Changes in Anthocyanin, Carotenoid, Chlorophyll, and Protein in Developing Florets of the Chrysanthemum

Florets of a purple cultivar (Fandango) of the horticulturalchrysanthemum (Chrysanthemum morifolium Ramat) were removedfrom flower heads at seven stages of opening (from unopenedbud to dying flower) and segregated into different lengths,each of which was analysed. Wet weight per floret increased from 0.25–1 mg in thebud to about 3 mg (tubular florets) or to 20–40 mg (rayflorets) in the fully open flower. Protein decreased from 6per cent of the wet weight in the bud to about 2 per cent inthe open flower. In the ray florets anthocyanin concentrationreached a maximum in the half-open flower and then decreasedsharply, whereas carotenoid and chlorophyll declined continuouslyfrom the bud stage. Almost no anthocyanin was formed by thetubular florets and chlorophyll declined as in the ray florets,but carotenoid concentration increased to a maximum in the half-openflower and then decreased. In another cultivar (Light Bronze Fandango) the content of anthocyaninwas lower and that of carotenoid higher but similar changesin pigment levels were observed except that carotenoid roseto a maximum in the ray florets. In two other cultivars, Redand Cerise Fandango, the anthocyanin content was the same asin Fandango but the carotenoid concentration was the same orabout half that in Light Bronze Fandango respectively.     

Freezing Avoidance Mechanisms by Supercooling in Some Rhododendron flower Buds with Reference to Water Relations

Excised florets of some hardy Rhododendron species did not toleratefreezing at –5°C when ice-inoculated due to intracellularfreezing. Florets in intact December buds, however, could besupercooled to about –30°C. When flower buds of R.japonicum were slowly cooled with daily decrements of 5°Cto temperatures ranging from 0 to –20°C, the exothermtemperatures of the florets drastically decreased. This wasaccompanied by a decrease in water content of florets and peduncleand an increase in that of scales. The water in florets andthe peduncle is thought to migrate to scales and other tissuesduring the early stages of freezing; the dehydrated floret hasa lower freezing point which enhances its supercooling abilityand the dehydrated peduncle helps to maintain the supercooledstate of the florets. This hypothesis would explain the dependenceon the cooling rate of supercooling in Rhododendron flower buds.Water migration within flower buds was observed in other hardyRhododendron species with some variation in ice formation siteand the quantity of migrated water. The exotherm temperatureof excised florets was inversely proportional to their watercontent. Dehydration of flower buds by wind at 0°C alsoenhanced their supercooling ability. Mechanisms of freezingavoidance by supercooling in Rhododendron flower buds and therelationship of supercooling to freezing tolerance are discussed. ¹ Contribution No. 2254 from the Institute of Low TemperatureScience ² This is a revised form of the master's thesis of the seniorauthor (M.I.) which is cited in the present and previous papers(Sakai 1979a, b, etc.). (Received August 11, 1980; Accepted June 1, 1981)     

Influence of Temperature on the Effects of Water Stress on Quercus Species

Quercus coccifera L. and Q. ilex L. leaves, collected in winter,when the plants were in full turgor, were treated with polyethyleneglycol (PEG) with a water potential of –48 and –65bar, at 15 °C and at 30 °C, for 2 days. The responseof each species differs with temperature as far as the plastidpigment, anthocyanin and soluble sugar content is concerned.Thus Q. ilex is affected more at 15 °C rather than at 30°C, while in Q. coccifera, apart from a small increase inanthocyanins, no significant change was observed at 15 °C.On the contrary, at 30 °C significant changes were observedin all studied parameters. Chloroplast pigments decreased generallyon the first day and their content either dropped further orincreased to reach the control values on the second day. Alltreatments resulted in an increase in anthocyanin content. Solublesugar content raised in Q. ilex at 15 °C and decreased inboth species at 30 °C. Quercus coccifera, Quercus ilex, oak, temperature, water stress, anthocyanin, chlorophyll, carotenoids     

