J.W.A. van Rheenen

Canopy structure and leaf nitrogen distribution in a stand of Lysimachia vulgaris L. as influenced by stand density

SummaryA hypothesis that a dense stand should develop a less uniform distribution of leaf nitrogen through the canopy than an open stand to increase total canopy photosynthesis was tested with experimentally established stands of Lysimachia vulgaris L. The effect of stand density on spatial variation of photon flux density, leaf nitrogen and specific leaf weight within the canopy was examined. Stand density had little effect on the value of the light extinction coefficient, but strongly affected the distribution of leaf nitrogen per unit area within a canopy. The open stand had more uniform distribution of leaf nitrogen than the dense stand. However, different light climates between stands explained only part of the variation of leaf nitrogen in the canopy. The specific leaf weight in the canopy increased with increasing relative photon flux density and with decreasing nitrogen concentration.

High levels of inter-ramet water translocation in two rhizomatous Carex species, as quantified by deuterium labelling.

We studied water trnaslocation between interconnected mother and daughter ramets in two rhizomatous Carex species, using a newly developed quantitative method based on deuterium tracing. Under homogeneous conditions, in which both ramets were subjected either to wet or dry soil, little water was exchanged between the ramets. When the ramet pair was exposed to a heterogeneous water supply, water translocation became unidirectional and strongly increased to a level at which 30–60% of the water acquired by the wet ramet was exported towards the dry ramet. The quantity of water translocated was unrelated to the difference in water potential between the ramets, but highly correlated to the difference in leaf area. In both species, the transpiration of the entire plant was similar under heterogeneous and homogeneous wet conditions. This was a direct result of an increase in water uptake by the wet ramet in response to the dry conditions experienced by the interconnected ramet. In C. hirta, the costs and benefits of integration in terms of ramet biomass paralleled the responses of water consumption. This species achieved a similar whole-plant biomass in heterogeneous and homogeneous wet treatments, and water translocation was equally effective in the acropetal and basipetal directions. In C. flacca, responses of biomass and water consumption did not match and, under some conditions, water translocation imposed costs rather than benefits to the plants of this species. It is concluded that enhanced resource acquisition by donor ramets may be of critical importance for the net benefits of physiological integration in clonal plants.

The interaction between water and nitrogen translocation in a rhizomatous sedge (Carex flacca)

Abstract In order to examine whether the translocation of water and nitrogen in clonal plants is interdependent, interramet translocation of these two resources was investigated in the greenhouse. Two-ramet systems of Carex flacca were imposed to different spatial patterns of water and nitrogen supply. The experimental design allowed to examine the effects of water heterogeneity on nitrogen sharing, and, vice versa, the effects of nitrogen heterogeneity on water sharing. Interramet translocation of both water and nitrogen was quantified by stable isotope labelling. If one of the ramets was deprived of water, nitrogen or both resources (parallel resource heterogeneity), resource translocation towards this ramet was markedly enhanced compared to a control treatment in which both ramets received ample water and ample nitrogen. Under these conditions, the amount of water or nitrogen translocated was not significantly affected by the pattern of heterogeneity of the other resource imposed on the two-ramet system. If one of the interconnected ramets was rooted in dry but nitrogen-rich soil and the other ramet was placed in nitrogen-deficient but well-watered soil (reciprocal resource heterogeneity), a significant amount of water was translocated towards the ramet in dry soil, while the low-N ramet hardly received any nitrogen. These results show that little nitrogen is translocated between ramets in a direction opposite to the transpiration stream within the rhizome. However, nitrogen may be translocated independently from water if both are transported in a similar direction within the clonal system. The effects of translocation on ramet performance (in terms of transpiration, nitrogen accumulation, and biomass) were assessed by comparing interconnected ramets with isolated (severed) ramets that were treated identically. Integration enhanced the performance of ramets deficient of one or both of the resources. In case of water translocation, the transpiration and growth of the water exporting (donor) ramets was similar to the transpiration and growth of their isolated counterparts. When nitrogen was heterogeneously supplied, however, nitrogen accumulation and growth of the donor ramet was reduced to the same extent as the performance of the nitrogen-deficient ramet was increased. Water translocation thus enhanced the performance of the whole plant, while nitrogen only reduced the differences in ramet performance within the plant. In the case of the reciprocal heterogeneity treatment, the benefits of translocation were strongly unidirectional towards the ramet in dry soil. The data for this treatment suggested that total nitrogen accumulation was enhanced by the acquisition of nitrogen from the dry pot as a result of “hydraulic lift” and water exudation in the dry soil. We conclude that nitrogen translocation in clonal plants, and the associated benefits in terms of resource utilization and growth, may strongly depend on the pattern of interramet water transport. The implications are discussed for studies of physiological integration in clonal plants and the patterns of interramet resource sharing in the field.

Androstenedione metabolism in the sea star Asterias rubens L. Studied in homogenates and intact tissue: Biosynthesis of the novel steroid fatty-acyl testosterone

1. 1. Androstenedione metabolism in ovary and pyloric caeca of Asterias rubens was studied in incubations both of homogenates and pieces of tissue under different experimental conditions. 2. 2. The major metabolites were testosterone and 5α-androstane-3,17-dione in homogenate incubations, and testosterone and a novel steroid conjugate: fatty-acyl testosterone in tissue incubations. 3. 3. The results of androstenedione incubations with homogenates were strongly dependent on the experimental conditions: addition of ATP to the incubation medium stimulated the production of testosterone, but did not affect testosterone esterification. 4. 4. Results were also dependent on the pretreatment of the animals. 5. 5. The possible physiological function of the fatty-acyl testosterone formation will be discussed.

