin was observed, even in low-light treatments. The increase in zeaxanthin can be related to previously observed photoprotective non-photochemical quenching of fluorescence in dehydrating stems ofS. lepidophylla. We hypothesize that under dehydrating conditions even low light levels become excessive and zeaxanthin-related photoprotection is engaged. We speculate that these processes, as well as stem curling and self shading (Eickmeier et al. 1992), serve to minimize photoinhibitory damage toS. lepidophylla during the process of dehydration.

Reduced photoinhibition with stem curling in the resurrection plant Selaginella lepidophylla.

Selaginella lepidophylla, the resurrection plant, curls dramatically during desiccation and the hypothesis that curling may help limit bright light-induced damage during desiccation and rehydration was tested under laboratory conditions. Restraint of curling during desiccation at 25° C and a constant irradiance of 2000 µmol m-2 s]t-1 significantly decreased PSII and whole-chain electron transport and the Fv/Fm fluorescence yield ratio following rehydration relative to unrestrained plants. Normal curling during desiccation at 37.5°C and 200 µmol m-2 s-1 irradiance did not fully protect against photoinhibition or chlorophyll photooxidation indicating that some light-induced damage occurred early in the desiccation process before substantial curling. Photosystem I electron transport was less inhibited by high-temperature, high-irradiance desiccation than either PSII or whole-chain electron transport and PSI was not significantly affected by restraint of curling during desiccation at 25°C and high irradiance. Previous curling also helped prevent photoinhibition of PSII electron transport and loss of whole-plant photosynthetic capacity as the plants uncurled during rehydration at high light. These results demonstrate that high-temperature desiccation exacerbated photoinhibition, PSI was less photoinhibited than PSII or whole-chain electron transport, and stem curling ameliorated bright light-induced damage helping to make rapid recovery of photosynthetic competence possible when the plants are next wetted.

Limited photosynthetic plasticity in Sedum pulchellum Michx.

Carbon dioxide exchange pattern, carboxylating enzyme specific activity and titratable acidity were measureed for field collected individuals of Sedum pulchellum Michx., a winter annual of xeric cedar glade communities in central Tennessee. Evidence of a weak-CAM mode of photosynthesis was found on 4 of 27 sampling dates during a three year period. Three of these dates occurred during 1978, the driest study season, and nocturnal CO2 uptake contributed a maximum of 10% of the plants daily CO2 uptake. Weak CAM in the field was associated with elevated phosphoenol-pyruvate (PEP) carboxylase activity and, to a lesser degree, with decreased ribulose bisphosphate (RuBP) carboxylase activity. In contrast, carboxylating enzyme specific activities of the sympatric, obligate CAM species Opuntia compressa, a dominant perennial of the cedar glades, were highly stable.Weak CAM was also induced in S. pulchellum upon water stress in the laboratory. Again, weak CAM was associated with elevated PEP carboxylase and decreased RuBP carboxylase activities and represented a relatively minor CO2 uptake contribution. The potential ecological significance of this limited photosynthetic flexibility in S. puchellum is not obvious but it may play an important role in successful yearly reproduction in this annual species at a time when water stress is frequent.

Photosynthetic recovery of the resurrection plant Selaginella lepidophylla (Hook. and Grev.) Spring: effects of prior desiccation rate and mechanisms of desiccation damage.

The effects of desiccation rate on photosynthetic recovery of the resurrection plant Selaginella lepidophylla (Hook. and Grev.) Spring were examined. Gas exchange over a 24 h rehydration period, ribulose bisphosphate carboxylase conservation in desiccated fronds, de novo protein synthesis rate during rehydration, and frond leakage rate during rehydration were measured following desiccation at various rates. Seven-day hydrated, fully recovered plants were dried at four rates. The initial decline in tissue fresh weight/dry weight ratio was essentially linear to a ratio value of 1.35, the point at which frond curling occurred, but slowed thereafter. Frond curling required 5.5, 52, 94, and 175 h of desiccation, respectively, for the four treatments.The rate of net photosynthesis after 24 h of rehydration was greatest for the two intermediate drying speeds; both very rapid and very slow drying were associated with significantly reduced rates. Electrolyte leakage was greatest following very rapid drying and de novo protein synthesis was impaired following very slow drying. Ribulose bisphosphate carboxylase conservation was not significantly affected by drying speed. These results support the hypothesis that desiccation injury is multicausal and that specific components of the photosynthetic recovery process respond differently to desiccation rate. However, in spite of these significant desiccation rate effects, overall photosynthetic recovery in S. lepidophylla appears to be relatively tolerant of widely varying rates of desiccation.

