PHYSIOLOGICAL INVESTIGATIONS ON ALARIA ESCULENTA (LAMINARIALES,PHAEOPHYCEAE). II. ROLE OF TRANSLOCATION IN BLADE GROWTH1,2

Laboratory studies on blade growth in Alaria esculenta (L.) Grev. showed 3 periods of rapid blade elongation during the year: October–November, February–April and late June. The first two periods are characteristic of many Laminariales; the unique June peak may reflect local nutrient conditions. While the distal blade functions as a source, supplying organic matter to the blade meristem, the stipe can be a source during periods of rapid growth or a sink during late summer when blade growth is slow. Maximum enhancement of elongation rate of blade meristems was observed in 40–50 cm blades; longer blades showed no further increase in growth rate. This blade length-growth promotion relationship may be independent of seasonal variations in meristematic activity. ¹⁴C tracer experiments suggested that separate growth promotion effects by distal blade, sporophylls and stipe were not additive in the intact thallus. The preferential source of assimilate for blade meristem growth was the distal blade. Secondary sources: sporophylls, which were activated following excision of the primary source; and stipe, which began to translocate assimilate when both sources were removed. The role of secondary sources in nature is discussed. Profiles of radioactivity in alcohol-soluble organic matter in blades are evaluated in relation to tracer profiles in higher plants and mechanisms of translocation.     

A Short Day Photoperiodic Response in Constantinea subulifera

SYNOPSIS. Constantinea subulifera, a perennial red alga witha bushy type growth, uses a short day (long night) photoperiodicresponse to initiate a new blade at the tips of several stipesin September to October. These blades complete the lag phaseof their growth during the winter via a food transfer from theold blade. Under a 9L–15D photoperiod it requires 21–28 daysto initiate a new blade. The critical photoperiod is 11–12hr. This is a phytochrome-mediated response which can be negatedby a low quantum dose light break of red or blue light in themiddle of a 16-hr dark period. Blades will initiate and growin complete darkness. The initiation of new blades in the fall and their slow growththroughout the winter (lag phase) gives Constantinea subuliferaan advantage in capture of habitat space.     

TRANSLOCATION OF PHOTOSYNTHATE IN THE BROWN ALGA NEREOCYSTIS

Translocation of photosynthates was found to occur when blades of Nereocystis were illuminated in the presence of C¹⁴ bicarbonates. Rates of translocation averaging 37 cm/hr in the laboratory were observed. Samples from the epidermis, cortex, and medulla of the stipes of plants with blades exposed to tracer showed that the radioactivity in organic compounds was confined to medulla where sieve filaments occurred. Girdle preparations of blades, interrupting the mucilage ducts and leaving the blade medulla intact, allowed translocation to take place. These data indicate that conduction of photosynthate takes place in the medulla. Similarities between the anatomy of algal sieve filaments and angiosperm sieve tubes, coupled with the continuity of the sieve filaments from blade medulla to stipe medulla suggested indirectly that the sieve filaments were conducting elements. Further support of this hypothesis was provided from collections of radioactive exudate from cells in the medulla of the lower stipe that were continuous with the sieve filaments. Tracer applied to the blades was partially recovered as organic material in a clear fluid that collected inside the pneumatocyst. Continued accumulation of radioactivity in this fluid was dependent on living blades; fluid with low specific activity that did not increase during the experiment accumulated slowly if blades were killed with ethanol after an exposure to tracer. It is possible that the system that produced the stipe fluid was part of (or a side effect of) the system responsible for maintaining volume in the conducting system. It may also provide an alternate route (other than the sieve filaments) for delivery of photosynthates to the base of the plant. Carbon-14 applied to blade tips as bicarbonate was recovered in part as radioactive mannitol in the translocation stream.     

