Allocation of Photosynthate to Individual Tubers of Solanum tuberosum L.: III. RELATIONSHIP BETWEEN GROWTH RATE OF INDIVIDUAL TUBERS, TUBER WEIGHT AND STOLON GROWTH PRIOR TO TUBER INITIATION

Potato plants (Solanum tuberosum L.) were grown in water culture.About 14 d after tuber initiation no significant differenceswere found between apical and basal tuber parts in ¹⁴C-uptakeand partitioning into various fractions from ¹⁴C-labelled photosynthate.Thus, the fresh weight of these tubers could be used as a parameterfor the sink size. The ¹⁴C-content per tuber (sink strength)20 h after ¹⁴CO₂-supply to the foliage was significantly correlatedwith the tuber fresh weight. No correlation was found betweenthe ¹⁴C-concentration of the tuber (sink activity; ct. min^–g^– fr. wt.) and tuber fresh weight. Consequently, tuberfresh weight (sink size) per se must have been a factor whichinfluenced sink strength. Stolon parameters characterizing theirgrowth prior to tuber initiation (e.g. stolon volume) and theircapacity for photosynthate transport (diameter, length) weremeasured at the time of tuber initiation. Significant correlationswere found between these stolon parameters and subsequent growthof individual tubers. Anatomical studies on the proportion ofvarious tissues in the cross sectional area of stolons supportthe idea of a negative relation between growth of individualtubers and transport resistance in the phloem of the stolons.It is concluded that in the initial phase of tuber growth, mainlyfactors outside of the tuber determine its growth rate. In laterstages of tuber growth, when the sink strength increases, thecompeting strength of individual tubers for photosynthate isdominated mainly by factors within the tuber itself, such astheir sink size and sink activity. Key words: Potato tuber, sink size, tuber initiation, transport resistance     

Allocation of Photosynthate to Individual Tubers of Solanum tuberosum L.: I. RELATIONSHIP BETWEEN TUBER GROWTH RATE AND ENZYME ACTIVITIES OF THE STARCH METABOLISM

Potato plants (Solanum tuberosum L.) were grown in water culturein a controlled environment. Cooling (+8°C) of individualtubers decreased their growth rates and increased the growthrates of non-cooled tubers of the same plant. The carbohydrateconcentration in non-cooled and cooled tubers did not differsignificantly, but ¹⁴C-import from labelled photosynthate waslower in cooled than in non-cooled tubers. The markedly lowerconversion rate of ethanol-soluble ¹⁴C to starch in cooled,in comparison to non-cooled tubers, was not associated withsignificant differences in the in vitro activities of starchsynthase, ADPG-pyrophosphorylase and starch phosphorylase understandard assay conditions (+30°C). However, the Q₁₀-valuesof the enzymes differed in vitro in the temperature range between30°C and 8°C, leading to a marked decrease in the activityratio of ADPG-pyrophosphorylase/starch phosphorylase in cooledtubers. In tubers differing in growth rates without manipulation, 14d after tuber initiation significant positive correlations werefound between ¹⁴C-concentration of tuber tissue and the in vitroactivities of starch synthase and ADPG-pyrophosphorylase anda significant negative correlation between ¹⁴C-concentrationand starch phosphorylase. In contrast, in tubers which wereanalysed 5 d after initiation, there were only small differencesbetween tubers in growth rate, ¹⁴C import and the activity ratioADPG-pyrophosphorylase/starch phosphorylase. From various directand indirect evidence it is concluded that the growth rate ofindividual tubers, and thus the sink strength, is at least inpart controlled by the activity of starch synthesizing enzymes. Key words: Potato tuber, cooling, starch synthesizing enzymes     

Interrelation Between Growth Rate of Individual Potato (Solanum tuberosum L.) Tubers and Their Cell Number

In potato plants fast and slow growing tubers develop on thesame plant. A hypothetical causality between tuber growth rateand tuber cell number was investigated by determining the tubercell number with the aid of an automatic counting procedure.Our data show a close correlation between tuber size and cellnumber over the whole range of tuber volumes considered (3–28cm³). If the influence of tuber size on cell number is eliminatedby means of a partial correlation analysis, the cell numberof the entire tuber is not significantly correlated with itsgrowth rate. An exclusive consideration of the smaller cells(10–30 µm) in the apical tuber region, where thecell division rate in potato tubers is highest, reveals a loosebut significant partial correlation to tuber growth rate (r= 0.383, P < 0.05). The growth rate of the slow growing tubers of any potato plantmay be enhanced by removing the fast growing tubers. In thefirst few days this enhanced growth rate is not due to a stimulationof cell division rate, but rather due to cell expansion. Potato, Solanum tuberosum L., tuber growth rate, tuber cell number     

