Distinct Diurnal Rhythm Research Articles

Diurnal variations in absorption and translocation of a ferrated phytosiderophore in barley as affected by iron deficiency

The release of phytosiderophore (PS) from roots of Fe-deficient graminaceous plants follows a distinct diurnal rhythm with maximum release rates occurring usually 3 to 4 hours after the onset of light. However, it remains to be determined whether absorption of the PS-Fe3+ complex shows a diurnal rhythmicity similar to that of PS release, Barley plants grown with or without 10 µM FeEDTA for 7 days were fed with ferreted PS (10 µM labelled with 59Fe) at 4-h intervals to study the diurnal variations in the absorption and transloca tion of 59Fe, The absorption of 59Fe, irrespective of the Fe nutritional status of the plants, was higher during the day and lower during the night but did not show any peak throughout the day-night cycle. On the other hand, the translocation of 59Fe into shoots of Fe-deficient plants was lower than that of Fe-sufficient plants, while the Fe nutritional status of the plants did not affect the absorption of 59Fe by roots, The formation of root apoplastic 59Fe was lower during the day and higher during the night, regardless of the Fe nutritional status of plants. Our results showed that the absorption of the PS-Fe3+ complex by roots did not follow the PS release pattern.

Characterization of phytosiderophore secretion under Fe deficiency stress in Festuca rubra

In the present study we investigated the response to iron (Fe) deficiency in two cultivars of Festuca rubra L. (Rubina and Barnica) used in correction of chlorosis of fruit trees cultivated on calcareous soils. We found that a Fe-chelating compound, identified as 2′-deoxymugineic acid (DMA), was secreted from the roots in response to Fe-deficiency in both cultivars. The amount of DMA secreted into solution increased with the development of Fe-deficiency. The secretion showed a distinct diurnal rhythm characterized by a secretion peak at between 2 and 5 hours after sunrise at 20 °C. However, this secretion peak was delayed by 3 hour at low temperature (<10 °C) and occurred 3 h earlier at high temperature (30 °C). When water used for the collection of root exudates was pre-warmed (25 °C) or pre-cooled (10 °C), this led to an earlier or a delayed secretion compared to control (15 °C) under the same air temperature, respectively. Short-term shading treatment did not affect the secretion pattern of DMA. These results demonstrate that the secretion time of DMA from the roots is, at least partly controlled by the temperature in the root environment. Overall, these findings suggest that the ability of Festuca rubra to prevent Fe chlorosis symptoms (`re-greening effect') of associated fruit trees is partially related to the secretion of DMA which increase Fe availability in calcareous soils.

CDNA microarray analysis of gene expression during Fe-deficiency stress in barley suggests that polar transport of vesicles is implicated in phytosiderophore secretion in Fe-deficient barley roots.

To acquire Fe from soil, graminaceous plants secrete mugineic acid family phytosiderophores (MAs) from their roots. The secretion of MAs increases in response to Fe deficiency, and shows a distinct diurnal rhythm. We used a microarray that included 8987 cDNAs of rice EST clones to examine gene expression profiles in barley roots during Fe-deficiency stress. Approximately 200 clones were identified as Fe-deficiency-inducible genes, of which seven had been identified previously. In order to meet the increased demand for methionine to produce MAs, Fe-deficiency enhances the expression of genes that participate in methionine synthesis, as well as recycling methionine through the Yang cycle. Of these 200 genes, approximately 50 exhibited different transcription levels in Fe-deficient roots at noon and at night. Northern blot analysis of time course experiments confirmed that five of these genes exhibited a diurnal change in their level of expression. The diurnal changes in the expression of these genes suggest that polar vesicle transport is involved in the diurnal secretion of MAs.

Rate of occurrence of transient magnetic field events in U.S. residences.

