Hydrilla stems and tubers as hosts for three Bagous species: two introduced biological control agents (Bagous hydrillae and B. affinis) and one native species (B. restrictus)

Field observations suggested that the introduced Hydrilla verticillata (L.f.) Royle biological control agent, a stem weevil, Bagous hydrillae O'Brien, would feed on hydrilla tubers and stems, and a native species, Bagous restrictus LeConte, would feed on hydrilla stems. In choice tests, B. hydrillae readily oviposited in hydrilla tubers. Larval development of B. hydrillae in hydrilla tubers was similar to that in stems; greater adult biomass was attained and less time was needed to complete development when the larvae were fed tubers. Larvae of the hydrilla tuber weevil, B. affinis Hustache, did not complete development in hydrilla stems. Larvae of B. affinis completed development more rapidly when fed new compared with old hydrilla tubers. The native B. restrictus successfully completed development in hydrilla stems, although the larvae required slightly more time compared with the biocontrol agent, B. hydrillae. These findings indicated that feeding on tubers by B. hydrillae may benefit the species particularly when hydrilla stems are seasonably absent or unsuitable especially in more northern climates. In terms of hydrilla control, damage to tubers by this species constitutes a reduction in future infestations of hydrilla propagated by tubers. Finally, hydrilla is suitable to the native weevil, B. restrictus, because larvae completed development in hydrilla stems.     

Overexpression of a bacterial 1-deoxy-D-xylulose 5-phosphate synthase gene in potato tubers perturbs the isoprenoid metabolic network: implications for the control of the tuber life cycle

Potato tubers were engineered to express a bacterial gene encoding 1-deoxy-D-xylulose 5-phosphate synthase (DXS) in order to investigate the effects of perturbation of isoprenoid biosynthesis. Twenty-four independent transgenic lines out of 38 generated produced tubers with significantly elongated shape that also exhibited an early tuber sprouting phenotype. Expression analysis of nine transgenic lines (four exhibiting the phenotype and five showing a wild-type phenotype) demonstrated that the phenotype was strongly associated with dxs expression. At harvest, apical bud growth had already commenced in dxs-expressing tubers whereas in control lines no bud growth was evident until dormancy was released after 56-70 d of storage. The initial phase of bud growth in dxs tubers was followed by a lag period of approximately 56 d, before further elongation of the developing sprouts could be detected. Thus dxs expression results in the separation of distinct phases in the dormancy and sprouting processes. In order to account for the sprouting phenotype, the levels of plastid-derived isoprenoid growth regulators were measured in transgenic and control tubers. The major difference measured was an increase in the level of trans-zeatin riboside in tubers at harvest expressing dxs. Additionally, compared with controls, in some dxs-expressing lines, tuber carotenoid content increased approximately 2-fold, with most of the increase accounted for by a 6-7-fold increase in phytoene.     

Mint essential oil can induce or inhibit potato sprouting by differential alteration of apical meristem

Sprouting of potatoes during storage, due to tuber dormancy release, is associated with weight loss and softening. Sprout-preventing chemicals, such as chlorpropham (CIPC), can negatively impact the environment and human health. Monthly thermal fogging with mint (Mentha spicata L.) essential oil (MEO) inhibited sprouting in eight potato cultivars during large-volume 6-month storage: the tubers remained firm with 38% lower weight loss after 140 days of storage. The sprout-inhibitory action may be nullified: treated tubers washed with water resumed sprouting within days, with reduced apical dominance. MEO application caused local necrosis of the bud meristem, and a few weeks later, axillary bud (AX) growth was induced in the same sprouting eye. MEO components analysis showed that 73% of its content is the monoterpene R-carvone. Tubers treated with synthetic R-carvone in equivalent dose, 4.5 μl l⁻¹, showed an inhibitory effect similar to that of MEO. Surprisingly, 0.5 μl l⁻¹ of MEO or synthetic R-carvone catalyzed AX sprouting in the tuber. To the best of our knowledge, this is the first report of an essential oil vapor inducing early sprouting of potato tubers. R-carvone caused visible damage to the meristem membrane at sprout-inhibiting, but not sprout-inducing doses, suggesting different underlying mechanisms. After 5 days’ exposure to R-carvone, its derivatives transcarveol and neo-dihydrocarveol were found in buds of tubers treated with the inhibitory dose, suggesting biodegradation. These experiments demonstrate the potential of MEO vapor as an environmentally friendly alternative to CIPC in stored potatoes and as a research tool for the control of sprouting in plants.     

