Allelopathy in Sorghum as a Possible Tool for Striga hermonthica Management

Abstract

The root parasitic weed Striga hermonthica constitutes a major constraint to cereals production in sub-Saharan Africa. Several control measures have been recommended, however, incompatibility with the prevalent low inputs production systems precludes their adoption. Resistant genotypes could provide an ideal solution; however, durability of resistance is contestable. A series of laboratory, greenhouse and field experiments was undertaken at the College of Agricultural Studies, Sudan University of Science and Technology (Shambat) to determine i) the influence of genotypes, growth stages and assay procedure on induction of germination, radicle length and haustorial initiation in S. hermonthica by root exudates and shoot and root powders ii) importance of resistance and tolerance in refuting Striga infection and damage as influenced by Striga seed bank size and iii) the feasibility of employing plant residues to deplete the parasite seed bank. Three sorghum genotypes, Wad Ahmed, Striga tolerant improved cultivar, Tetron, Striga resistant local landrace and Hakika, Striga resistant exotic genotype, were used. Root exudates of hydroponically grown plants collected 7- 28 days after sowing (DAS) displayed differential germination inducing activity. At 7 and 14 DAS mean germination was highest (33.7-29%) for Wad Ahmed and significantly lowest for Tetron and Hakika. At 21 DAS, root exudates from Wad Ahmed and Hakika displayed comparable and significantly higher mean germination than their congener from Tetron. At 28 DAS, Tetron root exudates induced the highest mean germination (34.7%), whereas root exudates from Wad Ahmed and Hakika were significantly less active. Radicle length and haustorium initiation were influenced by sorghum genotype, exudates volume and time of root exudates collection. Striga Root exudates from Wad Ahmed sustained the shortest radicle (0.3 and 1.2µm×10-2) and highest haustorium initiation (94.2%), whereas its congeners from Hakika and Tetron sustained the longest radicle length (4.5µm×10-2) and the lowest haustorium initiation (1.6 and 0.5 %). Investigations using the rhizotron technique revealed that the genotypes sustained about equal germination and attachment of the parasite. However, the parasite development on Tetron and Hakika was arrested subsequent to xylem-to-xylem connection. Irrespective of sorghum genotype, Striga emergence and dry weight increased with increasing seed bank size. Wad Ahmed sustained the highest Striga emergence and dry weight followed in descending order by Tetron and Hakika. Further, Wad Ahmed displayed the highest reductions in plant height (39.6%), relative leaf chlorophyll contents (61.7%) and shoot dry weight (89.5%), while Hakika showed the lowest. Powder from air-dried shoots collected from field growing sorghum plants at 40 DAS induced the highest (44.1-51.7%) germination while those collected at 120 DAS induced the lowest (6.8-17.1%). For roots powder, irrespective of amount and growth stage, mean germination was invariably maximal for Wad Ahmed (92.7-96.6 %) and minimal for Hakika (0.0-13.8%). Striga seedlings from seeds induced to germinate by sorghum shoot or root powder, irrespective of genotypes, powder amount or sampling date displayed radicle length of 0.1- 0.6 µm ×10-2. Root residue assayed subsequent to harvest (DSH), irrespective of genotype, induced low to moderate germination (10-50%). Germination increased with powder amount, reached a maximum and subsequently declined. Radicle length, affected by root residues collected after harvest, was invariably short (0.0- 0.04µm ×10-2) and varied with genotype and collection time. For roots residues collected 60 DSH Striga seedlings induced by powder from Wad Ahmed and Tetron displayed, significantly, longer mean radicle length (0.02µm ×10-2) than those elicited by Hakika (0.01µm×10-2). Mean radicle length was maximum and significantly the longest at 10 mg /well. Higher residues amount significantly suppressed radicle length. For roots residues collected 75 DSH radicle length was the longest (0.04µm×10-2) for seedlings induced by powder from Wad Ahmed root residues and significantly shorter (0.03µm × 10-2) for those elicited by powder from Tetron and Hakika. The study revealed that germination inducing activity of root exudates of hydroponically grown plants and that of shoots and roots powder collected from field growing plants together with their effects on radicle length and haustorium initiation were influenced by genotype, growth stage and growth medium. The differential germination inducing activity of the hydroponically and rhizotron grown plants taken in conjunction with the differential resistance observed in the green house experiment are in conformity with the reported production of hydroxylated Strigolactones (SLs) which are extremely labile in soil and water by Tetron and Hakika. Further, in addition to the reported pre-attachment resistance to Striga due to low stimulant production the study revealed, for the first time, that the genotypes Tetron and Hakika are endowed with post-attachment resistance which obstructs development of the parasite after establishment of xylem-to-xylem connection. The notable fluctuations of germination and radicle length with time and/or on increasing root exudates volume or shoot or root powder amount suggest changes in production and/or proportions of germination stimulants and inhibitors and radicle extension suppressors with growth stage. Due to low germination inducing activity sorghum roots residues of the tested genotypes are of no practical value for depletion of Striga seed bank. However, the high and persistence activity of the residues as haustorium inducers and radicle elongation suppressor deserves to be studied further as it may reduce contact between the parasite and the host roots.