Increased Chorismate Mutase Levels as a Response to Wounding in Solanum tuberosum L. Tubers

Discs excised from Solanum tuberosum L. cv White Rose tubers demonstrated a 4.5-fold increase in chorismate mutase activity 48 hours after excision. Incubation in the presence of cycloheximide (25 micromolar) or actinomycin D (100 micromolar) completely inhibited the wound response suggesting de novo synthesis of chorismate mutase. Ratios of activity in the presence of the activator tryptophan to that in the absence of tryptophan remained relatively constant during the induction period. This indicated either a constant ratio of tryptophan sensitive to tryptophan insensitive isozymes, or that only one form of chorismate mutase was present. Chromatography of crude extracts on three different columns yielded only one peak of chorismate mutase activity, activated by tryptophan in each case. Incubation under white light had no effect on chorismate mutase activity when compared to dark controls.     

The Effect of Gibberellic Acid on Starch Breakdown in Sprouting Tubers of Solanum tuberosum L.

The treatment of potato tubers with 150 µmol dm^–3gibberellic acid (GA₃) stimulated starch breakdown and hexoseaccumulation in tuber tissues and the transfer of dry matterto stems. These effects could not be accounted for by enhancedactivities of starch phosphorylase, amylase and acid invertase.Indeed enzyme activities either declined or remained relativelyconstant as starch degradation and hexose accumulation proceeded.Changes in the rate of starch depletion were related to changesin sink strength and sink type, the onset of tuber initiationin controls causing the rate of starch degradation to exceedthat in GA₃-treated tissues, in which tuberization was inhibited. Solanum tuberosum L., gibberellic acid, starch breakdown     

The Response of Groundnut (Arachis hypogaea L.) to Timing of Irrigation: II. 14C-PARTITIONING AND PLANT WATER STATUS

Finite quantities of water were applied at different growthstages of groundnut stands (Arachis hypogaea L.) grown in controlledenvironment glasshouses. Soil moisture deficits were imposedbetween sowing and pod initiation or between pod initiationand final harvest by withholding or applying water. Effectson assimilate production and partitioning and plant water relationswere examined. Leaves were the primary sites of ¹⁴CO₂ fixation, though theircontribution generally declined late in the season, whereasfixation by stems was initially low but increased sharply whenstress was released in the late-irrigated stands. ¹⁴C-fixationby stem apices and pegs also rose sharply following irrigationof the late-stressed stands. Leaves were the primary source of assimilates, though translocationtended to decrease as the season progressed, even in the late-irrigatedstands. Stems were initially the major sinks, but their sinkactivity disappeared almost completely when stress was releasedin the late-irrigated stands. Assimilate import by stem apicesdeclined progressively and pod sink activity was negligiblein the late-stressed stand, but both increased markedly whenearly-season stress was released. Leaf water status showed marked diurnal variation, whereas pegsshowed less variation and maintained much higher turgor levels,largely because of their lower solute potentials. Marked osmoticadjustment occurred in expanding but not in mature leaves, allowingthem to maintain higher turgor levels during periods of severestress. This adjustment was rapidly lost when stress was released.The observed changes in assimilate production and partitioningpreceded detectable changes in bulk turgor levels. Implications for growth, development and yield are discussed. Key words: Groundnut, irrigation, partitioning, water status     

Carbon Dioxide Exchange in Lichens : RELATIONSHIP BETWEEN NET PHOTOSYNTHETIC RATE AND CO(2) CONCENTRATION

The relationship between net photosynthesis and CO₂ concentration was investigated for four species of lichen using an infrared gas analyzer operating in a closed loop system. All species showed a linear relationship at low CO₂ levels (100 microliters per liter) with CO₂ saturation levels being in excess of 400 microliters per liter. Detailed studies of Sticta latifrons showed a strong influence of thallus water content which resulted in the net photosynthetic response at high water contents still being nearly linear at 1000 microliters per liter CO₂. Very low CO₂ compensation values (5 microliters per liter) were obtained under some conditions but the value varied between thalli and with thallus water content. The results differ from previous studies which reported low CO₂ saturation levels (200 microliters per liter) and no apparent effect of water content. It is suggested that some of these differences may result from the use of a discrete sampling injection infrared gas analyzer system in the earlier studies and an assessment is made of the influence of nonsaturating CO₂ levels, lack of cuvette ventilation, and data presentation for this technique.     

