Comparative Analysis of Tuber Development in Six Sweet Potato (Ipomoea batatas (L.) Lam) Cultivars: 1. Tuber initiation, tuber growth and partition of assimilate

Differences in tuber initiation, tuber growth and partitionof assimilate during development were investigated in six localsweet potato cultivars. Study of the quantitative morphogenesisof root types in the sweet potato root system indicated thattuber initiation was completed by 8 weeks after planting inmost cultivars and frustrated thereafter. Subsequent differencesin patterns of tuber development were not obviously relatedto final tuber yield. Thus, high yield resulted either froma short period of rapid tuber growth or a longer period of slowertuber growth. Maximum shoot growth was achieved by week 12 inall except one cultivar, and the cessation of shoot growth resultedin either increases or decreases in tuber growth rate or hadno affect on the rate of tuber growth (cv. A28/7). The partitionof assimilate to tubers at final harvest was the parameter mostclosely related to the yield of the six cultivars studied, butthere was some evidence that such partition was related to totaldry weight. It is suggested that limiting factors in sweet potatotuber yield, e.g. assimilate production and transport or capacityfor tuber growth varied with the cultivar studied.     

Biological Nitrogen Fixation in Pachyrhizus ahipa (Wedd.) Parodi

The patterns of plant growth and N₂ fixation capability in Pachyrhizusahipa (Wedd) Parodi inoculated with Bradyrhizobium ‘PachyrhizusSpec 1’ strains (Lipha Tech) were investigated in a zero-Nculture system under greenhouse conditions The P ahipa plantis day-neutral with respect to reproductive development Competitionoccurred between the two storage organs (legume and tuber) andprevented high tuber yield in P ahipa The symbiotic effectivenessof the association was high, as the profuse nodulation providedthe inoculated plants with adequate amounts of N Nodules werepresent throughout the cycle of P ahipa The change in rate ofN₂ fixation (RNF) and relative growth rate (RGR) was almostparallel during ontogenesis The developmental pattern of N₂fixation activity revealed that 65% of total N₂ fixation occurredafter N began to accumulate in the reproductive (pod wall plusseed) tissue During pod filling allocation of N compounds tothe seeds exceeded N₂ fixation, the pod walls being the primarysource of redistributed N, followed by the leaves. Pachyrhizus ahipa (Wedd) Parodi, ahipa, tuber crop, dinitrogen fixation, dry matter, N partitioning, reproductive growth     

An Analysis of the Relation between Dry Matter Allocation to the Tuber and Earliness of a Potato Crop

Compared with late cultivars, early potato cultivars allocatea larger part of the available assimilates to the tubers earlyin the growing season, leading to shorter growing periods andlower yields. A dynamic simulation model, integrating effectivetemperature and source –sink relationships of the crop,was used to analyse this relation, using data from experimentsin the Netherlands carried out over 5 years. Dry matter allocationto the tuber in these field experiments was simulated well whenthe tuber was considered as a dominant sink that affects earlinessof a potato crop in two ways: early allocation of assimilatesto the tubers stops foliage growth early in the season and reducesthe longevity of individual leaves. In a sensitivity analysisthe influence of tuber initiation, leaf longevity and the maximumrelative tuber growth rate (R_tb) on assimilate allocation andcrop earliness was evaluated. It was found that the maximumrelative tuber growth rate can influence crop earliness morethan the other two factors, but when conditions for tuber growthare optimal, the leaf longevity is most important. Solanum tuberosumL.; simulation model; source –sink relationships; cultivars     

A comparison of nitrate transport in four different rice (Oryza sativa L.) cultivars

