Minitubers TH E WOR LD OF P otato production is about harvesting the sun. It is about capturing solar energy with green leaves where photosynthesis takes place. Solar energy there is used to fuse water and carbon dioxide into sugars that alone may make starch and when combined with nitrogen can be made into proteins. Phosphorus is mainly needed in the processes of capturing, storing and transport of energy in plant cells. The driving force behind the development of a potato plant is temperature. Below a base temperature of about 3 degrees centigrade the tuber or plant is inert and does not show any development. At about 20 degrees most processes such as sprouting, sprout growth in the soil after planting and leaf area development after emergence are at their optimal rates. A few weeks after emergence when the crop has reached about 30% of ground cover the energy from the mother tubers is spent and further growth has to come from photosynthesis. Of each Mega Joule of solar radiation that the leaves intercept, the leaves produce about 1.5 grams of dry matter. The distribution of this dry matter over leaves, stems, roots and tubers is also determined by the temperature. At very high temperatures, of about thirty degrees, hardly any dry matter goes to the tubers and most of it goes to the foliage. But also day length and earliness – a short day with only twelve hours of light and early varieties favour the tubers so these factors influence the distribution of all the dry matter produced over the various organs. The aim of a grower is to enhance tuber growth but not too much as the leaves too need to be invested in to allow it to grow and perform until the end of the growing season. In a normal field crop about seventy five percent of all dry matter produced ends up in the tuber, say about 10 ton tuber dry matter (= 50 t fresh at 20 % dry matter concentration) of 13.3 ton total dry matter produced. But then growers and users are not just interested in a bulk of tubers. The fresh market has its requirements regarding colour, shape and size and so has the processing industry: long tubers for French fries and round ones for chips. The seed industry has its own needs with smaller tubers being offered a higher price since they represent more potential eyes delivering sprouts per kg of material. The most expensive seed tubers are minitubers derived from in-vitro plantlets. They usually are not sold per kilogram but per piece. The basic factors influencing potato production shown above also apply to production of minis. Van der Veeken and Lommen of the Wageningen Centre for Crop Systems Analysis recently published a scientific paper in Potato Research (2009, 50:105119) showing how different planting densities of in vitro plantlets in a commercial minituber glasshouse production system affect their number, size and yield. When planting 25, 62.5 and 145.8 of in vitro derived plantlets they found that when a lower number of plantlets were planted it took longer before full POTATO R E S EARCH ground cover with green leaves was reached. Consequently these crops initially intercepted less solar radiation but the efficiency with which they produced dry matter was slightly higher so final yields were not strongly influenced by low densities. Lower planting densities led to more stems per plant, less stolons per stem (leveling out both processes) and as more tubers per stolon were formed consequently it led to more tubers per plant The survival rates of the transplanted plantlets was also higher when planted at lower densities. At high plant densities a greater number of minitubers that were initially formed was resorbed again due to the higher degree of competition. Therefore at higher plant densities an earlier harvest (6 weeks after planting) could be considered than at low planting densities (10 weeks). Reducing the number of in vitro plants six fold increased the number of minitubers per plant two fold, so the same numbers of minitubers require three times as much greenhouse space. A better quality of minis produced at low densities and lower costs of the in vitro plants may help tip the balance. Anton Haverkort anton.haverkort@wur.nl Potato World 2009 • number 4 41 Pagina 40

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