Recent interest in the transient magnetic field events produced by electrical switching events in residential and occupational environments has been kindled by the possibility that these fields may explain observed associations between childhood cancer and wire codes. This paper reports the results of a study in which the rate of occurrence of magnetic field events with 2-200 kHz frequency content were measured over 24 h or longer periods in 156 U.S. residences. A dual-channel meter was developed for the study that, during 20 s contiguous intervals of time, counted the number of events with peak 2-200 kHz magnetic fields exceeding thresholds of 3. 3 nT and 33 nT. Transient activity exhibited a distinct diurnal rhythm similar to that followed by power frequency magnetic fields in residences. Homes that were electrically grounded to a conductive water system that extended into the street and beyond, had higher levels of 33 nT channel transient activity. Homes located in rural surroundings had less 33 nT transient activity than homes in suburban/urban areas. Finally, while transient activity was perhaps somewhat elevated in homes with OLCC, OHCC, and VHCC wire codes relative to homes with underground (UG) and VLCC codes, the elevation was the smallest in VHCC and the largest in OLCC homes. This result does not provide much support for the hypothesis that transient magnetic fields are the underlying exposure that explains the associations, observed in several epidemiologic studies, between childhood cancer and residence in homes with VHCC, but not OLCC and OHCC, wire codes.

Luminal fructose modulates fructose transport and GLUT-5 expression in small intestine of weaning rats

In neonatal rats, precocious introduction of dietary fructose significantly enhances brush-border fructose transport rates and GLUT-5 mRNA levels during early weaning. In this study, these rates and levels were more than two times higher in the anastomosed intestine compared with those in the bypassed loop of weaning pups that underwent Thiry-Vella surgery and consumed high-fructose (HF) diets. In Thiry-Vella pups fed fructose-free (NF) diets, uptake rates and mRNA levels in the anastomosed intestine were very low and similar to those in the bypassed loop. In sham-operated littermates, transport rates and mRNA levels were similar between intestinal regions that corresponded to anastomosed and bypassed loops in Thiry-Vella pups and were two to three times greater in pups fed HF than in those fed NF diet. In contrast, rates of brush-border glucose transport and levels of SGLT-1 and of GLUT-2 mRNA were independent of diet and were similar between bypassed and anastomosed regions. Changes in GLUT-5 expression did not follow a distinct diurnal rhythm. When pups were fed HF diet after 12 h of starvation to empty the intestinal lumen, fructose transport rates increased with feeding duration and reached a plateau 12-24 h after feeding; in contrast, GLUT-5 mRNA levels were highest within 4 h after arrival of chyme in the jejunum and then decreased gradually and returned to baseline levels 24 h later. In littermates fed NF diet, mRNA levels and uptake rates were each independent of feeding duration. Luminal, and not endocrine, signals regulate GLUT-5 expression in weaning pups.

No evidence for a diurnal vasoactive intestinal polypeptide (VIP) rhythm in the human suprachiasmatic nucleus

The mammalian suprachiasmatic nucleus (SCN) is implicated in the temporal organization of circadian rhythms in a variety of physiological, endocrine and behavioral processes. Since the environmental light-dark cycle is the main zeitgeber for many of these rhythms, photic information may have a synchronizing effect on the endogenous clock of the SCN by inducing periodic changes in the activity of certain groups of neurons. The present study was performed to investigate the diurnal profile of the vasoactive intestinal polypeptide (VIP)-producing neurons in the SCN of humans. No significant diurnal variations were found in the volume of the VIP subdivision of the SCN nor in the number of VIP-producing neurons. In contrast with the VIP cell population, the subdivision of the human SCN containing vasopressin-producing neurons has previously been reported to exhibit a distinct diurnal rhythm, with low values during the night and peak values during the early morning. These findings suggest that the expression of vasopressin, but not that of VIP, in the human SCN exhibits an endogenous circadian rhythm.

Endocrine and metabolic responses to realimentation in feed-restricted prepubertal gilts: associations among gonadotropins, metabolic hormones, glucose, and uteroovarian development.

Temporal relationships among plasma LH, FSH, metabolic hormones, and glucose concentrations were studied in nutritionally manipulated prepubertal gilts. Twenty-four growth-matched littermate gilts (73.6 +/- 1.2 kg) were fed at maintenance for 7 d (d 1 to 7). From d 8 to 14, littermates were divided between two treatments and were either fed twice daily to appetite or continued on feed restriction. Feeding to appetite increased episodic LH secretion during the first 6 h of sampling on d 8 (P < .05). Plasma FSH concentrations were not affected by treatment between d 8 and 14. Nocturnal increases in plasma LH (P < .01) and FSH (P < .002) concentrations were detected in feed-restricted gilts but not realimented gilts. Mean total plasma IGF-I concentrations gradually increased in gilts fed to appetite (d 12, P < .02; d 13 to 14, P < .001). Mean postprandial insulin concentrations were also greater in gilts fed to appetite (P < .01). Treatment differences in mean postprandial glucose concentrations on d 9 and 10 indicate that normoglycemia was established by d 9 in gilts fed to appetite. At slaughter on d 15, numbers of ovarian follicles (P < .01), follicular volume (P < .001), and uterine weight (P < .05) were greater in gilts fed to appetite. In conclusion, uteroovarian responses to realimentation after short-term feed restriction in prepubertal gilts are mediated by enhanced LH secretion. This rapid enhancement of LH secretion in response to dietary repletion might have been mediated by changes in glucose and insulin status. Ovarian responses to gonadotropins may also be potentiated by observed increases in plasma glucose, insulin, and IGF-I. During feed restriction, a distinct diurnal rhythm in gonadotropin secretion was evident.