As rice can use both nitrate (NO₃^-) and ammonium (NH₄⁺), we have tested the hypothesis that the shift in the pattern of cultivars grown in Jiangsu Province reflects the ability of the plants to exploit NO₃^- as a nitrogen (N) source. Four rice cultivars were grown in solution culture for comparison of their growth on NO₃^- and NH₄⁺ nitrogen sources. All four types of rice, Xian You 63 (XY63), Yang Dao 6 (YD), Nong Keng 57 (NK) and Si You 917 (SY917), grew well and produced similar amounts of shoot biomass with 1 mmol/L NH₄⁺ as the only N source. However, the roots of NK were significantly smaller in comparison with the other cultivars. When supplied with 1 mmol/L NO₃^-, YD produced the greatest biomass; while NK achieved the lowest growth among the four cultivars. Electrophysiological measurements on root rhizodermal cells showed that the NO₃^--elicited changes in membrane potential (ΔE_m) of these four rice cultivars were significantly different when exposed to low external NO₃^- (<1 mmol/L); while they were very similar at high external NO₃^- (10 mmol/L). The root cell membrane potentials of YD and XY63 were more responsive to low external NO₃^- than those of NK and SY917. The ΔE_m values for YD and XY63 rhizodermal cells were almost the same at both 0.1 mmol/L and 1 mmol/L NO₃^-; while for the NK and SY917 the values became larger as the external NO₃^- increased. For YD cultivar, ΔE_m was measured over a range of NO₃^- concentrations and a Michaelis-Menten fit to the data gave a K_m value of 0.17 mmol/L. Net NO₃^- uptake depletion kinetics were also compared and for some cultivars (YD and XY63) a single-phase uptake system with first order kinetics best fitted the data; while other cultivars (ND and SY917) showed a better fit to two uptake systems. These uptake systems had two affinity ranges: one had a similar K_m in all the cultivars (0.2 mmol/L); the other much higher affinity system (0.03 mmol/L) was only present in NK and SY917. The expression pattern of twelve different NO₃^- transporter genes was tested using specific primers, but only OsNRT1.1 and OsNRT2.1 expression could be detected showing significant differences between the four rice cultivars. The results from both the physiological and molecular experiments do provide some support for the hypothesis that the more popular rice cultivars grown in Jiangsu Province may be better at using NO₃^- as an N source.     

The use of chlorophyll fluorescence and leaf gas exchange as methods for studying the different responses to ozone of two bean cultivars

After exposure to 80 ppb of ozone for a single 4 h period, asignificant reduction in photosynthetic activity was rapidlyinduced in two cultivars of Phaseolus vulgaris, cv. Pinto, O₃-sensitiveand cv. Groffy, O₃-resistant. Pinto displayed a strong and irreversiblereduction in A_max accompanied by stomatal closure and an increasein intercellular CO₂ concentration. The fluorescence parametersindicated that the electron transport around PSII had been altered.The increase in non-photochemical quenching was related to anenergy dissipation through non-radiative mechanisms. Photosynthesiswas also reduced in the cv. Groffy, but the effect was reversiblewith time. Groffy showed alteration in the photochemical functioningof the thylakoids and specific disturbances of the water-splittingenzyme system of PSII, but these alterations disappeared only20 h after the end of fumigation. Recovery of photosyntheticability was more delayed with time and needed 7 d to show valuessimilar to the controls. Physiological tools, such as gas exchangeand chlorophyll fluorescence, play a major role in the earlydetection of environmental stress such as O₃ pollution, andin distinguishing the responses between cultivars with differentO₃-sensitivity. Key words: Gas exchange measurements, nonphotochemical quenching, Phaseolus vulgaris, photochemical quenching, stomatal conductance     

Ventilation in Plant Tissue Cultures and Effects of Poor Aeration on Ethylene and Carbon Dioxide Accumulation, Oxygen Depletion and Explant Development

A simple technique for comparing and quantifying the ventilationcapacity of vessels used for plant tissue culture is described.Ethylene was injected into culture vessels and its rate of lossmonitored by gas chromatography. From the resulting exponentialdecay curves, the time in hours for half the ethylene to belost (t₅₀) was calculated and used to compare different containersand sealing methods. Cultures of Ficus lyrata Warb. and Gerberajamesonii Bolus grown for up to 28 d in plastic vessels sufficientlywell-sealed to generate t₅₀ values of approx. 16 h, accumulatedethylene and carbon dioxide in association with depleted oxygen.The relationship between carbon dioxide accumulation and oxygendepletion within culture vessels indicated little if any anaerobicrespiration. Gerbera explants did not appear to be affectedby these gaseous environments. However, in Ficus, leaf expansionwas approximately halved, although fresh and dry mass of wholeshoots was not decreased. The smaller leaf size is attributedto the action of accumulated ethylene, because when the gaswas absorbed with 'Ethysorb' granules or its action inhibitedby 2,5–norbornadiene, leaf growth was normal. The removalof carbon dioxide with potassium hydroxide did not enhance theethylene effect, indicating little if any antagonism of ethyleneaction by carbon dioxide. Shoots of potato (Solanum tuberosumL. cv. Red Craig's Royal) were shortened in sealed culture vessels,in association with swelling, diageotropism and miniaturizationof the leaves. When tuber production was induced by decreasingthe photoperiod, increasing the sucrose concentration and includingcytokinin in the medium, partial sealing promoted conspicuoushypertrophy of the lenticels. These responses of potato wereprevented if the ethylene absorbant mercuric perchlorate wasenclosed together with the cultures. Plant tissue culture, poor aeration, ethylene, leaf expansion, Ficus lyrata Warb., Gerbera jamesonii Bolus, Solanum tuberosum L. cv. Red Craig's Royal     

