Effect of nitrogen form and root-zone pH on growth and nitrogen uptake of tea (Camellia sinensis) plants.

BACKGROUND AND AIMS: Tea (Camellia sinensis) is considered to be acid tolerant and prefers ammonium nutrition, but the interaction between root zone acidity and N form is not properly understood. The present study was performed to characterize their interaction with respect to growth and mineral nutrition. METHODS: Tea plants were hydroponically cultured with NH4+, NO3- and NH(4+) + NO3-, at pH 4.0, 5.0 and 6.0, which were maintained by pH stat systems. KEY RESULTS: Plants supplied with NO3- showed yellowish leaves resembling nitrogen deficiency and grew much slower than those receiving NH4+ or NH(4+) + NO3- irrespective of root-zone pH. Absorption of NH4+ was 2- to 3.4-fold faster than NO3- when supplied separately, and 6- to 16-fold faster when supplied simultaneously. Nitrate-grown plants had significantly reduced glutamine synthetase activity, and lower concentrations of total N, free amino acids and glucose in the roots, but higher concentrations of cations and carboxylates (mainly oxalate) than those grown with NH4+ or NH(4+) + NO3-. Biomass production was largest at pH 5.0 regardless of N form, and was drastically reduced by a combination of high root-zone pH and NO3-. Low root-zone pH reduced root growth only in NO(3-)-fed plants. Absorption of N followed a similar pattern as root-zone pH changed, showing highest uptake rates at pH 5.0. The concentrations of total N, free amino acids, sugars and the activity of GS were generally not influenced by pH, whereas the concentrations of cations and carboxylates were generally increased with increasing root-zone pH. CONCLUSIONS: Tea plants are well-adapted to NH(4+)-rich environments by exhibiting a high capacity for NH4+ assimilation in their roots, reflected in strongly increased key enzyme activities and improved carbohydrate status. The poor plant growth with NO3- was largely associated with inefficient absorption of this N source. Decreased growth caused by inappropriate external pH corresponded well with the declining absorption of nitrogen.

Self-reporting Arabidopsis expressing pH and [Ca2+] indicators unveil ion dynamics in the cytoplasm and in the apoplast under abiotic stress.

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(2+)]), 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(2+)] 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.

Is the infiltration-centrifugation technique appropriate for the isolation of apoplastic fluid? A critical evaluation with different plant species.

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.

The apoplast and its significance for plant mineral nutrition.

It 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 physico-chemical 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 Al3+ 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. Contents Summary 167 I. Introduction 168 II. The properties of the apoplast and its implication for solute movement 168 1. The middle lamella 168 2. The primary wall 168 3. The secondary cell wall 169 III. The root apoplast - nutrient uptake and short-distance transport 170 IV. The apoplast as a compartment for long distance transport 174 V. The apoplast - habitat for microorganisms 175 VI. The apoplast of leaves - a compartment of storage and of reactions 177 1. Transport routes in the leaf apoplast 177 2. Methods of studying apoplastic solutes 177 3. Solute relations in the leaf apoplast 178 4. Concentration gradients in the leaf apoplast 179 5. Ion relations in the leaf apoplast and symptoms of deficiency and toxicity 179 6. Ion relations in the leaf apoplast - influence of nutrient supply 180 7. The leaf apoplast - compartment for transient ion storage 180 8. Ion fluxes between apoplast and symplast 181 9. Apoplastic ion balance 181 10. Leaf apoplast - interaction with the atmosphere 183 VII. Conclusions 183 Acknowledgements 183 References 183.