Sprouting and Growth of Purple Nutsedge, Cyperus rotundus, in Relation to pH and Aeration

The results of experiments on the sprouting and growth of purple nutsedge (Cyperus rotundus L.) in relation to pH and aeration are described. All pH levels between 2.2 to 9.0 allowed high sprouting percentage but the sprouted tubers did not survive at pH below 3 and survival beyond pH 7 was very poor. Post-sprouting growth of nutsedge was optimal at pH 4. Pre-sprouting aeration of tubers stimulated early sprouting and produced vigorous shoots, whereas submersion of the tubers was inhibitory. Continuous aeration of the submerged tubers substantially removed the inhibitory influence on sprouting but the shoots remained weak. Seedlings grown in constantly aerated nutrient solution showed significantly better growth than in a non-aerated one.     

Ethanol breaks dormancy of the potato tuber apical bud

Growing potato tubers or freshly harvested mature tubers have a dormant apical bud. Normally, this dormancy is spontaneously broken after a period of maturation of the tuber, resulting in the growth of a new sprout. Here it is shown that in in vitro-cultured growing and maturing tubers, ethanol can rapidly break this dormancy and re-induce growth of the apical bud. The in vivo promoter activity of selected genes during this secondary growth of the apical bud was monitored, using luciferase as a reporter. In response to ethanol, the expression of carbohydrate-storage, protein-storage, and cell division-related genes are rapidly down-regulated in tuber tissue. It was shown that dormancy was broken by primary but not by secondary alcohols, and the effect of ethanol on sprouting and gene expression in tuber tissue was blocked by an inhibitor of alcohol dehydrogenase. By contrast, products derived from alcohol dehydrogenase activity (acetaldehyde and acetic acid) did not induce sprouting, nor did they affect luciferase reporter gene activity in the tuber tissue. Application of an inhibitor of gibberellin biosynthesis had no effect on ethanol-induced sprouting. It is suggested that ethanol-induced sprouting may be related to an alcohol dehydrogenase-mediated increase in the catabolic redox charge [NADH/(NADH+NAD+)].     

Effects of storage conditions on sprouting of microtubers of yam (Dioscorea cayenensis–D. rotundata complex)

The control of field tuber dormancy in the yam (Dioscorea cayenensis–D. rotundata complex) is poorly understood. Although studies have examined single environmental factors and chemical treatments that might prolong tuber dormancy and storage, only a few were focused on further tuber sprouting. The present study concerns microtubers obtained by in vitro culture. When microtubers were harvested (after 9 months of culture) and directly transferred on a new medium without hormones, the tubers rapidly sprouted in in vitro conditions. No dormancy was observed in this case. Harvested microtubers were also stored dry in jars in sterile conditions during 2 to 18 weeks before in vitro sprouting. In this case, microtubers stored during 18 weeks sprouted more rapidly than those stored 8 weeks. A constant “dormancy-like period” (storage duration + sprouting delay) was observed, between 20 and 28 weeks respectively for the more rapid and the slower microtubers. The size of the tubers used for the storage had great influence on further sprouting. The larger they were, the better they sprouted. Light during storage had no effect on the sprouting delay while a temperature of 25 °C permit a quicker sprouting than 18 °C. The medium used to obtain microtubers could also have an effect on sprouting rate.     

Organic nitrogen reserves and their mobilization during sprouting of purple nutsedge (Cyperus rotundus L.) tubers

Purple nutsedge is a perennial weed propagating vegetativelyby an extended network of rhizomes and tubers. Large quantitiesof starch are stored in the tubers, but the properties of nitrogenreserves and their importance for early plant growth have receivedlittle attention in the past. Organic nitrogen compounds werestudied in mature tubers by protein determination, SDS-PAGEand amino acid quantification and separation using reversed-phaseHPLC. Changes in these compounds were followed in sproutingtuber pieces fed with a complete nutrient solution (containingnitrate, control) or with a nutrient solution without nitrogensource. As judged by gel electrophoresis or direct protein quantification,some net protein degradation occurred after 2–4 weeksof sprouting in the presence or absence of exogenous N. Aminoacids decreased much faster, especially during the first 2 weeks.The major amino acids were arginine and asparagine, which togetheraccounted for 70% of total amino acids at day 0, and which hadalmost disappeared after 4 weeks of sprouting. Sprout growthdepended strongly on the availability of exogenous nitrogenduring the second week. The results indicate that amino acidsare the main N storage form of purple nutsedge tubers and, essentially,that no storage proteins are present. Future investigationson the N metabolism of sprouting nutsedge should focus on arginineand asparagine metabolism and eventually on nitrogen assimilation,which becomes important in an early phase of plant growth. Key words: Cyperus rotundus L., arginine, asparagine, nitrogen assimilation, storage proteins     

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.     

