Field observations of seedlings and saplings of Avicennia marina showed patterns that correlated with salinity, light and sediment. Models that account for these observations were subsequently tested in a series of field experiments. Establishment varied within an estuary under controlled conditions but was not related to salinity or sediment type. Seedling survival was uniform over 3 years regardless of position in estuary and sediment type. Seedling densities and survival under canopies or in canopy gaps were not significantly different. However, seedling growth and density of saplings were greater in canopy gaps. Experiments involving manipulations of canopies showed no differences in seedling survival under canopies or in light gaps, but addition of slow-release fertilizer enhanced growth and survival in canopy gaps and under canopies. Long-term comparison of areas denuded of a canopy and with sediment disturbance showed enhanced establishment and survival when compared with areas with canopy gaps but with undisturbed sediments. Overall there appears to be no restriction to establishment of propagules within mangrove stands other than the supply of propagules and tidal or wave action. In contrast, recruitment to the sapling stage appears to be restricted by light and sediment resources. We suggest that propagules need to establish in a regeneration niche for seedling recruitment to the sapling stage. This differs from the view that seedlings in the under-storey are analogous to a seed pool in the soil.

The influence of external phosphorus (P) on carbon (C) allocation and metabolism as well as processes related to P metabolism was studied in monoxenic arbuscular mycorrhiza cultures of carrot (Daucus carota). Fungal hyphae of Glomus intraradices proliferated from the solid minimal medium containing the colonized roots into C-free liquid minimal medium with different P treatments. The fungus formed around three times higher biomass in P-free liquid medium than in medium with 2.5 mm inorganic P (high-P). Mycelium in the second experiment was harvested at an earlier growth stage to study metabolic processes when the mycelium was actively growing. P treatment influenced the root P content and [13 C]glucose administered to the roots 7 d before harvest gave a negative correlation between root P content and 13 C enrichment in arbuscular mycorrhiza fungal storage lipids in the extraradical hyphae. Eighteen percent of the enriched 13 C in extraradical hyphae was recovered in the fatty acid 16:15 from neutral lipids. Polyphosphate accumulated in hyphae even in P-free medium. No influence of P treatment on fungal acid phosphatase activity was observed, whereas the proportion of alkaline-phosphatase-active hyphae was highest in high-P medium. We demonstrated the presence of a motile tubular vacuolar system in G. intraradices. This system was rarely seen in hyphae subjected to the highest P treatment. We concluded that the direct responses of the extraradical hyphae to the P concentration in the medium are limited. The effects found in hyphae seemed instead to be related to increased availability of P to the host root.Arbuscular mycorrhizal (AM) association is the only way for fungi in the order Glomales to proliferate and reproduce (Bécard and Fortin, 1988; Smith and Read, 1997; Bago et al., 2000). It is well known that C is transferred from colonized plants to AM fungi (Ho and Trappe, 1973), whereas the plants in many cases receive most of their P through hyphal uptake and fungal transfer to the host root (Pearson and Jakobsen, 1993). Colonization by AM fungi increases the C sink strength of roots (Douds et al., 1988). The fungal C demand upon root colonization can constitute a significant cost to the host plant, as indicated by reduced growth at high P levels (Peng et al., 1993). This also implies that there is an important connection between external P supply and the regulation of C allocation to the fungal partner in the symbiosis.C metabolism of the AM fungus Glomus intraradices has been studied using 13 C-NMR in monoxenic cultures (Pfeffer et al., 1999). Although no hexose uptake occurs in the extraradical mycelium, intraradical AM fungal mycelium takes up C supplied as hexoses to the root. Triacylglycerols are synthesized from this C in the fungus and transported to the extraradical mycelium. These triacylglycerols are substantial sinks for C in the AM fungal mycelium (Bago et al., 2000; Olsson and Johansen, 2000).Formation of AM is important for P acquisition in most plants. The adverse effect of high soil P le...

The level of malate in the epidermis from illuminated leaves of Vicia faba was greater than in that from dark-treated leaves. A difference in the malate level was still detected after the epidermis had been treated by rolling so that only the guard cells remained alive. The results suggest that malate may accumulate in guard cells on illumination. In subsequent experiments, stomatal apertures were measured, and potassium as well as malate was analysed in extracts of epidermis. In illuminated leaves, the potassium content of rolled epidermis increased from about 90 to about 335 picoequivalents mm(-2) of epidermis whele malate increased from about zero to about 71 pmoles mm(-2) and the stomata opened; in dark-treated leaves, the potassium content of rolled epidermis decreased slightly, the malate level remained about zero, and the stomata showed very slight further closure. The measured increase in potassium is likely to represent an increase in potassium concentration in the guard cells of about 0.4 Eq l(-1) with stomatal opening; the increase in malate could correspond to 0.23 Eq l(-1) (with respect to potassium) in the guard cells. Thus, malate accumulating in guard cells could balance about half of the potassium taken up by guard cells when stomata open in the light.