On the Sterols of Some Ascidians

The lipid content of sea squirts is low, namely less than a half percent of the fresh weight. Lipids consist of about seventy percent of saponifiable lipids and of about twenty percent of non-saponifiable lipids. Both types of these lipids, including sterols, can be synthesized from acetate by these animals. Small amounts of C30 sterols were observed only in Microcosmus sulcatus and Halocynthia papillosa, the species with a low content of C27 sterols and a high content of C28 sterols. In addition these species contained considerable higher amounts of sterols with a double bond at the C22 position than Ciona intestinalis and Styela plicata did.

Effects of different experimental conditions on progesterone metabolism in the sea star Asterias rubens L.

Abstract 1. 1. Progesterone metabolism in ovary and pyloric caeca of Asterias rubens was studied in incubations of homogenates and pieces of tissue at different times of the year and under different experimental conditions. 2. 2. The major metabolites were 5α-pregnane-3,20-dione and 3β-hydroxy-5α-pregnan-20-one. 3. 3. The results of progesterone incubations with homogenates were strongly dependent on the experimental conditions used. 4. 4. Evidence was obtained that progesterone metabolism varies during the year and may be in relation to the reproductive cycle. 5. 5. The consequences of these findings with respect to the design of experiments to obtain insight into the steroid metabolic processes going on under natural conditions, are discussed.

Short-term accumulation of organic matter and nutrient contents in two dry sand ecosystems

Inland dunes in northwestern Europe support a number of dry vegetation types. These ecosystems are poor in nutrients and it has been suggested that accumulation of nutrients triggers succession in such systems. We studied the accumulation of organic matter and N and P over a 30 months period in two adjacent ecosystems, the Spergulo-Corynephoretum and the Genisto-Callunetum. Amounts of plant matter and soil organic matter significantly accumulated during the sampling period in the Genisto-Callunetum but not in the Spergulo-Corynephoretum. While nutrient concentrations of live and dead phytomass in the Spergulo-Corynephoretum were significantly higher than in the Genisto-Callunetum, total nutrient contents in the systems showed the opposite pattern. N and P concentrations in litter were relatively high compared with the other fractions of plant matter and the amount of N significantly increased in both ecosystems during the sampling period. Soil moisture contents showed a seasonal pattern. It was highest in the top soil layer and higher in the Genisto-Callunetum than in the Spergulo-Corynephoretum. The estimated annual increase of total N in these two ecosystems was consistent with rates of atmospheric N deposition (wet fall + dry fall) measured in comparable Dutch sites.

On the biosynthesis and composition of sterols and sterolesters in some sea anemones (Anthozoa)

Abstract 1. 1. The incorporation of radioactivity from either sodium acetate-1-14C or dl -mevalonate-2-14C into some classes of lipids in Metridium senile, Anemonia sulcata and Cerianthus membrananaceus is investigated. 2. 2. Lipid contents in these animals are relatively very high. Structural lipids make up a relatively small portion of these lipids, suggesting that part of the lipids is stored and functions as a reserve. This is also reflected in the composition of the neutral lipids. 3. 3. It is demonstrated that these animals utilize the injected precursor for the biosynthesis of lipids. The high incorporation of radioactivity from mevalonate into phospholipids is rather enigmatic. 4. 4. There was only a slight incorporation of radioactivity into the sterols and it is supported that likely sea anemones can synthesize sterols. 5. 5. Sterols with twenty-seven carbon atoms were predominant, cholesterol in all cases being the main sterol. 6. 6. The composition of free and esterified sterols of each species, but also the sterol compositions of the different species closely resembled each other.

On the origin of the sterols in the sea star Asterias rubens.

1. 1. It was concluded from the results of experiments in which Asterias rubens had been injected with either radioactive acetate, mevalonate, cholesterol or methionine that this animal can synthesize only C27-sterols. It is assumed that C26, C28 and C29-sterols are of a dietary origin. 2. 2. It was found that Asterias ia able to introduce a double bond at C22 in both cholesterol and Δ7-cholestenol. 3. 3. Strong evidence was obtained that Asterias can synthesize Δ5-sterols. The significance of this possibility is discussed.

Carbohydrate content and composition of asterosaponins from different organs of the sea star Asterias rubens: Relation to their haemolytic activity and implications for their biosynthesis

Abstract 1. 1. Asterosaponins were isolated from the gonads, the pyloric caeca, the stomach and the bodywall of female specimens of the sea star Asterias rubens . 2. 2. Thin-layer chromatography showed that the saponins of each organ consisted of a mixture. 3. 3. For each of the saponins the sugar and sulphate content (in nmoles) was determined. The ratio of these contents, indicating the number of sugar units per saponin molecule, was calculated. 4. 4. Statistical analysis showed that the sugar content in the saponins of the pyloric caeca was significantly lower than those in the saponins of the stomach, the bodywall and the ovaries, which in their turn were not significantly different from each other. 5. 5. The number of haemolytic units per μg SO 4 2− and per μg carbohydrate was determined for the various saponins. Haemolytic activity was roughly correlated to their sugar content, but also other aspects of the saponin structure are responsible for their haemolytic activity. 6. 6. Sugar composition was determined for each of the saponin mixtures. Each had its own specific composition, but in all cases quinovose was the main sugar. 7. 7. Mechanisms which could explain the probably organ-specific composition of the saponin mixtures are discussed.