Hiroaki Samejima

Practicality of the suicidal germination approach for controlling Striga hermonthica.

BACKGROUND Purple witchweed (Striga hermonthica), Orobanchaceae, is an obligate root-parasitic weed of important cereal crops. The parasite is a copious seed producer, and a huge seed bank develops soon after the onset of the initial infestation. To germinate, a Striga seed requires a pretreatment in a moist warm environment and a subsequent exposure to an exogenous stimulant. One approach to reduce the seed bank is artificially to induce germination of the seeds in the absence of or away from the host roots. A newly developed germination stimulant for S. hermonthica, designated as T-010, was evaluated for efficacy in greenhouse and field experiments under artificial Striga infestation. RESULTS T-010 displayed germination-inducing activity in soil. Formulated T-010 applied at 0.1, 1 and 10 kg ai ha-1 to potted soil containing S. hermonthica seeds, previously conditioned by judicious irrigation, reduced Striga emergence by 94-100%. Results of a field trial showed that formulated T-010, at the same rates as for the pot experiment, delayed and reduced Striga emergence by 33% and increased sorghum shoot and head dry weight by 18.7-40.2% and 187-241% respectively. CONCLUSION These findings demonstrated, for the first time, the technical feasibility of suicidal germination for controlling S. hermonthica. Optimising structure, formulation and application protocol of germination stimulants should be the main goal for further improvement of the technology.

Identification of Striga hermonthica-Resistant Upland Rice Varieties in Sudan and Their Resistance Phenotypes

Rice has become a major staple cereal in sub-Saharan Africa. Currently, upland rice cultivation is expanding particularly in rainfed areas where the root parasitic weed Striga hermonthica, a major constraint to cereal production, is endemic. Laboratory, pot, and semi-controlled open air experiments were performed to evaluate resistance of selected rice varieties in Sudan to a resident S. hermonthica population. In the laboratory, 27 varieties were screened for post-attachment resistance using the rhizotron technique. Varieties displaying high post-attachment resistance, Umgar, NERICA5, and NERICA13 together with NERICA4, NERICA18, and Nipponbare, a lowland rice variety, were further evaluated for performance and Striga resistance in pot and semi-controlled open air experiments and for germination inducing activity in a laboratory. In addition, comparative studies on reaction of Umgar, Kosti1 and Kosti2, released varieties for commercial production in Sudan, to the parasite were performed in two pot experiments. In the pot experiments Umgar and NERICA5, consistently, sustained the lowest Striga emergence (<2.2 Striga plants per pot), while NERICA13 and NERICA4 supported 1.8–5.7 and 8.7–16.4 Striga plants per pot, respectively. In an artificially Striga-infested field, number of emergent Striga plants per 10 rice hills, at harvest, was 2.0, 2.0, 4.8, 13.5, 13.3, and 18.3 on Umgar, NERICA5, NERICA13, NERICA4, NERICA18, and Nipponbare, respectively. Striga had no adverse effects on total above-ground parts and panicle dry weight in Umgar and NERICA5. Germination-inducing activity of root exudates, at 14 days after sowing onward, was markedly lower for Umgar than for NERICA5, NERICA13, NERICA4, and NERICA18. Based on these findings, Umgar has both pre and post-attachment resistance to a resident Striga population in Sudan. Kosti1 and Kosti2 did not exhibit Striga-resistance at the same level as Umgar. Further the resistance of NERICA5, a variety reported to be endowed with a broad spectrum resistance to Striga species and ecotypes, at least to one resident Striga population in Sudan was clearly indicated.

