Abstract
It is vital to understand how rainfall onset, amount and distribution between planting dates affect sorghum yield and water use, in order to aid planting date and cultivar selection. This study investigated morphological, physiological, phenological, yield and water use characteristics of different sorghum genotypes in response to different planting dates under rainfed conditions. Four genotypes (PAN8816 [hybrid], Macia [open-pollinated variety, OPV], Ujiba and IsiZulu [both landraces]) were planted on 3 planting dates (early, optimal, and late) in a split-plot design, with planting dates as the main factor. Low soil water at the optimal planting date was associated with delayed crop establishment and low final emergence. Sorghum genotypes adapted to low and irregular rainfall at the late planting date through low leaf number, canopy cover, chlorophyll content index and stomatal conductance, and hastened phenological development. This resulted in low biomass and grain yields. Landraces exhibited grain yield stability across planting dates, whilst OPV and hybrid genotypes significantly reduced grain yield in response to low water availability when planted late. Biomass and grain yield water use efficiency (WUE) were highest at optimal planting date (30.5 and 9.2 kg∙ha -1 ·mm -1 ), relative to late (23.1 and 8.7 kg·ha -1 ·mm -1 ), and early planting dates (25.2 and 8.3 kg·ha -1 ·mm -1 ). For PAN8816 and Macia, biomass and grain WUE decreased in response to low soil water content, and irregular and disproportionate rainfall experienced during the late planting date. By contrast, biomass and grain WUE for Ujiba and IsiZulu improved with decreasing rainfall. PAN8816 is recommended when planting under low soil water availability to maximize crop stand. Cultivation of Macia is recommended under optimal conditions. Ujiba and IsiZulu landraces are recommended for low rainfall areas with highly variable rainfall. Repetition or modelling of genotype responses across environments different from Ukulinga is required for thorough water use characterisation of these genotypes. Keywords : planting dates, water use efficiency, rainfall variability, cultivar selection, landraces and improved sorghum varieties
Highlights
Sub-optimal availability of water for unrestricted plant growth and transpiration, i.e., drought, is a major limitation to agricultural production (Delmer, 2005)
Temperature and evapotranspiration In South Africa, Du Plessis (2008) reported that, for sorghum, the lower temperature threshold limit was 7°C, while the upper temperature threshold limit for sorghum grown in the semiarid tropics has been reported as 38°C (Huda et al, 1984)
For all planting dates, recorded rainfall was less than the crop water requirement for sorghum under South African conditions (450–650 mm) (Jewitt et al, 2009)
Summary
Sub-optimal availability of water for unrestricted plant growth and transpiration, i.e., drought, is a major limitation to agricultural production (Delmer, 2005). Establishment and high seedling emergence for all genotypes for late planted sorghum were attributed to high cumulative rainfall 7 days after sowing and high initial soil water content at sowing. Low initial SWC at sowing (Fig. 2) resulted in delayed crop establishment for early planted sorghum for all genotypes compared to optimal and late planting dates.
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