Abstract
The stems of bioenergy sorghum hybrids at harvest are > 4 m long, contain > 40 internodes and account for ~ 80% of harvested biomass. In this study, bioenergy sorghum hybrids were grown at four planting densities (~ 20,000 to 132,000 plants/ha) under field conditions for 60 days to investigate the impact shading has on stem growth and biomass accumulation. Increased planting density induced a > 2-fold increase in sorghum internode length and a ~ 22% decrease in stem diameter, a typical shade avoidance response. Shade-induced internode elongation was due to an increase in cell length and number of cells spanning the length of internodes. SbGA3ox2 (Sobic.003G045900), a gene encoding the last step in GA biosynthesis, was expressed ~ 20-fold higher in leaf collar tissue of developing phytomers in plants grown at high vs. low density. Application of GA3 to bioenergy sorghum increased plant height, stem internode length, cell length and the number of cells spanning internodes. Prior research showed that sorghum plants lacking phytochrome B, a key photoreceptor involved in shade signaling, accumulated more GA1 and displayed shade avoidance phenotypes. These results are consistent with the hypothesis that increasing planting density induces expression of GA3-oxidase in leaf collar tissue, increasing synthesis of GA that stimulates internode elongation.
Highlights
The stems of bioenergy sorghum hybrids at harvest are > 4 m long, contain > 40 internodes and account for ~ 80% of harvested biomass
To analyze bioenergy sorghum’s response to shading, the bioenergy sorghum hybrid TX08001 was planted in field plots with 0.76 m row spacing and plants within rows were thinned to a plant spacing of 1 m, 0.5 m, 0.25 m and 0.15 m (~ 20,000 to 132,000 plants/ha)
Plants were harvested at 60 days after emergence (DAE) and plant height, stem and internode morphology, and stem and leaf biomass were quantified (Fig. 1, see Supplementary Fig. S1)
Summary
The stems of bioenergy sorghum hybrids at harvest are > 4 m long, contain > 40 internodes and account for ~ 80% of harvested biomass. Prior research showed that sorghum plants lacking phytochrome B, a key photoreceptor involved in shade signaling, accumulated more GA1 and displayed shade avoidance phenotypes These results are consistent with the hypothesis that increasing planting density induces expression of GA3-oxidase in leaf collar tissue, increasing synthesis of GA that stimulates internode elongation. Increases in radiation use efficiency (RUE) were associated with canopy closure and the onset of rapid stem and internode e longation[23]. This suggested that increased sink strength due to stem growth could contribute to biomass yield. GA activates gene expression in part by interacting with and inducing the turnover of DELLAs, repressors of GA modulated gene expression[38]
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