Intraspecific variation in the response of rice (Oryza sativa L.) to increased CO2 and temperature: growth and yield response of 17 cultivars

Seventeen rice (Oryza sativa L.) cultivars of contrasting ecosystemsand origins were exposed to two CO₂ concentrations (373 [ambient]and 664 µl l^–1 CO₂ [elevated]) at two differentday/night temperatures (29/21 C and 37/29 C) in glasshousesat the International Rice Research Institute phytotron duringthe dry seasons of 1994 and 1995. Growth at elevated CO₂ (asdetermined by total plant biomass at maturity) increased byan average of 70% and 22%, respectively, for all cultivars forgrowth temperatures of 29/21 C and 37/29 C relative to theambient CO₂ treatment. At the 29/21 C optimal growth temperature,grain yield increased on average c. 50% with enriched CO₂. Incontrast, at the higher growth temperature (37/29 C), grainyield was almost zero, presumably due, in part, to temperature-inducedinfertility (i.e. the percentage of filled spikelets was <1%).Among cultivars, lAC 165, a tropical japonica from Brazil, showedthe largest relative increase in both biomass and grain yield.While the range of responses to increased CO₂ and/or temperaturewere quite large (e.g. 10–250%) and may not be applicableto field conditions, data indicate that lines are availablewhich could maximize productivity as CO₂ concentration increases.Additional work, however, would be needed to identify cultivarswhich would maintain maximum yields in a high CO₂, high temperatureenvironment. Key words: Plant growth analysis, experimental design, computational methods, relative growth rate, net assimilation rate     

Ribulose 1, 5-bisphosphate Carboxylase/ Oxygenase Activities in Leaves of Greenhouse Roses

An enzyme assay was developed to measure the initial and Mg²⁺–CO₂activated forms of Ribulose 1,5-bisphosphate Carboxylase/Oxygenase(Rubisco) in rose leaves. The assay was verified by co-extractionof the leaflets with partially purified spinach Rubisco andthrough correlation with net photosynthetic rates of individualleaflets (r²=0.7324). Changes in activities were measured asa function of depth of leaves in the canopy for two cultivarsof greenhouse hybrid tea roses. Initial Rubisco activity declinedwith increasing canopy depth for both cultivars. The activatedform of the enzyme, however, remained constant with canopy depthfor cv. Red Success; but increased with canopy depth, then declinedafter mid-canopy in the cv. Royalty. Rubisco activities werealso measured in the cv. Red Success grown in CO₂ enriched environments(100 mm³ dm^–3) at three humidity levels. The activitieswere not significantly affected by humidity treatment. However,there was a trend for plants grown at lower humidity to havehigher activated activities. Key words: Humidity, Rubisco, Rosa ? hybrida, Royalty, Red Success     

The Distribution of Nitrate Assimilation Between Root and Shoot in Vicia faba L.