Competition between perch (Perca fluviatilis) and ruffe (Gymnocephalus cernuus): the advantage of turning night into day

1. The outcome of interspecific competition for food resources depends both on the competitors’ sensory abilities and on environmental conditions. In laboratory experiments we tested the influence of daylight and darkness on feeding behaviour and specific growth rate (SGR) of two species with different sensory abilities. 2. We used perch (Perca fluviatilis) as a visually orientated, and ruffe (Gymnocephalus cernuus) as a mechano‐sensory oriented predator and tested their growth rates and behaviour under conditions of interspecific and intraspecific competition. Three different foraging conditions were used: food supplied (i) only during the day, (ii) only during the night or (iii) during both day and night. 3. In perch neither SGR nor feeding behaviour were influenced substantially by interspecific competition during daylight. During darkness their foraging behaviour changed markedly and their access to the food source as well as their SGR were negatively affected by the presence of ruffe. 4. Ruffe's foraging behaviour did not change during either day or night with interspecific competition. During the night ruffe's SGR was higher with interspecific competition, probably because of a release from intraspecific competition and the competitive inferiority of perch during the night. 5. Because of its seonsory abilities ruffe feeds predominantly at night, thereby reducing competitive interference from perch.     

CYTOPLASMIC ORGANIZATION AND RHYTHMIC STREAMING IN GROWING BLADES OF CAULERPA PROLIFERA

Light- and electron-microscopic studies of the growing blades and their meristematic tips in Caulerpa prolifera have been correlated with time-lapse photographic studies. The growing blade may be divided into three zones based on the level of maturation. A “meristemplasm” is present at the tip of the growing blade and at sites of wounding. The cytoplasm of these growing regions has an abundant endoplasmic reticulum, Golgi bodies, amyloplasts, and nuclei, but lacks chloroplasts and a central vacuole. An intermediate zone lies between the white meristemplasm and the green, mature basal zone. The basal zone contains a parietal cytoplasm with organelles typical of the Chlorophyta and a dominant central vacuole. Two distinct systems of cytoplasmic streams occur in the basal zone and both contain packets of microtubules orientated parallel to the axis of the streams. As the blade matures and growth ceases, the dominant central vacuole forms up to the tip. In developing blades the tips become white in the absence of light as a result of basipetal movement of the chloroplasts. In plants kept on a 12-hr L: 12-hr D cycle, blade growth and chloroplast migration at the blade tip are rhythmic, and the peak occurs about 2-4 hr after initiation of the dark phase. Experiments are reported using continuous light or darkness after three 24-hr periods of 12-hr L: 12-hr D phasing.     

Daily changes in uptake, reduction and storage of nitrate in spinach grown at low light intensity

Under poor light conditions, as normally used during winter production of greenhouse vegetables, the nitrate concentration in the shoot of spinach ( Spinacia oleracea L. cv. Vroeg Reuzenblad) showed a diurnal rhythm. This rhythm was mainly caused by a decrease during the day, followed by an increase during the night in the leaf blade nitrate concentration. Nitrate was mainly located in the vacuoles of the leaf blades. A strong correlation was found between net uptake of nitrate by the roots and the nitrate concentration in the leaf blade vacuoles. The nitrate concentration in the leaf blades increased during the initial hours of the night. This increase was caused by a marked increase in the net uptake rate of nitrate by the roots during the first hours of the dark period. During the second part of the night both net uptake rate of nitrate by the roots and the vacuolar nitrate concentration in the leaf blades remained constant.
We conclude that nitrate is taken up for osmotic purposes when light conditions are poor because of a lack of organic solutes. During the night, nitrate influx into the vacuole is needed for replacement of organic solutes, which are metabolized during the night, and possibly also for leaf elongation growth. During the day, vacuolar nitrate may be exchanged for newly synthesized organic solutes and be metabolized in the cytoplasm. A strong diurnal rhythm in nitrate reductase (NR; EC 1.6.6.1.) activity was absent, due to the poor light conditions, and in vitro NR activity was not correlated with nitrate flux from the roots. In vivo NR activity also lacked a strong diurnal rhythm, but it was calculated that in situ nitrate reduction was much lower during the night, so that the major nitrate assimilation took place during the day.     

