A SEASONAL COMPARISON OF CARBON,NITROGEN, AND PIGMENT CONTENT IN LAMINARIA SOLIDUNGULA AND L. SACCHARINA (PHAEOPHYTA) IN THE ALASKAN ARCTIC1

Laminaria solidungula and L. saccharina inhabit the Beaufort Sea in the Alaskan High Arctic. Laminaria solidungula is an Arctic endemic, whereas L. saccharina extends from north temperate Pacific and Atlantic waters to the Arctic. Previous studies have shown that the two species have different seasonal timing of growth, but little comparative physiological information exists. As a first step in characterizing these two species from a mixed Arctic population, we measured variations in carbon, nitrogen, and photosynthetic pigment content in blade tissue from plants collected under the fast ice in April and during the open water Period in late July, Both species exhibited seasonal differences in many measured variables; seasonal differences in L. solidungula were most pronounced in growing basal blades. For example, the molar CIN ratio of basal blades averaged 11 in April and 21 in July for L. solidungula and 11.5 in April and 28 in July for L. saccharina. Basal and mature second blades differed in pigment content in April but not in July: chlorophyll a + c in L. solidungula basal and mature second blades averaged 19 and 27 nmol.cm^?2 in April and 30 and 29 nmol. cm^?2 in July, respectively. The corresponding values for L. saccharina were 17 and 29 nmol.cm^?2 in April and 16 and 16 nmol.cm^?2 in July (95% confidence intervals approximately 1–3 nmol. cm ^?2). Carotenoids exhibited similar patterns. Species differences in pigments, carbon, and nitrogen were minor and were probably effects rather than causes of the different seasonal patterns of growth and development. The primary difference between the two species may be the ability of L. solidungula to retain multiple metabolically active blades and to fuel areal growth with stored carbohydrates during winter near-darkness, whereas L. saccharina growth is more closely tied to active photosynthesis in the growing basal blade. The cause of old blade retention in L. solidungula and the possibility of other physiological differences between the two species, including gametophytes, remain to be determined.     

A SPECTROPHOTOMETRIC ASSAY FOR RUBISCO ACTIVITY: APPLICATION TO THE KELP LAMINARIA SACCHARINA AND IMPLICATIONS FOR RADIOMETRIC ASSAYS1

We optimized a spectrophotometric method for measuring ribulose-l,5-bisphosphate carboxylase oxygenase (Rubisco) activity in crude extracts of the kelp Laminaria saccharina Lamour. Activity exhibited a decline during the assays, such that rates determined over a 10-min period were only 70% of activity during the first minute. Activity was significantly enhanced by addition of HCO₃^? to the extraction buffer and was stable for at least 4 h. Highest activities were determined using small extract loads (1–2.5 mg fresh weight·mL^?1 of assay mixture); extract loads of 10 mg·mL^?1caused a 25% decrease in activity. Rubisco activities determined for individual kelp plants using the spectrophotometric method were significantly correlated with total protein and gross photo synthetic capacity (P_max) and were 1.5–2 times higher than P_max. Low Rubisco activities relative to photosynthetic rates reported in many previous studies of macroalgae and microalgae may have resulted at least partly from underestimation of enzyme activity by radiometric assays using relatively large volumes of un-activated crude extract and assay periods longer than 1 min.     

EFFECTS OF EXTERNAL INORGANIC NITROGEN CONCENTRATION ON METABOLISM,GROWTH AND ACTIVITIES OF KEY CARBON AND NITROGEN ASSIMILATORY ENZYMES OF LAMINARIA SACCHARINA (PHAEOPHYCEAE) IN CULTURE1

The growth rate of Laminaria saccharina (L.) Lamour. is dependent on inorganic nitrogen in culture. Growth rates were saturated between 5 and 10 μmol · L^?1 nitrate. The activities of ribulose-1,5 bisphosphate carboxylase, phosphoenolpyruvate carboxykinase, mannitol-1-phosphate dehydrogenase, nitrate reductase and glutamine synthetase also varied with the concentration of inorganic nitrogen in the medium. All enzyme activities were lowest at 2.5 μmol · L^?1 nitrate (the lowest concentration used) increasing to a maximum activity between 10 and 30 μmol · L^?1 nitrate. Most enzyme activities followed a hyperbolic curve resembling those described by the Michaelis-Menten equation, with different half-saturation constants.     

