Effects ofSolanum glycoalkaloids on chemosensilla in the Colorado potato beetle

Steroidal glycoalkaloids, found in species of the Solanaceae, elicit bursting activity in galeal and tarsal chemosensilla of adult Colorado potato beetles. The effect has an average latency of 6–12 sec, depending on the sensillum/alkaloid combination. A 20-sec alkaloid treatment is often suffficient to render galeal sensilla unresponsive to gamma-aminobutyric acid, normally an effective stimulant. The alkaloids have similar effects on galeal sensilla of larval Colorado potato beetles and on labellar chemosensilla of the blowfly. It is concluded that these compounds act independently of any specialized chemoreceptor in the Colorado potato beetle, and that association of the Colorado potato beetle with solanaceous plants has not led to evolution of a specific receptor forSolanum glycoalkaloids.     

Glycoalkaloids in potato tubers: the effect of variety and drought stress on the α‐solanine and α‐chaconine contents of potatoes

Six varieties of Solanum tuberosum L potato grown in the Bolivian highlands under drought stress, with and without irrigation, were analysed for their content of glycoalkaloids (GAs). The plant material consisted of three drought‐tolerant varieties from a local breeding programme (PROINPA), Potosina, Chapaquita and Pampeña, and three control cultivated varieties, Malcacho, Sani Imilla and Desiree, either susceptible or relatively tolerant to drought. α‐Solanine and α‐chaconine were quantified in both the peel and flesh of the tubers. A significant increase in GA concentration (α‐solanine + α‐chaconine) was observed under drought stress conditions in most varieties; average concentration increases of 43 and 50% were registered in the improved and control cultivars respectively. In all tested cultivars, however, the GA concentration remained lower than the recommended food safety level (200 mg kg⁻¹ fresh tubers). It ranged from 52.4 to 100 mg kg⁻¹ fresh tubers in the improved cultivars and from 55.6 to 122.3 mg kg⁻¹ fresh tubers in the controls. In the improved and control varieties the α‐solanine content averaged 42.6 and 35.4% of the total potato GAs respectively and was not significantly affected by drought stress, except in Desiree. In all conditions the peel contained the greatest proportion of total GAs. The hybrid variety Pampeña (new drought‐tolerant variety) contained the lowest amounts of GAs, which were lower than those of the control varieties, with and without irrigation. © 2000 Society of Chemical Industry     

Variation in glycoalkaloid concentration of potato tubers harvested from mature plants

Individual tubers from mature plants of cultivars Pentland Dell and Estima were harvested from experimental plots in two successive years and analysed for α‐solanine, α‐chaconine and total glycoalkaloid concentration (α‐solanine and α‐chaconine combined) using high‐performance liquid chromatography. Mean tuber glycoalkaloid concentration per plant was strongly affected by genotype and was much higher in plants of Pentland Dell (10.0 and 16.3 mg per 100 g fresh weight in 1994 and 1995, respectively) than Estima (4.0 and 4.3 mg per 100 g fresh weight in 1994 and 1995, respectively). The variation between plants in mean tuber glycoalkaloid concentration, expressed as coefficient of variation, was similar for the two cultivars in both years. Variation within plants was higher for cultivar Pentland Dell than Estima for both years but it was statistically significant only in 1995. Tuber position within a plant had no effect on glycoalkaloid concentrations in tubers of similar size. An inverse relationship between total glycoalkaloid concentration and tuber fresh weight of individual tubers was found for both cultivars and small tubers of Pentland Dell (20–40 g fresh weight) exceeded the safety limit of 20 mg per 100 g fresh weight. The pattern of glycoalkaloid accumulation differed between genotypes and appeared related to tuber growth. The implications of these results in terms of food safety are discussed. © 1999 Society of Chemical Industry     

Bioassays of segregating plants

Solanum chacoense is a wild potato species resistant to the Colorado potato beetle,Leptinotarsa decemlineata. Most genotypes ofS. chacoense synthesize the glycoalkaloids solanine (sol) and chaconine (chac) and are hosts of the beetle. A few rare genotypes have a gene(s) for acetylation of carbon-23 of the steroid aglycone of sol and chac. Laboratory bioassays and replicated field tests of clones differing in the presence or absence of the acetyl moiety showed that acetylation of sol and chac markedly affects the response of both adults and larvae to the foliage. Adult feeding deterrency conferred by acetylated forms of sol and chac (leptines) in leaf-disk preference tests was consistent with the degree of antixenosis measured in the field. Development of larvae on foliage of clones with leptines was also inhibited. The studies support the validity of using laboratory bioassays of plants segregating for levels of a suspected defense compound to determine the role the compound has in defending the plant from attack by an insect predator in the field.     

Interactions of Aphid Herbivory and Nitrogen Availability on the Total Foliar Glycoalkaloid Content of Potato Plants

In plant growth room (PGR) and open-air pot (OAP) experiments, potato cvs King Edward and Maris Piper were grown under two nitrogen levels or two different nitrogen release patterns. Plants were subjected to infestation by peach potato aphids Myzus persicae (Homoptera: Aphididae). Total glycoalkaloid (GA) levels were measured in the foliage of both infested and non-infested plants, before, during and after aphid infestation. In the PGR experiment, aphid infestation reduced the amounts of total GAs in both cultivars. This reduction is attributed to the sugar deficiency induced in the plants owing to the dense aphid colonization. Results from the OAP experiment showed a temporal increase of GAs produced by potato cv. King Edward plants subjected to aphid infestation. Elevated amounts of nitrogen in the nutrient solutions (PGR experiment) reduced total GAs, while no differences were observed between manure and fertilizer treated plants (OAP experiment). It is concluded that the source of available nitrogen does not affect foliar GA synthesis in potatoes, and as a consequence, does not affect its endogenous chemical defense against insect herbivory. The case for insect-induced chemical defense mechanisms as triggered by low rates of aphid infestation is discussed.     

