Abstract
In this study, soybean root distribution in an inter-cropping system was influenced by various environmental and biotic cues. However, it is still unknown how root development and distribution in inter-cropping responds to aboveground light conditions. Herein, soybeans were inter- and monocropped with P (phosphorus) treatments of 0 and 20 kg P ha yr−1 (P0 and P20, respectively) in field experiment over 4 years. In 2019, a pot experiment was conducted as the supplement to the field experiment. Shade from sowing to V5 (Five trifoliolates unroll) and light (SL) was used to imitate the light condition of soybeans in a relay trip inter-cropping system, while light then shade from V5 to maturity (LS) was used to imitate the light condition of soybeans when monocropped. Compared to monocropping, P uptake and root distribution in the upper 0–15 cm soil layer increased when inter-cropped. Inter-cropped soybeans suffered serious shade by maize during a common-growth period, which resulted in the inhibition of primary root growth and a modified auxin synthesis center and response. During the solo-existing period, plant photosynthetic capacity and sucrose accumulation increased under ameliorated light in SL (shade-light). Increased light during the reproductive stage significantly decreased leaf P concentration in SL under both P-sufficient and P-deficient conditions. Transcripts of a P starvation response gene (GmPHR25) in leaves and genes (GmEXPB2) involved in root growth were upregulated by ameliorated light during the reproductive stage. Furthermore, during the reproductive stage, more light interception increased the auxin concentration and expression of GmYUCCA14 (encoding the auxin synthesis) and GmTIR1C (auxin receptor) in roots. Across the field and pot experiments, increased lateral root growth and shallower root distribution were associated with inhibited primary root growth during the seedling stage and ameliorated light conditions in the reproductive stage. Consequently, this improved topsoil foraging and P uptake of inter-cropped soybeans. It is suggested that the various light conditions (shade-light) mediating leaf P status and sucrose transport can regulate auxin synthesis and respond to root formation and distribution.
Highlights
Phosphorus (P) is a critical element for plant growth and development, but its availability in soil is often limited [1,2]
We examined whether the variable light condition and the auxin pathway were associated with the shallow distributed root of inter-cropped soybeans
Soybean root growth in SL in the as well as in the root biomass, total root length, and root surface area, all of which were significantly pot experiment was inhibited by shading at the V5 stage, as well as in the root biomass, total root length, and root surface area, all of which were significantly lower than LS treatment (Figure 3A,B)
Summary
Phosphorus (P) is a critical element for plant growth and development, but its availability in soil is often limited [1,2]. Inter-cropping is a farming practice that involves two or more crop species growing together or coexisting for a time. It is widely practiced worldwide and provides opportunities for the sustainable intensification of agriculture, as it offers a greater yield per unit land and a more efficient use of fertilizer than monocropping [5,6,7,8,9]. It is generally believed that the P uptake advantage of inter-cropping mainly comes from the following aspects: (1) under low P stress, legume root can release protons/hydroxyls, carboxylates, and enzymes to activate insoluble P in soil, and the P which is mobilized via legumes may be made available for cereal [12,13]
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