Waterlogging and fire impacts on nitrogen availability and utilization in a subtropical wet heathland (wallum)

Protein, amino acids and ammonium were the main forms of soluble soil nitrogen in the soil solution of a subtropical heathland (wallum). After fire, soil ammonium and nitrate increased 90- and 60-fold, respectively. Despite this increase in nitrate availability after fire, wallum species exhibited uniformly low nitrate reductase activities and low leaf and xylem nitrate. During waterlogging soil amino acids increased, particularly γ-aminobutyric acid (GABA) which accounted for over 50% of amino nitrogen. Non-mycorrhizal wallum species were significantly (P < 0.05) ¹⁵N-enriched (0.3–4.3‰) compared to species with mycorrhizal associations (ericoid-type, ecto-, va-mycorrhizal) which were strongly depleted in ¹⁵N (-6.3 to -1.8‰). Lignotubers and roots had δ¹⁵N signatures similar to that of the leaves of respective species. The exceptions were fine roots of ecto-, ecto/va-, and ericoid type mycorrhizal species which were enriched in ¹⁵N (0.1–2.4‰). The 5¹⁵N signatures of δ¹⁵N_{total soil N} and δ¹⁵N_{soil NH4+} were in the range 3.7–4.5‰, whereas δ¹⁵N_{soil NO3?} was significantly (P < 0.05) more enriched in ¹⁵N (9.2–9.8‰). It is proposed that there is discrimination against ¹⁵N during transfer of nitrogen from fungal to plant partner. Roots of selected species incorporated nitrogen sources in the order of preference: ammonium > glycine > nitrate. The exception were proteoid roots of Hakea (Proteaceae) which incorporated equal amounts of glycine and ammonium.     

Free Proline and Prolamin Protein in the Grain of Three Barley Varieties Subjected to a Gradient of Water Supply

Three barley varieties (Atem, Arabic Abiad and Beecher) weregrown on a line source sprinkler to establish a gradient ofwater supply from rain-fed (218 mm) to fully irrigated (330mm). Developing ‘seeds’ were analysed for free grainproline at three stages up to maturity. At the milk stage allthree varieties showed a strong inverse correlation (r>0–95)between free proline and irrigation treatment. A pronouncedinter-varietal difference was evident with the relatively drought-intolerantAtem having 213 nmol seed^–1 free proline when rain-fedas compared with Arabic Abiad or Beecher which had about 120nmol seed^–1 free proline at this stage. By maturity thiscorrelation was lost in all varieties, with most of the freeproline being incorporated into protein. Prolamin protein production was not impaired by drought in anyof the varieties, when measured on a per seed basis. There wassome indication that it may even rise in concentration in severelydrought-stressed plants. The results are discussed in relation to the decline in finalgrain yield with increased water deficit. Each of the threevarieties tested showed different yield responses along theirrigation gradient. These may reflect the physiological tolerancesto drought of each variety. At the milk stage the measurementof free ‘grain’ proline as a diagnostic for droughtstress tolerance may be of some value. Key words: Drought stress, seed physiology, proline