Photoperiod and Temperature Effects on Late Developmental Stages of Stylosanthes guianensis

Seedlings of Stylosanthes guianensis var. guianensis cv. Cookand S. guianensis var. pauciflora cv. Bandeirante were defoliatedand placed in a naturally lit glasshouse at 23/18 °C, 28/23°C or 33/28 °C (day/night). After exposure to 14 h daysand after floral induction with 30 cycles of 11 h, plants wereallocated to 11, 12, 13 or 14 h during flowering and seed formation. Floral initiation occurred after 10–15 short-day cycles.Flower appearance was hastened by warm temperatures and spikenumber per plant at 20 d after flower appearance was negativelyrelated to temperature and greater in Cook than in Bandeirante.Exposure to 13- and 14-h days reduced the continued differentiationof inflorescences in Bandeirante, and in Cook in warm temperatures.Floret number per spike was greatest at 23/18 °C and a higherproportion of florets aborted in Bandeirante at 33/ 28 °C.Variations in seed setting of the bi-articulate loment of Bandeiranteare described. Highest potential seed yield occurred if afterfloral induction 11 or 12 h days were maintained with 23/18°C or 28/23 °C temperatures. Photoperiod, temperature, development, Stylosanthes guianensis, flowering     

Supercooling ability of Rhododendron flower buds in relation to cooling rate and cold hardiness

The freezing process and supercooling ability in flower budsof 11 native Rhododendron species were examined with referenceto the cooling rate and cold hardiness by differential thermalanalysis. The freezing patterns of the excised whole buds variedwith the season: in autumn, buds froze as whole units, whilein winter, freezing was initiated in the scales and propagatedto each floret. The supercooling ability of florets was enhancedduring winter. The freezing patterns in winter buds were stronglyinfluenced by the cooling rate (1 to 30°C/hr). Althoughthe first exotherm in scales occurred at –5 to –10°Gand was rate-independent, the occurrence of several floret exothermsshifted considerably to lower subzero temperatures at slowerrates. The most reliable cooling rate for testing maximum supercoolingability was l°C/hr. The exotherm in florets of hardier speciesoccurred at –20 to –25°C and at –7 to–20°C for less hardy ones, and were well correlatedwith their killing temperatures. Water relations within budtissues in response to freezing are briefly discussed. (Received June 26, 1980; )     

Supercooling ability of Rhododendron flower buds in relation to cooling rate and cold hardiness

The freezing process and supercooling ability in flower budsof 11 native Rhododendron species were examined with referenceto the cooling rate and cold hardiness by differential thermalanalysis. The freezing patterns of the excised whole buds variedwith the season: in autumn, buds froze as whole units, whilein winter, freezing was initiated in the scales and propagatedto each floret. The supercooling ability of florets was enhancedduring winter. The freezing patterns in winter buds were stronglyinfluenced by the cooling rate (1 to 30°C/hr). Althoughthe first exotherm in scales occurred at –5 to –10°Gand was rate-independent, the occurrence of several floret exothermsshifted considerably to lower subzero temperatures at slowerrates. The most reliable cooling rate for testing maximum supercoolingability was l°C/hr. The exotherm in florets of hardier speciesoccurred at –20 to –25°C and at –7 to–20°C for less hardy ones, and were well correlatedwith their killing temperatures. Water relations within budtissues in response to freezing are briefly discussed. (Received June 26, 1980; )     

Induction and Growth of 'Microspore-Derived' Embryos of Brassica napus ssp. oleifera

