Remobilisation of vacuolar stored nitrate in barley root cells

Abstract

Double-barrelled nitrate-selective microelectrodes have been used to measure the time course of the remobilisation of vacuolar stored nitrate in barley (Hordeum vulgare L. cv. Klaxon) root cells during 24 h of nitrate deprivation. These measurements showed that there are dierent time courses for this process in epidermal and cortical cells of the same root. The remobilisation was much slower from cortical cell vacuoles and had a time course which was similar to that obtained for tissue digests of the roots. The microelectrodes were also used to measure the nitrate concentration in sap exuding from detopped seedlings. These measurements showed that there was a gradual decrease in the delivery of nitrate to the shoot during this time. Root nitrate reductase activity of neither shoots nor roots changed signi®cantly during the ®rst 24 h. Direct measurement of the cytosolic nitrate in a root epidermal cell showed that during short-term changes, such as a 20-min exposure to zero external nitrate supply, cytosolic nitrate was maintained relatively unchanged. Net nitrate eux from the roots was measurable during the initial 5 h of the zero-nitrate incubation period; after this time no further nitrate eux was detectable. These measurements are discussed in relation to the nitrate budget of a root cell and we conclude that during the ®rst 24 h of nitrate withdrawal vacuolar nitrate can be readily mobilised to supply the nitrogen demands of the seedling and to maintain the cytosolic nitrate concentration.

Light-Dark Changes in Cytosolic Nitrate Pools Depend on Nitrate Reductase Activity in Arabidopsis Leaf Cells

“…The only previous dynamic microelectrode measurements investigating changes in cytosolic nitrate activity under changing environmental conditions did not demonstrate changes during the removal of the external nitrate supply in barley (Hordeum vulgare) root epidermal cells (van der Leij et al, 1998). In this study, cytosolic nitrate activity changes were also not detected in epidermal cells of Arabidopsis leaves.…”

“…Zhen et al, 1991;Miller and Smith, 1992;van der Leij et al, 1998). Values for the cytosol in leaf cells are similar to those in root cells and are in the low millimolar range (Table I).…”

“…Mesophyll cells were impaled by maneuvering the microelectrode tip through the stomata; guard cells were easily identified on the leaf surface. Under these growth conditions two populations of microelectrode measurements can be easily identified and these have been shown to be randomly distributed and correspond to the cytosol and vacuole Walker et al, 1995;van der Leij et al, 1998).…”

A High Affinity Fungal Nitrate Carrier with Two Transport Mechanisms

“…When the cytosolic pool of nitrate is depleted, the concentration gradient alone can drive uptake into the cell. In contrast to plants (31), fungi have small vacuolar stores of nitrate to maintain the cytosolic nitrate pools, so the concentration may be depleted more readily. An evolutionary advantage is provided by a nitrate uptake system that requires less energy input by the cell.…”

Organik ve kimyasal azot kaynağının ıspanak bitkisinin bazı besin içeriği ve nitrat birikimi üzerine etkileri

“…Şekilden de görüleceği üzere, azotlu gübre uygulaması bitkideki nitrat içeriğinin oldukça yükselmesine neden olmuştur. Azot gübrelemesi ile bitki dokularında azot tüketiminin azalması sonucunda nitrat birikimi kolaylaşmaktadır (Van der Leij et al, 1998). Artan azotlu gübre dozlarına bağlı olarak ıspanak bitkisinde NO 3 -N kapsamının arttığını, CAN gübresi ile olan bu artışın diğer gübrelerden daha fazla olduğu (Güneş, 1994), İnal ve Tarakçıoğlu (2001) Organik gübrelerle yetiştirilen sebzeler, mineral olarak gübrelenmiş veya konvansiyonel olarak yetiştirilen sebzelere kıyasla düşük nitrat içeriğine sahiptir ve bu etki saha koşullarından bağımsızdır (Raupp, 1996).…”