Water storage and osmotic pressure influences on the water relations of a dicotyledonous desert succulent

Abstract Water storage and nocturnal increases in osmotic pressure affect the water relations of the desert succulent Ferocactus acanthodes, which was studied using an electrical circuit analog based on the anatomy and morphology of a representative individual. Transpiration rates and osmotic pressures over a 24-h period were used as input variables. The model predicted water potential, turgor pressure and water flow for various tissues. Plant capacitances, storage resistances and nocturnal increases in osmotic pressure were varied to determine their role in the water relations of this dicotyledonous succulent. Water coming from storage tissues contributed about one-third of the water transpired at night: the majority of this water came from the nonphotosynthetic, water storage parenchyma of the stem. Time lags of 4 h were predicted between maximum transpiration and maximum water uptake from the soil. Varying the capacitance of the plant caused proportional changes in osmotically driven water movement but changes in storage resistance had only minor effects. Turgor pressure in the chlorenchyma depended on osmotic pressure, but was fairly insensitive to doubling or halving of the capacitance or storage resistance of the plant. Water uptake from the soil was only slightly affected by osmotic pressure changes in the chlorenchyma. For this stem succulent, the movement of water from the chlorenchyma to the xylem and the internal redistribution of water among stem tissues were dominated by nocturnal changes in chlorenchyma osmotic pressure, not by transpiration.     

Environmental influences on the productivity of three desert succulents in the south-western United States

Abstract Net CO₂ uptake over 24 h periods for shoots of Agave deserti, Ferocactus acanthodes, and Opuntia ficus-indica was measured under the ranges of water status, air temperature, and photo-synthetically active radiation (PAR) that occur in the south-western U.S.A. An environmental productivity index (EPI) was constructed indicating the overall influence of these three factors on net CO₂ uptake. Using growth chambers whose conditions were changed monthly to simulate the environmental conditions at a field site, the observed shoot dry weight increases per unit surface area changed in concert with monthly changes in EPI. The observed dry weight gain of the shoot was 17–19% lower than the predicted shoot net CO₂ uptake, which could be accounted for by carbon diversion to the roots. Mean monthly EPI was also predicted for 21 sites in the south-western U.S.A. All three species had low EPIs in the Colorado River basin, which has low annual rainfall and high summer temperatures, and in the north-central part of the region, which has low temperatures and low PAR during winter when water is available. The two native species, A. deserti and F. acanthodes, had high EPIs beyond their range in coastal southern California, where competition by other vegetation for PAR may limit net CO₂ uptake. Such regions as well as south-central California and south-central Arizona had high EPIs for all three species, indicating that these areas would be appropriate for the cultivation of O. ficus-indica.     

Water flow and water storage in Agave deserti: osmotic implications of crassulacean acid metabolism

Abstract Water flow and water storage were investigated for Agave deserti, a desert succulent showing crassulacean acid metabolism (CAM). The anatomy and water relations of the peripheral chlorenchyma, where CAM occurs, and the central water-storage parenchyma were investigated for its massive leaves so that these tissues could be incorporated as discrete elements into an electrical-circuit analogue of the whole plant. The daily cycling of osmotic pressure was represented by voltage sources in series with the storage capacitors. With soil water potential and leaf transpiration rate as input variables, axial water flow through the vascular bundles and radial flows into and out of storage during the day/night cycle were determined. The predominantly nocturnal transpiration was coincident with increases in cell osmotic pressure and in titratable acid of the leaf chlorenchyma. In the outer layers of the chlorenchyma, water potential was most negative at the beginning of the night when transpiration was maximum, while the water-storage parenchyma reached its minimal water potential 9 h later. The roots plus stem contributed 7% and the leaves contributed 50% to the total water flow during maximal transpiration; peak water flow from the soil to the roots occurred at dawn and was only 58% of the maximal transpiration rate. Over each 24-h period, 39% of the water lost from the plant was derived from storage, with flow into storage occurring mainly during the daytime. Simulations showed that the acid accumulation rhythm of CAM had little impact on water uptake from the soil under the conditions employed. In the outer chlorenchyma, water potential and water flows were more sensitive to the day/night changes in transpiration than in osmotic pressure. Nevertheless, cell osmotic pressure had a large influence on turgor pressure in this tissue and determined the extent to which storage was recharged during the latter part of the night.     