Three cultivars of spring rape (Brassica napus ssp. oleifera),Tower, Willi and Duplo, were used for a study of induction andgrowth of ‘microspore-derived’ embryos, Buds, 2.0mm in length, containing uninucleate microspores were harvestedand stored for 14 d at 4 ?C in darkness. Anthers were then removedand cultured on a liquid medium based upon that of Murashigeand Skoog and containing 8% sucrose, 0.5 mg l^–1 naphthylaceticacid and 0.05 mg l^–1 benzylaminopurine. Cultures werepre-incubated at 35 ?C for 0–3 d and then incubated at30 ?C. After a total of 42 d incubation, cultures were scoredfor the presence of macroscopic embryos (1–2 mm in length)and for the presence of anthers containing abortive embryoidswhich had not developed further. The cultivars differed greatly in terms both of the frequencyof anthers showing induced embryoids and of the final yieldof embryos. Tower showed the highest frequency of induction(maximum 38% of cultured anthers with induced embryoids) whereasthe highest yield (equivalent to 1.1 embryo per cultured anther)was obtained from anthers of the cv. Duplo after a 3 d treatmentat 35 ?C. Yields from the other cultivars were much lower andwere relatively unaffected by the 35 ?C treatment. Key words: Brassica napus, Rape, Anther culture, Pollen, Haploid     

Supercooling Ability, Water Content and Hardiness of Rhododendron Flower Buds during Cold Acclimation and Deacclimation

The relationship between supercooling ability and water contentand killing temperature of flower buds during cold acclimationand deacclimation were studied using R. kiusianum and R. x akebono.The occurrence of multiple floret exotherms and their shiftto a narrow range at lower subzero temperatures, as well asthe marked decrease of florets water content, were observedas the symptoms of cold acclimation occuring in flower budsfrom fall to winter, and vice versa in spring buds during deacclimation.In R. kiusianum, the fully acclimated period was from Novemberto March and two months longer than that of R. x akebono. Thesupercooling ability of the former was about –25°Cand about –20°C in the latter. Although the watermigration within bud tissues during the freezing process wasdetermined in the acclimated and deacclimated buds for R. xakebono, no significant water changes could be observed, evenin the acclimated buds. Thus, it is conceivable that deep supercoolingin florets may result not necessarily from water migration fromflorets and bud axes to scales in response to freezing, butfrom low water content in situ of cold-acclimated or artificiallydehydrated flower buds. (Received July 29, 1981; Accepted October 12, 1981)     

The Effect of Temperature on the Production and Abscission of Flowers and Pods in Snap Bean (Phaseolus vulgaris L.)

The effect of temperature on production and abscission of flowerbuds, flowers and pods was studied in a determinate snap-beancultivar (cv. Tenderette). Under moderate temperature (e.g.27/17 °C) the onset of pod development was associated withcessation of flower bud production and with enhanced abscissionof flower buds. Raising night temperature from 17 °C to27 °C strongly reduced pod production, mature pod size andseeds per pod, while an increase in day temperature from 22°C to 32 °C had smaller and less consistent effects.Pod production under high night temperature was not constrainedby flower production since 27 °C at night promoted branchingand flower bud appearance. Under 32/27 °C day/night temperaturethe large reduction in pod set was due to enhanced abscissionof flower buds, flowers and young pods (< 3 cm). Flowershad the highest relative abscission followed by young pods andflower buds. Therefore, the onset of anthesis and of pod developmentwere the plant stages most sensitive to night temperature. Podslarger than 3 cm did not abscise but usually aborted and shrivelledunder high night temperature. The effects of 32/27 °C werenot due to transient water stresses and were observed even undercontinuous irrigation and mist-spraying. High temperature, flower production, pod set, seed set, abscission, snap bean, Phaseolus vulgaris L., cv. Tenderette     

Sucrose-Modulated Morphogenesis in Anagallis arvensis L.