Melafen effect on ATP-dependent accumulation of calcium in plasma membrane vesicles from cells of potato tubers

Synthetic growth regulator melafen (10⁻⁵–10⁻¹⁰ M) was tested for aneffect on the Ca²⁺ accumulation in plasma membrane vesicles (PMVs) isolated from potato Solanum tuberosum L. tubers at forced rest and sprouting. Melafen proved to regulate the Ca²⁺ accumulation in PMVs by changing the activity of Ca²⁺, Mg²⁺-ATPase of the plasma membrane, while no effect was observed with respect to Ca²⁺ outflow from vesicles. The melafen effect on Ca²⁺, Mg²⁺-ATPase activity depended on the physiological condition of tubers and the melafen concentration.     

Antisense inhibition of cytosolic phosphorylase in potato plants (Solanum tuberosum L.) affects tuber sprouting and flower formation with only little impact on carbohydrate metabolism

To determine the function of cytosolic phosphorylase (Pho2; EC 2.4.1.1), transgenic potato plants were created in which the expression of the enzyme was inhibited by introducing a chimeric gene containing part of the coding region for cytosolic phosphorylase linked in antisense orientation to the 35S CaMV promotor. As revealed by Northern blot analysis and native polyacrylamide gel electrophoresis, the expression of cytosolic phosphorylase was strongly inhibited in both leaves and tubers of the transgenic plants. The transgenic plants propagated from stem cuttings were morphologically indiscernible from the wild-type. However, sprouting of the transgenic potato tubers was significantly altered: compared with the wild-type, transgenic tubers produced 2.4 to 8.1 times more sprouts. When cultivated in the greenhouse, transgenic seed tubers produced two to three times more shoots than the wild-type. Inflorescences appeared earlier in the resulting plants. Many of the transgenic plants flowered two or three times successively. Transgenic plants derived from seed tubers formed 1.6 to 2.4 times as many tubers per plant as untransformed controls. The size and dry matter content of the individual tubers was not noticeably altered. Tuber yield was significantly higher in the transgenic plants. As revealed by carbohydrate determination of freshly harvested and stored tubers, starch and sucrose pools were not noticeably affected by the antisense inhibition of cytosolic phosphorylase; however, glucose and fructose levels were markedly reduced after prolonged storage. These results favour the view that cytosolic phosphorylase does not participate in starch degradation. The possible links between the reduced levels of cytosolic phosphorylase and the observed changes with respect to sprouting and flowering are discussed.     

Effects of Temperature on Bud-Sprouting and Early Growth of Cyperus esculentus in the Dark

Cyperus esculentus tubers and early growth of their sprouts. Percent sprouting increased with increasing temperature within the range of 12 to 38 C, while no sprouting occurred at 10 C and few tubers sprouted at 42 C. The rate of sprouting also increased with temperature up to 35 C. A base temperature of 11.4 C was determined for bud-sprouting of tubers in this species. Higher temperatures led to larger sprouts and greater survival rate. In particular, increased temperature favored root growth, and hence resulted in high root: shoot ratio of the sprouts. Larger tubers produced larger sprouts as a consequence of mobilizing a greater amount of their reserves, but they tended to utilize a smaller proportion of their reserves. The efficiency of reserve utilization significantly differed among the incubation temperatures, and its relation with temperature followed a quadratic pattern. This pattern is different from that documented for the bud-sprouting of rhizomes and stolons of other perennials. Our results demonstrate that temperature is crucial to the successful establishment of C. esculentus. Received 24 June 1999/ Accepted in revised form 14 December 1999     

Persistence of Genetically Modified Potatoes in the Field

Volunteers from genetically modified (GM) potatoes may pose an environmental problem if allowed to grow in the field after the annual crop is harvested. We tested whether they are more likely to produce volunteers than non-GM potatoes. Specifically, we compared the number of volunteers, number of tubers per plant, tuber size, and their vertical distribution in the soil. More volunteer plants came from non-GM potatoes than from GM potatoes, but the number and size of tubers were similar between the two. Vertical distribution of the tubers differed significantly, with most non-GM tubers being found in shallower soil (<2 cm deep). Our results suggest that spontaneous GM volunteers may emerge and produce tubers to a degree similar to that of the non-GM plants. No viable volunteers emerged from GM tubers in the next growing season, probably deterred by winter frost and a period of low soil temperatures (below −2°C) at our study site. However, in regions with warmer climates, such GM volunteers may survive Winter and produce more plants the following year.     

