Timing of larval release by Porites astreoides in the northern Florida Keys

 Annual and lunar patterns of larval release by the coral Porites astreoides in the Florida Keys were investigated. Corals were collected monthly and maintained in outdoor, flow-through systems to quantify nightly release of larvae. Planulae were released during a period from 10 days prior to the new moon through 11 days after the new moon, with a peak in release centered on the new moon. The average duration of larval release by a single colony was 9 nights. Although the correlation was low (r=0.292), there was a positive relationship between colony size and number of larvae released. The majority of larval release occurred in April and May, although smaller numbers of larvae were observed from June through as late as September. Larval release appeared to be correlated with the average water temperatures over the 30 days prior to the new moon, peaking when water temperatures were 24.5–27.5 °C. Accepted: 3 August 1998     

Assessing between-colony oxygen isotope variability in the coral Porites lobata at Clipperton Atoll

The eastern Pacific warm tongue is a region of stable and elevated sea surface temperature (SST) located just north of the equator in the eastern Pacific. This warm water mass is thought to influence the position of the Intertropical Convergence Zone (ITCZ) in the eastern Pacific and to directly influence climate in Central America. To assess the use of corals in the development of paleoclimatic reconstructions in this region, we have developed oxygen isotope (δ¹⁸O) time-series from multiple specimens of the massive coral Porites lobata collected at Clipperton Atoll (10°18′N, 109°13′ W). Six near-monthly δ¹⁸O records from different sized (age) colonies where produced for the interval 1986–1994, and three of these were extended back to 1969. All corals sampled were found to contain numerous fish-grazing skeletal scars (～0.5?mm deep scallop shaped hiatuses). Samples collected at 1?mm intervals showed anomalous ¹⁸O/¹⁶O in the area of a bite scar, with 2 to 2.5?mm sampling intervals (10–12/year) minimizing these effects. Our results show that the average δ¹⁸O disequilibrium offset (vital effect) from equilibrium seawater composition for individual corals can vary by up to 0.4‰. However all δ¹⁸O results suggest that the vital effect offset is constant over time. Similar “offsets” are observed in the tops of old (age=～100?y) and young (age=～10?y) colonies, further suggesting that the biologically mediated vital effect offset does not change as a Porites colony ages. A 6-coral average composite δ¹⁸O record was constructed from 1985–1994 and a 3-coral average δ¹⁸O_anomaly record was constructed from 1969–1994. Regression analysis between monthly SST and the 6 coral average δ¹⁸O records yields an r ² correlation of 0.54 (individual r ²-values ranged from 0.27–0.55). The 3 coral δ¹⁸O_anomaly average record has an even lower correlation to SST, with an r ² of 0.40. Potential causes of the only moderate correlation to SST are explored and we find that inferred salinity effects, fish grazing scars, and slight chronology imperfections have all contributed to a reduced correlation to SST. Nevertheless, all El Niño events in this time period appear to be recorded by coral skeletal δ¹⁸O.     

Water circulation in a fringing reef located in a monsoon area: Kabira Reef, Ishigaki Island, Southwest Japan

 Kabira Reef is a well-developed fringing reef situated in a monsoon area where the dominant wind direction changes seasonally: south in summer and north in winter. Circulation in this reef shows a marked wind influence. The circulation pattern under calm wind conditions is characterized by an inflow of ocean waters into the moat over the reef crest and an outflow through a prominent channel. Current vectors change according to wind conditions, and this pattern is weakened and strengthened under southern and northern wind conditions, respectively. We establish a simple model to explain these circulation patterns with two factors: wind and a fundamental circulation pattern under calm conditions. We estimate the ratios of the component of wind to that of the fundamental circulation. The ratios reach 3 and 10 in absolute values under southern and northern wind conditions, respectively. These results can be applied to water circulation throughout the year, with the southern wind-driven circulation dominant in the summer, and the northern wind-driven circulation dominant in the winter. While trade wind conditions often result in a constant circulation pattern, monsoonal wind conditions make the circulation pattern vary according to the seasons. Accepted: 12 September 1997     

The effects of tail loss on survival, growth, reproduction, and sex ratio of offspring in the lizard Uta stansburiana in the field

