| Soybean (Glycine max [L.] Merr.),as one of the five major economic crops,is an important source of human quality protein and edible oil. Water demand of soybean is high,but the soybean root system is not developed. Hence, soybeab is very sensitive to water deficit. a-Naphthaleneacetic acid (NAA), synthetic auxin, plays a role in alleviating stress.Two types of soybean, Nannong 99-6 and Kefeng 1, were used to study the effect of NAA on plant growth, antioxidant system, and carbon and nitrogen metabolism under gradual drought in a pot experiment,so as to clarify the mechanism of improvement of drought tolerance by NAA. The results were as follows:1. Compared with stressed Nannong 99-6, stressed Kefeng 1 and NAA pretreatment have higher leaves ability to keeping moisture and water uptake ability of the root. Leaf relative water content was thus effectively inhibited in these two treatments, which reduced effect of drought on photosyntheticrate (Pn). Due to higher Pn in NAA treatment when compared with the drought treatments, NAA application better maintained plant growth under drought.Drought at flowering stage, therefore, had less effect on NAA-treated plants. Water stress decrease significantly pod number per plant, and after re-watering the decline in yield could be offset by the increase in 100-seed weight caused by compensatory growth. However, the compensation for 100-seed weight first was not enough to make up for the yield loss with stress time extended. Spraying NAA obviously reduced the difficulty of offsetting yield loss by increasing pod number per plant, leading to the gain in yield of two soybean cultivars subjected to drought stress, especially Nannong 99-6.2. In the pot experiment, the accumulated ROS in Kefeng 1 during 2-4 days triggered higher antioxidant capacity relative to stressed Nannong 99-6 under subsequent drought, which reduced the damage of drought on cells. During leaf in vitro culture, sodium diethyldithiocarbamatre (DDTC) and NaN3 pretreatment significantly decreased ROS and malondialdehyde (MDA) levels in Nannong 99-6 during the late stage of drought by improving preceding ROS accumulation, while 1,1-dimethylurea (DMTU) and 1,2-dihydroxybenzene-3,5-disulfonic acid disodium salt (Tiron) pretreatment inhibited the early ROS accumulation in Kefeng 1, causing the increase of ROS and MDA doses under subsequent stress. These results showed previous ROS accumulation played the role in improving drought resistance of soybean. In the pot experiment, NAA induced ROS burst by changing endogenous auxin (IAA) and abscisic acid (ABA) content at 4 DAT. The inceased ROS obviously alleviated the damage of late drought on soybean. Because it could more significantly inhibit the increase in MDA content and decrease in plant dry weight in Nannong 99-6 than in Kefeng 1, NAA can better improve drought tolerance of Nannong 99-6. During leaf in vitro culture, diphenyleneiodoniumcloride (DPI) reduced the effect of NAA on soybean for drought resistance by inhibiting the previous ROS accumulation induced by NAA, showing preceding ROS accumulation play a big part in improvement of soybean for drought resistance by NAA.3. Kefeng 1 and NAA pretreatment could sustain higher Pn than stressed Nannong 99-6 under the condition of same water deficit, which led to higher photosynthate production rate. Due to the greater degree of increase of sucrose phosphate synthase (SPS), sucrose synthetase synthesis direction, acid invertase,neutral invertase,and sucrose synthetase decomposition direction activities, sucrose production rate and soluble sugar accumulation rate were higher in stressed Kefeng 1 and NAA treatment, which could improve osmotic adjustment ability under drought. In stressed Kefeng 1 and NAA treatment, higher rate of sucrose synthesis and sink demand caused the transport of sucrose from leveas to root, to better maintain plant moisture balance. Moreover, glycolate oxidase activity were lower in the early stage of drought in stressed Kefeng 1 and NAA treatment, which decreasing photosynthetic product consumption, while the activity was higher in the late stage of drought, decreasing the harm of glycollic acid and excess reducing power on cells.4. Compared with stressed Nannong 99-6, NO3- content and glutamine synthetase (GS)activity were higher in stressed Kefeng 1 and NAA treatment, which reducing the concentrations of NH4+ generated by NR, glycolate oxidase, endopeptidase and glutamic dehydrogenase (GDH) and favouring amino acid accumulation and protein synthesis.Relative to stressed Nannong 99-6, stressed Kefeng 1 and NAA treatment had a lower accumulation rate of proline (Pro) and higher increasing extent of soluble protein, due to the lower rate of protein degradation. Less accumulation of Pro reflected drought caused smaller damage to Kefeng 1 and NAA treatment, while increased soluble protein could maintain the normal metabolism of cells and improve the cell water holding capacity. |
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