Effects of reducing sugar concentration on in vitro tuber formation and sprouting in yam (<Emphasis Type="Italic">Dioscorea cayenensis–D. rotundata</Emphasis> complex)

The effects of reducing sucrose level on tuber formation (% of cultures with microtubers), development (length and fresh weight of microtubers) and sprouting in yam Dioscorea cayenensis–D. rotundata complex in vitro were investigated. Only 29% of the explants showed tuber formation after 3 weeks in the presence of 1% sucrose in contrast to 100% with 3%. After 120 days of culture, the length and the weight of the tubers obtained in the presence of 1% sucrose were less than with 3% sucrose. Addition of sorbitol to keep osmolarity at the same level did not restore normal rate of tuber formation. Similar results were obtained with the use of reduced fructose or glucose level. Microtuber sprouting was also affected by sucrose level incorporated into the tuberisation medium. Tubers obtained on reduced sucrose level sprouted later and the increase of osmolarity with sorbitol did not restore normal sprouting. The bigger tubers obtained on high sucrose media could contain more carbohydrate reserves that could partially explain a higher sprouting rate. These results can be used for optimising in vitro conditions for mass production of microtubers in yam and especially in Dioscorea cayenensis–D. rotundata complex, a very important species in West Africa. They specially showed the importance of tuberisation conditions on precocity of tuberisation, on tuber length and weight and on their further sprouting.     

Hydrilla tuber formation in response to single and sequential bensulfuron methyl exposures at different times

Hydrilla (Hydrilla verticillata (L.f.) Royle) grown in outdoor tanks was exposed to bensulfuron methyl concentrations of 25, 50, or 100 ppb on June 16, August 20 or October 15; 50 ppb June 16 and August 20, or 25 ppb on June 16, July 21, August 20, and October 15, 1990, with a 35-day contact time. Hydrilla was also exposed to the compound on August 9, 1991 at concentrations of 10, 20, 30, 40, or 50 ppb. In 1990, the August 20 exposure resulted in the greatest inhibition of tuber production for a single application. Exposure in June caused hydrilla to produce at least twice as many tubers as unexposed plants by April 10, 1991. Exposure in October arrested tuber production, which had already begun. Exposure in June and August delayed tuber formation until after February 9, 1991. Exposure in June, July, August, and October inhibited tuber formation for the entire growing season. Hydrilla treated with all concentrations of bensulfuron methyl on August 9, 1991 produced tubers only sporadically through March 16, 1992, compared to unexposed hydrilla, which produced an average of 48 tubers/531 sq cm by January 4, 1992. With the onset of warmer weather after March 16, tubers produced by unexposed hydrilla more than doubled, and comparable numbers of new tubers were produced by plants that were exposed to 10 or 20 ppb. Tuber production was inhibited for the entire growing season by exposure to 50 ppb on August 9, 1991. In spite of the promise that bensulfuron methyl showed for use in aquatic plant management, the Experimental Use Permit was not renewed in 1992 and efforts to register the compound were discontinued.     

Nature of enhanced respiration during sprouting of aged potato seed-tubers

Respiration of 18-month-old Solarium tuberosum L. tubers was about 53% greater than that of 6-month-old tubers during sprouting at 23°C; yet, a significant loss of sprout vigor in the older tubers was apparent. Involvement of alternative oxidase (AO) in the age-induced difference in tuber respiration was assessed. AO was only detected in immunoblots if tissue disks from tubers were pre-incubated for 24 h prior to isolation of submitochondrial membrane particles (SMPs). No AO¹ was detected in SMPs from nonincubated tuber tissue of either age, indicating that it was not contributing to tuber respiration during sprouting as previously thought. Respiratory control and ADP/O ratios indicated that oxidative phosphorylation was fully coupled to electron transport in mitochondria isolated from 6- and 18-month-old tubers. Cytochrome c oxidase (EC 1.9.3.1) activities of intact mitochondria were also not affected by tuber age. The difference in respiration during sprouting was unique to whole tubers, as oxygen consumption by mitochondria from young and oid tubers was equal on a milligram protein basis. Sprouting 18-month-old tubers had 15% more mitochondrial protein per gram fresh weight than did 6-month-old tubers. Older tubers also produced more ATP than younger tubers prior to and during sprouting, through a fully coupled, Cyt-mediated respiratory pathway, reduced sprout vigor notwithstanding. From 5 to 10 days of sprouting, coinciding with development of the age-induced difference in whole-tuber respiration, ATP concentration in 18-month-old tubers increased to become 52% higher than that in 6-month-old tubers. ATP synthase (EC 3.6.1.34), assessed by SDS-PAGE and immunoblots of β- and oligomycin-sensitivity conferring protein-subunits, also increased as a proportion of SMP protein in older tubers during this period. Relative to 6-month-old tubers, the increased respiration and associated oxidative phosphorylation of 18-rnonth-old tubers during sprouting were probably in response to a lower adenylate energy charge (AEC) prior to sprouting (from 0 fo 5 days). From 5 to 10 days of sprouting, AEC of 18-rnonth-old tubers increased to equal that of 6-month-old tubers and the two tuber ages maintained the same AEC for the remainder of the 20-day sprouting interval. Higher respiration and lower AEC of older tubers in storage at 4°C, along with the fact that older tubers respired at a higher rate to achieve the same AEC as younger tubers during sprouting, indicate greater utilization of ATP by older tubers.     