Tail autotomy is a defense against predators used by many lizard species but is associated with various costs, most of which have been measured only in the laboratory. We conducted a field experiment in which we induced tail autotomy to approximately half (58%) of a marked sample (n=326) of Uta stansburiana from western Texas in the fall, and left the other half with intact tails. The following spring we determined survival, measured growth, and brought females to the laboratory to allow them to oviposit their eggs, which we incubated until hatching. Based on past studies, we anticipated inferior survival, growth, and reproduction following tail autotomy. We also predicted that females with tail loss would be energetically compromised and would alter the sex ratio of their offspring toward more daughters (as predicted by the Trivers-Willard hypothesis). Tailless lizards experienced significantly reduced survivorship, but those that survived grew the same as their tailed counterparts. Tailed and tailless females produced clutches equivalent in number of eggs and total mass. Whereas tailed females showed a significant positive relationship between average egg mass and snout-vent length, tailless females did not. Contrary to our expectations, tailless females produced heavier hatchlings than tailed ones, and sex ratios of hatchlings were equivalent for tailed and tailless females. In this population, tail loss in subadults leads to an increased risk of death, but apparently does not impose an energetic handicap such that later growth and reproduction suffer. We suggest that because tailless females are faced with decreased reproductive value, they respond by growing as much and laying as many eggs of the same mass as tailed females, despite the fact that they are also regenerating the tail. In addition, they somehow produce larger hatchlings than tailed females. Nevertheless, tailless females probably end up with lower overall lifetime fitness than tailed females, and tail loss thus induces the conditional reproductive strategy ”make the best of a bad situation”. Because tailless females produce larger, not smaller, hatchlings, they do not produce more daughters as predicted; i.e., we found no evidence for the Trivers-Willard effect following tail autotomy. Received: 29 November 1998 / Accepted: 17 September 1999     

Multiple activities of a novel substance,dictyopyrone C isolated from Dictyostelium discoideum,in cellular growth and differentiation

Summary.  We report that a novel substance named dictyopyrone C (DPC) has remarkable effects on growth and differentiation of Dictyostelium discoideum Ax-2 cells, in a dose-dependent manner. In the presence of 3–15 μM DPC, differentiation of starving Ax-2 (clone MS) cells was greatly enhanced in submerged culture, when vegetative MS cells were harvested at the mid-late-exponential growth phase (>3 × 10⁶ cells per ml) and starved. In contrast, DPC above 30 μM markedly impaired the progression of differentiation including cell aggregation, most of starved cells being round after 3–4 h of DPC application and then lysed during further incubation. In the presence of 30 μM DPC however, MS cells that had been harvested at the early exponential growth phase (<5 × 10⁵ cells per ml) and starved became neither round nor lysed and exhibited rather enhanced differentiation. Essentially the same results were obtained in cultures of starved cells on nonnutrient agar. With respect to the DPC effect on MS cells growing in axenic medium, cell lysis and growth inhibition by DPC at concentrations higher than 15 μM were realized in the mid-late-exponential-growth-phase cells (>3 × 10⁶ cells per ml) but not in the early-exponential-growth-phase cells (<5 × 10⁵ cells per ml). Moreover, analysis using synchronized MS cells has demonstrated that the DPC effect changes in a cell-cycle-dependent manner. In contrast to such unique DPC actions, the pyrone ring of DPC had no effects on growth and differentiation within the range of 3–120 μM tested. These findings strongly suggested the importance of the combined structure of the pyrone ring and the linear carbon chain in revelation of the DPC activities. Received August 5, 2002; accepted November 11, 2002; published online April 8, 2003 RID="*" ID="*" Correspondence and reprints: Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Aoba, Sendai 980-8578, Japan. E-mail: ymaeda@mail.cc.tohoku.ac.jp     

Developmental quantitative genetics of growth in Populus

 Nursery growth and dry weight were analyzed for F₂ genotypes derived from Populus trichocarpa×P. deltoides that have been field tested with clonal replicates in three different environments. The correlations between nursery and plantation performance differed among the environments, with higher values at Boardman and Clatskanie (both planted with rooted cuttings) than Puyallup (planted with unrooted cuttings). At Puyallup, nursery height was more strongly associated with plantation growth than were nursery diameter and dry weight. Yet, this finding was not supported by QTL mapping. A single overdominant QTL on linkage group G affected the stem height of both seedlings and resprouts in the nursery but showed nonsignificant LOD scores for plantation height from ages 1 to 5 at Puyallup. A total of four QTLs were identified for nursery diameter, one of which on linkage group O also controlled plantation basal area at all ages. Two important nursery QTLs on linkage groups B and G were used to estimate the relative efficiency of marker-assisted selection for plantation productivity. Despite the fact that they were not detected in the plantation stage, these two QTLs could significantly increase the proportion of the phenotypic variance explained by plantation QTLs. Received: 5 February 1998 / Accepted: 1 April 1998     