Diurnal rhythm of plasma delta-sleep-inducing peptide in humans: evidence for positive correlation with body temperature and negative correlation with rapid eye movement and slow wave sleep.

Since delta-sleep-inducing peptide (DSIP) was isolated in 1977, numerous reports have suggested that this nonapeptide stimulates delta-sleep [slow wave sleep (SWS)]. Although DSIP-like immunoreactivity (DSIP-LI) has been found in the serum of many animals and man, its diurnal rhythm and relation to sleep stages have not been well defined. We hypothesized that circulating levels of this putative sleep hormone would be highest at night and would probably be elevated before or during episodes of SWS. We, therefore, measured plasma DSIP-LI levels every 30 min for 24 h in 12 normal volunteers in whom we obtained simultaneous polygraphic recordings. We found a distinct diurnal rhythm for plasma DSIP-LI levels, with the maximum at 1500 h and the minimum at 0100 h. DSIP-LI levels were substantially lower in rapid eye movement sleep (P < 0.005) and somewhat lower in SWS (P < 0.05) compared to awake values. DSIP-LI levels did not rise before, during, or after a significant percentage of episodes of SWS. We found, however, that the diurnal rhythm of DSIP-LI closely followed that of body temperature with a high degree of correlation (r2 = 0.66; P < 0.0001). We conclude that endogenous elevations of circulating DSIP may be associated with suppression of slow wave and rapid eye movement sleep, and that the circadian rhythm of this peptide is coupled directly or indirectly to that of body temperature.

A Distinct Diurnal Rhythm in Hydroxyindole-O-methyltransferase (HIOMT) Activity in the Male Albino Rat

Pineal hydroxyindole-O-methyltransferase (HIOMT) has been reported to show a diurnal rhythm activity in the rat, although this is not fully confirmed. The present study reexamines this issue and reports a distinct diurnal rhythm in rat pineal HIOMT. Peak enzyme activity occurs 5 hr after the onset of darkness, followed by two lesser bursts of activity preceeding each change in the photoperiod. The approximately 18-fold increase in methylating capacity at night represents the greatest variation in HIOMT activity thus far reported in the rat and may be attributed to the age, weight, and housing regimen of the animals used.

Activity of vasoactive intestinal peptide and serotonin in the paraventricular nucleus reflects the periodicity of the endogenous stimulatory rhythm regulating prolactin secretion.