Sucrose was found to have a modulating effect on the morphogenesisof Anagallis arvensis L. leaves cultured in a Murashige-Skoogmedium. Root formation and growth seem to be more independentthan other morphogenetic expressions. Roots formed without exogenoussugars at 25°C but sucrose seemed to be necessary at 32and 35°C. Sucrose at 3% improved shoot formation at 25°Cand had an inhibitory effect at 6%concentration and 35°C.Shoot growth (internode length) is inhibited by sucrose concentrationshigher than 3%. Sucrose could also replace light irradiancein regulating shoot and leaf growth. A higher sucrose concentration,than that required for roots and shoots formation, is necessaryfor flower and fruit formation, but sucrose could not replacethe photoperiod requirement for flowering in culture medium. (Received June 17, 1985; Accepted December 24, 1985)     

Influence of Temperature and Floret Age on Nectar Secretion in Trifolium repens L.

The influence of temperature on nectar secretion in non-pollinatedflorets of Trifolium repens was investigated in growth chambersat 10, 14, 18 and 22°C. The effect of temperature on therate of nectar secretion was significant in all clones. Theoptimum temperature for secretion in three clones varied from10°C for a clone of Icelandic origin, to 18°C in a cloneselected from a Danish variety. Similarly, the average nectaryield varied significantly among clones of different geographicalorigin. One clone secreted two to four times more than othersat 10°C. The optimum day temperature for nectar secretionwas higher when the plants were exposed to low night temperature,presumably a result of decreased night respiration. Nectar accumulatedat the floret base until senescence. Evidence for reabsorptionof nectar was obtained in four clones. Sucrose, fructose andglucose were identified as the major sugars in the nectar. Highnight temperatures led to decreased sucrose percentage in favourof glucose and fructose. The frequency of new florets openingper day was not influenced by temperatures between 10 and 22°Cin one clone, whereas low temperatures significantly decreasedthe number of new florets in another. Few or no modified stomatawere observed in the epidermis of the nectary. The high variationwith respect to nectar secretion at low temperatures, alongwith the high heritability of this quality, suggests that breedingfor high nectar production at low temperature is plausible.The significance of nectar yield in pollination biology is discussed.Copyright1994, 1999 Academic Press Trifolium repens, white clover, nectar, temperature, floret age, flowering, nectary     

Partitioning of [14C]sucrose and Acid Invertase Activity in Reproductive Organs of Pepper Plants in Relation to Their Abscission Under Heat Stress

The effect of heat stress on processes related to carbohydratepartitioning was investigated in young bell pepper (Capsicumannum L. cv. Maor) plants in relation to abscission of theirreproductive organs at different stages of development. None of the reproductive organs abscised after 5 d in a normalday/night temperature regime (25/18 °C). With a temperatureregime of 35 °C day, 25 °C night, abscission occurredin only a small portion of the flower buds and none of the flowersand fruitlets. However, when temperatures in the day and nightwere reversed (25/35 °C, day/night) all the buds and someof the flowers abscised during that time period. The young fruitat the first node did not abscise under any temperature regime.The abscission rate of the flower buds was reduced under heatstress if the developing fruit at the first node had been removed. High temperature during either the light or dark periods reducedthe export of [¹⁴C]sucrose from the source leaf (fed for 48h with [¹⁴C]sucrose). Both heat stress and fruit presence reduced the relative amountof [¹⁴C]sucrose which was exported to the flower buds, flowersand roots. Likewise, these treatments reduced the concentrationof reducing sugars in the reproductive organs. Concomitantly,the heat stress and fruit presence on the first node reducedthe activity of soluble acid invertase in the flower buds andthe roots, but not in young leaves. Overall, the results show that heat stress causes alternationin sucrose distribution in the plant, but may also have specificeffects on metabolic activities related to sucrose import andutilization in flower buds and flowers which in turn may enhancetheir abscission. Bell pepper, (Capsicum annuum L. cv. Maor), abscission, acid invertase, heat stress, reproductive organs, sink leaves     

Partitioning of [14C]sucrose and Acid Invertase Activity in Reproductive Organs of Pepper Plants in Relation to Their Abscission Under Heat Stress