Comparative ecophysiology of CAM and C3 bromeliads. IV. Plant water relations

Abstract An investigation was carried out into the water relations of CAM and C₃ bromeliads in their natural habitat during the dry season in Trinidad. Measurements were made of xylem tension with the pressure chamber and of cell-sap osmotic pressure and titratable acidity on crushed leaf samples. A steady-state CO₂ and H₂O-vapour porometer was also used so that changes in leaf water relations during individual day-night cycles could be directly related to gas-exchange patterns in situ. Xylem tension changed in parallel with transpiration rate and in general reached its maximum value in CAM bromeliads at night and in C₃ bromeliads during the day. In addition, large nocturnal increases in cell-sap osmotic pressure and titratable acidity (ΔH⁺) typically occurred in the CAM bromeliads. The C₃-CAM intermediate Guzmania monostachia showed slight nocturnal acidification, but had higher values of xylem tension during the day. Very high values of AH+ were observed in the CAM species when the tanks of the epiphytic bromeliads contained water: Aechmea nudicaulis showed a mean maximum ΔH⁺ of 474 mol m^?3, the highest value so far observed for CAM plants. On some nights dew formed on the leaf surfaces of the epiphytes, partially curtailing gas exchange and leading to a marked decrease in xylem tension in both C₃ and CAM species. Between-site comparisons were also made for a wide range of habitats from arid coastal scrub to montane rain forest. Compared with values characteristic of other life-forms, xylem tension and cell-sap osmotic pressure were low for all bromeliads, and did not differ significantly in co-occurring CAM and C₃ bromeliads. Mean maximum xylem tension (10 species in total) ranged from 0.29 M Pa at the montane sites to 0.67 MPa at the most arid site, and mean minimum osmotic pressure (17 species) from 0.51 to 0.97 MPa. At the arid sites the bromeliads were exclusively CAM species, two of which (Aechmea aquilega and Bromelia plumieri) grew terrestrially in the undergrowth of the coastal scrub. Xylem tension in these species was low enough to indicate that they must be functionally independent of the substratum during the dry season. In the wetter part of Trinidad, no between-site differences in leaf water relations were found along an altitudinal gradient in the Northern Mountain Range; seasonal differences in this area were also small. Overall, leaf water relations and gas exchange in the bromeliads were strongly affected both by short-term changes in water availability and by longer-term climatic differences in the various regions of the island.     

Oxygen isotope composition of CAM and C3 Clusia species: non-steady-state dynamics control leaf water 18O enrichment in succulent leaves

Leaf gas exchange and leaf water (18)O enrichment (Delta(18)O(L)) were measured in three Clusia species under field conditions during dry and wet seasons and in Miconia argentea during the dry season in the Republic of Panama. During the dry season, all three Clusia species used crassulacean acid metabolism (CAM); during the wet season Clusia pratensis operated in the C(3) mode, while Clusia uvitana and Clusia rosea used CAM. Large departures from isotopic steady state were observed in daytime Delta(18)O(L) of the Clusia species, especially during the dry season. In contrast, daytime Delta(18)O(L) was near isotopic steady state in the C(3) tree M. argentea. Across the full data set, non-steady-state predictions explained 49% of variation in observed Delta(18)O(L), whereas steady-state predictions explained only 14%. During the wet season, when Delta(18)O(L) could be compared with Clusia individuals operating in both C(3) and CAM modes, steady-state and non-steady-state models gave contrasting predictions with respect to interspecific variation in daytime Delta(18)O(L). The observed Delta(18)O(L) pattern matched that predicted for the non-steady state. The results provided a clear example of how non-steady-state control of leaf water (18)O dynamics can shift the slope of the relationship between transpiration rate and daytime Delta(18)O(L) from negative to positive.     

