Morphological, phenological and water-use dynamics of sorghum varieties (Sorghum bicolor) under Striga hermonthica infestation

Leaf number was unaffected due to Striga infestation in the course of crop development in susceptible and resistant sorghum cultivars. Striga effect on stem height of IS 9302 was significant particularly during flowering and grain filling periods. No significant reduction in stem height was observed in SRN 39. Leaf area index (LAI) of IS 9302 Striga infested plants was significantly lower during panicle initiation and flowering. In SRN 39 reduction in LAI started relatively late and was significant only at peak flowering and at final harvest. Specific leaf area (SLA) in both sorghum cultivars was unaffected by Striga infestation. However, the leaf weight ratio (LWR) in the Striga infested plants of both cultivars was increased. Both SLA and LWR indicate that the Striga effect on the assimilatory surface and on the portion of dry matter allocated to leaves was small. The change in allometric coefficient was not high in both Striga infested sorghum cultivars. Only about 14% more dry matter was allocated to roots in IS 9302 Striga infested plants. Both the dynamic changes in leaf morphology and allometric coefficient indicate that the Striga effect may not be mainly through the interference with water balance of the infested host plants. Under Striga infestation there was a delay of about 120 degree-days to reach booting stage in both sorghum cultivars. The delay in developmental shift from booting to flowering under Striga infestation was also more pronounced. Unlike SRN 39. most of IS 9302 Striga infested plants failed to flower. Possible reasons for this delay are discussed. Water use efficiency (WUE) of Striga infected IS 9302 sorghum plants were reduced by about 72% compared to the control. The reduction in WUE of SRN 39 was about 24%. However, the evapotranspiration of Striga infected IS 9302 plants was even larger than the control and almost equal in the SRN 39 infected plants. This implies that dry matter accumulation was reduced, thereby reducing WUE significantly. In this case, the resistant cultivar SRN 39 showed less reduction in LAI, no more dry matter partitioning to roots and a significant improvement in WUE compared to IS 9302 under Striga infestation.