EFFECTS OF NITRATE CONCENTRATION ON THE GROWTH AND PHYSIOLOGY OF LAMINARIA SACCHARINA (PHAEOPHYTA) IN CULTURE1,2

Laminaria saccharina Lamour. sporophytes were grown in enriched and synthetic media through a range of nitrate concentrations, There was an approximately linear relationship between growth and nutrient concentration up to 10 μ substrate concentration. The half-saturation constant (K_{2) was ca. 1.4 μ NO3-. The internal levels of NO3- increased at substrate concentrations above 10 μM b>3- and reached levels several thousand times higher than the surrounding medium. Thus there is evidence for luxury consumption of NOsb>3}^-. The chlorophyll content and photosynthetic capacities of plants also increased with increasing external NO₃^- The ecological implications of this work are considered.     

RESPONSE OF LAMINARIA SACCHARINA (PHAEOPHYTA) GROWTH AND PHOTOSYNTHESIS TO SIMULTANEOUS ULTRAVIOLET RADIATION AND NITROGEN LIMITATION1

The brown macroalga Laminaria saccharina (L.) J. V. Lamour. was grown in large outdoor tanks at 50% ambient solar radiation for 3–4 weeks in July and August of 2000, 2001, and 2002, in either ambient or nitrogen (N)–enriched seawater and in either ambient light [PAR + ultraviolet radiation (UVR)] or ambient light minus UVR. Growth, N‐content, photosynthetic pigments, and RUBISCO content increased in N‐enriched seawater, indicating N‐limitation. UVR inhibited growth, but this inhibition was ameliorated by N‐enrichment. The response of growth to UVR could not be explained by changes in respiration and photosynthesis. Gross light‐saturated photosynthesis (P_max) remained unaffected by UVR but was significantly higher under N‐enrichment, as was dark respiration (R_d). UVR had no effect on pigments or N content. However, RUBISCO contents were low in the presence of UVR, reflecting the overall change in soluble cellular protein. Overall, our data indicate that the response to UVR in L. saccharina depends on other environmental factors, such as N, and these effects need to be considered when evaluating the response of macroalgae to increased UVR.     

BLUE LIGHT PHOTOREACTIVATION IN ULTRAVIOLET-IRRADIATED YOUNG SPOROPHYTES OF ALARIA ESCULENTA AND LAMINARIA SACCHARINA (PHAEOPHYTA)1

Exposure of ultraviolet (UV)-irradiated young sporophytes of Alaria esculenta (L.) Grev. and Laminaria saccharina (L.) Lamour. to visible light resulted in recovery from UV damage that would otherwise cause much higher mortality. For this photoreactivation, blue light was highly effective, whereas negligible reactivation was produced in green or red light. The blue quantum requirements for a 50% response were 1.9 mol·m^?2 in A. esculenta and 1.2 mol·m^?2 in L. saccharina, which were comparable to those reported for other blue light responses requiring high energy found in brown algae.     

ADAPTATION OF PHOTOSYNTHESIS IN LAMINARIA SACCHARINA (PHAEOPHYTA) TO CHANGES IN GROWTH TEMPERATURE1

The effect of growth temperature on photosynthetic metabolism was studied in the kelp Laminaria saccharina (L.) Lamour. Photosynthesis was subject to phenotypic adaptation, with almost constant photosynthetic rates being achieved at growth temperatures between 0 and 20° C. This response involved: (1) an inverse relationship between growth temperature and photosynthetic capacity, (2) a reduction in the Q₁₀ value for photosynthesis of L. saccharina grown at 0 and 5° C compared with 10, 15 and 20° C grown sporophytes, and (3) an acquired tolerance of photosynthesis to temperatures between 15–25° C (which inhibited photosynthesis in 0 and 5° C grown L. saccharina) in sporophytes grown at 10, 15 and 20° C. The physiological basis of these adaptations is discussed in terms of observed changes in activities and kinetics of the Calvin cycle enzyme ribulose-1, 5-bisphosphate carboxylase (oxygenase) and efficiency of light harvesting-electron transport systems.     

ON THE TAXONOMY OF CALIFORNIA LAMINARIA (PHAEOPHYTA)1

It is suggested that Laminaria setchellii Silva be retained as a species separate from L. dentigera Kjellman as there is no evidence suggesting they are conspecific. Further, L. saccharina (L.) Lamouroux and L. groenlandica Rosenvinge should also continue to be recognized as valid components of the California flora.     