Nitrate assimilation was examined in two cultivars (Banner Winterand Herz Freya) of Vicia faba L. supplied with a range of nitrateconcentrations. The distribution between root and shoot wasassessed. The cultivars showed responses to increased applied nitrateconcentration. Total plant dry weight and carbon content remainedconstant while shoot: root dry weight ratio, total plant nitrogen,total plant leaf area and specific leaf area (SLA) all increased.The proportion of total plant nitrate and nitrate reductase(NR) activity found in the shoot of both cultivars increasedwith applied nitrate concentrations as did NO₃^–: Kjeldahl-Nratios of xylem sap. The cultivars differed in that a greaterproportion of total plant NR activity occurred in the shootof cv. Herz Freya at all applied nitrate concentrations, andits xylem sap NO₃^–: Kjeldahl-N ratio and SLA were consistentlygreater. It is concluded that the distribution of nitrate assimilationbetween root and shoot of V. faba varies both with cultivarand with external nitrate concentration. Vicia faba L., field bean, nitrate assimilation, nitrate reductase, xylem sap analysis     

Shoot Formation from Explant Cultures of Fourteen Potato Cultivars and Studies of the Cytology and Morphology of Regenerated Plants

Plants were regenerated in quantity from cultured excised stem,rachis or leaf pieces of 13 potato (Solarium tuberosum L.) cultivarsusing modifications of a two-step procedure. Pronounced differenceswere observed in the response of different cultivars and explantsources to medium formulations containing different auxins orcytokinins. A few plants were recovered from tuber pieces ofseven cultivars by a different two-step procedure. Morphologicaldifferences from control plants were observed in some of theprimary regenerants. Anthocyanin pigmentation of tubers wasscored and stable changes to red-splashed white or white tuberswere found in some plants regenerated from cv. Desiree. Regenerantsfrom the particoloured cultivars Cara and King Edward oftenlost this trait but it returned to varying extents in subsequentgenerations. Most of the regenerated plants (87 per cent) containedthe euploid number of chromosomes (2n = 4x = 48). The methodsare suitable for production of plants for assessment of somaclonalvariation in a range of genotypes. Potato breeding, somaclonal variation, chromosome variation, skin colour, explant culture, plant regeneration, solanum tuberosum L. potato     

Wheat Cultivars Respond Differently to a Drying Top Soil and a Possible Non-Hydraulic Root Signal

Research has shown that when plant roots are exposed to a dryingsoil a non-hydraulic (chemical) signal is produced in the rootand transported to the shoot, causing stomatal closure and growthretardation. This study was designed to reveal genetic diversityin wheat response to soil conditions which elicit a root signal,as the first step in the investigation of the genetic controlof the production of and the response to the root signal. Five spring wheat (Triticum aestivum L.) cultivars were establishedin the growth chamber in soil-filled polyvinyl chloride tubes,120 cm long and of an internal diameter of 10·2 cm. Soilwas well fertilized and wet to field capacity at emergence whentwo treatments were imposed: (1) tubes were watered from thetop as needed to eliminate stress (control); and (2) tubes hada constant water table at a soil depth of 100 to 120 cm, withno applied water. Measurements were performed on five dateson leaf water status and stomatal diffusive resistance. Above-groundbiomass and grain yield per plant were determined at maturity. The water table treatment resulted in dry and hard top soilconditions which were previously indicated to elicit a possibleroot signal. Under these experimental conditions, cultivarsdiffered in their leaf water status, stomatal diffusive resistance(R_s) and plant production. In the control treatment, R_s of cultivarsincreased with reductions in their relative water content (RWC)and leaf water potential (LWP), indicating the expected controlof R_s by leaf water status. Under conditions of a drying topsoil, relative water content (RWC) and leaf water potential(LWP) increased in cultivars that had a higher R_s, indicatingthat stomatal activity was controlling leaf water status. Itwas therefore suggested that the drying top soil elicited aroot signal which caused stomatal closure and reduced plantproduction. Under such conditions, two cultivars (Bethlehemand V748) consistently maintained relatively low R_s and highplant production, despite their relatively lower RWC and LWP,as compared with cvs C97, V747 and V652. Limited observationssuggest that in these two cultivars relatively fewer roots mayhave been exposed to the drying top soil, as compared with theother three cultivars. Key words: Triticum aestivum, cultivars, soil moistrue, drought stress, root, root signal, stomata, relative water content, leaf water potential, biomass, yield     

Effects of CO2 Enrichment at Different Irradiances on Growth and Yield of Wheat: I. EFFECTS OF CULTIVAR AND OF DURATION OF CO2 ENRICHMENT