CIRCADIAN GROWTH RHYTHM IN JUVENILE SPOROPHYTES OF LAMINARIALES (PHAEOPHYTA)1

A circadian rhythm in growth was detected by computer-aided image analysis in 3–4-cm-long, juvenile sporophytes of the kelp species Pterygophora California Rupr. and in seven Laminaria spp. In P. californica, the free-running rhythm occurred in continuous white fluorescent light, had a period of 26 h at 10°or 15°C, and persisted for at least 2 weeks in white or blue light. The rhythm became insignificant in continuous green or red light after 3 cycles. Synchronization by white light-dark regimes, e.g. by 16 h light per day, resulted in an entrained period of 24 h and in a shift of the circadian growth minimum into the middle of the light phase. A morning growth peak represented the decreasing portion of the circadian growth curve, and an evening peak the increasing portion. The circadian growth peak was not visible during the dark phase, because growth rate decreased immediately after the onset of darkness. At night, some growth still occurred at 16 or 12 h light per day, whereas growth stopped completely at 8 h light per day, as in continuous darkness. During 11 days of darkness, the thallus area became reduced by 3.5%, but growth rate recovered in subsequent light–dark cycles, and the circadian growth rhythm reappeared in subsequent continuous light.     

TRANSLOCATION OF 14C IN MACROCYSTIS INTEGRIFOLIA (PHAEOPHYCEAE)1,2

Translocation patterns in the giant kelp, Macrocystis integrifolia Bory, were investigated in situ using ¹⁴C tracer; sources and sinks were identified. Export was first detected after 4 h of labeling; experiments were routinely 24 h continuous ¹⁴C application. Mature blades exported ¹⁴C to young blades on the same frond and on younger fronds, as well as to sporophylls and frond initials at the bases of the fronds. Blades <0.3 m from the apex imported and did not export; this distance did not change seasonally. In spring export from blades 0.3–1.25 m from the apex was exclusively upwards; older blades also exported downwards. In fall downward export began 0.5 m from the apex, and blades >2 m from the apex exported exclusively downwards. Carbon imported by frond initials, young fronds, and sporophylls in fall may partly be stored for growth in early spring. No translocation was seen in very young plants until one blade (secondary frond initial) bad been freed from the apical blade; this blade exported to the apical blade for a time, but imported when it began to develop into a frond. The second and third formed blades on the primary fronds (sporophylls also exported when <0.3 m from the apex, and later stopped. Frond initials and sporophylls on later-formed fronds did not export at all. The translocation pattern in M. integrifolia differs from that previously reported in M. pyrifera in seasonal change and in distances from the apex at which the changes take place.     

The inhibition of potato sprout growth by light.

When potato seed tubers (Solanum tuberosum cv. Pentland Javelin) were stored in darkness or diffuse daylight at 9°C and transferred at intervals to conditions suitable for sprouting, their capacity for sprout growth was unaffected by the presence or absence of light during previous storage. When similar tubers were stored at 10°C, 18°C or 25°C, sprout growth commenced earliest at 25°C, but the date was unaffected by fluorescent light. It was concluded that light did not affect the length of the dormant period, but only the rate of sprout elongation after that period had ceased. When tubers with growing sprouts at 10°C or 18°C were transferred from darkness into fluorescent light, sprout growth virtually ceased. Transfer from light into darkness resulted in immediate sprout growth, at a rate comparable with tubers stored continuously in the dark. Tubers of three Peruvian cultivars, stored in farm-scale diffuse-daylight stores, grew progressively shorter sprouts with increasing daily exposure to light from 30 min to 12 h. Storage of cv. Wilja under 21 Wm^-2 (total) of white fluorescent light for 10 h per day maintained the sprouts at the same length as ten times this light intensity for 1 h per day. In a subsequent experiment with cv. Bintje the 10 h, low-intensity light regime gave slightly shorter sprouts. It appeared that the total light energy falling on the tubers was the dominant factor controlling sprout growth.     

Rhabdom size and photoreceptor membrane turnover in a muscoid fly

Summary The cross-sectional area of rhabdomeres in the compound eye of the blowfly, Lucilia, was found to remain constant under 12 h light/12 h dark cyclic lighting, and 10 days constant light or darkness. It was reduced only slightly during 3 h light after 10 days darkness (by 21%), or on exposure to 2h darkness + 1.5 h light after 10 days light (by 10%). Morphological evidence indicates that shedding of photoreceptor membrane during turnover is achieved by an extracellular route, and by pinocytosis from the bases of the microvilli. The photoreceptor membrane shed by both mechanisms appears to accumulate in multivesicular bodies. The amount of photoreceptor membrane shedding, as indicated by numbers of multivesicular bodies, is constant throughout the day and night on cyclic lighting, decreases in constant darkness, but returns to its normal level after an exposure to 3 h light subsequent to 10 days darkness.     

