Mechanism And Technology Study Of Nitrogen Topdressing Recommendation At Spike Differiatiation Stage Based On Leaf Color (SPAD-502) For Rice

In this study, different nitrogen (N) pot experiment involving four japonica rice varieties (95-38, Wuyujing3, zhendao5394, 9915) and 1 indica variety in 2003, pot experiment with N15 labelled urea applied to soil and 4th leaf from the top on rice ningjing2 in 2004, field experiment with different nitrogen rate at spike differentiation stage on wuyujing 3 and different basal-dressing ratio on ningjing2 in 2003 were carried out. The objectives of this study were to: (1) determine the optimum measuring leaf site and leaf position with leaf chlorophyll meter (SPAD-502) for diagnosing the nitrogen status; (2) analysis the translocation and distribution of N between different plant parts and different leaves; (3) investigate the relationship of spad value between third leaf and 4th leaf from the top under different N supply level and explore the feasibility of nitrogen spike-dressing rate recommendation based on RSPAD value (the ratio of SPAD value of third leaf to that of 4th leaf). Results showed as following:(1) SPAD value varied with the measuring position on the same leaf, and distribution pattern can be fitted by quadratic (y=ax2+bx+c, a<0) or linear (y=ax +b, a>0) curve very well. At spike differentiation stage, the SPAD value measured at about 20-50% from the tip of leaf blade correlated best with leaf nitrogen content, variation coefficient of SPAD value measured at 30-50% from the tip of leaf blade was lowest, and the average SPAD value was located at 30% from the tip of leaf blade regardless of the leaf and measuring date. Thus, 30% from the tip of leaf blade was the best measuring position.(2) Leaf N/ chlorophyll content and SPAD value increased with the increasing N fertilizer, and the lower leaves were more sensitive to the soil N supply than the upper leaves. N content of different leaf positions were exponentially related with the corresponding SPAD values during the whole growth cycle, the determinationcoefficient was as high as 0.919, significant at 0.01 level. The SPAD value of third leaf from the top was most correlated with the whole leaf and plant N concentration among the top four leaves at all growth stages. The CV of SPAD value of third leaf from the top was smallest at fore-and-aft spike differentiation under optimal N level. Thus, third leaf from the top was the most ideal indicator leaf when using SPAD value or leaf color difference to diagnose N status and make N fertilization suggestion for spike in rice.(3) Nitrogen concentration and SPAD value of third leaf and 4th leaf varied with nitrogen supply level, third leaf with higher value under low N supply level and vice versa. Stronger was the soil N supply level, higher was the amount and distribution ratio of N15 uptake from soil to 4th leaf from the top, and more output from the 4th leaf to other leaves. Relative change of nitrogen concentration or SPAD value of third and 4th leaf not only indicated the plant nitrogen status, but also reflected the soil nitrogen supply level, thus their ratio (RSPAD) can be used to diagnosis the rice nitrogen status.(4) RSPAD value changed in rule after nitrogen topdressing at spike differentiation stage. It is feasible for N topdressing rate recommendation based on the RSPAD value at n-4 stage. RSAPD can be divided into two types through clustering analysis, leaf SPAD value of third leaf from the top higher (RSAPD>0) or lower than the 4th leaf (RSPAD<0). In order to achieve high yield and high nitrogen use efficiency (NUE), the optimal N rate was 120-135 kg N ha-1 for RSPAD<0 and 180 kg N ha-1 for RSAPD>0.