Inbreeding in Pinus radiata. I. The effect of inbreeding on growth, survival and variance

 The effects of inbreeding on growth, survival and variance in a 12-year-old radiata pine trial were studied in five populations each inbred to one of five different levels: outcross (F=0), half-sib (F=0.125), full-sib (F=0.25), selfing (S₁, F=0.5), and two-generations of selfing (S₂, F=0.75). These five populations were derived from a founder population of eight clones. Inbreeding reduced diameter, growth, and survival but increased the variance for diameter. Inbreeding depression at F=0.125, 0.25, 0.5, and 0.75 was 5%, 6%, 15%, and 19% respectively for DBH; −3%, 1%, 7%, and 11% respectively, for survival. The standard deviation for diameter increased by 10%, 10%, 30%, and 25% respectively for F=0.125, 0.25, 0.5, and 0.75 and, similarly, the coefficient of variation increased by 17%, 16%, 53%, and 55% respectively. There were significant differences among the eight founder clones in their response to inbreeding. The best clone in the trial showed no inbreeding depression. Overall, inbreeding depression was found to be linearly related to the inbreeding coefficient F with no significant quadratic effects for any trait at any population level. However, two individual clones had a quadratic relationship with F for DBH and one clone had a similar relationship for survival. A significant correlation (r=0.96) between S₂ and the breeding values of founder clones was observed while the correlation (r=0.58) between S₁ and breeding values was insignificant. The low inbreeding depression in radiata pine relative to other conifers may indicate that historical purging of detrimental alleles through small geographic populations, a higher degree of population subdivision, and the relative high fecundity of inbred progenies has rendered radiata pine an ideal species to use inbreeding as a breeding tool. Received: 10 March 1998 / Accepted: 19 May 1998     

Importance of foraminifera for the formation and maintenance of a coral sand cay: Green Island, Australia

 CaCO₃ production by reef-building organisms on Green Island Reef in the Great Barrier Reef of Australia is estimated and compared with the contribution of benthic foraminifera to the sediment mass of the vegetated sand cay. Major constituents of the cay are benthic foraminifera (mainly Amphistegina lessonii, Baculogypsina sphaerulata, and Calcarina hispida), calcareous algae (Halimeda and coralline algae), hermatypic corals, and molluscs. Among these reef-building organisms, benthic foraminifera are the single most important contributor to the sediment mass of the island (ca. 30% of total sediments), although their production of CaCO₃ is smaller than other reef-building organisms. Water current measurements and sediment traps indicate that the velocity of the current around Green Island is suitable for transportation and deposition of foraminiferal tests. Abundant foraminifera presently live in association with algal turf on the shallow exposed reef flat, whose tests were accumulated by waves resulting in the formation and maintenance of the coral sand cay. Accepted: 30 June 1999     

Corals in a non-reef setting in the southern Arabian Gulf (Dubai, UAE): fauna and community structure in response to recurring mass mortality

 Reef coral communities in a non-reef setting on shallow, flat hardgrounds were quantitatively sampled in Dubai Emirate (UAE, Southern Arabian Gulf) before and after a coral mass mortality in 1996. The coral fauna consisted of 34 scleractinian species before and 27 after the event, which removed virtually all Acropora. No alcyonacea were recorded. Five community types were identified and characterized by the dominant species: (A) a sparse Porites lutea community in sandy areas, (B) a dense Acropora clathrata community in areas with little sand, (C) a faviid community in muddy areas, (D) a Siderastrea savignyana community in sandy areas, and (E) a Porites compressa community, which built a framework in sandy areas. These communities are comparable to those described from other areas of the Gulf, where a stable pattern of community differentiation appears to exist. The spatial distribution and dynamics of the coral communities appears to be strongly influenced by mass mortality events recurring every 15 to 20 y. A combination of extreme water temperatures and high sedimentation/turbidity appear to be the major cause of mortality. Accepted: 13 June 1998     

Effect of NaCl salinity on growth and mineral partitioning in Quercus robur L., a rhythmically growing species