The effect of heat stress on processes related to carbohydratepartitioning was investigated in young bell pepper (Capsicumannum L. cv. Maor) plants in relation to abscission of theirreproductive organs at different stages of development. None of the reproductive organs abscised after 5 d in a normalday/night temperature regime (25/18°C). With a temperatureregime of 35°C day, 25°C night, abscission occurredin only a small portion of the flower buds and none of the flowersand fruitlets. However, when temperatures in the day and nightwere reversed (25/35°C, day/night) all the buds and someof the flowers abscised during that time period. The young fruitat the first node did not abscise under any temperature regime.The abscission rate of the flower buds was reduced under heatstress if the developing fruit at the first node had been removed. High temperature during either the light or dark periods reducedthe export of [¹⁴C]sucrose from the source leaf (fed for 48h with [¹⁴C]sucrose). Both heat stress and fruit presence reduced the relative amountof [¹⁴C]sucrose which was exported to the flower buds, flowersand roots. Likewise, these treatments reduced the concentrationof reducing sugars in the reproductive organs. Concomitantly,the heat stress and fruit presence on the first node reducedthe activity of soluble acid invertase in the flower buds andthe roots, but not in young leaves. Overall, the results show that heat stress causes alternationin sucrose distribution in the plant, but may also have specificeffects on metabolic activities related to sucrose import andutilization in flower buds and flowers which in turn may enhancetheir abscission. Bell pepper, (Capsicum annuum L. cv. Maor), abscission, acidinvertase, heat stress, reproductive organs, sink leaves. Bell pepper, (Capsicum annuum L. cv. Maor), abscission, acid invertase, heat stress, reproductive organs, sink leaves.     

Environmental Factors Controlling Flower Opening and Closing in aPortulacaHybrid

To examine flower opening and closing of aPortulacahybrid, flowerbuds were placed in darkness for 12 h (2030–0830 h) at20 °C and then exposed to various light-temperature conditions.Flower buds exposed to light at 25, 30 or 35 °C opened within1 h, and wilted 10–14 h later. Flower buds exposed tolight at 20 °C started to open after 4 h but opened slowlyand not completely. Flower buds subjected to 25, 30 or 35 °Cin darkness also opened rapidly, but did not reach full opening.Flowers opened at 30 °C in light, and partially closed andopened repeatedly in response to cycles of a 2-h exposure to20 °C and a 2-h exposure to 30 °C at any time between1000 to 1600 h. Similar phenomena were observed when the flowersopened at 30 °C in light and then were subjected to darknessand light alternately at 30 °C, although the effect of lightwas less obvious than that of alternating temperature. Floweropening and closing were not affected by relative humidity.These results indicate that a rise in temperature is requiredfor rapid flower opening in the buds kept at 20 °C, andthat light intensifies the effect of high temperature. Exposureto light without a temperature change delayed and slowed floweropening which was never complete. The involvement of an endogenousrhythm in flower opening byPortulacais indicated. Portulacahybrid, flower opening, flower closing, temperature shift, endogenous rhythm.     

Influence of Genotype, Plant Growth Temperature and Anther Incubation Temperature on Microspore Embryo Production in Brassica napus ssp. oleifera

Eleven F₁ hybrid genotypes of winter rape (Brassica napus ssp.oleifera) were used in a study of induction and growth of microspore-derivedembryos. Plants of each genotype were grown in controlled environmentsat either a constant 15°C or a constant 20°C, both witha 16 h photoperiod. Equal numbers of buds, approximately 2.5mm in length, containing uninucleate microspores were harvestedfrom each genotype and either pretreated (14 d at 4°C) ordissected immediately after harvest. Anthers were cultured onliquid medium based upon that of Murashige and Skoog (1962)and containing 8% sucrose, 0.5 mg dm^–3 naphthylaceticacid and 0.05 mg dm^–3 benzylaminopurine. Anthers fromequal samples of buds were incubated at 35°C for 0, 1, 2or 3 d before transfer to 30°C (21 d) and then 25°C.After a total of 42 d incubation, cultures were scored for thepresence of macroscopic embryos (1–2 mm in length) andfor the presence of anthers containing aborted embryoids whichhad not developed further. The results showed first that bud pretreatment completely inhibitedinduction and secondly that anthers of all genotypes had anabsolute requirement for a 35°C treatment (optimal duration2 d) in order to induce embryoid formation. In the great majorityof genotypes plants grown at 15°C provided more productiveanthers than plants grown at 20°C. However, within eachtreatment there were great differences both in the frequencyof anthers showing induced embryoids and of the final yieldof embryos. There was evidence that hybrids with a common parentresponded similarly under certain treatments. This confirmedthe importance of genotypic control for some components of embryoyield. Key words: Brassica napus, Rape, Anther culture, Pollen, Haploid     