Fluoride-Induced Inhibition of Starch Biosynthesis in Developing Potato, Solanum tuberosum L., Tubers Is Associated with Pyrophosphate Accumulation

Pretreatment of discs excised from developing tubers of potato (Solanum tuberosum L.) with 10 millimolar sodium fluoride induced a transient increase in 3-phosphoglycerate content. This was followed by increases in triose-phosphate, fructose 1,6-bisphosphate and hexose-phosphate (glucose 6-phosphate + fructose 6-phosphate + glucose 1-phosphate). The effect of fluoride is attributed to an inhibition of glycolysis and a stimulation of triose-phosphate recycling (the latter confirmed by the pattern of ¹³C-labeling [NMR] in sucrose when tissue was supplied with [2-¹³C]glucose). Fluoride inhibited the incorporation of [U-¹⁴C] glucose, [U-¹⁴C]sucrose, [U-¹⁴C]glucose 1-phosphate, and [U-¹⁴C] glycerol into starch. The incorporation of [U-¹⁴C]ADPglucose was unaffected. Inhibition of starch biosynthesis was accompanied by an almost proportional increase in the incorporation of ¹⁴C into sucrose. The inhibition of starch synthesis was accompanied by a 10-fold increase in tissue pyrophosphate (PPi) content. Although the subcellular localization of PPi was not determined, a hypothesis is presented that argues that the PPi accumulates in the amyloplast due to inhibition of alkaline inorganic pyrophosphatase by fluoride ions.     

The Metabolism of Fructose Polymers in Plants: II. EFFECT OF TEMPERATURE ON THE CARBOHYDRATE CHANGES AND MORPHOLOGY OF STORED TUBERS OF HELIANTHUS TUBEROSUS L.

It is confirmed that the change in optical rotation of extractsobtained from tubers during dormancy is correlated with theconversion of fructosan to oligosaccharides of lower molecularweight, and that there is an associated increase in the combinedglucose content. Storage at 2° C. both accelerated and greatlymagnified this process; storage at 20° C. considerably depressedit. The change is not initiated or controlled by the dormantbuds. Tubers kept at 20° C. throughout the dormancy periodsprouted to give daughter tubers; new plants did not develop,even after some months. Normal sprouting was induced by keepingthe dormant tubers at 2° C. for 7 weeks.     

Regulation of Bud Rest in Tubers of Potato, Solanum tuberosum L: VI. Biochemical Changes Induced in Excised Potato Buds by Gibberellic Acid

The rest period of the potato tuber was studied in relation to certain biochemical changes that are induced by gibberellic acid (GA₃). The concentration of reducing sugars in excised plugs with buds treated with 10⁻⁴m GA₃ decreased in the first 4 hours after treatment and then rapidly increased up to 70 hours. The pattern in control buds was similar, but the changes occurred more slowly. The response to GA₃ is temperature-dependent and is not limited to any particular tissue of the tuber. The concentration of reducing sugars in excised buds increased proportionally to the log of the concentration of GA₃ in a range from 10⁻⁸ to 10⁻⁴m. At 10⁻³m, GA₃ slightly inhibited production of reducing sugars. Malonate inhibits the initial decrease and the subsequent increase in reducing sugars in control buds, but not the increase induced by GA₃.     

