Studies on the factors inducing cracking in tuberous roots of sweet potatoes (Ipomoea batatas Lam.).

Abstract

In the present paper the factors associated with the cracking of tuberous roots of sweet potatoes (Ipomoea batatas Lam.) were analyzed. Emphasis was placed on the observation of cracking under low temperature and dry conditions during the early stage of tuber formation, using rooted leaf pot culture. The development of cracking in the field was also studied. The results obtained are summarized as follows : 1. Three typical symptoms consisting of white-cracking (Fig. 2A), white-flatness (Fig. 2B) and red-flatness (Fig. 2C) were observed in the tuberous roots of the cv. Beniazuma that were exposed to low temperature and/or dryness for 5 days at 20 and 25 days after transplanting. Low temperature treatments (16/12°C, 12/12h) during periods ranging from 20-25 days and 25-30 days after transplanting increased the incidence of white-cracking (Fig. 3). The dry treatments associated with moisture percentages of the vermiculite medium below 120% on a dry weight basis (Fig. 1) during a period ranging from 20-25 days after transplanting also promoted white-cracking (Fig. 3). An increase in the incidence of cracking was observed when the normal tuberous roots developed under the low temperature and dry treatments were put in water, and especially those subjected to dryness during a period from 20-25 days exhibited considerable cracking 24 hours after being placed into water (Fig. 3). Those tuberous roots subjedted for 5 days to the low temperature and/or dry conditions described above showed thin cortices (Table 1). 2. White-cracking, which is considered to be a symptom occurring immediately after cracking, was observed on the 20th day after transplanting, and white-flatness, which is considered to be an early healing symptom, was observed on the 25th day in plants not subjected to the low temperature and dry treatments. Furthermore, on the 30th day some of the white-flatness symptoms on the tuberous roots are considered to be changed to red-flatness symptoms (Fig. 4). Based on these findings it is considered that the cracking of the tuberous roots of sweet potatoes which occurs easily even without exposure of the plants to low temperature and/or dry conditions, may heal through the occurrence of flatness symptoms under favorable conditions. 3. The white-cracking symptoms on the 25th day after transplanting remained until the 35th day under the low temperature treatment and/or dryness induced by 10% solution of polyethylene glycol (PEG 6, 000) treatments (Fig. 5). This result suggests that low temperature and dry conditions delay the healing of cracking. 4. It was also found that cracking occurred and healed at the early stages of tuber formation in the cvs. Beniaka, Kokei No.14 and Beniazuma in the field. The white-cracking symptoms observed in the 5th week after transplanting was considered to be changed to red-cracking and red-flatness symptoms in the 10th week. The red-cracking symptoms, which correspond to cracking at harvesting time, may represent an incomplete healing symptom of cracking. Red-cracking of the tuberous roots grown under the polyethylene-mulch treatment in the 10th week was more frequently observed than in the nontreated materials (Fig. 6). It is considered that the healing can not take place easily due to the rapid swelling of tuberous roots and the cracking symptoms persisted under the polyethylene-mulch treatment. From these results, it is suggested that cracking is induced when tuberous roots grow fast in the early stages of tuber formation, and heals readily under favorable conditions. Thus low temperature and dry conditions may prevent healing, and consequently increase the incidence of cracking.