No abstract

The major aluminum (Al) tolerance gene in wheat ALMT1 confers. An Al-activated efflux of malate from root apices. We determined the genomic structure of the ALMT1 gene and found it consists of 6 exons interrupted by 5 introns. Sequencing a range of wheat genotypes identified 3 alleles for ALMT1, 1 of which was identical to the ALMT1 gene from an Aegilops tauschii accession. The ALMT1 gene was mapped to chromosome 4DL using 'Chinese Spring' deletion lines, and loss of ALMT1 coincided with the loss of both Al tolerance and Al-activated malate efflux. Aluminium tolerance in each of 5 different doubled-haploid populations was found to be conditioned by a single major gene. When ALMT1 was polymorphic between the parental lines, QTL and linkage analyses indicated that ALMT1 mapped to chromosome 4DL and cosegregated with Al tolerance. In 2 populations examined, Al tolerance also segregated with a greater capacity for Al-activated malate efflux. Aluminium tolerance was not associated with a particular coding allele for ALMT1, but was significantly correlated with the relative level of ALMT1 expression. These findings suggest that the Al tolerance in a diverse range of wheat genotypes is primarily conditioned by ALMT1.

Diverse element toxicities (or deficiencies) that are exacerbated during waterlogging are proposed as a major reason why waterlogging tolerance at one site is often not replicated at another. Recommendations for germplasm improvement for waterlogging tolerance include use of inductively coupled plasma analyses of soils and plants.

The present paper describes the effects of growth of roots of wheat {Triticum aeslivum cv. Gamenya) in hypoxic nutrient solutions on aerenchyma formation and Oj movement from shoots to roots Two types of roots were investigated: (1) seminal roots of 4-7-d-old seedlings, and (2) seminal and nodal roots of 10-28-d-old plants. Gas-filled porosity of seminal and nodal roots increased from 3 to 12% and from 5-7 to 11-15%, respectively, when the roots emerged in stagnant or Nj-flushed solutions (0.003 mol m ^ O2) compared with growth in continuously aerated solutions (0.26 mol m-^ O2). However, neither root type increased in porosity when they were longer than 100-200 mm at the start of the exposure to these stagnant or N2-flushcd treatments, A vernier microscope and cylindrical platinum-electrode were used to examine the relationship betwen root extension and transport of O2 from shoots to roots via the gas spaces. Measuretnents were made when the roots were in an anoxic medium and were dependent solely on O2 supplied from the shoots. For seminal roots of 5-7-d-old seedlings raised in stagnant solutions (90-100 mm), internal O2 transport was sufficient to support a rate of root elongation in the O2-free medium of between 0.03 and 0.17 mm h '. When the O2 pressure around the shoots was increased from 20 to 100 kPa O2, the O2 concentrations at the walls of the expanding zone (2-7 mm from the tip) of these roots increased from 0.006 mol m"^ to between 0.04 and 0.26 mol m"'', and the rate of root extension increased five-fold. Oxygen transport to roots grown continuously in aerated solutions was considerably less than for roots raised in stagnant solutions; this dilTerence was greater for seminal than for nodal roots. When the aerated seminal roots were longer than 100 mm and transferred to an O2-free root medium, O2 concentrations became zero at the root tip causing elongation to cease. After 24 h of anoxia, none of these roots were able to resume elongation following a return to aerated solutions.

S U IVl .VI .\ R YThe effects of shoot submergence on root aeration were examined using floodwater flushed with 1 or 2 kPa CO., plus 1 0 or 21 kPa ().,, or with air; the rooting media were stagnant. Oxygen regimes within the roots were evaluated from measurements ot radial ()., loss to polarographic electrodes sleeving the extension zone of nodal roots, or, from dissolved O., and ethanol in the rooting medium.With shoots submerged, root O^ and extension, as well as O., and ethanol concentrations in the rooting medium underwent marked diurnal changes. In the dark, the radial ()., loss fell rapidly until root surface O., became (0-001 mol m"'' (gas-phase equivalent <()'l kPa) and root extension slowed down or ceased. Oxygen, previously accumulated in the rooting medium, also declined markedly, whilst ethanol accumulated rapidly. When the lights come on, radial O., loss resumed within 3 min and surged to a peak at c. 30 min before decreasing gradually to a lower level. Root extension also resumed, and a steady rise in dissoh'ed ()., was accompanied by a decline in ethanol concentration. The expression of a diurnal cycling of O,, was attributed to high boutidary-layer resistance to gas exchange between leaf and water, to CO,^ supply and to a buffering eftect by oxygen in the floodwater. Thus, during the day the escape of photosynthetic ()., was hindered, while at night, ()., How from floodwater to leaf was restricted. Fluctuations in ethanol were attributed to the generation and subsequent consumption of ethanol by the roots in response to lower and higher internal O,, concentration. The O., surge at dawn was partly attributed to enhanced photosynthesis from accumulated internal CO.,, and partly to a decline in O,, demand during the night as a result of substrate depletion in the plant. Stagnant floodwaters around the shoots led to much higher daytime rates of radial O,^ loss from the roots, and to unexpected oscillations in radial O., loss attributed to pressurisation and de-pressurization during the expansion and subsequent release of O.,-enriched bubbles from the leaves.With non-submerged plants, the diurnal cycles in O., and ethanol concentrations were smaller and in the case of O,,, different in pattern: although, initially at night, radial O., loss declined, it often rose again later. T his trend would be consistent with a reduction in respiration during the night following rapid substrate depletion at the high temperatures. Short roots grew day and night, but as roots became longer, apical O., concentration declined and, because of this, growth ceased at c. 80 mm. The considerable influence of high temperatures on root aeration was confirmed by rapid increases in radial O., loss when temperatures were lowered from 32 to 23 °C.The ecological significance of the landings is discussed and it is concluded that {a) a diurnal periodicity ot root growth and of localized internal anoxia may be a normal feature of roots in submerged and even non-submerged rice, and (b) ethanol production may play an important role in ...