Energy Metabolism in Rhizomes of Acorus calamus (L.) and in Tubers of Solanum tuberosum (L.) With Regard to their Anoxia Tolerance

Rhizomes of the marsh plant Acorus calamus (L.) and tubers of the flooding-intolerant Solanum tuberosum (L.) var. Bintje, both kept under strict anoxia, differ markedly in their fermentation properties. The fermentation capacities as measured by ADH and LDH activities and their respective product concentrations were estimated. While rhizomes of Acorus calamus, having high ADH and low LDH activities, accumulate mainly ethanol, tubers of Solanum tuberosum tend towards lactic acid fermentation. The total amount of adenine nucleotides is quite stable in Acorus calamus, whereas they show a sharp decline in S. tuberosum during the first 6h of anoxia. The adenylate energy charge of A. calamus recovers after a short initial drop (AEC > 0.8). AEC values of S. tuberosum decrease rapidly and remain at very low values (AEC ～ 0.3). Tuber tissues became soft and lost viability after about 48–72 h of anoxia at 25 °C. This might be due to tissue acidification and impaired energy metabolism, but not to the lack of energy reserves. Energy metabolism of A. calamus is well adapted to anoxia.     

THE FITNESS CONSEQUENCES OF MULTIPLE-LOCUS HETEROZYGOSITY: THE RELATIONSHIP BETWEEN HETEROZYGOSITY AND GROWTH RATE IN PITCH PINE (PINUS RIGIDA MILL.)

Positive correlations between measures of “fitness” and the number of electrophoretic loci for which an individual is heterozygous have been observed in many species. Two major hypotheses have been proposed to explain this phenomenon: inbreeding depression and overdominance. Until recently, there has been no way to distinguish between these hypotheses. The overdominance model devised by Smouse (1986) is used here in a reanalysis of Ledig et al.‘s (1983) study of heterozygosity and growth rate in eight populations of pitch pine and is contrasted with an inbreeding-depression analysis. Ledig et al. (1983) regressed mean growth rate per heterozygosity class on the number of heterozygous loci, a method of analysis which, although it points to general trends in the data, does not differentiate between hypotheses. The correlations they obtained in four populations were significant only because regressing on the means eliminates most of the sum of squares for error and does not weight the unequally sized heterozygosity classes. Reanalysis of Ledig et al.‘s data using individuals, not means, showed no significant correlations between heterozygosity and fitness. A major assumption of Smouse's overdominance model is that genetic polymorphism is in part a reflection of selection for heterozygotes at genetic equlibrium. The homozygote for the most frequent allele at a locus should be more fit than a homozygote for a less frequent allele, with the heterozygote superior to both homozygotes. Smouse's model predicts a negative, linear relationship between fitness and “adaptive distance,” a variable that for a heterozygote is zero and for homozygotes is equal to the inverse of the frequency of the corresponding allele. The adaptive-distance model accounted for between 15% and 50% of the variation in growth rate within eight P. rigida population samples by accounting for genotypic differences at eight polymorphic loci. This is over twice as much of the variation in growth rate accounted for by Ledig et al.'s (1983) analysis using individuals. Significant correlations were found between adaptive distance and growth rate in four of the eight populations, but in only two of the populations were more of the partial coefficients negative than positive, as would be predicted by the overdominance hypothesis. The remaining two populations in which correlations were significant did not lend themselves to such clear-cut interpretation, as the majority of the partial coefficients were positive. Positive partial coefficients indicate that the growth rate of the heterozygote is inferior to that of at least one of the homozygotes. The adaptive-distance analysis provides evidence that specific genotypes do play a role in determining growth rate in pitch pine. The correlation between growth rate and adaptive distance increased significantly with the age of the population, possibly reflecting competition subsequent to crown closure.     

Regulation of Bud Rest in Tubers of Potato, Solanum tuberosum L: VII. Effect of Abscisic and Gibberellic Acids on Nucleic Acid Synthesis in Excised Buds

The effect of gibberellin A₃ (10⁻⁴m) and abscisic acid (10⁻⁴m), applied separately and together, on incorporation of ³H-thymidine and ³H-uridine into DNA and RNA of buds from freshly harvested potatoes was investigated. In some treatments apical buds in intact tubers were treated three times daily for 3 days with test solution before the buds were excised and treated an additional 12 hours in Petri dishes. In other treatments, untreated buds were excised and treated 12 hours. Irrespective of length of treatment, gibberellin A₃ slightly promoted synthesis of DNA and RNA, and abscisic acid essentially blocked such synthesis, in both the presence and absence of gibberellin A₃.     