Effects of Irradiance and Temperature on Flowering of Chrysanthemum morifolium Ramat. in Continuous Light

The short-day plant Chrysanthemum morifolium cv. Polaris initiatedflower buds in all irradiances of continuous light from 7.5to 120 W m^–2. As the irradiance increased, the transitionto reproductive development began earlier and the number ofleaves initiated before the flower bud was reduced. The autumn-floweringcultivars Polaris and Bright Golden Anne, and the summer-floweringGolden Stardust were also grown in continuous light at differenttemperatures; all initiated flower buds at temperatures from10 to 28 °C but only the buds of Golden Stardust developedto anthesis and then only at 10 and 16°C. Flower initiationbegan earliest at 16–22 °C, and the number of leavesformed before the flower bud was increased at 28°C. GoldenStardust was exceptional in that the number of leaves formedwas also increased at 10 °C. Axillary meristems adjacentto the terminal meristem initiated flower buds rapidly at 10°C but not at 28 °C in all three cultivars. These resultsare discussed in relation to the autonomous induction of flowerinitiation and the effects of the natural environment on floweringof chrysanthemum. Chrysanthemum morifolium Ramat, flowering, irradiance, temperature     

Low Temperature Treatments Induce an Increase in the Relative Content of Both Linolenic and {lambda}3-Hexadecenoic Acids in Thylakoid Membrane Phosphatidylglycerol of Squash Cotyledons

The effect of low temperatures on the fatty acid compositionof phosphatidylglycerol (PG) in thylakoid membranes, in particularon the ratios of nmol% 16:1(3t) (mg fresh weight)^–1 ofcotyledons and nmol 16:1(3t) (mg chlo rophyll)^–1 weremeasured during squash seedling growth. Plants were germinatedand grown for one day at 30°C then were either kept at 30°C(control plants) or trans ferred to low temperatures (18, 14or 10°C). When plant were transferred from 30°C to lowtemperatures, the increase in fresh weight was gradually limited.The lowe the temperature, the smaller was the fresh weight.In contrast, the relative content of 16:1(3t) and 18:3, as wella the ratios of nmol 16:1(3t) (mg chlorophyll)^–1 and mol%16:1(3t) (mg cotyledon fresh weight)^–1 increased indicatingthat the increase of fresh weight and chlorophyll was mor sensitiveto low temperature than PG desaturation in thyla-koid membranes.Furthermore, low temperatures inducei an increase in 16:1(3t)and 18:3 (the final products of PC synthesis) at the expenseof 16:0 and 18:1 (the initial products of PG synthesis). However,within a range of temperature from 10 to 18°C, the extentof these changes (nmol% of 18:3 or 16:1(3t) per day) was graduallylimited by lower temperatures. We therefore propose that lowtemperature inhibit both fatty acid synthesis and desaturationactivities. However, at low temperatures the fatty acid synthesisis likely to be more strongly inhibited than the desaturationactivities, thus explaining the observed increase in the relativecontent of PG-18:3 and PG-16:l(3t). Results an discussed interms of the mechanism which could be in volved in the metabolismof PG in squash cotyledons. (Received July 5, 1996; Accepted March 10, 1997)     

Bulbing in Long-day Onion (Allium cepa L.) Cultured In Vitro: Comparison Between Sugar Feeding and Light Induction

