Objectives Researching physiological effects of nitrogen and sulfur application on related enzymes activities involved their assimilation will provide theoretical reference for garlic fertilization and improving product quality.
Methods A pot experiment using perlite and vermiculite as cultural substrate was performed. Two factors and three levels of the complete design were included. N was applied in levels of 5, 10 and 20 mmol/L and S in levels of 2, 4 and 8 mmol/L. Leaves and plants were sampled at the seedling stage, stem elongation stage, and bulb expansion preliminary and middle stages.
Results The nitrogen content of garlic was on the rise, and achieved the highest level at the bulb expansion stage. The nitrate reduction (NR) activity and glutamine synthetase (GS) activity were increased at first and then dropped later, and the enzymatic activities were higher from the stem elongation stage to bulb expansion preliminary stage. The sulfur content of garlic showed a trend of rise and then steady. At the stem elongation stage, the ATP sulfurylase (ATPS) activity reached its maximum value, while O-acetylserine thiollyase (OAS-TL) activity tended to rise after declining and reached its minimum level at the stem elongation stage. The nitrogen and sulfur interaction affected the N and S assimilation significantly before the bulb expansion stage, compared with the N or S alone. On the contrary, single factor effect was obvious at all the growth stages. The NR and GS activities showed increased first and then decreased in the development of garlic. N affected NR activity significantly. S affected NR activity significantly just at stem elongation stage and bulb expansion preliminary stage, while there was a significant interaction between N and S for NR and GS. The activities of key enzymes involved nitrogen and sulfur metabolism showed a minor impact with the N or S single fertilizer, but extremely significant with the N and S interaction. There were significantly positive correlations between the NR activities and N contents at the stem elongation stage and bulb expansion early stage and the Pearson correlation coefficients were 0.690 and 0.847, respectively, and the Pearson correlation coefficients between ATPS activities and S contents were 0.662 and 0.816. At the bulb expansion early and middle stages, the GS activities and N contents existed negative correlations, and the Pearson correlation coefficients were –0.857 and –0.693, respectively. The negative correlation was observed as a whole between the OAS-TL activity and S content, the Pearson correlation coefficient was 0.646 at the bulb expansion early stage.
Conclusions There is an interaction relationship between nitrogen and sulfur at growth stages of garlic. With the increase of activities of NR and ATPS, nitrogen and sulfur assimilation ability of garlic plant is promoted. On the contrary, GS accelerates nitrogen assimilation when the enzyme activity tends to decrease. Under the combined application of N and S, the nitrogen and sulfur assimilation is regulated by key enzymes before the bulb expansion stage, and the plant growth is stimulated. At the bulb expansion stage, the single application of nitrogen or sulfur fertilizer can affect the nitrogen or sulfur contents in garlic plants.
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