TRANSLOCATION OF IODINE IN LAMINARIA SACCHARINA(PHAEOPHYTA)1

Long-distance translocation of ¹²⁵I in Laminaria saccharina (L.) Lamour. followed a “source to sink” pattern. When the source of ¹²⁵I was placed on the distal mature part of the blade, the translocation was unidirectional, basipetal and directed towards the meristematic region at the blade-stipe junction. When the source was placed directly at the meristem there was no movement of label distal to the meristem. The velocity of¹²⁵I transport ranged from 2 to 3.5 cm · h^?1. The anion I^? seemed to be the only species of¹²⁵I transported. An assay of iodine content in different parts of L. saccharina plant showed much higher levels of iodine in the meristem, stipe and holdfast than in the blade. This distribution concurs well with the pattern of I^? translocation.     

PHOTOINHIBITION AND RECOVERY AFTER HIGH LIGHT STRESS IN DIFFERENT DEVELOPMENTAL AND LIFE-HISTORY STAGES OF LAMINARIA SACCHARINA (PHAEOPHYTA)1,2

The capacity to cope with high light stress was investigated in different life-history and developmental stages of Laminaria saccharina Lamour. sporophytes and gametophytes. Changes in photosynthetic efficiency and in the level of photoinhibition were measured by in vivo fluorescence changes of photosystem II. Pigment content was studied using high performance liquid chromatography. Additionally, the morphology of the various developmental stages during the life cycle was studied by light microscopy in relation to the photosynthetic parameters. High light stress (2 h, 500 μmol.m-².s^?1) induced photoinhibition of photosynthesis with fast kinetics in older sporophytes and gametophytes. In contrast, the absolute degree of photoinhibition after light stress was higher in younger than in older sporophytes. Photosynthesis recovered faster in older sporophytes and gametophytes compared to young sporophytes. In very young sporophytes, photosynthesis did not recover fully even after 12 h exposure to low light, indicating severe photodamage. Kinetics of recovery in old sporophytes and in gametophytes showed a fast and a slow phase, whereas younger sporophytes recovered only with a slow phase, The fast phase is indicative of a decline of the photoprotective process, whereas the slow phase indicates a recovery from photodamage. The capacity to cope with high light stress in Laminaria sporophytes increased with increasing age of the thalli. The gametophytes are less sensitive to high light stress and may be selected to endure unfavorable white light conditions. Investigation of the xanthophylls showed that the higher resistance to high light is not caused solely by a higher content of xanthophyll cycle pigments. Additionally, changes in the thallus structure during the development of the sporophytes seemed to cause a higher resistance to high light. The observed changes in the ability to cope with high light in the different life-history and developmental stages of Laminaria saccharina may influence the distribution of the species on the shore.     

TRANSIENT PHOTOSYNTHETIC RESPONSES OF NITROGEN-DEPRIVED PETALONIA FASCIA AND LAMINARIA SACCHARINA (PHAEOPHYTA) TO AMMONIUM RESUPPLY1

Petalonia fascia (Müller) Küntze and Laminaria saccharina (L.) Lamouroux cultured in N-free medium exhibited transient depressions in light-limited and light-saturated gross O₂ evolution within 0.3 h following ammonium resupply. Recovery of photosynthesis to pre-resupply rates required 24 h. After 6-9 d of ammonium resupply, photo synthetic rates of N-deprived algae were similar to those of control plants grown with nitrogen. This study demonstrates that N-deprived macroalgae exhibit suppressed photosynthesis upon nitrogen resupply similar to many microalgal species. The apparent slowness of short-term recovery from nitrogen-elicited photo synthetic suppression may have ecological consequences for macroalgae under conditions of patchy nutrient supply.     