The effect of nitrogen availability and water conditions on competition between a facultative CAM plant and an invasive grass

Abstract Plants with crassulacean acid metabolism (CAM) are increasing their abundance in drylands worldwide. The drivers and mechanisms underlying the increased dominance of CAM plants and CAM expression (i.e., nocturnal carboxylation) in facultative CAM plants, however, remain poorly understood. We investigated how nutrient and water availability affected competition between Mesembryanthemum crystallinum (a model facultative CAM species) and the invasive C₃ grass Bromus mollis that co‐occur in California's coastal grasslands. Specifically we investigated the extent to which water stress, nutrients, and competition affect nocturnal carboxylation in M. crystallinum. High nutrient and low water conditions favored M. crystallinum over B. mollis, in contrast to high water conditions. While low water conditions induced nocturnal carboxylation in 9‐week‐old individuals of M. crystallinum, in these low water treatments, a 66% reduction in nutrient applied over the entire experiment did not further enhance nocturnal carboxylation. In high water conditions M. crystallinum both alone and in association with B. mollis did not perform nocturnal carboxylation, regardless of the nutrient levels. Thus, nocturnal carboxylation in M. crystallinum was restricted by strong competition with B. mollis in high water conditions. This study provides empirical evidence of the competitive advantage of facultative CAM plants over grasses in drought conditions and of the restricted ability of M. crystallinum to use their photosynthetic plasticity (i.e., ability to switch to CAM behavior) to compete with grasses in well‐watered conditions. We suggest that a high drought tolerance could explain the increased dominance of facultative CAM plants in a future environment with increased drought and nitrogen deposition, while the potential of facultative CAM plants such as M. crystallinum to expand to wet environments is expected to be limited.     

Natural deuterium and oxygen-18 enrichment in leaf water of cotton plants grown under wet and dry conditions: evidence for water compartmentation and its dynamics

Abstract. Significant differences in leaf water oxygen and hydrogen isotopic composition were observed between cotton plants grown under wet and dry conditions. The magnitude of the differences could be fully explained by the conventional model that describes the isotopic composition of an evaporating water pool under steady state conditions. The results indicate that leaf water isotopic composition is strongly influenced by transpiration rate via its effects on relative humidity adjacent to the leaf surface and on the isotopic composition of the air moisture. Our application of the model, however, provides evidence that leaf water must consist of a mixture of several isotopically distinct pools. These pools are suggested to reside in the symplast, in the cell walls and intercellular spaces and in the veins. A model is proposed suggesting that only the water residing in the cell walls and the intercellular spaces (the transpiration pool) interacts directly with the external environment. The large symplastic pool responds to the external environment to a limited extent via its relatively slow exchange with water in the transpiration pool. It is likely that the isotopic composition of water in the symplastic pool is strongly buffered against shortterm environmental variations, a possibility that would have important implications for the isotopic conditions under which organic matter biosynthesis occurs.     

Effects of mucilage on seed germination of the desert ephemeral plant Plantago minuta Pall. under osmotic stress and cycles of wet and dry conditions

To test the role of the seed mucilage of Plantago minuta Pall. in regulating germination under osmotic stress and cycles of hydration and dehydration, two experiments were carried out using seeds with intact mucilage and mucilage‐free seeds. In Experiment 1 seeds were immersed in a range of iso‐osmotic polyethylene glycol solutions (?1.15 to 0 MPa) for 14 days; any ungerminated seeds were transferred to deionized water to investigate the recovery germination. In Experiment 2 seeds were immersed in deionized water for 24 h, and were then incubated on filter paper for an additional 13 days to ensure complete desiccation before reimbibition to test the germination recovery percentage. Under mild osmotic stress (?0.73 to 0 MPa), the intact seeds with mucilage were shown to have higher germination rates than the mucilage‐free seeds, indicating that the mucilage led to a “fast sprouting” germination strategy under mild osmotic stress. However, when seeds were exposed to high osmotic stress (?1.15 MPa), the mucilage apparently slowed the germination rate, resulting in a “risk‐balancing” germination strategy. Extreme drought induced by polyethylene glycol solution and the desiccation pretreatment accelerated germination rates compared to non‐pretreated seeds; both germination potential and recovery percentage of the mucilage seeds were significantly higher than that of the mucilage‐free seeds. Our results revealed that the seed mucilage of P. minuta plays a crucial role in regulating seed germination rates and the germination strategies adopted by controlling seed water absorption when the seeds experience different osmotic stresses or alternating wet and dry conditions.