<p>After harvest, potato tubers are usually stored at a temperature of 6-8°C in combination with the application of a synthetic sprout inhibitor. Frequently used sprout inhibitors are isopropyl N-phenyl-carbamate (propham or IPC), isopropyl N-(3-chlorophenyl)carbamate (chlorpropham or CIPC) or a combination of both compounds. There are several reasons for the development of alternative, natural sprout inhibitors. First, the Scandinavian market, for example, requires potato tubers free of (C)IPC residues, and the so-called "green" market, for which no or very little synthetic chemicals are allowed, does not yet have alternative sprout inhibitors. Secondly, governmental policy is directed towards a reduction of the amount of synthetic pesticides used in agricultural practice (Meerjarenplan Gewasbeschermingsmiddelen, MJPG).<p>Natural potato sprout inhibitors were already used in the ancient Inca cultures. After harvest, the potato tubers were stored in boxes or bins together with the twigs of muña plants <em>(Minthostachys</em> species). Treating the tubers in this way controlled sprouting as well as insect attack during a prolonged storage. Volatiles emanating from the muña leaves during the storage were responsible for the insect repellent and sprout inhibitory effects.<p>The monoterpene S-carvone is a related volatile compound which can be isolated from the seeds of caraway <em>(Carum carvi</em> L.) or dill ( <em>Anethum graveolens</em> L.), for example; also this compound has good potato sprout growth inhibitory effects. Application of S-carvone, derived from caraway seed, as a potato sprout inhibitor can stimulate the demand for caraway and therefore the need to grow it. This can be beneficial for Dutch growers, since cultivation of caraway is suitable on heavy clay soils in which crop rotation is limited to only a few crops. The research described in this thesis has been performed within the Dutch Caraway Research Programme in which nine research groups were amalgamated with the objective to reduce the problems with respect to the cultivation of caraway and to stimulate possible new applications of its essential oil or of S-carvone.<p>S-carvone inhibits the sprouting of potato tubers and the sprout growth reversibly: removal of S-carvone allows sprouting and regrowth of the individual sprouts. A high dosage leads to necrosis, but the side buds remain their viability and they start to sprout again when the concentration of S-carvone in the atmosphere comes below a threshold value. The enantiomer of S-carvone, R- carvone, can be isolated from spearmint ( <em>Mentha spicata</em> L.) <em></em> and possesses almost the same sprout growth inhibitory properties as S-carvone. Current research is focussed on the practical application of S-carvone to seed potatoes as a reversible sprout growth inhibitor.<p>In addition to the inhibitory effects just mentioned, the growth of several storage pathogens is also reduced by S-carvone. However, the susceptibility of fungi to S-carvone, e.g. <em>Fusarium</em> species that cause dry-rot, differs between (sub)species. <em>F.</em><em>solani</em> var. <em>coeruleum</em> is able to grow on tubers treated with S- carvone, whereas <em>F</em> . <em>sulphureum</em> cannot withstand it. This difference was not found <em>in vitro;</em> both fungi were susceptible to the same range of S-carvone concentrations, they were both able to convert S-carvone with the same rate, and into almost the same conversion products. Therefore, the difference in susceptibility <em>in situ</em> must be found in, for example, a specific interaction of the fungi with the potato tubers.<p>Carvone is stereoselectivily converted into other compounds by potato tissue: R-carvone mainly into neodihydrocarveol, and S-carvone into neoisodihydrocarveol. The bioconversion only takes place in easily accessible tissues, such as sprouts and tuber wound tissue. More than 90% of the amount of S-carvone found in intact tubers, is located on or in the skin. In addition to the chloroform-soluble bioconversion products, water-soluble carvone derived compounds were detected in potato tissue, using <sup><font size="-2">13</font></SUP>C-labelling studies. The identity of the conjugated compounds has not been established yet, but S-carvone is found after addition of HCl to the aqueous phase containing the conjugates. The induction of glutathione S-transferase may point to the conjugation of S-carvone to glutathione. Conjugation to saccharides may be an alternative explanation.<p>The sprout growth inhibition is correlated strongly with a decreasing 3- hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) activity, a key enzyme providing building blocks for the synthesis of various essential plant metabolites. Using specific potato HMGR antibodies, it was found that the decrease of activity correlated well with the disappearance of HMGR protein signals on Western blots derived from samples of proteins from organelle fractions and microsomal membranes.<p>S-carvone inhibits the healing of wounded tubers temporarily; in particular, suberization is delayed for about 10 days. The formation of a cambium layer is almost completely inhibited, which indicates that S-carvone interferes with cell division processes during the healing of wounded tissue. The suberization is correlated with the activity of phenylalanine ammonia Iyase (PAL). This enzyme catalyses the first step that leads to the synthesis of suberin, and in S-carvone treated wound tissue, the induction of PAL is delayed for about 10 days. This implies that tuber wound tissue is able to adapt to the exposure to S-carvone.<p>In conclusion, based on the research described in this thesis, it can be stated that S-carvone is a compound with a great potential because of its sprout growth inhibitory effect, possibly partly due to an inhibition of HMGR. Since S-carvone inhibits sprouting reversibly, it may also be useful as a temporary inhibitor of seed potatoes. In addition, S-carvone reduces the development and growth of several storage pathogens. These effects make the chances of an application of S-carvone as a natural potato sprout growth inhibitor even better.
|Qualification||Doctor of Philosophy|
|Award date||11 Oct 1995|
|Place of Publication||S.l.|
|Publication status||Published - 1995|
- carum carvi
- solanum tuberosum
- essential oils