Ontogeny of the tuber crop Plectranthus edulis (Lamiaceae)

Abstract

Plectranthus edulis (Lamiaceae) is one of the economically important tuber crops of the genus Plectranthus. It is grown in mid and high altitude areas in parts of Ethiopia. The structure of this plant and its development in time has not been described in detail. Two similar experiments were carried out at Awassa and Wondogenet (Southern Ethiopia) with two cultivars (Lofuwa and Chankua) and 20 harvest dates [from 14 to 280 days after planting (DAP)]. The plants were grown from seed tuber pieces and attained a maximum height of about 1.5 m. Plants produced main stems and primary, secondary and tertiary branches, with primary and secondary branches and their leaves constituting the main part of the canopy. Plant components were the seed tuber pieces, sprouts, main stems, branches, leaves, flowers, fruits, seeds, roots, stolons and tubers. Five, partly overlapping, ontogenetic vegetative phases were observed during one crop cycle: emergence, canopy development, stolon initiation and development, tuber initiation and growth, and a phase of canopy senescence. Stolons were formed on main stems and primary branches and originated below ground or above ground (aerial stolons). Aerial stolons were initiated later than below-ground stolons and were much longer (up to 2.5 m). Tubers usually were produced as a swelling on the tip of the stolon and sometimes as a swelling of the middle part of stolons. Tubers were stem tubers with pairs of ''eyes'' (compound buds) being arranged in the same decussate pattern as the axillary buds on stolons and stems. Tubers of cv. Lofuwa were up to 25 cm in length, those of cv. Chankua up to 20 cm, and both with a diameter of about 2 cm. The tubers in the middle of the stolon were longer than the ones at the tip. After tuber initiation, the total number of tubers increased almost linearly during 12 to 14 weeks, and maximum numbers of tubers were attained around 238 DAP, at crop senescence. Also, the number of smaller tubers (< 10 or 20 g) increased until 238 DAP. In the period of tuber initiation, the average weight per tuber increased up to 20 to 25 g per tuber. The increase in tuber fresh weight with time was therefore realized by an increase in both tuber number and in average weight per tuber. After crop maturity, farmers keep the tubers unharvested in the soil until they need them. This practice dramatically reduces tuber yield and number, because decreases of 36 to 59% were found in total tuber fresh weight and of 18 to 48% in number of tubers when tubers were kept in the soil in the 6 weeks period between 238 and 280 DAP.