Interactions between light intensity and phosphorus nutrition affect the phosphate-mining capacity of white lupin (Lupinus albus L.)

Abstract

Light intensity affects photosynthetic carbon (C) fixation and the supply of carbon to roots. To evaluate interactions between carbon supply and phosphorus (P) supply, effects of light intensity on sucrose accumulation, root growth, cluster root formation, carboxylate exudation, and P uptake capacity were studied in white lupin (Lupinus albus L.) grown hydroponically with either 200 µmol m(-2) s(-1) or 600 µmol m(-2) s(-1) light and a sufficient (50 µM P) or deficient (1 µM P) P supply. Plant biomass and root:shoot ratio increased with increasing light intensity, particularly when plants were supplied with sufficient P. Both low P supply and increasing light intensity increased the production of cluster roots and citrate exudation. Transcripts of a phosphoenol pyruvate carboxylase gene (LaPEPC3) in cluster roots (which is related to the exudation of citrate), transcripts of a phosphate transporter gene (LaPT1), and P uptake all increased with increasing light intensity, under both P-sufficient and P-deficient conditions. Across all four experimental treatments, increased cluster root formation and carboxylate exudation were associated with lower P concentration in the shoot and greater sucrose concentration in the roots. It is suggested that C in excess of shoot growth capabilities is translocated to the roots as sucrose, which serves as both a nutritional signal and a C-substrate for carboxylate exudation and cluster root formation.

Keywords: Photosynthesis; citrate exudation; cluster roots; phosphorus deficiency; sucrose; white lupin..

Fig. 1.

Effects of P supply and light intensity on plant dry weight (DW) (A and B), and on the root:shoot of dry weight ratio (C). Plants were grown in nutrient solution at two levels of P supply (P1=1 µM P, P50=50 µM P) and two levels of light intensity (L200=200 µmol m^–2 s^–1, L600=600 µmol m^–2 s^–1), and harvested after 28 d growth. Data are averages of three replicates and bars represent standard errors. Data with different letters are significantly different (P<0.05).

Fig. 2.

Effects of P supply and light intensity on (A) total root length, (B) root surface area, (C) primary root length, and (D) lateral root number. Plants were grown in nutrient solution at two levels of P supply (P1=1 µM P, P50=50 µM P) and two levels of light intensity (L200=200 µmol m^–2 s^–1, L600=600 µmol m^–2 s^–1), and harvested after 28 d growth. Data are averages of three replicates and bars represent standard errors. Data with different letters are significantly different (P<0.05).

Fig. 3.

Effects of P supply and light intensity on root formation. Shown are: cluster root number (A) and percentage of cluster root dry weight in whole root system (B) under different treatments. Plants were grown in nutrient solution at two levels of P supply (P1=1 µM P, P50=50 µM P) and two levels of light intensity (L200=200 µmol m^–2 s^–1, L600=600 µmol m^–2 s^–1), and harvested after 28 d growth. Data are averages of three replicates and bars represent standard errors. Data with different letters are significantly different (P<0.05).

Fig. 4.

Effects of P supply and light intensity on (A) carboxylate exudation and (B) the expression of LaPEPC3. Plants were grown in nutrient solution at two levels of P supply (P1=1 µM P, P50=50 µM P) and two levels of light intensity (L200=200 µmol m^–2 s^–1, L600=600 µmol m^–2 s^–1), and harvested after 28 d growth. Data are averages of three replicates and bars represent standard errors. Data with different letters are significantly different (P<0.05). For LaPEPC3 expression, data are expressed as relative values based on the expression of LaPEPC3 in roots of plants grown with P50 and L200 referenced as 1.0. DW represents dry weight.

Fig. 5.

Effects of P supply and light intensity on plant P status (A: shoot P concentration, C: shoot P content, and D: root P content) and the expression of the P transporter gene LaPT1 (B). Plants were grown in nutrient solution at two levels of P supply (P1=1 µM P, P50=50 µM P) and two levels of light intensity (L200=200 µmol m^–2 s^–1, L600=600 µmol m^–2 s^–1), and harvested after 28 d growth. Data are averages of three replicates and bars represent standard errors. Data with different letters are significantly different (P<0.05). For LaPT1 expression, data are expressed as relative values based on the expression of LaPT1 in roots of plats grown with P50 and L200 referenced as 1.0. DW represents dry weight.

Fig. 6.

Effects of P supply and light intensity on photosynthetic efficiency (A), and sucrose concentration in leaves (B) and roots (C). Plants were grown in nutrient solution at two levels of P supply (P1=1 µM P, P50=50 µM P) and two levels of light intensity (L200=200 µmol m^–2 s^–1, L600=600 µmol m^–2 s^–1), and harvested after 28 d growth. Data are averages of three replicates and bars represent standard errors. Data with different letters are significantly different (P<0.05). FW represents fresh weight.

Fig. 7.

Relationships between cluster root formation (open squares; expressed as percentage contribution of cluster root to total root dry weight (DW)) or citrate exudation (black circles), and shoot P concentration (A) or sucrose concentration in roots (B). (C) Relationship between cluster root formation and citrate exudation. FW represents fresh weight.