DAY AND NIGHT KINETICS OF GROWTH RATE IN GREEN,BROWN, AND RED SEAWEEDS1

Growth rates in terms of area increase per 30 min were measured in flat thalli of several seaweed, species by means of computer-assisted image analysis, at 12 h light per day and a photon fluence rate of 20 μmol · m^-2· s^?1. Light fields included white fluorescent, imitated underwater, blue, green, and red light. In the green alga Ulva pseudocurvata Koeman et Hoek, blue light caused an immediate reduction of thallus area and growth rate after the onset of light, whereas green light and red light resulted in an initial peak in growth rate followed by inhibition 60 min after the onset of light. More growth was observed in darkness than in blue light in U. pseudocurvata. All brown and red algae tested, with Laminaria saccharina (L.) Lamour. and Palmaria palmata Stackh. as the main investigated species, grew faster during the day than during the night, irrespective of light quality during the main light phase. The upper intertidal red alga Porphyra umbilicalis (L.) J. Ag. achieved most of its thallus expansion per 24 h during the first 3 h of the light phase, with maximum growth rates of 2–3% increase in area per hour. Maximal growth rates were 0.7% for juvenile laminarian sporophytes and were lower than this in Palmaria palmata and other perennial red algae. The temporary growth inhibition by light in Ulva pseudocurvata suggests photomorphogenetic events, similar to the kinetics of stem elongation in higher plant seedlings after blue or red light pulses in darkness.     

VENTILATION BY FLOATING LEAVES IN NUPHAR

The network of internal gas spaces in the yellow waterlily constitutes a pressurized flow-through system which forces oxygen to the rhizome buried in anaerobic sediment. This ventilation has been documented previously in a subspecies of Nuphar luteum with aerial leaves, and appeared to occur only during daylight when the leaves are warmed by the sun's radiation. In this study we have found the ventilation system operating in two different subspecies of N. luteum growing in Alaska and in Massachusetts. These plants have floating leaves so that during certain times of the year the leaves are warmed not only by the sun in daylight but also by lake water at night, allowing the ventilation to continue during darkness.     

Influence of light on ochratoxin biosynthesis by <Emphasis Type="Italic">Penicillium</Emphasis>

Light has a profound influence on ochratoxin biosynthesis by Penicillia. When incubated under constant daylight of a certain intensity, ochratoxin A biosynthesis is decreased by about 20–30% compared to incubation under constant darkness. Under day/night oscillation, the ochratoxin A polyketide synthase gene, a key gene of the ochratoxin A biosynthesis pathway, is rhythmically expressed, and moreover, the amount of ochratoxin also oscillates between the amounts produced either during constant darkness or during constant light. This indicates a partial degradation of ochratoxin A (20–30%) under light conditions until a certain lower limit is reached. This behavior is dependent on the light intensity. At 1,600 Lux, only weak effects could be observed; however, at 2,800 Lux, the effects became significant. After growth under constant light conditions, Penicillium produced ochratoxin B at amounts which are 5 times higher than after growth in constant dark or in alternating light/dark conditions. Growth experiments in the dark on medium with increasing amounts of ochratoxin A revealed that externally applied ochratoxin is moderately toxic. However, if the same growth experiments are carried out under light conditions, the growth inhibiting activity of ochratoxin A is greatly increased, indicating that light amplifies the toxic activity of ochratoxin. Because of the oscillation of the concentration of ochratoxin A during night and day incubation, Penicillium seems to have developed an adaptive mechanism to reduce the amount of ochratoxin A during daylight below a toxic level.     