EFFECTS OF ULTRAVIOLET RADIATION ON PHOTOSYNTHESIS IN THE SUBTROPICAL MARINE DIATOM,CHAETOCEROS GRACILIS (BACILLARIOPHYCEAE)1

Acclimation to ambient ultraviolet radiation (UVR) was examined in a subtropical marine diatom, Chaetoceros gracilis Schutt. Short-term exposure to UVR (<24 h) reduced the efficiency of photosynthetic energy conversion, carbon fixation, activity of 1,5-bisphosphate carboxylase-loxygenase (RUBISCO), and the rapid turnover of the putative Dl reaction center (32 kda) protein, whereas longer-term exposure to ambient UVR (24–48 h) revealed a steady-state acclimation, defined as recovery of carbon fixation and RUBISCO activity to rates equivalent to treatments without exposure to UVR. The turnover of D1 and chlorophyll a (Chl a) remained high during exposure to UVR. Efficiency of energy conversion by photosystem II, measured with double flash (pump and probe) fluorometry, increased by 24% in cells acclimated to UVR. Acclimation to UVR had no detectable effect on the functional absorption cross-section or cellular concentrations of Chl a, Chl c, or total carotenoids. However, the maximum rate of carbon fixation was reduced by UVR on a Chl a basis but remained unaffected on a per-cell basis. Response to UVR exposure in this subtropical diatom has two components: a short-term inhibitory response and a longer-term acclimation process that ameliorates the inhibition of carbon fixation.     

BIOLOGICAL WEIGHTING FUNCTIONS FOR UV INHIBITION OF PHOTOSYNTHESIS IN THE KELP LAMINARIA HYPERBOREA (PHAEOPHYCEAE)1

Different wavelengths of sunlight either drive or inhibit macroalgal production. Ultraviolet radiation (UVR) effectively disrupts photosynthesis, but since UVR is rapidly absorbed in coastal waters, macroalgal photoinhibition and tolerance to UVR depend on the depth of attachment and acclimation state of the individual. The inhibition response to UVR is quantified with a biological weighting function (BWF), a spectrum of empirically derived weights that link irradiance at a specific wavelength to overall biological effect. We determined BWFs for shallow (0 m, mean low water [MLW]) and deep (10 m) Laminaria hyperborea (Gunnerus) Foslie collected off the island of Finnøy, Norway. For each replicate sporophyte, we concurrently measured both O₂ evolution and ¹³C uptake in 48 different light treatments, which varied in UV spectral composition and irradiance. The relative shape of the kelp BWF was most similar to that of a land plant, and the absolute spectral weightings and sensitivity were typically less than phytoplankton, particularly in the ultraviolet radiation A (UVA) region. Differences in BWFs between O₂ and ¹³C photosynthesis and between shallow (high light) and deep (low light) kelp were also most significant in the UVA. Because of its greater contribution to total incident irradiance, UVA was more important to daily loss of production in kelp than ultraviolet radiation B (UVB). Photosynthetic quotient (PQ) also decreased with increased UVR stress, and the magnitude of PQ decline was greater in deepwater kelp. Significantly, BWFs assist in the comparison of biological responses to experimental light sources versus in situ sunlight and are critical to quantifying kelp production in a changing irradiance environment.     

REPRODUCTIVE PHENOLOGY OF LAMINARIA SACCHARINA (L.) LAMOUR. (PHAEOPHYTA) AT THE SOUTHERN LIMIT OF ITS DISTRIBUTION IN THE NOTHWESTERN ATLANTIC OCEAN1,2

Laminaria saccharina (L) Lamour. Sporophytes were monitored monthly from October 1982 to September 1983 to investigate reproduction phenology and relationshiops to growth paatterns aaat its southern limit of distribution in the northwest Atlantic Ocean (Long Island Soundd). Plants exhibited an annual growth pattern. Growth raate, bladelehgth, maximum width, area, stiipe lehgth an wet weight swhoed the same seasonal pattern and reached maximum values between May and June. Only blade thickness continued to increase tthrooughout the ovservation period. Blade dissintegration occurred dduring August and September. Reproductive sporophytes occurred throughout the observation period; the greatest frequency of appearance occurred in October (43.8%) and June (37.8%). The blade area covered by sori ranged from 2.4% (Janaury) to 6.1% (August). Meiospore release under laboratory conditions was maximum in May and minimum in July. No meio-spores were released in August Sporulation was not correlated with meristematic growth of nitrogen content How ever, reproductive plants were generally larger and thicker throughout the sporulation period, and had a greater carbort content is spring than nonreproductive plants. Fecundity and reproductive success of female gametophytes were maximum in spring and minimum in winter. The growth of early sporophyte stages in the laboratory was greatest in early spring; however, juvenile macroscopic stages were hardly observed in the field during summer months due to warm water temperatures. “Over-summering” of gamelophytes and / or microscopic sporophytes may account for the annual cycle of Laminaria at its southern limit of distribution.     