The apoplastic permeability of Avicennia marina (Forsk.) Vierh. (grey mangrove) roots was investigated by examining the composition of xylem sap collected under pressure from root systems immersed in 25% seawater containing 0.01% trisodium 3-hydroxy-5,8,10-pyrenetrisulfonate (PTS), a mobile fluorescent dye which is confined to ihe apoplast. Xylem sap from intact root systems contained between 3 and 18% of the NaCl, and less than 0.2% of the PTS concentrations present in the external bathing medium. The low levels of PTS in the xylem sap indicate that apoplastic transport was minimal. By contrast, xylem sap from disturbed roots that had been removed from the sand in which they were growing and transferred to the PTS treatment solution contained an average of 57% of the NaCl and 12% of the PTS concentrations present in the external solution, demonstrating the importance of using undisturbed root systems for such studies. Using Cellufluor, an apoplastic fluorescent dye which binds to cellulose, an apoplastic barrier was located at the periderm of mature roots, and at the exodermis and endodermis of progressively younger regions of fine roots. Uranin, a symplastic tracer dye, was taken up only by cells of the distal 17 mm of fine roots, the region of the root where the periderm had not yet developed. These results suggest that the uptake of ions and water in intact roots of A. marina occurs mainly via the symplast of younger regions of the third-order and fourth-order roots.

Abaxial epidermal strips, containing guard cells as the only viable cells, were prepared from leaves of Vicia faba following a period in darkness, and floated, under CO2-free air, on 2 mM RbCl + 0.1 mM CaCl2 labeled with 'Rb4. Under white light (high pressure mercury vapor lamp), stomatal opening in these strips approached its maximum at less than 0.02 calorie per square centimeter per minute. Under light of different wavelengths, 20 nanometers apart, and at a low quantum flux density of 7 X 1014 quanta per square centimeter per second, Rb+ uptake and stomatal opening were activated only in the blue and long ultraviolet regions, with a peak at 420 to 460 nanometers. The action spectrum suggests that the underlying process is not photosynthesis. At higher quantum flux density (38 X 10's quanta per square centimeter per second), uptake and opening also responded to red (600-680 nanometers) and somewhat to green light, with a minimum at 540 to 560 nanometers, indicating a possible involvement of the photosynthetic process. This light-induced opening appeared not to be mediated by a lowering of CO2 concentration, since C02-free air was used in all treatments and controls. Stomatal opening paralleled Rb+ uptake in all cases. This constitutes further evidence for the potassium transport hypothesis of stomatal movement.In the abaxial surface of leaf discs under air of normal CO2 concentration, stomatal opening in white light approached its maximum at an intensity similar to that for epidermal strips. At both quantum flux densities, the action spectra for opening in leaf discs were very similar to those for epidermal strips. Thus, these light-linked processes for stomatal opening are likely to be the same in leaves as in epidermal strips.large increases in guard cell K+. On the other hand, closing in darkness is associated with a loss of K+ (11, 29). With stomata in isolated epidermal strips of Vicia faba, exogenous K-must be supplied to obtain substantial opening in response to light (7,10). Therefore, opening appears to be the result of K+ uptake, not the cause. In this role, K+ can be replaced by Rb', but other monovalent cations are ineffective (10) unless supplied in concentrations much higher than expected to occur in the leaf apoplast (10,33).The dependence of a light-driven process on wavelength can be indicative of the pigment and mechanism involved. Action spectra for stomata have been studied previously. To date, however, such data are scanty for opening without the complication introduced by the underlying mesophyll. Also, there are no data on the dependence on wavelength of K+ or Rb+ uptake associated with opening in light.Most earlier studies of the effects of wavelength on stomatal opening indicated that blue light was most effective and green least, with red in between (21). Since leaf pieces were often used, the light effect might have been indirect, mediated through the mesophyll. For There is considerable evidence that a mechanism underlying stomatal opening is the uptake of K+ by guard cell...