Gas exchange of root hemi-parasite Striga hermonthica and its host Sorghum bicolor under short-term soil water stress

The gas exchange of the upper fully expanded leaf of the root parasite Striga hermonthica and of its host Sorghum bicolor was measured under wet and dry conditions to identify the mechanisms of the devastating effects of the parasite on its hosts under drought. The short-term water stress severely reduced photosynthetic rate in infected sorghum, but less in S. hermonthica. Soil water stress did not affect leaf respiration rate in either S. hermonthica or infected sorghum. This suggests that under dry conditions both infected sorghum and S. hermonthica decreased autotrophic carbon gain. The transpiration rate of S. hermonthica, a major driving force for assimilate uptake from the host, was higher and less affected by water stress than that of infected sorghum. Stomatal density on the abaxial surfaces of the leaves was higher in S. hermonthica than in sorghum. Both S. hermonthica infection and water stress decreased stomatal conductance of the sorghum leaves. S. hermonthica, irrespective of soil water status, had greater stomatal aperture on the adaxial and abaxial surfaces of its leaves than infected sorghum. These results indicate that the higher transpiration rate of S. hermonthica even under water stress, achieved through higher stomatal density on the abaxial surfaces of the leaves and greater stomatal aperture on both surfaces of the leaves, may induce the maintenance of water and solute transfers from the host to the parasite leading to severe damage to the host under drought.

Grain yield responses of lowland rice varieties to increased amount of nitrogen fertilizer under tropical highland conditions in central Kenya

Abstract Tropical highland conditions in Mwea Kenya, ensure the high radiation and the large day–night temperature differences. Such conditions are generally believed to promote rice growth and yield, but the current grain yield is lower than the expectation. In the current standard N fertilizer practice in Mwea, 75 kg nitrogen (N) ha−1 is applied in three splits at fixed timing. The effects of increases in N fertilizer amount (125, 175, and 225 kg N ha−1) on rice growth and yield were evaluated to test the hypothesis that unachieved high rice grain yield in Mwea is due to insufficient amount of N fertilizer. Two popular lowland varieties in Mwea (Basmati 370 and BW196) and two varieties reported as high yielding in other countries (Takanari and IR72) were used. Shoot dry weight (DW) increased with increases in the amount of N fertilizer applied in three splits at fixed timing, irrespective of variety. It reached approximately 20 t ha−1 under increased N conditions (>75 kg N ha−1) in several cases, indicating that high biomass production could be achieved by increasing N application rate. However, the increased biomass did not increase grain yield, due to decreased grain filling under high N conditions in all varieties. Thus, N amounts above 75 kg ha−1 were ineffective for increasing grain yields in Mwea, where N fertilizer was applied in three splits at fixed timing. Increasing influence of low temperature under high N conditions may be one of the reasons for the decreased grain filling in Mwea.

Identification and validation of QTLs for cold tolerance at the booting stage and other agronomic traits in a rice cross of a Japanese tolerant variety, Hananomai, and a NERICA parent, WAB56-104

Abstract In Africa, cold temperatures occur in the highlands of East and Southern Africa and in some areas of the Sahel region of West Africa leading to substantial rice yield losses. Cold tolerance (CT) at booting stage on basis of spikelet fertility after cold water irrigation was evaluated using F2 population derived from a cross between temperate japonica, Hananomai, and tropical japonica, WAB56-104. Two Quantitative trait loci (QTLs) for CT were detected on chromosome 8 and 10 with enhanced effects on the trait coming from Hananomai and WAB56-104 allele, respectively. The QTLs explained 30% and 33% of phenotypic variation in spikelet fertility, respectively. CT was negatively correlated with panicle number (r = −0.35, p < 0.01) and positively correlated with panicle weight (r = 0.61, p < 0.001). Selected BC1F4 and BC1F5 genotypes having homozygous alleles for both CT QTLs exhibited higher spikelet fertility under cold stress. The identified QTLs will be useful in the development of cold-tolerant varieties for production in high altitude areas through marker-assisted selection.