THE EFFECT OF IRON NUTRITION ON PHOTOSYNTHESIS AND NITROGEN FIXATION IN CULTURES OF TRICHODESMIUM (CYANOPHYCEAE)1

Cultures of Trichodesmium NIBB 1067 were grown in the synthetic medium AQUIL with a range of iron added from none to 5 × 10^?7 M Fe for 15 days. Chlorophyll-a, cell counts, and total cell volume were two or three times higher in medium with 10^?7 M Fe than with no added Fe. Oxygen production rate per chlorophyll-a was over 60% higher with higher iron. Increased iron stimulated photosynthesis at all irradiances from about 12–250 μE · m^?2· s^?1. Nitrogen fixation rate, estimated from acetylene reduction, for 10^?7 and 10^?8 M Fe cultures was approximately twice that of the cultures with no added Fe. The range of rates of O₂ production and N₂ fixation in cultures at the iron concentrations we used were similar to the rates from natural samples of Trichodesmium from both the Atlantic, and the Pacific oceans. This similarity may allow this clone to be used, with some caution, for future physiological ecology studies. This study demonstrates the importance of iron to photosynthesis and nitrogen fixation and suggests that Trichodesmium plays a central role in the biogeochemical cycles of iron, carbon and nitrogen.     

PHOTOSYNTHETIC RESPONSE OF THE GIANT KELP MACROCYSTIS PYRIFERA (PHAEOPHYCEAE) TO ULTRAVIOLET RADIATION1

Solar ultraviolet radiation (UVA + UVB) impairs photosynthesis in marine algae. Canopy blades of the giant kelp Macrocystis pyrifera (L.) C. Agardh are exposed to high levels of solar UV in the field. To determine the effects of UV radiation on photosynthesis in the giant kelp and to identify sites of UV damage, O₂ evolution, reaction center organization, light harvesting, and energy transfer efficiency were measured in canopy blades that had been exposed to elevated levels of UV in the laboratory. UV treatment reduced both the light-saturated rate and the light-limited rate of photosynthesis by 50% but produced no significant change in the rate of dark respiration. A significant impairment of photosystem II (PSII) reaction center function was observed, suggesting that PSII is a major site of damage in chromophytes. Reduced quantum efficiency of photosynthesis and loss of energy transfer from light-harvesting pigments (fucoxanthin, chlorophyll a, and chlorophyll c) to PSII indicate that the major light-harvesting complex of M. pyrifera, the fucoxanthin-chlorophyll protein complex (FCPC), was another site of UV damage. These measures provide the first evidence of a direct effect of UV radiation on specific sites in the photosynthetic apparatus of chromophytes and indicate that in situ fluorescence excitation analysis may be a simple means to detect UV stress in algae.     

CIRCANNUAL GROWTH RHYTHM AND PHOTOPERIODIC SORUS INDUCTION IN THE KELP LAMINARIA SETCHELLII (PHAEOPHYTA)1

Growth and reproduction of laboratory-grown sporophytes of Laminaria setchellii Silva were investigated in a tank system with controlled conditions of daylength, temperature, and nutrients (N and P). A circannual growth rhythm of the frond was detected under constant laboratory conditions. In continuous long-day and night-break conditions the period τ of the free-running rhythm varied between 11.3 and 17.3 months; in short-day conditions the frond grew indefinitely. The growth rhythm of individual plants could be synchronized by a simulated annual cycle of day-length with a period of T = 12 months. The four seasons of the year were simultaneously simulated by phase shifting the annual cycle of daylength by 3, 6, or 9 months in three out of four tanks. The annual growth cycle followed these phase shifts, and initiation of the new blade always started just after the winter daylength minimum. The formation of sori was induced by a genuine photoperiodic short-day reaction in 1- to 2-year-old plants. Sori became, visible 9–14 weeks after transfer of individual plants from long-day to short-day conditions, whereas plants cultured in continuous long-day or night-break conditions remained sterile. Sporophytes with or without blades were able to continue growth or produce new blades in continuous darkness.     