PRL secretion in the female rat is regulated by an endogenous stimulatory rhythm (ESR) of prolactin-releasing factors of hypothalamic origin which has a bimodal periodicity with distinct nocturnal (N) and diurnal (D) phases. The N phase reaches peak magnitude by 0300 h and the D phase reaches peak magnitude by 1700 h. This rhythm was first unmasked in ovariectomized rats by correctly timed injection of a dopamine antagonist. OT, vasoactive intestinal peptide (VIP), and serotonin (5-HT) are differentially involved in generating the ESR. Pharmacological studies suggest that OT is the neurohormone and VIP and 5-HT are neuromodulators which act to stimulate OT release. Recently, we reported that activity of OTergic neurons in the paraventricular nucleus (PVN) and OT concentrations in the anterior pituitary mirror the periodicity of the ESR. The present experiments were conducted to determine if VIP and 5-HT activity in the hypothalamus also mirrors the periodicity of the ESR. Push-pull cannulae were surgically implanted in the PVN of ovariectomized female rats. Following recovery, push-pull perfusion was conducted from either 0600-1400 h, 1400-2200 h, or 2200-0600 h. VIP was measured in perfusates by RIA. There was no difference in VIP pulse frequency between rats perfused during the three periods studied. However, animals perfused from 2200-0600 h had significantly greater pulse amplitude as compared to rats at either 0600-1400 h or 1400-2200 h. Activity of 5-HTergic neurons in the hypothalamus was studied by estimating the turnover of 5-HT 10 min following the injection of pargyline. Hypothalamic nuclei were dissected using Palkovits' punch technique and 5-HT concentration assayed by HPLC in conjunction with electrochemical detection. Turnover of 5-HT was estimated by calculating the slope of the accumulation of 5-HT over 10 min at differing times of day using least squares regression analysis. There was a distinct diurnal rhythm of 5-HT accumulation in the PVN. Rats killed at 1700 h had significantly greater slopes of 5-HT accumulation in the PVN than rats killed at either 0300 or 1200 h. Similarly, there was a diurnal rhythm of 5-HT turnover in the suprachiasmatic nucleus. Rats sampled at either 1200 or 1700 h had significantly greater slopes of 5-HT accumulation in the suprachiasmatic nucleus than rats sampled at 0300 h. There was no diurnal rhythm of 5-HT turnover evident in either the median eminence or the supraoptic nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Diurnal rhythm of release of phytosiderophores and uptake rate of zinc in iron-deficient wheat

Abstract The diurnal rhythm of release of phytosiderophores and uptake rate of zinc (Zn) was studied in iron (Fe) deficient wheat (Triticum aestivum L. cv. Ares) plants grown in nutrient solution under controlled environmental conditions. Different forms of Zn (e.g. ZnSO4, ZnEDTA) were used to obtain different degrees of loading of the root apoplasmic pool with Zn. In the Fe-deficient plants the release of phytosiderophores from the roots followed a distinct diurnal rhythm with a steep peak about 4 h after the onset of the light period. These plants also showed a similar pattern in the rates of Zn uptake over the 24 h day-night cycle. During the light period there was a steep transient peak (factor 3.8) in Zn uptake rate in the Fe-deficient plants supplied with ZnSO4. This transient peak was much less distinct in plants supplied with ZnEDTA (factor 1.8) and absent in plants supplied with ZnEDTA plus free chelator (+ NaEDTA) in excess. The peak in Zn uptake coincided with the maximum rate of phytosideropho...

Lack of diurnal rhythm of low molecular weight insulin-like growth factor binding protein in patients with Cushing's disease

A specific radioimmunoassay with antibodies raised against the 25 kD insulin-like growth factor binding protein (25 kD IGFBP) in amniotic fluid was used to measure levels of cross-reacting protein in human serum and plasma. Plasma samples collected continually at 20-min intervals during 24-h in 6 healthy adults revealed a distinct diurnal rhythm in the concentration of 25 kD IGFBP. The lowest levels (9-13 micrograms/l) were found between 13.00 and 24.00 h with a rise after midnight to maximum levels (23-71 micrograms/l) between 03.00 and 09.00 h. There was no relation between the patterns of GH and 25 kD IGFBP. In 3 patients with active Cushing's disease, the levels of 25 kD IGFBP in plasma samples collected during 12 h, 19.00-07.00 h, were generally low and without nocturnal variations. One of the patients studied after extirpation of a pituitary adenoma displayed a nocturnal rhythm with maximum levels of 25 kD IGFBP between 03.00 and 07.00 h. Eight patients treated with stereotactic pituitary irradiation owing to Cushing's disease also showed a distinct nocturnal increase of 25 kD IGFBP. The results indicate the existence of a diurnal rhythm of 25 kD IGFBP in adults. Further, low levels and lack of diurnal rhythm of 25 kD IGFBP are demonstrated in Cushing's disease.