Photosynthetic recovery of resurrection spikemosses from different hydration regimes.

Patterns and mechanisms of recovery of photosynthetic activity of two resurrection spikemoss species from different hydration environments in the Chihuahuan desert of southwest Texas, USA were compared in the laboratory. Selaginella lepidophylla Hook. and Grev. is the dominant poikilohydric plant found in hot, arid low-elevation desert scrub communities and S. pilifera A. Br. is one of many poikilohydric species found in more mesic, high-elevation woodland communities in Big Bend National Park, Texas. Infrared gas analysis of CO2 exchange during hydration of desiccated plants indicated that photosynthetic competence was achieved significantly more rapidly in S. lepidophylla tha S. pilifera. Chloramphenicol, a chloroplast protein synthesis inhibitor at 100 µg·ml-1 in the hydration medium, significantly inhibited the resumption of CO2 uptake in both species, but the level of inhibition was significantly greater in S. pilifera. The level of conserved ribulose 1,5 bis phosphate carboxylase specific activity in desiccated plants was significantly greater in S. lepidophylla. These results support the hypothesis that poikilohydric plants from increasingly xeric environments are capable of more rapid photosynthetic recovery, due to increased conservation of the photosynthetic system during desiccation and a reduced requirement for photosynthetic system repair during hydration, than those from more mesic environments.

Photosynthetic recovery in the resurrection plant Selaginella lepidophylla after wetting.

Photosynthetic recovery (PR) in a southwest Texas, USA population of Selaginella lepidophylla (Hook and Grev.) (Selaginellaceae), a poikilohydric spikemoss, was examined in the laboratory. Infrared CO2 gas analysis and ribulose 1,5-bisphosphate (RuBP) carboxylase activity measurements indicated that optimal temperature for PR was near 25°C in terms of: (1) rapidity of net CO2 uptake after hydration (5.4 h), (2) maximum net photosynthetic rate at 2000 µE·m-2·s-1 (2.44 mg CO2·g(DWT)-1·h-1), and (3) maximum net CO2 assimilation per 30 h hydration event (43.8 mg CO2·g(DWT)-1·30 h-1). The PR was much slower at both 15° and 35° C, with lower photosynthetic rates and net carbon gains per hydration event. High respiratory costs were incurred at 45°C and no net photosynthesis was observed. Increases in RuBP carboxylase activity and chlorophyll content during 24 h hydration were also greatest near 25°C. Dry plants had 60% of the enzyme activity of fully recovered (24 h hydration) plants, indicating enzyme conservation. Actinomycin D and cycloheximide did not appear to inhibit PR, but chloramphenicol appeared to totally inhibit RuBP carboxylase activity increases over levels conserved in dry plants. Therefore, rapid PR in S. lepidophylla was achieved by both rapid increase in RuBP carboxylase activity, possibly via de novo synthesis, and conservation of the photosynthetic enzyme. Both mechanisms are essential to maximize assimilation in S. lepidophylla in an environment where hydrated periods are rare and of short duration.

Carbon isotope ratios of crassulacean acid metabolism species in relation to climate and phytosociology.

The δ 13C, ‰ values of the two principal, widely distributed CAM species in Big Bend National Park, Texas, were examined and related to phytosociologic and macroclimatic parameters. Samples of Agave lecheguilla and Opuntia engelmannii were collected along an elevational gradient covering 1370 m, and δ values for both total and insoluble fractions were measured. Linear regressions were used to correlate changes in δ values with changes in community importance and mean annual precipitation for both species to determine whether or not they became increasingly C3-like in their photosynthetic mode at more mesic, upper elevations. In addition, δ values were measured for other Agave and Opuntia species from different locations and for different seasons and for A. lecheguilla following a short laboratory experiment. Results show that there are very small changes in δ values for both species along the pronounced temperature and precipitation gradients associated with elevational change. The ranges in δ values for O. engelmannii are only 1.5‰ (n=7) and 1.2‰ (n=6) for total and insoluble fractions respectively. The ranges in values for A. lecheguilla are 2.6‰ (n=9) and 3.1‰ (n=9). The results for other species confirm this low variability. We conclude that the species examined are obligate CAM species and that wide elevational distributions are not due to shifts in photosynthetic mode.