Hydropriming and seed additives in soybean as measures of improving early plant development

Abstract

Soybean production is increasing because it provides high-quality protein for human consumption and animal feed, especially for monogastric animals. Successful soybean production in the temperate climate of central Europe is challenged by late field emergence with a short vegetative growth period. Germination can be improved by seed hydropriming, a process of starting the first stage of germination under controlled conditions to reach a certain point then the process is stopped. If these seeds are sown later, the germination process is accelerated. The technique is used commercially in different crops for example, in sugar beet and vegetables. Another option to improve plant growth is the use of seed additives, namely plant growth-promoting bacteria (PGPR) and arbuscular mycorrhizal fungi (AMF). They mainly improve the nutrient supply and can improve yield in various crops. In addition, isoflavonoids, for instance, genistein play a crucial role in the process of symbiosis development between Bradyrhizobia and soybean and can be used for co-inoculation, leading to improved symbiotic nodulation under low soil temperatures. This study aimed to find a hydropriming protocol suitable for improving soybean emergence under cold growing conditions and to test commercially available seed additive products. Five early maturing soybean varieties commonly used by German farmers were selected. Seeds were first hydroprimed in distilled water for 4, 8, 12, and 16 hours, followed by air drying for 25 hours at room temperature, while no hydropriming served as a control. Seed size increase and water uptake were recorded. In the pot experiment done with field soil and sand, carried out in climate chambers at 9, 12, 15, and 18°C, the time to emergence was studied after the above-mentioned hydropriming durations. In another pot experiment at 15°C, the effects of a PGPR product, an AMF product, and genistein as seed additives on emergence and early plant development were studied. In a subsequent two-year field study, the best options for hydropriming and seed additives were tested with three soybean varieties. Various plant characteristics relating to emergence, root and shoot development, and nodule development were recorded during the growing season. Plant development was also measured at harvest, including the number of pods per plant, the height of the first pod above ground, yield, and N content of beans. Hydropriming treatments reduced the number of seeds germinated and emerged, without improving the overall speed of germination. Only under 12°C the seed emerged faster after 12 h of hydropriming compared to the control. The seed additives under study did not significantly improve emergence or early growth, but the AMF product tended to favour nodule development. Under field conditions, after hydropriming fewer seeds emerged and the overall plant development and yield was not improved. The addition of the AMF product did not influence plant development and yield. All experiments showed challenging results for hydropriming. Contrary to the initial hypothesis, hydropriming did not improve germination and emergence but reduced the germinability of the soybean seeds under study. Further research is needed to develop a priming protocol suitable for the soybean varieties used in Central Europe, either by aerating the seeds or using other priming techniques such as osmopriming, solid matrix priming, or biopriming. The AMF, PGPR, and genistein products under study also showed no improvement in plant development or yield. These results are also contrary to the initial hypothesis, but the validity of these results is limited by the number of products and replications in this study. Further research should focus on seed additives for soybeans under cold conditions. At this stage further research on hydropriming and seed additives for soybean is needed before these techniques can be transferred to practical farming.