ROLE OF ADENOSINE 3'', 5''-MONOPHOSPHATE IN THE REGULATION OF CIRCADIAN OSCILLATION OF SEROTONIN N-ACETYLTRANSFERASE ACTIVITY IN CULTURED CHICKEN PINEAL GLAND

—When pineal glands of 10–12-day-old chicks were organ-cultured in darkness, serotonin N-acetyltransferase activity was low during the daytime, increased at midnight and then decreased to the daytime level the next morning. The pattern of increase and decrease of enzyme activity in cultured pineal glands was comparable to the circadian rhythm of N-acetyltransferase activity in vivo. When pineal glands were kept at a low temperature for 5 h prior to culture, the phase of autonomous rhythm of enzyme activity was delayed. When chicken pineal glands were cultured during the daytime for 6 h, derivatives of adenosine 3′, 5′-monophosphate (cyclic AMP), cholera toxin, a high concentration of KCl and phosphodiesterase inhibitors increased N-acetyltransferase activity 3–7-fold, indicating an involvement of cyclic AMP in the regulation of N-acetyltransferase activity in chicken pineal gland as has been shown in rat pineal gland. When pineal glands were cultured at night in darkness, cholera toxin or a high KCl did not enhance the night-time increase of the enzyme activity. Derivatives of cyclic AMP or phosphodiesterase inhibitors enhanced the autonomous night-time increase of N-acetyltransferase activity in an additive or more than additive manner in cultured pineal glands. These observations suggest that adenylate cyclase of pinealocytes is inactive during daytime, but is activated at night in darkness, which is transduced to the synthesis of N-acetyltransferase molecules. Catecholamines suppressed the basal level and the nocturnal increase of N-acetyltransferase activity via α-adrenergic receptor. The nocturnal increase of enzyme activity was prevented by cycloheximide or actinomycin D. Cocaine, which stabilizes cell membrane potential or light exposure, blocked the nighttime increase of N-acetyltransferase activity in cultured chicken pineal glands.     

Chloroplast development and the synthesis of chlorophyll and protochlorophyllide in Zostera transferred to darkness

Intact plants and isolated leaves of Zostera capricornii Martens ex Aschers were transferred from daylight to darkness. Substantial amounts of chloropyll a and b continued to accumulate in immature and mature tissue in the same ratio as in the light and were incorporated into chlorophyll-protein complexes in the thylakoids. A small amount of protochlorophyllide also accumulated in immature tissue in the dark. Proplastids and immature chloroplasts continued to develop into mature chloroplasts in the dark in the normal manner but prolamellar bodies, which are a conspicuous feature of immature chloroplasts, took longer to disperse than in the light. Protochlorophyllide accumulation and prolamellar-body formation were not correlated. The results indicate that Zostera has a genetic capacity for dark chlorophyll synthesis which is expressed in immature and mature leaf tissue and enables this plant to continue synthesising chlorophyll and assembling chloroplasts at night.Abbreviations Chl chlorophyll - T _o time of transfer to darkness     

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.     

VARIABILITY OF NITRATE UPTAKE CAPACITY IN MACROCYSTIS PYRIFERA (LAMINARIALES,PHAEOPHYTA) WITH NITRATE AND LIGHT AVAILABILITY1

The nitrate uptake capacity of mature blade tissue of the giant kelp, Macrocystis pyrifera (L.) C. Ag., was examined as a function of the availability of light and nitrate. Time course measurements indicated that nitrate uptake rate, as measured by the incorporation of ¹⁵N, was significantly increased by N starvation. The response was linear over the first hour of exposure regardless of the N status of the tissue indicating that surge uptake was not responsible for the increase. The Michaelis-Menten parameters V_max and K_s, however, were not significantly changed by either growth nitrate concentration or growth irradiance as a result of high variability among blades. Similarly, the initial slope (α) of the nitrate uptake kinetics curves was unaffected. Concentration of photosynthetic pigments increased in response to increased nitrate availability but not to increased growth irradiance. Time course and pigment data demonstrated that mature blade tissue responds to increased N availability by decreasing its capacity to take up nitrate and by increasing its investment in photosynthetic pigments, perhaps for N storage or enhanced light-harvesting capabilities and the increase in reducing power available for N assimilation. This study provides evidence for a dynamic regulatory system that responds to changes in nitrate availability in an integrated manner.     

