Nitrate and ammonium have different effects on many biochemical and physiological processes in plants, and at high concentrations this can lead to markedly different growth responses. Most plant species show reduced growth, smaller leaves and a stunted root system when exposed to high ammonium concentrations, and in severe cases this leads to chlorosis. Although well known, ammonium toxicity is poorly understood and is generally considered to be the result of one or more of the following effects; (i) ammonium‐induced mineral nutrient deficiency, arising from the impaired uptake of metal ions; (ii) secondary growth inhibition arising from the acidification of the rooting medium; (iii) alterations in intracellular pH and osmotic balance; (iv) uncoupling of photophosphorylation from electron transport, following the accumulation of ammonium in leaves; and (v) altered polyamine and phytohormone metabolism. These hypotheses are reviewed in the light of the available literature and experimental evidence from own experiments. It is concluded that no mechanism on its own provides an adequate explanation of the available data.

The suitability of the infiltration-centrifugation method for collection of apoplastic fluid from intact leaves was evaluated for different plant species. Large differences with respect to infiltrability of the leaves, which correlated inversely with stomatal and mesophyll resistance, became apparent. Osmolality of infiltration medium (deionised water, 0.2 mM CaCl2, 10 mM KCl, 180 mM 2-[N-morpholino]ethane-sulphonic acid) and incubation time, time passed between onset of infiltration and end of centrifugation, revealed relatively little influence on the composition of the apoplastic washing fluid (AWF). In contrast, the pH of the infiltrated solution highly influenced the concentration of sucrose and hexoses. With increasing centrifugation force, hexosephosphate isomerase (HPI) activity in the AWF, which was taken as an indication for cytoplasmic contamination, increased. At the same time, Ca2+ concentration in the AWF increased even more. Since Ca2+ cannot originate from the cytoplasm, the suitability of HPI as marker for cytoplasmic contamination is questioned. From the composition of the AWF, it is concluded that, if centrifugation force does not exceed 1 000 g, cytoplasmic contamination is negligible and that the infiltration-centrifugation technique reveals an easy and inexpensive way to study apoplastic solutes. The infiltration-centrifugation method was also suitable to determine apoplastic air volume (Vair) and apoplastic water volume (Vwater), which are necessary for the calculation of the ion concentration in the leaf apoplast. It could be shown that the leaves of different species and the apical and basal leaves of single plants differ in Vair and Vwater.

SummaryIt has only recently become apparent that the apoplast plays a major role in a diverse range of processes, including intercellular signalling, plant-microbe interactions and both water and nutrient transport. Broadly defined, the apoplast constitutes all compartments beyond the plasmalemma -the interfibrillar and intermicellar space of the cell walls, and the xylem, including its gas-and water-filled intercellular space -extending to the rhizoplane and cuticle of the outer plant surface. The physicochemical properties of cell walls influence plant mineral nutrition, as nutrients do not simply pass through the apoplast to the plasmalemma but can also be adsorbed or fixed to cell-wall components. Here, current progress in understanding the significance of the apoplast in plant mineral nutrition is reviewed. The contribution of the root apoplast to short-distance transport and nutrient uptakes is examined particularly in relation to Na + toxicity and Al 3+ tolerance. The review extends to long-distance transport and the role of the apoplast as a habitat for microorganisms. In the leaf, the apoplast might have benefits over the vacuole as a site for short-term nutrient storage and solute exchange with the atmosphere.

For noninvasive in vivo measurements of intra-and extracellular ion concentrations, we produced transgenic Arabidopsis expressing pH and calcium indicators in the cytoplasm and in the apoplast. Ratiometric pH-sensitive derivatives of the green fluorescent protein (At-pHluorins) were used as pH indicators. For measurements of calcium ([Ca 2ϩ ]), luminescent aequorin variants were expressed in fusion with pHluorins. An Arabidopsis chitinase signal sequence was used to deliver the indicator complex to the apoplast. Responses of pH and [Ca 2ϩ ] in the apoplast and in the cytoplasm were studied under salt and "drought" (mannitol) stress. Results are discussed in the frame of ion flux, regulation, and signaling. They suggest that osmotic stress and salt stress are differently sensed, compiled, and processed in plant cells.

The root system of permanent grasslands is of outstanding importance for resource acquisition. Particularly under semi-arid conditions, the acquisition of water and nutrients is highly variable during the vegetation growth period and between years. Additionally, grazing is repeatedly disturbing the functional equilibrium between the root system and the transpiring leaf canopy. However, very few data is available considering grazing effects on belowground net primary productivity (BNPP) and root-shoot dry mass allocation in natural grassland systems. We hypothesise that grazing significantly reduces BNPP due to carbon reallocation to shoot growth. Root biomass and BNPP were estimated by soil coring in 2004, 2005 and 2006 and from ingrowth cores in 2005 and 2006 at one site which has been protected from grazing since 1979 (UG79), at one winter grazing (WG), and one heavily grazed (HG) site. BNPP was estimated from the summation of significant increments of total and live root biomass and from accumulated root biomass of ingrowth cores. Belowground biomass varied from 1,490-2,670 g m −2 and was significantly lower under heavy grazing than at site UG79. Root turnover varied from 0.23 to 0.33 year −1 and was not significantly different between sites. Heavy grazing significantly decreased live root biomass and BNPP compared to site UG79. Taking BNPP estimates from live root biomass dynamics and ingrowth cores as the most Plant Soil (2008) 307:41-50 reliable values, the portion of dry mass allocated belowground relative to total net primary productivity (BNPP/NPP) varied between 0.50-0.66 and was reduced under heavy grazing in 2005, but not in 2006. The positive correlation between cumulative root length density of ingrowth cores and leaf dry matter suggests that the ingrowth core method is suitable for studying BNPP in this semi-arid steppe system. Grazing effects on BNPP and BNPP/NPP should be considered in regional carbon models and estimates of belowground nutrient cycling.