Seed Coating with Hydro-Absorbers as Potential Mitigation of Early Season Drought in Sorghum (Sorghum bicolor L. Moench)

Climate change poses a threat to sorghum production systems by shifting the onset of the rainy season to a later date, increasing the risk of crop failure during crop establishment. The effects of drought on sorghum during seedling establishment have not been determined. Coating seeds with a water absorbing substance offers a way to buffer the seed against insufficient moisture in the surrounding soil. Seeds of two different sorghum varieties were coated with one of two commercially available hydro-absorbers: Stokosorb® and Geohumus®. These hydro-absorbers have the capacity to store water several times their own weight. The aim of this study was to compare the effects of the cited hydro-absorbers on early seedling growth of two sorghum landraces under different levels of soil water deficit. Seedlings were grown for 12 days under three water availability levels (Field capacity (FC), 50% of FC, and 25% of FC). The seedlings under water limited treatments were subsequently re-watered. Biomass, root length, plant height, leaf area, and leaf extension rate were monitored in two-day intervals for 24 days. Coating strongly affected seedling growth both under fully watered and water deficit conditions. Sorghum varieties differed in their responses to both soil water deficit and coating materials. In general, Stockosorb improved seedling performance under water limited conditions particularly by promoting root growth, whereas Geohumus did not.

Seed Coating Increases Seed Moisture Uptake and Restricts Embryonic Oxygen Availability in Germinating Cereal Seeds

Seed coating is a technology to improve germination and homogenize stand establishment. Although coating often results in lower germination rates, seeds that do germinate grow more vigorously and show strongly reduced respiratory losses during reserve mobilization. We hypothesize that the higher mobilization efficiency is due to a shift in the enzymatic cleavage of sucrose from invertase to sucrose synthase in the embryonic tissue caused by a reduced oxygen availability induced by oversaturation with water caused by the coating during early germination. We investigated the effect of coating on barley, rye, and wheat seed imbibition during the first 30 h after seeds were placed in moisture. We profiled oxygen in the embryos and measured sucrose and acid invertase levels as imbibition progressed. We found that seeds within coatings absorbed significantly more moisture than uncoated seeds. Coating resulted in near anoxic oxygen concentrations in the developing embryonic tissues in all three species. In barley, sucrose was not cleaved via the invertase pathway, despite the fact that invertase activity in coated seeds was increased. In rye and wheat, invertase activities were significantly lower in embryos from coated seeds without significantly changing the sugar composition.

Seed coating reduces respiration losses and affects sugar metabolism during germination and early seedling growth in cereals

Seed germination and the successful establishment of young seedlings is an important aspect of plant life. Seed coats are used to improve stand establishment and early seedling vigour. Seedlings growing from hydro-absorber coated barley, rye and wheat with coat-shares greater than 75% of the average seed have been shown to promote better seedling growth compared with those seedlings growing from uncoated seeds. We investigated how and why these seedlings performed better by analysing the proportion of grain reserves mobilised for growth and respiration as well as how both sucrose and glucose available in the embryo translated into seedling growth in the presence or absence of seed coats containing hydro-absorber gel. We found that mobilisation efficiency was higher, resulting in higher biomass in these cereals when they were coated. The relationship between sucrose and glucose available to the seedling as well as its correlation with early seedling growth indicate a switch in the enzymatic cleavage of embryonic sucrose from invertase to sucrose synthase. This in turn indicates that in coated seeds, embryonic tissue must be hypoxic leading to a more efficient use of glucose and thus reduced respiration losses during germination.