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     

Sprouting success of shrubs after fire: height-dependent relationships for different strategies

The sprouting success of co-occurring populations of shrub species in a temperate woodland of semi-arid Australia was investigated and related to population survival strategies. Straw was added to 21 × 15 m plots in the woodland, burnt and the pre-fire characteristics of shrubs were used to determine the basis for sprouting success. Species differed widely (4–94%) in sprouting success; a high percentage of established seedlings of all species were killed by fire but survival increased with height reaching a maximum at 25–60 cm (depending on the species). Thickness of bark at stem bases increased with height growth but sprouting success was not related to bark thickness; sprouting success of shrubs at similar thickness varied greatly between species. All species were able to initiate sprouts after cutting through their basal stems, so lack of active meristems was not a limitation. Species differed in the height at which shrubs began flowering but this was always after maximum sprouting success was reached. It is proposed that differences between individual shrubs in supply of nutrients, carbohydrates, and/or water to activated meristems would account for patterns of in ter- and intra-specific sprouting success. The data are consistent with recognised fire survival strategies. `Sprouters', the species relying more on sprouting than recruitment for population persistence, maintained maximum sprouting success with height growth and gained sprouting ability along stems once they reached 1 m in height. In contrast, `non-sprouters', the species largely relying on recruitment from seed to maintain populations, were either not able to sprout after seedling establishment or steadily lost the ability to maintain sprouts with growth beyond 60 cm and did not develop axillary buds along stems at any height. Received: 19 July 1997 / Accepted: 8 February 1998     

Gibberellic acid-induced prolongation of the dormancy in tubers or rhizomes of several species of East Asian Dioscorea

Effects of the application of gibberellic acid on sprouting of tubers or rhizomes were tested in seven species of the genus Dioscorea that are native to the temperate regions of East Asia. The lowest concentrations for significant inhibition of sprouting in these species varied from 0.1–1 µM Application of gibberellic acid at 100 µM inhibited sprouting for more than 500 days at 20 °C. Some responses to the application of gibberellic acid differed between species and between sections of the genus. In D. japonica, the application of gibberellic acid inhibited sprouting of tubers and bulbils while it promoted seed germination.     

Temperature and Light Requirements for the Sprouting of Chilled and Unchilled Tubers of the Purple Nutsedge, Cyperus rotundus

Mature and immature tubers of purple nutsedge (Cyperus rotundus L.) chilled at 0°C in dry and wet conditions, were sprouted along with fresh, unchilled tubers over a range of temperatures (10°C-45°C) in light and darkness. Fresh immature tubers showed a high sprouting percentage at all temperatures between 20°C and 40°C, while the mature ones did so only at 30°C and 35°C. Chilling of dry tubers stimulated early sprouting and increased the maximum sprouting percentage of both the mature and immature tubers. Dry chilling also lowered the limit of favourable temperatures to 15°C in the case of mature tubers. Chilling of wet tubers had a depressing effect and no sprouting occurred below 30°C. At all temperatures, light apparently favoured the sprouting of both the mature and immature tubers (except mature wet-chilled ones at 35°C and 40°C). Immature tubers showed relatively higher sprouting percentage than the mature ones, both in light and darkness. Alteration of temperature requirements due to dry and wet chilling of the tubers is regarded as significant and functional in relation to the ecology of the species.     

Conversion of potato phosphorylase isozymes

The conversion of a slow moving potato phosphorylase isozyme to a fast one, in sprouting tubers, either on freezing the whole tubers or on storage of their crude extracts, is due to limited proteolysis. High protease inhibitor concentration seems to be the primary factor preventing this conversion in freshly harvested tubers under similar conditions. Though MW determinations on both isozymes show the removal of a peptide during conversion, it is also likely that the enzyme may take up a different conformation due to the removal of this peptide.