Improvement of Food Security in Semiarid Regions of Sudan Through Management of Root Parasitic Weeds

The root parasitic weed, Striga hermonthica, has long been recognized as the greatest biological constraint to food production in sub-Saharan Africa. Development of simple, easy-to-implement, sustainable, durable, and inexpensive solutions is imperative to fully exploit the vast arable areas in the region. To combat S. hermonthica, a project entitled “Improvement of Food Security in Semiarid Regions of Sudan Through Management of Root Parasitic Weeds” was implemented from 2009 to 2015 under the auspices of JICA/JST SATREPS. In the project, suicidal germination approach to reduce S. hermonthica seed bank in soil by inducing germination of the parasite in the absence of host plants through artificial application of germination stimulant was demonstrated, for the first time, as a technically feasible countermeasure. In addition, NERICA5 and Umgar were selected as S. hermonthica-resistant rice varieties under the growth conditions in Sudan. A new SATREPS project on S. hermonthica management in Sudan entitled “Developing Counter Measures Against Striga to Conquer Poverty and Improve Food Security” started in July 2017. The project is to take over successful achievements in the former project and is to introduce an additional activity based on the concept of management through utilization.

Development of Rice Breeding and Cultivation Technology Tailored for Kenya’s Environment

In many sub-Saharan African countries, it is fundamental to boost rice production because the increased rice consumption exceeds the growth in domestic rice production. Kenya has currently implemented a policy to double its rice production which, however, has been suffering from serious constraints. Nagoya University and Kenya Agricultural and Livestock Research Organization, in collaboration with other Japanese and Kenyan research institutions, have identified several biotic and abiotic stresses such as drought, cold temperatures at high elevations, high salinity, low soil fertility, and rice blast disease as such constraints and then initiated an international joint research project titled “The project on rice research for tailor-made breeding and cultivation technology development in Kenya” from May 22, 2013, to May 21, 2018. Using technologies such as molecular breeding and DNA marker-assisted selection, the project has advanced the development of rice varieties carrying useful genes/QTL to overcome stress conditions in Kenya. For phenotyping and examination of incorporated gene/QTL expression, we have developed unique evaluation systems for plant adaptability against drought, low temperatures, high salinity, low fertility, and rice blast disease in Kenya. After assessing current cultivation practices and environments, the project aimed to develop cultivation technologies that maximize the potential of rice varieties carrying useful genes/QTL. Eventually, superior varieties and customized cultivation technology will be disseminated to the farmers. The project has also assisted the development of human resources for rice research both in Kenya and Japan. Based upon our research, we will persist in our efforts to establish an international network as a framework for variety improvement and cultivation technology development customized to the sub-Saharan African environment.

Recent research progress in combatting root parasitic weeds

ABSTRACT The obligate root parasitic Orobanchaceae plants Striga, Orobanche and Phelipanche spp. parasitize economically important crops, vegetables and oil plants. They are the most devastating agricultural weed pests worldwide. Based on an analysis of the climatic requirements of these parasites, very large areas of new territory are at risk of invasion if care is not taken. Recent research in combatting root parasitic weeds was reviewed based on scientific papers reported from 2010 and onwards. The countermeasures fell into eight kinds: resistant varieties, tolerant varieties, microbiological approach, cultural practices, chemical controls, host-induced gene silencing, integrated management and dissemination of technologies including the current situation survey. The development of practical, feasible and economical control technologies against root parasitic weeds would be expected by advancing and combining the countermeasures.

Genotypic variation in rice varieties screened for deep rooting under field conditions in West Africa

Abstract In this study, to identify deep rooting accessions, we assessed the differences in root depth based on the length of the longest primary root among 586 different rice accessions: 511 Oryza sativa and 75 O. glaberrima. Malagkit Pirurutong and Binicol were identified as the two rice accessions with deepest roots through four field experiments conducted at two different locations in West Africa. For these two accessions, root depths reached 35.6 and 41.4 cm, respectively, in the first experiment at Bamako; on the other hand, their depths only reached 22.6 and 18.6 cm, respectively, in the second and third experiments at Ibadan, leading to inconsistent genotypic ranking based on root depth between the two locations. However, Malagkit Pirurutong was identified as deep rooting in both locations; in addition, it showed deep rooting in the fourth experiment in a 20-mm irrigation treatment, even when compared with the deep rooting reference Azucena. Nonetheless, this pattern was not found under a 10-mm irrigation treatment. Malagkit Pirurutong kept developing deep roots even following 60 days after sowing (DAS), whereas other shallower rooting accessions ceased deepening by 60 DAS. The longer period for deepening roots would be beneficial for terminal drought stress.