Water yam produces a single or a few tubers per plant and the rate of clonal propagation by tuber cutting is extremely low. Therefore, it is important to develop a more efficient method of clonal propagation by the use of vine cuttings. It is generally recognized that in the vine cuttings of water yam, axillary buds pre-differentiated on leaf axils remain dormant and that shoots or mini-tubers tend to be formed at the base of the axillary buds. The objective of the present study was to characterize the sprouting and shoot formation processes of rooted cuttings of water yam. These processes in an accession, Obukosumbori, were investigated in a micro-anatomy study using a stereoscopic microscope and a transmission optical microscope. The following results were obtained. While an axillary bud differentiating on leaf axils buried in the ground remained dormant, enlarged tissue appearing like multiple shoots was newly developed at the base of the axillary bud. When no shoots were formed, a spherical body appearing like a bulbil with several shoot primordia developed from the enlarged tissue at the base of axillary buds. The effects of the collection time of cuttings on shoot formation of rooted cuttings were investigated in 8 accessions of water yam. This is the first report in which it was shown that the tendency of shoot formation from rooted cuttings varied among the accessions and could be categorized into three types. The results disclosed that the rates of survival of the cuttings and shoot formation of rooted cuttings were higher when the cuttings were collected from younger plants and tended to decrease with the progression of the growth stages of mother plants.

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To examine the effect of NPK application on the growth, yield and nutrient uptake by sugarcane (Saccharum officinarum L.) and to elucidate the limiting factors of cultivation on sandy soils in the tropics, a field experiment was carried out in 2005 on a sandy soil in Khon Kaen, Northeast Thailand. Six treatments were applied: NPK (300 kg ha^-1 each), PK, NK, NP, control and NPK+manure (300 kg ha^-1 each + 30 t ha^-1) treatments. Plants were harvested and the dry weight, yield and NPK contents determined. Root distribution in the soil profile was also examined. Aboveground biomass values in the PK, NP, NPK and NK treatments were 26, 37, 85 and 87% higher than in the control (31.4 t ha^-1), respectively, indicating that N was the dominant limiting factor, followed by K, with little effect of P. Aboveground biomass value in the NPK + Manure treatment was not significantly different from that of the NPK treatment, suggesting that the advantage of organic matter application was limited in the present study. More than 80% of the roots were distributed within the surface 30cm. Nutrient removal in the control treatment amounted to 39 kgN ha^-1, 12 kgP ha^-1 and 39 kgK ha^-1, which corresponded to 83, 5 and 15% of the soil available nutrients. In conclusion, adequate N and K fertilization in combination with effective erosion control to conserve surface soil would be essential for sustainable production of sugarcane on sandy soils in the tropics.

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Using two rice lines, namely 2F₁₈-7-32 (32) a Rhizoctonia solani - resistant rice line and one, 2F₂₁-21-29 (29) a susceptible rice line, changes in the time course of mRNA expression of 30 metabolic enzyme genes in R. solani-infected samples were examined. These consisted of eight metabolic enzyme genes of the glycolytic pathway; three metabolic enzyme genes of the oxidative pentose phosphate pathway (OPPP), and six metabolic enzyme genes of the reductive pentose phosphate cycle (RPPC); sucrose phosphatase (SPase, EC 3.1.3.24), participating in sucrose synthesis; ADP-glucose pyrophosphorylase (ADPase, EC 2.7.7.9) and starch synthase (SS, EC 2.4.1.21) which are involved in starch synthesis; six metabolic enzyme genes of the tricarboxylic acid (TCA) cycle and four metabolic enzyme genes of the secondary metabolism. The results showed that significant changes in the time course of mRNA expression occurred at 1 dpi and that mRNA expression of glycolytic enzymes; 6-phosphofructokinase, phosphoglycerate kinase, enolase and pyruvate kinase and secondary metabolism enzymes; 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase and phenylalanine ammonialyase was significantly higher (P < 0.05) in the R. solani-infected samples of the resistant rice line, compared to that of the susceptible rice line. The results suggested that R. solani infection led to activation of the glycolytic pathway, OPPP, TCA cycle and secondary metabolism. Time course of mRNA expression of RPPC genes, ADPase and SS suggested that starch synthesis was low in R. solani-infected samples of both R. solani-resistant and susceptible rice lines. It appeared that R. solani infection was associated with the activation of the glycolytic pathway, OPPP, secondary metabolism and TCA cycle and low starch synthesis.

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Water yam (Dioscorea alata L.) is a tuber crop widely cultivated in the tropics and sub-tropics in Africa as well as in Asia. It is clonally propagated by tubers in conventional cultivation due to the difficulty in sexual reproduction. However, a wide intra-specific variation is observed in various phenotypic traits and in somatic chromosome numbers ranging from 30 to 80. The authors investigated the intra-specific variation of ploidy and the effects on the phenotypic expression of a water yam collection from Myanmar. In the present report, the basic chromosome number of 20 (x=20) was adopted to determine the level of ploidy, according to the recent proposal by Arnau et al. (2009). Among 39 accessions, 24 (61.5%) consisted of diploids (2x=40), 11 of (28.2%) triploids (3x=60) and 4 of (10.3%) tetraploids (4x=80). Triploid accessions showed the largest leaves. The shape of the leaf became roundish and the SPAD value of leaf almost linearly increased along with the ploidy level. Size of stoma tended to enlarge as the ploidy level increased. In contrast, the stoma density decreased as the ploidy level went up. The intra-specific variation of phenotypic traits of the water yam collection from Myanmar is considered to be mainly due to ploidy variation. It is suggested that the information obtained in the present study could be not only useful to improve water yam but also significant for further investigation of genetic diversity within the species of water yam.

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