Effect of zinc deficiency in wheat on the release of zinc and iron mobilizing root exudates

AbstractThe effect of Zn deficiency in wheat (Triticum aestivum L. cv. Ares) on the release of Zn mobilizing root exudates was studied in nutrient solution. Compared to Zn sufficient plants, Zn deficient plants had higher root and lower shoot dry weights. After visual Zn deficiency symptoms in leaves appeared (15–17 day old plants) there was a severalfold increase in the release of root exudates efficient at mobilizing Zn from either a selective cation exchanger (Zn‐chelite) or a calcareous soil. The release of these root exudates by Zn deficient plants followed a distinct diurnal rhythm with a maximum between 2 and 8 h after the onset of light. Re‐supply of Zn to deficient plants depressed the release of Zn mobilizing root exudates within 12 h to about 50%‐, and after 72 h to the level of the control plants (Zn sufficient plants).The root exudates of Zn deficient wheat plants were equally effective at mobilizing Fe from freshly precipitated FeIII hydroxide as Zn from Zn‐chelite. Furthermore, root exudates from Fe deficient wheat plants mobilized Zn from Zn‐chelite, as well as Fe from FeIII hydroxide. Purification of the root exudates and identification by HPLC indicated that under Zn as well as under Fe deficiency, wheat roots of the cv. Ares released the phytosiderophore 2′‐deoxymugineic acid. Additional experiments with barley (Hordeum vulgare L. cv. Europa) showed that in this species another phytosiderophore (epi‐3‐hydroxymugineic acid) was released under both Zn and Fe deficiencies.These results demonstrate that the enhanced release of phytosiderophores by roots of grasses is not a response mechanism specific for Fe deficiency, but also occurs under Zn deficiency. The ecological relevance of enhanced release of phytosiderophore also under Zn deficiency is discussed.

Presence of a diurnal rhythm in fetal prolactin secretion and influence of maternal nutrition.

The possible effect of altered maternal nutrition on fetal prolactin (PRL) secretion and the possible presence of a diurnal rhythm in fetal PRL secretion has been investigated in 11 fetuses aged between 132 and 140 days of gestation. Fetal PRL levels in 4 of 5 ewes fed once a day were high (greater than 40 ng/ml) and exhibited a distinct diurnal rhythm in PRL secretion. Fetal PRL levels in those fetuses where ewes fed 4-hourly (n = 6) were all low (less than 10 ng/ml) and did not exhibit a diurnal rhythm. This study suggests that frequent maternal feeding results in low fetal PRL concentrations and that maternal undernutrition results in high fetal PRL levels that exhibit a distinct diurnal rhythm.

Seasonal and environmental influences upon the diurnal heart-rate pattern in wild rabbits living under seminatural conditions

Diurnal patterns in the heart-rates (HR) of 16 wild European rabbits have been studied, based on more than 1200 days of radiotelemetrical HR recordings. HR shows a distinct diurnal rhythm which is closely coupled to the diurnal pattern of activity for free ranging animals. The rabbits' activity phase, accompanied by high HRs extends from late afternoon to early morning. It is followed by diurnal rest with low HRs, irrespective of burrow utilization. This diurnal resting phase is often interrupted by short excursions outside the burrow. The duration of the activity phase and the times of onset and cessation of activity are very stable across different individuals and over three years of measurement. Disturbances of this stable seasonal pattern are by predation and by weather influences. The importance of light cycle and human disturbances as synchronizers for diurnal HR pattern in wild rabbits is discussed.

Reproductive state and motor activity in female ferrets

The pattern of motor activity in female ferrets ( Mustela putorius furo) housed in cages mounted upon rocker platforms was followed under conditions of short-(8 hr) and long-(16 hr) day illumination. A distinct diurnal rhythm was evident that remained unchanged after lesioning of the anterior hypothalamus. The level of activity did not differ between animals kept under short days and long days, between anoestrous and oestrous females, or during the development or waning of oestrus. Ovariectomy did not affect the pattern of activity. These findings are at variance with those of a previous study where wheel-running activity was followed and oestrous females found to be more active than anoestrous ferrets.

Diurnal rhythm of HMG CoA reductase activity in canine intestine is independent of luminal contents.

Activity of the enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (EC 1.1.1.34) measured in isolated segments of canine intestinal mucosa showed a distinct diurnal rhythm. Total activity changed over a twofold range with a peak occurring during midday, shortly after feeding. Since the isolated segments had no contact with luminal contents, the rhythm was not directly related to food components or bile salts. Humoral or neural influences must mediate the rhythm. The diurnal rhythm persisted for at least 3-5 mo, but was lost by 10 mo following formation of the isolated segment, possibly because of mucosal involution.