 Sodium salt sensitivity of common oak (Quercus robur L.) was evaluated in hydroponic culture using INRA-Morizet solution. Addition of NaCl to the nutrient solution reduced only length and weight of roots and first flush stems. In contrast, the second flush was properly expanded even in the presence of 40 mM of NaCl in culture medium. Both leaf number and leaf area were not affected by increasing salt concentration in medium culture while this increase induced significant leaf damage especially in first flush leaves. Stem starch storage was reduced only at 40 mM NaCl treatment. Common oak seedlings seemed to be able to better compartmentalize sodium than chloride when the NaCl concentration increased in the medium culture. Chloride presented a lower uptake than sodium. Sodium was preferentially accumulated in roots and this accumulation occurred at the expense of potassium uptake. The decrease of ATP content in leaves of common oak seedlings submitted to NaCl treatments could indicate that it was used for sodium exclusion out of the leaves, especially in second flush leaves. Relationships between growth responses, starch and mineral element distribution in common oak seedlings will be discussed. Received: 20 November 1997 / Accepted: 3 March 1998     

Cytogenetic characteristics and the breeding system in six Hibiscus species

Cytogenetic characteristics confirm that Hibiscus acetosella and Hibiscus cannabinus are outbreeders, while Hibiscus asper, Hibiscus physaloides, Hibiscus sabdariffa and Hibiscus surattensis have evolved into inbreeders. The inbreeding species appear to have co-evolved a floral structure in which some anthers abut on the stigma prior to anthesis. Received: 19 February 1999 / Accepted: 17 June 1999     

Leaf structure and anatomy as related to leaf mass per area variation in seedlings of a wide range of woody plant species and types

The structural causes of variation in leaf mass per area, and of variations in leaf structure accounted for by leaf habit and life form, were explored in a set of laboratory-grown seedlings of 52 European woody species. The leaf traits analysed included density, thickness, saturated mass/dry mass, and leaf nitrogen per mass and per area. Other traits described the anatomy of leaves, most of them relating to the lamina (proportions of palisade and spongy parenchymata, epidermis, air space and sclerified tissues, expressed as volume per leaf area, and per-cell transversal areas of epidermis and parenchymata), and another referring to the mid rib (transversal section of sclerified tissues). Across the whole set of species leaf mass per area was correlated with leaf density but not with thickness, and this was confirmed by taxonomic relatedness tests. Denser leaves corresponded with greater proportion of sclerified tissues in the lamina, smaller cells and lower water and N contents, but no relation was found with the proportion of air space in the lamina. Taxonomic relatedness analysis statistically supported the negative association of leaf density with saturated to dry leaf mass ratio. Thicker leaves also exhibited greater volume per leaf area and greater individual cell area in each of the tissues, particularly parenchyma. Mean leaf mass per area and leaf thickness were lower in deciduous than in evergreen species, but no significant differences in leaf density, proportion of sclerified tissues in the lamina or cell area were found between the two groups. Leaf mass per area was higher in trees and subshrubs than in shrubs and climbers-plus-scramblers, this rank being equal for leaf density and proportion of sclerified tissues in the lamina, and reversed for cell area. Given the standardised environment and ontogenetic phase of the seedlings, we conclude that variation in leaf structure and anatomy among species and species groups has a strong genetic basis, and is already expressed early in the development of woody plants. From an ecological viewpoint, we can interpret greater leaf mass per area across this species set as greater allocation to support and defence functions, as shown predominantly by species from resource-poor environments. Received: 16 August 1999 / Accepted: 29 March 2000     

Effect of liana cutting on water potential and growth of adult<Emphasis Type="Italic"> Senna multijuga</Emphasis> (Caesalpinioideae) trees in a Bolivian tropical forest

