Experimental set-up for sterol feeding studies.

<p>Young potato shoots were trimmed to a final length of 10 cm, fed labelled sterols through the cut stem, and incubated for either three or five weeks. After the feeding period, shoots were divided into four parts consisting of upper leaves, lower leaves, upper stem, and lower stem. Materials from up to three separate shoots were pooled when necessary. </p

Chemical structures of deuterium (D) -labelled sterols used in the present study.

<p>Cut potato shoots were fed D₅- or D₆-cholesterol (labelled in the sterol ring structure), or D₇-cholesterol (labelled in the sterol side chain). Side chain-labelled D₆-27-hydroxycholesterol and D₇-sitosterol were used as controls.</p

A model for the conversion of different deuterium (D) -labelled cholesterol substrates to D-labelled glycoalkaloids.

<p>Cut potato shoots were fed D₅- or D₆-cholesterol (labelled in the sterol ring structure), or D₇-cholesterol (labelled in the sterol side chain). All these labels were converted <i>in </i><i>vivo</i> to D-labelled SGA. Labels in the ring structure were intact after conversion to SGA, but two D atoms were lost from the side-chain label, indicative of an aldehyde formation, likely at the C-26 position. R signifies either OH (solanidine), chacotriose (α-chaconine), or solatriose (α-solanine).</p

Formation of deuterium (D) -labelled SGA in potato shoots after feeding with D-labelled sterols solubilised in Tween-80 or methyl-β-cyclodextrin (MBD).

<p>Cut potato shoots (cv. King Edward) were fed 200 µg D₆- cholesterol, D₇-cholesterol, or D₇-sitosterol solubilised in Tween-80 or MBD, and incubated during either three or five weeks. After the incubation period, the levels of D-SGA (α-solanine and α-chaconine) were analysed in upper leaves (black bars) and lower leaves (white bars) using LC-MS/MS. (A) Graphical illustration of the difference between Tween-80 (T-80) and MBD feedings. Mean value ± SD for the amount of D-SGA formed in leaves up to 5 weeks. A difference between the treatments (n=4 full plants each) was significant at p<0.001 (paired Student's <i>t</i>-test). (B) to (E) individual analyses, each representing a single analysis of materials from 2-3 pooled plants: (B) Tween-80, 3 weeks; (C) Tween-80, 5 weeks; (D) MBD, 3 weeks; (E) MBD, 5 weeks. Note the different scales for Tween-80 and MBD. No D-SGA was formed in parallel analyses of blank controls, or shoots fed D₇-sitosterol. </p

Glycoalkaloid and Calystegine Levels in Table Potato Cultivars Subjected to Wounding, Light, and Heat Treatments

Potato tubers naturally contain a number of defense substances, some of which are of major concern for food safety. Among these substances are the glycoalkaloids and calystegines. We have here analyzed levels of glycoalkaloids (α-chaconine and α-solanine) and calystegines (A₃, B₂, and B₄) in potato tubers subjected to mechanical wounding, light exposure, or elevated temperature: stress treatments that are known or anticipated to induce glycoalkaloid levels. Basal glycoalkaloid levels in tubers varied between potato cultivars. Wounding and light exposure, but not heat, increased tuber glycoalkaloid levels, and the relative response differed among the cultivars. Also, calystegine levels varied between cultivars, with calystegine B₄ showing the most marked variation. However, the total calystegine level was not affected by wounding or light exposure. The results demonstrate a strong variation among potato cultivars with regard to postharvest glycoalkaloid increases, and they suggest that the biosynthesis of glycoalkaloids and calystegines occurs independently of each other