Wheat, Triticwn aestivum L., the winter cultivars Hobbit andCappelle-Desprez, and the spring cultivars Sicco and KJeiber,were grown in normal air or air enriched with CO₂ either outdoorsin a glass-roofed cage or in controlled environment rooms. Inneither the winter nor the spring wheat was growth increaseddue to enrichment with CO₂ before anthesis. Enrichment of thetwo winter wheat cultivars increased shoot dry weight significantlyat 15 d after anthesis but produced no significant increasein grain yield. With the spring cultivars there was a significantincrease in shoot dry weight by 18 d after anthesis and thegrain yield was also larger due to an increase in grain size.Shoot weight increased because the stems were larger, and therewas a diversion of assimilate from grain growth to late tillerproduction. Root tissue comprised less than 20% of the totaldry matter at anthesis (for all cultivars); effects of CO₂ enrichmenton root growth appeared to be less important than effects onshoot and ear growth. Growth and yield responses to CO₂ enrichmentwere observed (for the spring cultivars) at irradiances of both250 and 635 µE m^–2 s^–1, but the effects weregreater at the lower irradiance. Key words: CO₂ enrichment, Wheat, Cultivar     

Levels of endogenous ethylene, carbon dioxide, and soluble pectin, and activities of pectin methylesterase and polygalacturonase in ripening tomato fruits

In 4 cultivars of tomato (Lycopersicon esculentum Mill.), theearly detachment of fruits advanced ripening and considerablyreduced the threshold value of endogenous C₂H₄. This indicatesa supply from the vegetative parts of (a) labile ripening-inhibitingsubstance(s) antagonizing the action of C₂H₄. The endogenous level of CO₂ increased shortly after the risein C₂H₄, and maximum levels of C₂H₄ and CO₂ occurred almostsimultaneously. The activity of PE showed no connection with ripening, but PGactivity did not occur until the onset of ripening. However,this activity increased at considerably higher C₂H₄ concentrationsthan the rise in WSP, and was independent of the possible presenceof ripening inhibitor(s). Hence PG is considered not to be involvedin the primary events leading to fruit ripening. Exposure of fruits to different C₂H₄ concentrations in the ambientatmosphere also showed PG activity to increase only after therise in WSP had started. Other pectin degrading or synthesizingenzymes may be involved. In the non-ripening Rin mutant of cv. Rutgers, no rise occurredin C₂H₄, CO₂, WSP, and PG activity. ¹ Present address: Department of Agricultural Chemistry, Facultyof Agriculture, Kochi University, Otsu 200 Monobe, Nangoku City,Kochi Prefecture 783, Japan. (Received February 16, 1978; )     

Distribution of Endogenous Gibberellins in Vegetative Shoots of Rice

Levels of endogenous gibberellins in rice seedlings (Oryza sativaL., cv. Nipponbare) were compared between young and old leavesat the 4- and 5-leaf stages. The levels of GA₁, GA₁₉ and GA₅₃were higher in the youngest leaf than in older leaves at the5-leaf stage, but they did not differ significantly betweenthe leaf sheath and the leaf blade. At the 4-leaf stage, thelevel of GA₁, was highest in the third leaf sheaths which containedyoung elongating tissues. These results indicate that gibberellinsare synthesized in young vegetative tissues to promote theirelongation growth. The levels of GA₁ in the youngest leaf sheathsof two cultivars of dwarf rice, Tan-ginbozu and Waito-C, wereapproximately 10% of that in the normal rice at the 5-leaf stage.This result could explain the retardation of shoot elongationin these dwarf cultivars. (Received February 15, 1995; Accepted June 1, 1995)     

NaCl-induced Senescence in Leaves of Rice (Oryza sativa L.) Cultivars Differing in Salinity Resistance

The influence of NaCl on senescence-related parameters (proteinand chlorophyll concentrations, membrane permeability and chlorophyllfluorescence) was investigated in young and old leaves of fiverice cultivars differing in salt resistance. NaCl hastened thenaturally-occurring senescence of rice leaves which normallyappears during leaf ontogeny: it decreased chlorophyll and proteinconcentrations and increased membrane permeability and malondialdehydesynthesis. Such an acceleration of deteriorative processes affectedall leaves in salt-sensitive cultivars while it was more markedin oldest than in youngest leaves of salt-resistant genotypes.NaCl-induced senescence also involved specific modifications,such as an increase in basal non-variable chlorophyll fluorescence(F ₀) recorded in all cultivars or a transient increase in solubleprotein concentration recorded in salt-resistant genotypes only.Alteration of membrane permeability appeared as one of the firstsymptoms of senescence in rice leaves and allowed discriminationamong cultivars after only 7 d of stress. In contrast, F _v/F _mratio (variable fluorescence/maximal fluorescence) was thesame for all cultivars during the first 18 d of stress and thuscould not be used for identifying salt-resistant rice exposedto normal light conditions. Relationships between parametersinvolved in leaf senescence are discussed in relation to salinityresistance of rice cultivars. Chlorophyll concentration; chlorophyll fluorescence; electrolyte leakage; magnesium; malondialdehyde; membrane permeability; NaCl; Oryza sativa L.; protein; rice; salinity resistance; senescence; UV absorbing substances     

