Mechanism of maize intercropping peanut improving iron nutrition to increase photosynthetic performance of peanut

Objectives The mechanism of improving photosynthetic performance of peanut through intercropping of maize and peanut was studied in terms of iron nutrition, light energy absorption, trapping and electron transport, and CO₂ fixation in functional leaves of peanut.

Methods A random complete design of field experiment with two factors and two levels was conducted in the Farm of Henan Science and Technology University. The two cropping patterns were intercropping and sole cropping of peanut. The factor of P application included no applying (P0) and applying P₂O₅ 180 kg/hm² (P1). The leaves of sole cropping peanut appeared yellowish in 14 July and became serious yellowing in 2 August, while those of intercropping peanut did not appear yellowish at all. Both the yellowing and normal peanut plant samples were collected, and the photosynthesis response to light intensity and CO₂ concentration were measured, and the related parameters in functional leaves of peanuts were analyzed. The kinetic curve of chlorophyll fluorescence and the related parameters were drawn using JIP-test method.

Results Compared with the yellowing peanut, the intercropping obviously increased the light energy absorption (ABS/CS_o), trapping (TR_o/CS_o), electron transport (ET_o/CS_o) per unit area, the energy of electron reduction at the photosystem I receptor side (RE_o/CS_o), the number of reaction center per unit area (RC/CS_m) of peanut, enhanced the electron transport ability of the photosynthetic electron transport chains in functional leaves of peanut, and significantly increased (P < 0.05) the maximum quantum yield for primary photochemical of photosystem Ⅱ (Ψ_Po), probability of trapped excitons moving electrons into the electron transport chain beyond Q_A^– (Ψ_o), quantum yield for electron transport (Ψ_Eo), efficiency of electron transport from the reduction system to the photosystem I electron receptor side (δ_R), and quantum yield of PS I terminal receptor reduction (Ψ_Ro) in the functional leaves, with the increments of 36.7%–39.6%, 79.6%–92.2%, 151%–163%, 16.3%–20.0% and 177%–215%, respectively; and significantly improved the photosystem I photochemical activity (ΔI/I_o) and the coordination between photosystem I and photosystem II (Φ_PSⅠ/PSⅡ) of intercropping peanut. Intercropping significantly increased (P < 0.05) the SPAD value, net photosynthetic rate at light saturation (LSP_n), light saturation point (LSP), carboxylation efficiency (CE), net photosynthetic rate at the CO₂ saturation (A_max), and the maximum rate of Rubisco carboxylation (V_{c, max}), electron transport driving regeneration of RuBP (J_max) and triose phosphate utilization (TPU) at the CO₂ saturation. P application significantly increased the SPAD value, Ψ_Po, Ψ_o, Ψ_Eo, δ_R, and Ψ_Ro, iron content, net photosynthetic rate of functional leaves and the dry matter accumulation of the intercropped peanut (P < 0.05), but aggravated the symptoms of iron deficiency of sole cropping peanut, and significantly reduced all the SPAD value, Ψ_Po, Ψ_o, Ψ_Eo, δ_R, and Ψ_Ro, iron content, photosynthetic rate of functional leaves and the dry matter accumulation. Compared with the no yellowing peanut, intercropping significantly reduced the ABS/CS_o, TR_o/CS_o, ET_o/CS_o, LSP_n in functional leaves and dry matter weight per plant of intercropping peanut, while significantly improved the Ψ_Eo of intercropping peanut functional leaves.

Conclusions Intercropping with maize improved greatly the iron nutrition of peanut, and prevented from yellowing. When intercropped with maize, the light energy absorption, trapping and electron transport of PS II, photochmical activity of PS I, coordination between PS II and PS I, and stability of electron transport chain were all improved significantly, the capacity of CO₂ carboxylation fixation in functional leaves of peanut was also enhanced greatly. As a result, the net photosynthetic rate and biomass of peanut were increased. Phosphorus application would aggravate the symptom of iron deficiency in sole peanut, while enhance the interspecific effect of the intercropping.