Lianas, or woody climbing plants, are a major constituent of seasonally dry tropical forests, and are thought to impact negatively their host trees. In this study we evaluated whether liana presence was associated with reduced leaf water potentials and growth in adult Senna multijuga trees during the dry season in a lowland Bolivian forest. We used leaf water potentials in trees as a first approach to assess trees’ water status, under the assumption that leaf water potentials become more negative when water losses (via transpiration) exceed gains (by uptake). We measured relative growth in girth at 1.5 m height (gbh) to quantify tree growth. At the beginning of the 1996 dry season (early June), we selected 20 S. multijuga trees 10–20 cm dbh, and measured their gbh. We also recorded pre-dawn and mid-day leaf water potentials in these trees. In ten experimental trees all lianas were then cut, while the remaining trees were used as controls. Pre-dawn and mid-day water potentials were re-measured 1 day after liana-cutting, and then every week in all trees for 1 month and then at 3 and 5 months, until the beginning of the next rainy season (November); gbh was measured again in July 1997 to estimate relative growth rate. Liana removal was associated with less negative pre-dawn (–0.3 vs –0.4 MPa) and mid-day (–0.5 vs –0.7 MPa) water potentials in trees during the dry season. This difference appeared as early as 1 day after cutting, and disappeared once the rainy season began. Liana-cut trees grew more (0.4 mm/mm year) than liana-uncut trees (0.2 mm/mm year). These findings suggest that lianas may interfere with water availability to these trees during the dry season, and may also hinder tree growth. Received: 16 November 1999 / Accepted: 23 March 2000     

Effects of seasonal rainfall on radial growth in two tropical tree species

 Seasonal drought may limit primary productivity in most of the tropics, but the determinants of tree growth are not well known. A 10-year study of the deciduous trees Cochlospermum vitifolium (Willd.) Spreng. (Cochlospermaceae) and Cnidoscolus spinosus Lundell (Euphorbiaceae) in southwestern México showed radial growth to be highly correlated (both r>0.85) only with precipitation during an interval of <2 months in the mid-wet season. Growth was not affected by total annual precipitation or by an early starting or late ending of the wet season, or by heavy rainfall in the dry season. Annual mean girth increments ranged from 0.03 to 3.31 cm and −0.1 to 2.01 cm, respectively. The best model for growth (r ²>0.85) was a linear combination of mid-summer precipitation (positive coefficient) and total precipitation over the previous 2 years (negative coefficient). Comparison with other species showed heterogeneous responses of wood production to climate variation, and suggests that the range of functional types of dry forest trees is still unknown. Received: 9 September 1996 / Accepted: 4 March 1997     

Daily stem growth patterns in irrigated Eucalyptus globulus and E. nitens in relation to climate

Daily increments in stem radius were determined from hourly dendrometer measurements in each of three irrigated Eucalyptus nitens and E. globulus trees. Multiple regressions determined from daily weather variables accounted for 40–50% of the variance in increment. The use of weather variables lagged by 1–2 days increased the variance explained. The diurnal variation in stem radius was resolved into three mathematically defined phases: shrinkage, recovery and increment. The positive daily net increment in stem radius, by definition, occurred in the increment phase. Average weather conditions during this phase (predominantly night-time) did not explain any more variance in increment than the average daily conditions, determined over a 24 h period. Daily increment was resolved into a rate of stem radius increase during the increment phase and the duration (hours) of that phase. Significant species by month interactions were evident with growth in summer characterised by faster rates of stem expansion over shorter time periods within each diurnal cycle. E. nitens tended to have longer increment phases in spring and autumn, and faster phase rates in autumn than E. globulus. Interactions between weather variables and cambial growth were complicated and varied over the year. The correlation between temperature and stem growth varied from positive in spring to zero or negative during summer. The data indicate a need to understand weather-by- climate interactions at the level of whole tree physiology in order to fully understand the effect of weather on cambial activity and therefore stem increment and wood properties. Received: 12 April 1999 / Accepted: 6 July 1999     

The effect of Tiron, a water soluble radical scavenger, on growth, morphology and alkaloid content of adventitious roots in Atropa belladonna

 The effect of Tiron (disodium 1,2-dihydroxybenzene-3,5-disulfonate) on the growth, morphology and alkaloid content of adventitious roots in Atropa belladonna was investigated. High concentrations of Tiron had an inhibitory effect on growth of the root. The appearance of cultured roots was significantly changed from rough roots accompanied with callus-like tissue in control cultures to fine roots without callus formation. Alkaloid content was drastically increased by the addition of 1 mM Tiron to the medium. The influence of NAA, which has an inhibitory effect on alkaloid production, was partially restored by Tiron treatment, indicating that this radical scavenger may affect the production of alkaloids through modulation of the mode of action of auxin. Glutathione content of the root was not influenced by Tiron. Received: 3 June 1999 / Revision received: 28 September 1999 / Accepted: 30 September 1999     

Inter-decadal patterns of population and dietary change in sea otters at Amchitka Island, Alaska

