Relationship of physiological and biochemical indicators with SPAD values in maize leaves at different layers

Objectives The variation of nitrogen indicator of leaves at different layers were measured and the most suitable leaves to the variation at different growth stage were selected, for realizing timely and accurately diagnosis of nitrogen nutrition.

Methods Pot experiments were conducted in 2017 and 2018, using maize as tested materials. Six N levels were setup with triplicate. At the jointing, booting, anthesis-silking and filling stages of maize, the photosynthetic rate (P_n) of leaves at different layers were measured, and leaf samples at different layers were collected for measuring the leaf thickness, analyzing the content of N (LNC), chlorophyll (Chl.), soluble protein, and soluble sugar. The relationships of above indexes with SPAD readings were compared.

Results 1) Within the tested N application levels, the changes in LNC, Chl., soluble protein, leaf thickness and P_n in maize leaves at all layers were similar, showing trend of increasing first and then keeping stable, and all of the indexes were positively correlated with SPAD values, except soluble sugar content which showed decreasing initially and then stable trend with a negative correlation with SPAD values. 2) The content of LNC, Chl., and soluble protein, the leaf thickness and P_n at upper layer leaves were higher than at lower layer ones, while soluble sugar content was opposite. 3) The linear regression models established between SPAD values and LNC were highly significant. The coefficients of determination (R²) between SPAD values and LNC in top canopy leaves at the jointing and filling were higher (0.768 and 0.865), while those in leaves of the middle canopy leaves at the booting and anthesis-silking stages were higher (0.893 and 0.924).

Conclusions The leaf physiological and biochemical indicators are significantly affected by growing stages, leaf layers and N application, and show vertical heterogeneity. When using SPAD values to predict the leaf nitrogen content, the top canopy leaves have priorities at the jointing and filling stages, and the middle layer leaves should be selected at the booting and anthesis-silking stages.