Effects of Soil Temperature and Nitrogen Status on Kinetics of 15NO3 Uptake by Roots of Field-grown Agropyron Desertorum

Plant NO8‐ acquisition is largely determined by root uptake capacity. Although root uptake capacity has been shown to be sensitive to both root temperature and previous nitrogen (N) supply in hydroponic systems, the uptake capacity response to similar environmental factors under field conditions has not been investigated. Using 15NO3−, root uptake capacities were determined in excised roots of Agropyron desertorum (Fisch. ex Link) Schult grown in the field at two soil temperatures and two N fertilization treatments. Variation in soil and root temperatures was achieved by application of clear plastic film or insulating mulch to the soil immediately around the target plants. Uptake rates were measured at six different assay solution concentrations (from 1 to 1000 μM external 15NO3− concentration range). Two months after the imposition of soil N and temperature treatments, a biphasic transport system (a high‐affinity) saturable phase and a low‐affinity transport phase) was apparent in low N‐treated plants. Nitrate uptake capacity in the low‐concentration range (1–500μM) was significantly reduced in N‐fertilized plants compared with unfertilized control plants and the effect was more pronounced at high (27 °C) than low (17 °C) soil and assay temperatures. Furthermore, high soil N status inhibited the expression of a low‐affinity NO3− transport system which was clearly apparent at external NO3− concentration ranges between 500 and 1000 mM in plants grown at low soil N. Prior soil N and temperature history may ultimately determine root ability to exploit NO3− flushes which can result from changes in soil environmental conditions

Chloride transport and compartmentation within main and lateral roots of two grapevine rootstocks differing in salt tolerance

Root Cl⁻ transport was investigated using ³⁶Cl⁻ flux analysis in two grapevine (Vitis sp.) rootstock hybrids differing in salt tolerance; 1103 Paulsen (salt-tolerant) and K 51–40 (salt sensitive). Initial ³⁶Cl⁻ influx to the root was greater in Paulsen than K 51–40. This flux, attributed to the Cl⁻ influx to the cytoplasm (Φ ₒc) increased with increasing external concentrations of Cl⁻ for plants adapted to growth in 30� mM NaCl. The concentration kinetics in this high concentration range could be fit to a Michaeils–Menton equation. There was no significant difference between genotypes in Km (28.68� ±� 15.76 and 24.27� ±� 18.51� mM for Paulsen and K 51–40, respectively), but Paulsen had greater V ₘₐₓ (0.127� ±� 0.042) compared to K 51–40 (0.059� ±� 0.026� μm� g⁻¹� FW� min⁻¹). In Paulsen, the main root had greater contribution to ³⁶Cl⁻ uptake than lateral roots, there being no significant difference in lateral root influx between the genotypes. ³⁶Cl⁻ transport to the shoot of K 51–40 was greater than for Paulsen. It was estimated that efflux rate from the xylem parenchyma cells to the xylem vessels (Φ cₓ) in K 51–40 was twice that of Paulsen. Compartmental analysis from ³⁶Cl⁻ efflux kinetics confirmed the larger Φ ₒc and the higher ratio of main to lateral root Φ ₒc for Paulsen. Efflux from the cytoplasm (Φ cₒ) was higher than 95� % of Φ ₒc indicating a high degree of cycling across the plasma membrane in roots at these high external Cl⁻ concentrations. Paulsen appears to keep the cytoplasmic Cl⁻ concentration in roots lower than K 51–40 via greater efflux to the vacuole and to the outside medium. The difference in salt tolerance between the genotypes can be attributed to different Cl⁻ transport properties at the plasma membrane and tonoplast and particularly in Cl⁻� efflux to the xylem.Nasser Abbaspour, Brent Kaiser, Stephen Tyerma

Mineralisation of carbon and plant uptake of phosphorus from microbially-derived organic matter in response to 19 years simulated nitrogen deposition

Here we test the hypotheses that 19 years of simulated pollutant N deposition increases both losses of carbon (C) and the ability of plants to access P from organic material in upland heathland. The grass, Dactylis glomerata, and the dwarf shrub, Calluna vulgaris, were grown in soil containing microbial-derived organic matter labelled with 14C and 33P. We found that both soil and root-surface phosphatase activity increased significantly in response to N deposition. We also found a significant positive relationship between root-surface phosphatase activity and 33P uptake for Calluna, but a negative relationship for Dactylis. Efflux of 14C from the microbial-derived organic matter was strongly dependent on an interaction among plant presence, plant species and N deposition. Our results show that mineralisation of C and P, and subsequent plant uptake of P from organic sources is decoupled. In our experimental conditions, stimulation of P turnover coupled with subsequent plant uptake through up-regulation of root phosphatases is little affected by N addition. However, our data indicate that root-surface phosphatases are likely to be more important for uptake of P derived from organic sources for Calluna than for Dactylis