After having been hunted to near-extinction in the Pacific maritime fur trade, the sea otter population at Amchitka Island, Alaska increased from very low numbers in the early 1900s to near equilibrium density by the 1940s. The population persisted at or near equilibrium through the 1980s, but declined sharply in the 1990s in apparent response to increased killer whale predation. Sea otter diet and foraging behavior were studied at Amchitka from August 1992 to March 1994 and the data compared with similar information obtained during several earlier periods. In contrast with dietary patterns in the 1960s and 1970s, when the sea otter population was at or near equilibrium density and kelp-forest fishes were the dietary mainstay, these fishes were rarely eaten in the 1990s. Benthic invertebrates, particularly sea urchins, dominated the otter’s diet from early summer to mid-winter, then decreased in importance during late winter and spring when numerous Pacific smooth lumpsuckers (a large and easily captured oceanic fish) were eaten. The occurrence of spawning lumpsuckers in coastal waters apparently is episodic on a scale of years to decades. The otters’ recent dietary shift away from kelp-forest fishes is probably a response to the increased availability of lumpsuckers and sea urchins (both high-preference prey). Additionally, increased urchin densities have reduced kelp beds, thus further reducing the availability of kelp-forest fishes. Our findings suggest that dietary patterns reflect changes in population status and show how an ecosystem normally under top-down control and limited by coastal zone processes can be significantly perturbed by exogenous events. Received: 23 March 1999 / Accepted: 19 January 2000     

God Is Nothing but Talk: Modernity, Language, and Prayer in a Papua New Guinea Society

This article brings together theories of local modernity and of linguistic ideology to analyze the way the Urapmin of Papua New Guinea have encountered modern linguistic ideology through their Christianization. Against the prevailing anthropological focus on the indigenization of modernity, this article argues the importance of attending to cases in which people grasp the content of modernity on its own terms. Studying this kind of local modernity allows us to model an important kind of contemporary cultural change and discover neglected aspects of modernity as refracted through the experiences of people new to it. Here, an analysis of the Urapmin encounter with modern linguistic ideology reveals that ideology's rootedness in a model that ties meaning to intention and truthfulness and favors the speaker over the listener in the construction of meaning. It is suggested that an awareness of the biases of this ideology can open up new topics in linguistic anthropology. [ modernity, linguistic ideology, religion, Christianity, Melanesia ]     

Decrease of phosphoribulokinase activity by antisense RNA in transgenic tobacco: definition of the light environment under which phosphoribulokinase is not in large excess

 To test the hypothesis that the contribution of phosphoribulokinase (PRK) to the control of photosynthesis changes depending on the light environment of the plant, the response of transgenic tobacco (Nicotiana tabacum L.) transformed with antisense PRK constructs to irradiance was determined. In plants grown under low irradiance (330 μmol m⁻² s⁻¹) steady-state photosynthesis was limited in plants with decreased PRK activity upon exposure to higher irradiance, with a control coefficient of PRK for CO₂ assimilation of 0.25 at and above 800 μmol m⁻² s⁻¹. The flux control coefficient of PRK for steady-state CO₂ assimilation was zero, however, at all irradiances in plant material grown at 800 μmol m⁻² s⁻¹ and in plants grown in a glasshouse during mid-summer (alternating shade and sun 300–1600 μmol m⁻² s⁻¹). To explain these differences between plants grown under low and high irradiances, Calvin cycle enzyme activities and metabolite content were determined. Activities of PRK and other non-equilibrium Calvin cycle enzymes fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase and ribulose-1,5-bisphosphate carboxylase-oxygenase were twofold higher in plants grown at 800 μmol m⁻² s⁻¹ or in the glasshouse than in plants grown at 330 μmol m⁻² s⁻¹. Activities of equilibrium enzymes transketolase, aldolase, ribulose-5-phosphate epimerase and isomerase were very similar under all growth irradiances. The flux control coefficient of 0.25 in plants grown at 330 μmol m⁻² s⁻¹ can be explained because low ribulose-5-phosphate content in combination with low PRK activity limits the synthesis of ribulose-1,5-bisphosphate. This limitation is overcome in high-light-grown plants because of the large relative increase in activities of sedoheptulose-1,7-bisphosphatase and fructose-1,6-bisphosphatase under these conditions, which facilitates the synthesis of larger amounts of ribulose-5-phosphate. This potential limitation will have maintained evolutionary selection pressure for high concentrations of PRK within the chloroplast. Received: 15 November 1999 / Accepted: 27 January 2000