Abstract:

Relay-planting soybean pattern is dominant in southern China. However, studies on relay-planting soybean are so defective that it may influence the development of soybean. Soybean accumulates vast protein by itself; and grows well with adequate nitrogen fertilizer. Most of studies have been concentrated on the effects of nitrogen on the aerial parts, but less on the roots. The present experiment was carried out to study the effects of different nitrogen levels (0, 45, 90, 135, 180, 225 N kg ha^-1) on morphological and physiological characteristics in the relay-planting soybean roots by root-excavating method. The results showed that the optimal treatments on the dry root weight, root nodule number, length of first lateral root and bleeding sap were the low nitrogen levels (45 and 90 N kg ha^-1) at the growth stages of V3–R5. Length of first lateral root, dry root weight and bleeding sap all were positive correlated with the application amount of nitrogen at the growth stage of R7. The relation between the root-top ratio and nitrogen levels was a quadratic curve at growth stages of V3–R5, but root-top ratio had a linear relationship with the nitrogen levels at the very late growth stage. The relation between bleeding sap-top ratio (y=Bleeding sap/dry matters) and days (x) after V3 (11th, Aug, 2005) was a negative exponential correlation (y=1.0357e-0.03823x). The content of NO₃^--N in bleeding sap was more than that of NH₄⁺-N; it was also raised with the increasing of nitrogen amount. Root activity had a peak during the whole growth stage of relay-planting soybean. The activity of soybean roots was enhanced by low nitrogen levels (45 and 90 N kg ha^-1). High nitrogen levels(180 and 225 N kg ha^-1) postponed the appearance of the peak value of root activity at the growth stage R3 and maintained a high level(＞116.00 μg g^-1 FW h^-1) at the very late growth stage(R7). However, whether high nitrogen levels are propitious to increasing the yield of relay-planting soybean is not clear. The relationship between characteristics of roots and the aerial parts should be studied in the future.

Key words: Relay-planting soybean, Nitrogen levels, Morphological and Physiological Characteristics