Regulation of Bud Rest in Tubers of Potato, Solanum tuberosum L: VIII. Early Effects of Gibberellin A(3) and Abscisic Acid on Ultrastructure

Using the electron microscope, we compared the effects of abscisic acid and gibberellin A₃ on excised buds from resting potato (Solanum tuberosum L.) tubers. Cells of abscisic acid-treated buds became progressively more vacuolated during a 12-hour time course study as compared with control (water) and gibberellin A₃-treated buds. Concentric configurations of endoplasmic reticulum were present in apical cells of freshly excised buds. After about 6 hours these configurations began to open and disperse, and after 12 hours, intact concentric configurations were no longer evident. Both abscisic acid and gibberellin A₃ induced opening and dispersal of the concentric configurations, sometimes as early as 0.5 hour after excision and treatment with hormones.     

ALLOZYME VARIATION WITHIN AND BETWEEN LASTHENIA MINOR AND ITS DERIVATIVE SPECIES,L. MARITIMA (ASTERACEAE)

Enzyme electrophoresis was employed to examine genetic variation at 20 loci in 16 populations of Lasthenia minor and 18 populations of its presumed derivative species L. maritima. The purposes of the study were to ascertain levels of genetic variation in each species, to assess how the variation at enzyme-coding genes is apportioned within and among populations of each species, and to determine the level of divergence between the two species. The two species are both diploid annuals, similar morphologically, and produce fertile F₁ hybrids when crossed. Lasthenia minor is self-incompatible and restricted to mainland California, whereas L. maritima is self-compatible and probably largely autogamous; it occurs on seabird rocks from central California to British Columbia. Mean genetic identities for pair-wise comparisons of populations of the two species are similar to values for populations of the same species, indicating they have not diverged at the 20 genes coding for soluble enzymes. Despite its more extensive geographical range, L. maritima exhibits only 50% of the genetic diversity of L. minor. The latter species apportions a greater amount of its diversity within populations, whereas the former harbors more diversity among populations than within them. This is probably a reflection of the different breeding systems of the two species. Six unique alleles were detected in L. minor, whereas only one novel allele was found in a single individual of L. maritima. The electrophoretic data are concordant with the suggestion that L. maritima is relatively recently derived from L. minor. The switch from outcrossing to selfing and selection of genotypes adapted to the chemically and physically unusual substrate on the seabird rocks are considered the critical steps in the evolution of L. maritima.     

Translucent Tissue Defects in Solanum tuberosum L. : II. Alterations in Lipolytic Acyl Hydrolase, Lipoxygenase, and Morphology of Mitochondria and Amyloplasts

We analyzed a physiological defect that involved translucent-like tissue which occurred randomly in potato tubers (Solanum tuberosum L., cv Kennebec) after 8 months of storage. The translucent areas had reduced lipoxygenase (0.73-fold) and lipolytic acyl hydrolase (0.27-fold) activities. The effect(s) of these reduced enzyme activities in vivo is uncertain, but they may have influenced composition, turnover and permeability of membranes because potato lipid is primarily membranous in nature. Electron micrographs of the translucent tissue revealed a discernible decrease in the number of starch granules compared to normal/healthy tissue. A few remaining amyloplasts, which still contained starch granules, possessed large electrondense areas (stroma) within the organelle. Mitochondria in translucent tissue appeared to be present in increased numbers, were aggregated, had fewer but swollen cristae, and, morphologically, were of irregular size and shape suggestive of division. The result of this tuber defect appeared, in part, to be an exaggerated or accelerated form of senescence.     

The Geoelectric Effect in Plant Shoots:: IV. INTER-RELATIONSHIP BETWEEN GROWTH, AUXIN CONCENTRATION AND ELECTRICAL POTENTIALS IN ZEA COLEOPTILES