Biological studies in the vicinity of a shallow-sea tidal mixing front III. Seasonal and spatial distribution of heterotrophic uptake of glucose

Heterotrophic incorporation and respiration of 14C-labelled glucose (tracer approach) and natural concentrations of glucose were measured, as part of a multidisciplinary research projection five cruises in the western Irish Sea from M arch to September 1980. The investigations were carried out along a transect across a shallow-sea tidal mixing front and its adjacent stratified and vertically mixed water masses. The spatial distribution pattern in relation to hydrographical conditions, diurna changes observed at drogue stations, and seasonal developments are described. High turnover rates of [14C]glucose were strongly associated with stratification, both spatially as well as seasonally, starting with low rates early in spring at the beginning of stratification, increasing to maxim um rates in July after the phytoplankton bloom, and subsequent y declining in autumn when stratification weakened. Turnover rates were consistently and significantly higher in the waters above the pycnocline than below it or in the vertically mixed water masses to the east of the front. No distinct diurnal rhythm s were recognized. The mixed water column, in particular, was totally uniform in heterotrophic uptake of glucose whereas the surface of the stratified water showed greater variability. A fairly constant proportion of, on average, 32 % of the glucose carbon was respired. Natural glucose concentration ranged from less than 30 to 322 nM, mean 116 nM. No particular pattern in its distribution could be detected in the different water masses despite considerable changes in use of glucose. Turnover rates of glucose were unrelated to numbers of bacterial cells or their biomass. Glucose uptake per bacterial cell (uptake index) was estimated and showed pronounced seasonal increase during sum m er in the surface stratified water mass, especially, in the vicinity of the front. The importance of the surface waters of the stratified water body and the frontal zone in respect to carbon flux and potential bacterial biomass production is discussed.

Diurnal periodicity of plasma Cortisol and corticosterone in desert bighorn sheep demonstrated by radioimmunoassay

A sensitive and specific radioimmunoassay (RIA) for Cortisol and corticosterone was validated for desert bighorn sheep (Oviscanadensis cremnobates) plasma. Statistical and physiological validation of assay accuracy, precision, sensitivity, and specificity demonstrated the RIA to be reliable. Coefficients for intra- and inter-assay variation were 1.20 and 2.35% for Cortisol and 3.50 and 3.97% for corticosterone, respectively. Sensitivity was 3.3 and 4.9 pg for cortisol and corticosterone, respectively. A distinct diurnal rhythm in plasma cortisol and corticosterone synchronous with photoperiod was found in free-ranging and captive bighorn sheep. A significant difference (P &lt; 0.01) existed between the mean 24-h plasma cortisol concentration for captive (33.04 ± 5.97 ng/mL) and free-ranging (49.20 ± 8.17 ng/mL) bighorn. Corticosterone levels were not significantly distinct between captive and free-ranging bighorn. The mean scotophase cortisol concentration in free-ranging class III rams (60.97 ± 3.21 ng/mL) was significantly greater (P &lt; 0.05) than the mean scotophase cortisol concentration (54.03 ± 5.97 ng/mL) of the free ranging group. Absence of group interactions in solitary rams may be sufficiently stressful so as to entrain increased plasma cortisol concentrations.

Characterization of Diurnal Changes in Activities of Enzymes Involved in Sucrose Biosynthesis

Experiments were conducted with vegetative soybean plants (Glycine max [L.] Merr., ;Ransom') to determine whether the activities in leaf extracts of key enzymes in sucrose metabolism changed during the daily light/dark cycle. The activity of sucrose-phosphate synthase (SPS) exhibited a distinct diurnal rhythm, whereas the activities of UDP-glucose pyrophosphorylase, cytoplasmic fructose-1,6-bisphosphatase, and sucrose synthase did not. The changes in extractable SPS activity were not related directly to photosynthetic rates or light/dark changes. Hence, it was postulated that the oscillations were under the control of an endogenous clock. During the light period, the activity of SPS was similar to the estimated rate of sucrose formation. In the dark, however, SPS activity declined sharply and then increased even though degradation of starch was linear. The activity of SPS always exceeded the estimated maximum rate of sucrose formation in the dark. Transfer of plants into light during the normal dark period (when SPS activity was low) resulted in increased partitioning of photosynthate into starch compared to partitioning observed during the normal light period. These results were consistent with the hypothesis that SPS activity in situ was a factor regulating the rate of sucrose synthesis and partitioning of fixed carbon between starch and sucrose in the light.