Bulbs were obtained on onion plants cultured in vitro. No bulbinghappened under long days with fluorescent light and 30–40g l^–1 sucrose. Bulb formation was mainly dependent eitheron sucrose concentration in the culture medium, or on lightspectral composition. Raising the sucrose concentration from40 to 120 g l^–1 increased plant basal swelling and stoppedfurther vegetative development. These plants were not dormant.When fluorescent light was enriched in incandescence duringa long day period, bulbs were obtained in two months and underwenta consecutive dormancy. Bulb, dormancy, light spectral quality, photoperiod, R: FR ratio, sugar, tissue culture     

The Freezing Response of an Arctic Cushion Plant, Saxifraga caespitosa L.: Acclimation, Freezing Tolerance and Ice Nucleation

Cold hardiness in actively growing plants of Saxifraga caespitosaL., an arctic and subarctic cushion plant, was examined. Plantscollected from subarctic and arctic sites were cultivated ina phytotron at temperatures of 3, 9, 12 and 21 °C undera 24-h photoperiod, and examined for freezing tolerance usingcontrolled freezing at a cooling rate of 3–4 °C eitherin air or in moist sand. Post-freezing injury was assessed byvisual inspection and with chlorophyll fluorescence, which appearedto be well suited for the evaluation of injury in Saxifragaleaves. Freezing of excised leaves in moist sand distinguishedwell among the various treatments, but the differences werepartly masked by significant supercooling when the tissue wasfrozen in air. Excised leaves, meristems, stem tissue and flowerssupercooled to –9 to –15 °C, but in rosettesand in intact plants ice nucleation was initiated at –4to –7 °C. The arctic plants tended to be more coldhardy than the subarctic plants, but in plants from both locationscold hardiness increased significantly with decreasing growthtemperature. Plants grown at 12 °C or less developed resistanceto freezing, and excised leaves of arctic Saxifraga grown at3 °C survived temperatures down to about –20 °C.Exposure to –3 °C temperature for up to 5 d did notsignificantly enhance the hardiness obtained at 3 °C. Whenwhole plants of arctic Saxifraga were frozen, with roots protectedfrom freezing, they survived –15 °C and –25°C when cultivated at 12 and 3 °C, respectively, althougha high percentage of the leaves were killed. The basal levelof freezing tolerance maintained in these plants throughoutperiods of active growth may have adaptive significance in subarcticand arctic environments. Saxifraga caespitosa L., arctic, chlorophyll fluorescence, cold acclimation, cushion plant, freezing stress, freezing tolerance, ice nucleation, supercooling     

The Growth and Development of the Wheat Apex: The Effects of Photoperiod on Spikelet Production and Sucrose Concentration in the Apex

Spring wheat (Triticum aestivum cv. Warimba) plants were grownin a controlled environment (20°C) in two photoperiods (8or 16 h). In the first instance, plants were maintained in eachof the photoperiods from germination onwards at the same irradiance(375 µE m^–2 s^–1). In the second case, allplants were grown in a long photoperiod until 4 days after double-ridgeinitiation when half the plants were transferred to a shortphotoperiod with double the irradiance (16 h photoperiod at225 or 8 h at 475 µE ^–2 s^–1). The rates of growth and development of the apices were promotedby the longer photoperiod in both experiments. Shoot dry weightgain was proportional to the total light energy received perday whereas the dry weight of the shoot apex increased withincreasing photoperiod even when the total daily irradiancewas constant. The principal soluble carbohydrate present in the shoot apexwas sucrose, although low concentrations of glucose and fructosewere found in the apices of long photoperiod plants late indevelopment. Sucrose concentration was invariably greater inthe slow-growing apices of short photoperiod plants, but roseto approach this level in the long photoperiod plants when theterminal spikelet had been initiated. Triticum aestivum, wheat, apex, spikelet initiation, photoperiod, flower initiation