Studies on agronomy and crop physiology of Plectranthus edulis (Vatke) Agnew

Keywords: Development, morphology, plant density, potato, radiation interception, radiation use efficiency, seed size, seed tuber, spacing, stolon, tipping, tuber Plectranthus edulis (Vatke) Agnew (Lamiaceae) is an ancient Ethiopian tuber crop grown in mid and high altitude areas in the north, south and south-west of Ethiopia. Cultivation dates back from c. 3000 BC, but in recent years its acreage and production have declined. Renewed interest to conserve the crop and increase its production is limited by absence of accurate information on growth, development and cultural practices of P. edulis. This project aimed at providing the basic knowledge needed to direct further applied research. A standard production technique was developed after interviewing farmers in Chencha and Wolaita in southern Ethiopia, and was used in later experiments. The standard planting material chosen were de-sprouted tuber pieces, prepared from a medium (12–15 cm) sized mother tuber broken into three pieces. Three pieces were planted per hole, at a hole spacing of 75  90 cm. Shoot tipping (pinching; the removal of the apices with 12 leaf pairs) was carried out when the crop was 10–15 cm high. The general structure of the crop was similar to that of Irish potato. Plant components were: the seed tuber pieces, sprouts, main stems, branches, leaves, inflorescences, fruits, seeds, roots, stolons and tubers. The crop had a long growing period. In two growth studies, maximum fresh tuber yields were attained c. 34 weeks after planting (WAP). Above-ground development was characterised by a late emergence (c. 4 weeks), a slow development of the canopy after emergence until full ground cover was attained (c. 20 weeks), a very short period during which ground cover was full (c. 2 weeks) and a relative fast decline in ground cover thereafter (6−8 weeks). Primary and secondary branches constituted the major part of the canopy. The first stolons were formed c. 1012 WAP on below-ground nodes of main stems and primary branches. Tubers were first recorded at 18 WAP as a swelling on the tip of the stolon and sometimes as a swelling of the middle part of stolons. Tubers attained a maximum length of 2025 cm, and a maximum diameter of c. 2 cm. Aerial stolons were initiated 1216 weeks later than below-ground stolons and could be up to 2.5 m long. The increase in tuber fresh weight with time was realized by an increase in both number of tubers and in average weight per tuber over the entire tuber formation period. Fresh tuber yields at 34 WAP were 4549 Mg ha1. Yield levels in other sets of experiments in which the harvest date was chosen arbitrarily were c. 21 Mg ha1 (29.7 WAP) and c. 30 Mg ha1 (34.7 WAP). Experimental yields were very high compared to those reported by farmers. Nevertheless, in growth studies, the average daily dry matter production of the crop over the whole growing period was only 4.2−4.6 g m2 day1. The dry matter production was limited by a poor radiation interception by the canopy – only one third of the incident radiation was intercepted − and a low radiation use efficiency (RUE) – on average only 1.59 g MJ1 photosynthetically active radiation (PAR). RUE gradually increased after emergence to about 2.7 g MJ1 PAR when tuber formation was still in an early stage (24−26 WAP), but then declined because of a stagnation or decline in total crop dry weight, that lasted several weeks. Dry matter production decreased in that period because the decrease in canopy dry matter – especially stem dry matter − was not yet compensated for by the increase in tuber dry matter. This was attributed partly to a still limited capacity of the tubers to convert and / or store assimilates in this stage. Later this changed and total dry weight and RUE increased again. Harvest index was 8199% at the moment when tuber yield was maximum. Shoot tipping significantly increased ground cover and delayed canopy senescence. Tipping also had a positive – though not always significant – effect on tuber yield. Tipping enhanced early stolon formation, but did not consistently affect the number of stolons later in the growing season Because differences among tipped treatments were not large, limiting the tipping frequency to one will help to save time, labour and money. Across experiments in which the number and size of the tuber pieces planted per hole were varied, the tuber fresh weight increased when the number of main stems per m2 increased up to 2.53 main stems per m2. This sufficiently high stem number could usually be achieved by planting sufficient seed tuber material (equalling at least one medium-sized mother tuber per hole) and breaking it into two or three pieces. This confers with the farmers practice. Over all treatments, an increase in fresh tuber yield was never realized by merely increasing the individual tuber weight, but either by combined effects on number of tubers and individual tuber fresh weight or by an effect on number of tubers alone. A further increase in radiation interception by advancing and improving canopy development could likely be achieved by planting larger seed pieces, pre-sprouting the seed tuber pieces and using a higher plant density. However, the below ground development should be geared to that. At present the late initiation and formation of tubers already seems to limit production, and this should be improved when an enhanced canopy cover should result also in higher tuber yield. On short term notice, however, the major constraints to concentrate on will be the shortage of seed tubers and the poor storability of the progeny tubers. Shortage of seed tubers was mentioned by the interviewed P. edulis farmers as a major constraint and the principle reason for the decline in production of P. edulis. The present practice by farmers of storing tubers in situ in the ground was shown to reduce tuber fresh weights by 3659% and the number of tubers by 1848% in 6 weeks.