SEASONAL LIGHT AND TEMPERATURE INTERACTION EFFECTS ON DEVELOPMENT OF LAMINARIA SACCHARINA (PHAEOPHYTA) GAMETOPHYTES AND JUVENILE SPOROPHYTES1

In previous studies, Laminaria saccharina L. (Lamour.) sporophytes were found to exhibit two major peaks of sporogenesis and an annual life cycle in Long Island Sound, New York. Young sporophytes were observed shortly after the sporogenesis peaks in early autumn and spring, but most of the mature sporophytes decayed during summer. A new study was conducted to determine if the spring sporogenesis activity contributed to the recruitment observed in autumn through oversummering of gametophytic and juvenile sporophytic stages, as previously suggested. Reproduction and growth in gametophytes and growth in juvenile sporophytes were studied under crossed gradients of light and temperature. Periodic outplantings of substrata seeded with gametophytic and sporophytic stages to the field were conducted to assess actual survival. The optimum temperature and light conditions for gametophyte development, growth and reproduction varied with the time of year meiospores were obtained. Most of this variation was attributable to temperature. A seasonal adaptation to temperature in most developmental stages was observed. Higher temperatures resulted in greater numbers of male gametophytes. Gametophytic stages could develop at all times, suggesting that oversummering in this stage was possible. Juvenile sporophytes had a narrower optimum temperature range and again photon fluence rate contributed little to observed variances. Out planting of sporophytic stages at various times during the year indicated only sporophytes prepared from autumn and winter could survive summer conditions. The thalli of these plants grew rapidly in spring and eroded back to the meristematic region in summer. Most of these plants then quickly became reproductive, resulting in another autumn sporogenesis peak. Gametophytic and sporophytic outplantings prepared from spring meiospores did not survive the summer. Thus, the recruitment observed in autumn can only be the result of the previous autumn's sporogenesis activity. The sporogenous activities of spring and early summer appear to be unimportant, despite the fact that all reproductive indices are superior at those times.     

A SCAR MOLECULAR MARKER SPECIFICALLY RELATED TO THE FEMALE GAMETOPHYTES OF SACCHARINA (LAMINARIA) JAPONICA (PHAEOPHYTA)1

PCR amplification was employed to identify female or male gametophyte associated markers in Saccharina japonica (Aresch.) C. E. Lane, C. Mayes et G. W. Saunders (=Laminaria japonica Aresch.). One pair of the primers, P5, was screened from five pairs designed based on a specific sequence (GenBank accession no. AB069714 ) of Marchantia polymorpha Y chromosome, resulting in a differential band ～500 bp in size between female and male gametophytes of Rongfu strain of S. japonica. According to the SCAR (sequence‐characterized amplified regions) strategies, one pair of primers, P51, was designed on the basis of the sequence of this band that was only present in female gametophytes. A SCAR marker, designated FRML‐494 (494‐bp Female‐Related Marker of S. japonica, GenBank accession no. EU931619 ), was developed successfully by PCR amplification using the designed P51 primer pair. The SCAR marker was verified to be present only in female gametophytes of another variety 901 of this kelp that was a hybrid between S. japonica as paternal and S. longissima (Miyabe) C. E. Lane, C. Mayes, Druehl et G. W. Saunders (=Laminaria longissima Miyabe) as maternal, suggesting that the FRML‐494 marker was specifically related to female gametophytes of the genus. This marker is the first molecular tool reported for sex identification in kelps. This study was beneficial for identifying gametophyte gender during vegetative growth and for judging whether the monogenetic sporophytes came from exclusive male or female gametophytes, as well as for further research on sex determination at the molecular level in kelps.     

EVIDENCE FOR INHIBITORS OF SPORANGIUM FORMATION IN LAMINARIA DIGITATA (PHAEOPHYCEAE) DURING THE SEASON OF RAPID GROWTH

The pathway from the intercalary frond meristem to distal frond portions was blocked in experimental sporophytes of Laminaria digitata (Huds.) Lamour. in January by cutting 2 or 3 holes (15–25 mm diameter) 5 cm from the base of the frond. This procedure resulted in the formation of sori adjacent to the distal edge of the holes within 5–10 weeks. Frond portions that were cut at least 20 cm away from the stipe–frond transition readily formed sori within 5–7 weeks after isolation from the rest of the thallus. These findings suggest that the basal actively dividing and expanding part of the laminarian frond is the source of inhibitors of sporangium formation that move in a distal direction and keep the young frond free of sori during the season of rapid growth (i.e. during the first part of the year). The natural occurrence of sori during the season of slow growth (i.e. the second part of the year in the Northern Hemisphere) may be due to reduced synthesis and export of inhibitors of sporangium formation from the more or less resting basal meristem.