Light‐induced glycoalkaloid accumulation of potato tubers (Solanum tuberosum L)

The total glycoalkaloid concentration of non‐sprouted potato tubers, cvs Marfona, Fianna, Maris Piper, Kestrel and Golden Wonder, was measured following 15 days of continuous illumination (250 µmol m⁻² s⁻¹ photosynthetically active radiation) using high‐performance liquid chromatography. Comparisons were made of the influence of four sources of illumination: fluorescent tube‐type warm white, high‐pressure sodium, high‐pressure mercury types MB/U and MBFR/U. Irrespective of cultivar, glycoalkaloid concentrations of tubers placed in the dark remained relatively constant from day 0 to day 15 and did not exceed food safety values of 200 mg kg⁻¹ FW. A highly significant cultivar, light and cultivar × light (P < 0.01) interaction was recorded. Cultivar Kestrel was shown to be light‐sensitive with high rates of glycoalkaloid accumulation irrespective of light source, and cv Maris Piper was demonstrated to be light‐insensitive. In the majority of cultivars tested, maximal rates of glycoalkaloid accumulation were recorded following exposure of tubers to sodium or fluorescent light, and minimal rates of accumulation were recorded following exposure of tubers to mercury (MB/U and MBFR/U) illumination. In virtually all cases glycoalkaloid concentrations steadily increased with time during light exposure with no indication of cessation. In some instances the α‐chaconine/α‐solanine ratio decreased over 15 days of illumination (P < 0.05), indicating enhanced α‐solanine synthesis over that of α‐chaconine. The implications of these results in terms of consumer safety are discussed. © 1999 Society of Chemical Industry     

Effect of environmental stress during tuber development on accumulation of glycoalkaloids in potato (Solanum tuberosum L)

The effects of low temperature and of combined stresses, cool temperature and waterlogging and also warm temperature and drought, on potato tuber glycoalkaloid levels were investigated, with emphasis on comparison of tubers of similar size. The early maturing cultivars Home Guard, Rocket and British Queen were grown under favourable controlled environment conditions (18/14 °C day/night temperature, 14 h photoperiod) until the imposition of treatments after the start of tuber initiation. Transfer to low temperature (12/9 °C) did not increase total glycoalkaloid concentration in any of the three cultivars. However, waterlogging at 12/9 °C increased total glycoalkaloid concentration during the later stages of development of British Queen. Drought stress at 24/18 °C also increased total glycoalkaloid concentration in British Queen, although the response varied between individual tubers of similar size. Total glycoalkaloid concentrations in tubers of Rocket were not affected by the combined stress treatments, and any effects on Home Guard were very much less than on British Queen. © 1999 Society of Chemical Industry     

Potato glycoalkaloids: formation and strategies for mitigation

With the increasing production and consumption of potato and its products, glycoalkaloid (GA) formation and toxicity are likely to become an important focus for food safety researchers and public health agencies. Not only the presence of GA, particularly in the form of α‐solanine and α‐chaconine, but also the changes occurring as a result of various post‐harvest handling practices and storage, are critical issues influencing the quality of stored potatoes. Studies on various factors (pre‐harvest, during harvest and post‐harvest) affecting GA have been carried out from time to time, but it is difficult to compare the results of one study with another due to wide variation in the parameters chosen. This review aims to develop a clear understanding of these issues. Published information on the types of GA, their effects on health, their typical concentrations in potatoes, their formation mechanisms, and how their levels can be controlled by following appropriate post harvest practices and storage regimes are critically analysed. The levels of GA in potato can be controlled effectively by adopting appropriate post‐harvest practices. Further studies are necessary, however, to investigate best practices, which either check completely or retard substantially their formation. Copyright © 2008 Society of Chemical Industry     

Colorado Potato Beetle Toxins Revisited: Evidence the Beetle Does Not Sequester Host Plant Glycoalkaloids

The Colorado potato beetle feeds only on glycoalkaloid-laden solanaceous plants, appears to be toxic to predators, and has aposematic coloration, suggesting the beetle may sequester alkaloids from its host plants. This study tested 4th instars and adults, as well as isolated hemolymph and excrement, to determine if the beetles sequester, metabolize, or excrete alkaloids ingested from their host plants. HPLC analysis showed: that neither the larvae nor the adults sequestered either solanine or chaconine from potato foliage; that any alkaloids in the beetles were at concentrations well below 1 ppm; and that alkaloids were found in the excrement of larvae at approximately the same concentrations as in foliage. Analysis of alkaloids in the remains of fed-upon leaflet halves plus excreta during 24 hr feeding by 4th instars, as compared to alkaloids in the uneaten halves of the leaflets, showed that equal amounts of alkaloids were excreted as were ingested. The aposematic coloration probably warns of a previously-identified toxic dipeptide instead of a plant-derived alkaloid, as the Colorado potato beetle appears to excrete, rather than sequester or metabolize, the alkaloids from its host plants.