Patterns of Growth Costs and Nitrogen Acquisition in Cytisus striatus (Hill) Rothm. and Cytisus balansae (Boiss.) Ball are Mediated by Sources of Inorganic N

Abstract

Nitrogen-fixing shrubby legumes in the Mediterranean area partly overcome nutrient limitations by making use of soil N and atmospheric N2 sources. Their ability to switch between different sources lets them adjust to the carbon costs pertaining to N acquisition throughout the year. We investigated the utilization of different inorganic N sources by Cytisus balansae and Cytisus striatus, shrubby legumes under low and a sufficient (5 and 500 µM P, respectively) levels of P. Plants grew in sterile sand, supplied with N-free nutrient solution and inoculated with effective Bradyrhizobium strains; other treatments consisted of plants treated with (i) 500 µM NH4NO3; and (ii) 500 µM NH4NO3 and inoculation with effective rhizobial strains. The application of NH4NO3 always resulted in greater dry biomass production. Carbon construction costs were higher in plants that were supplied with mineral and symbiotic N sources and always greater in the endemic C. striatus. Photosynthetic rates were similar in plants treated with different sources of N although differences were observed between the two species. Non-fertilized inoculated plants showed a neat dependence on N2 fixation and had more effective root nodules. Results accounted for the distribution of the two species with regards to their ability to use different N sources.

Phosphorus and Nitrogen Modulate Plant Performance in Shrubby Legumes from the Iberian Peninsula

“…In the four species, in the water treatment, the greatest number of nodules was observed in the Inoc + low P treatment and the highest N accumulation occurred in the Inoc + high P plants (Figure 5), which coincides with a peak in plant biomass production in the four species (Figure 1). However, in the water-stressed plants, N and P accumulation did not differ between Inoc + high P and Inoc + low P and that can only be explained by a more efficient nodulation in the water-stressed plants following the same pattern as in other leguminous species [4,34,35,36,37]. This behavior has been already described in other species in the Genistea family under several stresses [38,39].…”

“…In fact, the root allocation of nodulated plants is the highest. This proves the requirement for the plants to maintain all the structures that guarantee N acquisition as it has been observed in Virgilia divaricata [5] and other legumes from nutrient poor ecosystems, where the belowground organs have a higher resource allocation under low P values [34,45]. The BNF in legumes is a costly process, very much dependent on phosphate supply.…”

“…BNF is a very expensive process for the legume; when P and N are sufficient in the soil, plants prefer these mineral nutrients, avoiding the expensive biological fixation. In our study, under high P conditions, the four species (Figure 5) did accumulate more P in both the high P and the Inoc + high P treatments, thus avoiding expenses in BNF [34,44]. Contrary to that, in the control and low P treatments, plants produced very efficient nodules, resulting in greater nitrogen accumulation in the whole plant.…”

“…G. cinerea and E. barnadesii, the two species from very poor and acidic soils, responded better to the combination of inoculation and low P, whereas G. florida and R. sphaerocarpa , naturally growing in basic soils, had better responses to the combined inoculation with high P. The former species have the ability to acquire P from soil and use it efficiently, increasing plant performance in the short term and even when P is present in a very low concentration. This can be understood as an evolutionary response to the acidic soils where these plants grow, as has been observed in other legumes [4,10,34,37,40]. The other two species, G. florida and R. sphaerocarpa , whose distribution in natural systems corresponds with richer soils, show lower values of relative growth and do not show signs of need for a quick mineral nutrition acquisition [41].…”

Regulatory Effect of Light and Rhizobial Inoculation on the Root Architecture and Plant Performance of Pasture Legumes

“…The benefits of symbiotic interactions include increased plant growth in terms of increases in height [11,12], total biomass production [13,14] shoot-root ratio [14][15][16], and the production of flowers. Increases in seed yield and total crop biomass Agronomy 2023, 13, 2058 2 of 15 have been observed in Cicer arietinum, Phaseolus vulgaris, and Glycine max, among others [1,11,15,17]. Similarly, the presence of legumes and their symbionts critically enhances the sustainability of agroforestry systems [18] and pastures [19].…”