Incubation of Zea coleoptiles in 0.5 M mannitol totally inhibitsgrowth and geotropic curvature, but does not affect the developmentof the geoelectric effect. This pre-treatment also inhibitsthe curvature induced by the asymmetrical application of IAAto the apical end of decapitated vertical coleoptiles, but itdoes not prevent the IAA from giving rise to an electropotentialdifference between the two sides of the coleoptile. Neitherthe normal geoelectric effect, nor the auxin-induced potentialdifference in vertical coleoptiles, can therefore arise as theresult of the different rates of cell extension in the two halvesof the organ. They must be the result of the change of IAA concentrationaffecting some other aspect of the cell's physiology or metabolism. The abolition of the electrical responses in coleoptiles whichhave been plasmolysed in 1.0 M mannitol strongly suggests thatboth longitudinal and lateral transport of IAA are severelydepressed by this degree of plasmolysis. Asymmetrical application of 10^-5 M mersalyl and several othersubstances to the apical end of a decapitated vertical coleoptilegave rise to a marked electropotential difference between thetwo sides of the coleoptile, the side beneath the donor beingpositively charged with respect to the other side. Mersalyldoes not promote the growth of Zea coleoptiles. These resultsprovide additional evidence that the electropotentials do notarise from differential growth, and suggest that such substances,especially the diuretics used in clinical medicine, may provideuseful tools in the further study of the induction of surfaceelectropotentials in plant tissues at the cellular level.     

COMPARATIVE PHYSIOLOGICAL STUDY OF MARINE DIATOMS AND DINOFLAGELLATES IN RELATION TO IRRADIANCE AND CELL SIZE. II. RELATIONSHIP BETWEEN PHOTOSYNTHESIS,GROWTH, AND CARBON/CHLOROPHYLL a RATIO1,2

Photosynthetic rates, growth rates, cell carbon, cell protein, and chlorophyll a content of two diatom and two dinoflagellate species were measured. The microalgae were chosen to have one small and one large species from each phylogenetic group; the two size categories differed from each other by 1.5 orders of magnitude in terms of cell carbon or cell protein. The cultures for the experiments were grown under continuous light at an irradiance high enough for the light-saturation of growth for all four species. The four species were found to have similar maximum photosynthetic rates per unit chlorophyll a. The diatom species showed lower carbon/chlorophyll a ratios and higher photosynthetic rates per unit carbon than the dinoflagellates. The higher growth rates of the diatoms were shown to be related to their higher photosynthetic rates per unit carbon. The ecological significance of the physiological difference between these two groups of microalgae is discussed.     

Devlopmental Physiology of Sugar-Beet: II. EFFECTS OF TEMPERATURE AND NITROGEN SUPPLY ON THE GROWTH, SOLUBLE CARBOHYDRATE CONTENT AND NITROGEN CONTENT OF LEAVES AND ROOTS

Plants were grown at temperatures of 15 and 25 ?C with two ratesof nitrogen supply. The changes in dry weight, leaf area, cellnumber, mean cell volume, soluble carbohydrate, and total nitrogenconcentration of the cotyledons, the first and second pair oftrue leaves, and the storage root were measured. Changes incell number and cell volume of the first pair of true leavesand storage root of plants were also measured at 11, 18, 25,and 32 ?C. Leaf growth before unfolding was chiefly by increase in cellnumber and after unfolding by increase in mean cell volume,while the growth of the storage root was almost entirely byincrease in cell number. The rates of cell division and cellexpansion were fastest at 25 ?C, but the initially high ratesof cell division in the terminal bud and in individual leavesdecreased rapidly and greater rates were maintained at the sub-optimaltemperatures, i.e. 15 and 18 ?C. After an initial period ofslow growth, the first-formed leaves grew faster and becamelarger at 15 than at 25 ?C. Leaves were produced, unfolded,grew faster, and became larger with increase in the externalconcentration of nitrogen, because cells divided and expandedfaster, so that nitrogen increased the number and size of cells. Sugar concentration was greater at 15 than at 25 ?C in leavesbut not in the storage root. Sugar concentration in the petiolesof the first and second pair of true leaves increased to 1.2and 2.0 per cent fresh weight respectively. Decreased nitrogensupply temporarily increased the sugar concentration of cotyledonpetioles and the seedling hypocotyl, but later decreased itin the leaves and storage root. Nitrogen concentration was greaterin the leaves and storage root at 15 than at 25 ?C with thelarger nitrogen supply. Nitrogen concentrations were similarin young leaves of all treatments but as the size of leavesincreased nitrogen concentrations decreased most rapidly at25 ?C with the smaller nitrogen supply. It is suggested that when increased leaf production and storage-rootgrowth occurs at temperatures below the growth optimum (25 ?C),they may be due to an effect of increased carbohydrate supplyon cell division and sugar storage.