The influence of nitrogen, phosphate and microbial associations on photosynthesis, respiration and growth in Vicia faba L.
This project represents the development of a comprehensive description of growth in Vicia faba L. Particular attention has been paid to the impact in the tripartite legume-Rhizobium-AMF on the growth. The development of the description was divided into two parts. The first one was made up of (i) different N supply (0, 10, 25, 50, 100, 250 and 500ppm soil N) with normal P supply and (ii) different N supply (0, 10, 25, 50, 100, 250 and 500ppm soil N) with two phosphorus (0.05 and 1.6 mmol P) concentration applied and the Vicia faba seeds were planted pots filled with autoclaved river sand in order to non-producing the nodules just like normal cereal crops. As leaf nitrogen concentration (NL) increased, the quantum yield efficiency (α)，carboxylation efficiency (Ce), photon saturated net photosynthetic rats (PNmax) were converged onto a maximum asymptotic value，the Ci value fell to an asymptotic minimum. A monotonic decline in the steady-state value of Rf occurred with increasing N supply. Specific leaf area (δL) increased with increasing N supply or with increasing NL. An increase in P supply was consistently associated with an increase in N accumulation and N productivity in terms of biomass and leaf area production. Furthermore, P increased the photosynthetic N use efficiency in terms of Pmax and α. An increase in P was also associated with an increase in Ce and a decrease in Ci.. Under variable daily meteorological conditions, the values for NL, specific leaf phosphorus content (PL), specific leaf phosphorus content (PL), specific leaf area (δL), root mass fraction (Rf), PNmax and α remained constant for a given N supply during the stage of steady-state exponential growth. This study tests the hypothesis that P supply positively affects both N demand and photosynthetic NUE by influencing the upper limit of the asymptotic values for Pmax, Ce, and the lower limit for Ci in response to increasing N. The short-term photosynthetic responses to the increasing concentrations of CO2 were observed to be co-limited by both N and P supply. These findings support the proposal that the N:P supply ratio controls the plant photosynthetic capacity in response to elevated CO2 concentrations. Also, the short-term photosynthetic responses to the increasing concentrations of CO2 were observed to be co-limited by both N and P supply. The second part is tripartite symbiosis experiment with the concentration of nitrate-N for (a) the low N (LN, 10ppm N) and the high N treatments (HN, 250ppm N) without any microbial symbiotic associations;; (b) two different N supply rates plus AMF association, LNM and HNM; (c) two different N supply rates plus Rhizobium association, LNR and HNR; and (d) two different N supply rates plus both AMF and Rhizobium symbiotic associations: LNMR and HNMR. All treatments received a low level of phosphorus supply with 0.05mg P L-1 (1.61 mM NaH2PO4 ). AMF promoted biomass production and photosynthetic rates by increasing the ratio of P to N accumulation. An increase in P was consistently associated with an increase in N accumulation and N productivity, expressed in terms of biomass and leaf area. Photosynthetic N use efficiency, irrespective of the inorganic source of N (e.g. NO3‾ or N2 ), was enhanced by increased P supply due to AMF. The presence of Rhizobium resulted in a significant decline in AMF colonization levels irrespective of N supply. Without Rhizobium, AMF colonization levels were higher in low N treatments. Presence or absence of AMF did not have a significant effect on nodule mass but high N with or without AMF led to a significant decline in nodule biomass. Plants with the Rhizobium and AMF symbiotic associations had higher photosynthetic rates per unit leaf area and increased plant productivity. The plants colonized with both microbial symbionts had significantly higher total biomasses, leaf areas, the whole plant photosynthesis and respiration rates than plants with only one or no microbial symbionts. Similarly, plants with both microbial symbionts also had significantly higher growth yield (Yg ) values than all the other treatments. Maintenance respiration rates were also highest in plants with two microbial symbionts. In low N plants colonized by both microbial symbionts there was evidence of compensatory increases in the photosynthetic rates in response to the carbon sink demands of the microbial symbionts. It was shown that the plant potential photosynthetic capacity exceeds the carbon demand of the Rhizobium–AMF symbiotic associations.