Hormonal Regulation of Morphogenesis and Cold-resistance: II. EFFECT OF COLD-ACCLIMATION AND OF EXOGENOUS ABSCISIC ACID ON GIBBERELLIC ACID AND ABSCISIC ACID ACTIVITIES IN ALFALFA (Medicago sativa L. SEEDLINGS

The endogenous levels of gibberellin and abscisic acid weredetermined in extracts from seedlings of alfalfa (Medicago sativaL.) ov. Hairy Peruvian and cv. Ranger, growing under long daysand high temperatures (not inducing cold-hardiness), or shortdays and low temperatures (inducing cold-hardiness). Under inductiveconditions only Ranger was coldacclimated, exhibiting a rosettegrowth; non-acclimated seedlings of Ranger and Hairy Peruviandeveloped an elongated shoot. Under non-inductive conditionsgibberellin A₃ (GA₃)-like activity was found in both cultivars.Under inductive conditions GA3-like activity increased in HairyPeruvian and was almost non-existent in Ranger. In spite ofmorphological modification, ABA-like activity was hardly affectedby thermophotoperiod conditions. Addition of ABA to the nutrientsolution of seedlings growing under non-inductive conditionssimulated the effects of short days and of low temperatures.It diminished GA3 content, and affected morphological modificationof the seedlings. It is concluded that the modification of theABA/GA balance, through the decrease of the GA level, monitorsthe capacity of the twoalfalfa cultivars to become cold-acclimatedwhen exposed to low temperatures.     

Mixed Nitrogen Nutrition and Productivity of Wheat Grown in Hydroponics

The objective of this study was to study the effects of nitrogen(N) supplied as either mixtures of NO₃ and NH₄ or as all NO₃on the final yield of spring wheat. Two separate greenhouseexperiments evaluated the durum spring wheat (Triticum durumL.) cultivar ‘Inbar’ in 1986, and the hard red springwheat (Triticum aestmum L.) cultivar ‘Len’ in 1987.Nitrogen treatments consisted of all NO, or mixtures (75/25or 50/50) of NO₃ and NH₄. At maturity, plants were harvested,separated into leaves, stems, roots, and grain, and each partanalysed for dry matter and chemical composition Compared to plants receiving only NO₃ as the source of N, mixedN nutrition resulted in greater accumulation of whole plantreduced-N (49 to 108% more), phosphorus (38 to 69% more), andpotassium (25% more) for both cultivars. In all cases, plantsproduced higher grain yields (28% for Len to 78% for Inbar)when grown with mixed N nutrition than with only NO₃. The yieldincrease was not associated with heavier grains or more grainsper ear, but rather with an increase in the number of ear-bearingtillers per plant. For both cultivars, the higher yields withmixed N resulted from the production of more total biomass (36to 76%) as the partitioning of dry matter between plant partswas not altered by N treatment. Under the hydroponic conditionsof this experiment, the utilization of both NO₃ and NH₄ resultedin greater growth, nutrient absorption, and yield than NO₃ alone,which was primarily associated with an enhancement in tillerdevelopment Triticum aestivum L., Triticum durum L., spring wheat, hydroponics, ammonium nutrition, nitrate nutrition, tillering, yield components, partitioning     

Seed Production Environment, Time of Harvest, and the Potential Longevity of Seeds of Three Cultivars of Rice (Oryza sativa L.)