FIELD AND CULTURE STUDIES OF A POPULATION OF ENDARACHNE BINGHAMIAE (PHAEOPHYTA) FROM SOUTHERN CALIFORNIA1

Abundances of the erect, blade phase of Endarachne binghamiae J. Ag. (Scytosiphonales, Phaeophyta) varied seasonally at a southern California rocky intertidal site. Blade cover and density were much greater in the fall through early spring; blades were mostly absent from quadrats during the summer. Blade abundances were negatively correlated with both seasonal variations in seawater temperature and photoperiod. Laboratory culture studies failed to provide evidence for sexual reproduction. The life history appears to be of the “direct” type with plurangia-produced zooids germinating into crustose disks. Most disks developed erect blade clusters under spring/fall (17° C) and winter (13° C) temperatures over the range of natural photoperiods employed (14:10, 12:12, 10:14 h LD). In contrast, cultures held under the summer temperature (21° C) produced almost entirely crustose growths regardless of photoperiod. Similar results were obtained for cultures grown at 100 and 200 μE · m^?2· s^?1. E. binghamiae blades were fertile throughout the year and produced viable zooids indicating that reproductive seasonality did not influence the seasonal pattern of blade abundance. Culture and field studies suggest that the initiation of new erect blade clusters from crustose disks is confined to the cooler months of the year (winter and spring). The summer reduction or absence of E. binghamiae blades appears to be due to increased mortality rates and temperature constraints on the development of new erect bladed thalli. Hypothetical causes of mortality are desiccation stress, sand burial, increased grazing activity and a genetically-based short life span.     

Effect of one night with "low light'on uptake, reduction and storage of nitrate in spinach

During the night, shoot nitrate concentration in spinach (Spinacia oleracea L. cv. Vroeg Reuzenblad) increased due to increased uptake of nitrate by the roots. When the plants were subjected to a one night “low light’period at 35 μmol m^?2 s^?1, the shoot nitrate concentration did not increase and was reduced by 25% compared to control plants in the dark. The major contribution to this decrease was located in the leaf blades, where the nitrate concentration was decreased by 60%, while the petiole nitrate concentration decreased by only 9%. Nitrate accumulated in the leaf blade vacuoles during a dark night, but this was not the case during the “low light’period. This decrease in vacuolar nitrate concentration, compared to control plants in the dark, was not caused by increased amounts of leaf blade nitrate reductase (NR; EC 1.6.6.1). During a “low light’night period, the cytoplasmic soluble carbohydrate concentration was increased compared to the control plants in the dark. Calculations showed in situ NR activity to be higher than in the control plants in the dark. This increase in NR activity, however, was not large enough to account for the total difference found in the shoot nitrate concentration. Net uptake of nitrate by the roots was increased during the initial hours of the dark night, while vacuolar nitrate concentration in the leaf blades increased at the same time. During the “low light’night period, however, net uptake of nitrate by the roots did not increase, and vacuolar nitrate concentration did not change. We conclude that nitrate uptake by the roots and vacuolar nitrate concentration in the leaf blades are tightly coupled. The decreased shoot nitrate concentration is mainly caused by a reduction in net uptake of nitrate by the roots. During the “low light’night period, carbohydrates and malic acid partly replaced vacuolar nitrate. A “low light’period one night prior to harvest provides a valuable tool to reduce shoot nitrate concentrations in spinach grown in greenhouses in the winter months.     

Expansion growth of isolated leaf blades of Todea barbara

Summary Etiolation of Todea barbara sporophytes and the subsequent deetiolation of excised leaf blades have been studied. In etiolated plants leaf blade growth is arrested, petiole and stem growth is enhanced, and root growth is decreased. De-etiolation permits the resumption of leaf blade growth and differentiation, and the resulting de-etiolated blade appears comparable to its light-grown counterpart in every respect but cell number. Only two factors are required to attain maximum surface area growth in cultured leaf blades; these are light and sucrose. The addition of plant growth substances does not increase the final area attained. However, the inhibition of leaf blade growth with growth retardants and its partial reversal by exogenously supplied gibberellic acid demonstrates a role for gibberellins in leaf expansion in Todea.