Changes in seed quality (assessed by potential longevity, i.e.the value of the seed lot constant K₁ of the seed viabilityequation) in three contrasting cultivars of rice (Oryza sativaL.) were monitored during seed development and maturation intwo temperature regimes, viz 28/20°C and 32/24°C (12/12h), provided by controlled environments. Mass maturity (definedas the end of the seed-filling phase) varied only between 18and 20 d after 50% anthesis. In five of the six treatment combinationsmaximum potential longevity was not achieved until 12-19 d aftermass maturity. In contrast, the maximum potential longevityof seeds of a japonica rice cultivar produced in the warmerregime was obtained in the first harvest after mass maturity.After mass maturity, the potential longevity of the japonicarice seed lots produced in the warmer environment was much lessthan that for the cooler environments. Maximum potential longevitywas also consistently greater in the cooler than the warmerregime for the two indica cultivars, although the differencein K₁ was small (0·3-0·5). The deleterious effectof increase in temperature on seed quality development was notdetected until after mass maturity. Maximum potential longevityin the cooler regime was greatest in the glutinous indica (K₁= 3·9) and least in the japonica cultivar (K₁ = 3·1).It is concluded that the japonica cultivar is not as well adaptedto warm seed production regimes as the indica cultivars. Consequently,subject to confirmation, this research suggest that the seedproduction of japonica cultivars for long-term genetic conservationshould be undertaken, whenever possible, in warm temperate environments.Copyright1993, 1999 Academic Press Oryza sativa L., rice, genebanks, seed development, seed storage, seed longevity, temperature     

A Modified Micro-Drop Bioassay Using Dwarf Rice for Detection of Femtomol Quantities of Gibberellins

The sensitivity of the micro-drop assay with dwarf rice (Oryzasativa L., cv. Tan-ginbozu and cv. Waito-Q to gibberellins (GAs)was increased conspicuously by the use of assay plants thathas been treated with uniconazole (S-3307), an inhibitor ofthe biosynthesis of GAs. The Tan-ginbozu plants treated withS-3307 responded to 10 fmol/plant of GA₃ (ca. 3.5 pg/plant)and to 30 fmol/plant of gibberellins A₁, A₄, A₇, A₁₉ and A₂₀.Waito-C plants treated with S-3307 responded to 10 fmol of GA₃and to 30 fmol/plant of gibberellins A₁, A₄ and A₇. GibberellinsA₉, A₁₉ and A₂₀ had much less of an effect on the treated Waito-Cplants than did gibberellins A₁, A₃, A₄ and A₇. Furthermore,treatment with S-3307 counteracted the inhibition of growthof both cultivars by abscisic acid. Thus, the modified micro-dropassay should prove very useful for the detection of minute amountsof GAs in plant extracts. (Received October 3, 1988; Accepted March 29, 1989)     

Thermal Kinetics of Glutathione Reductase and their Relation to Thermotolerance in Diverse Cultivars of Maize

The characteristics of glutathione reductase from a number ofmaize cultivars with contrasting thermotolerance have been investigated.The apparent K_m (Michaelis constant) for oxidised glutathione(GSSG) was measured between 10 and 45°C at constant pH.The enzyme from highland cultivars adapted to cool environmentshad a slightly lower apparent K_m for GSSG than that from lowlandtropical cultivars at low assay temperatures. Similarly theenzyme from lowland tropical cultivars had a lower apparentK_m for GSSG at high assay temperatures. However these effectswere small and regression lines plotted through the data werenot significantly different in slope or intercept. There wasa strong correlation (r = 0·939) between apparent K_mand V_max (maximum initial velocity) as assay temperature wasvaried. The interpretation of apparent K_m/temperature relationshipsis discussed with hypothetical examples of the effects of temperatureon enzyme activity/substrate concentration plots. It is demonstratedthat an increase in apparent K_m at higher assay temperaturesneed not necessarily reflect any temperature-dependent impairmentof enzyme function. The apparent K_m for GSSG of glutathionereductase from maize increased over four-fold when the temperaturewas raised from 10 to 45°C, but it is concluded that invivo rates of reaction are likely to be increased rather thandecreased by this same change in temperature. Glutathione reductasewould thus appear to be equally well adapted to function atall these temperatures. This suggests that the potential ofenzyme thermal kinetics to predict thermotolerance may be limited.Copyright1994, 1999 Academic Press Zea mays L., maize, glutathione reductase, thermal kinetics, thermotolerance