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 ...
For 18 sugarcane cultivars, four,.distinct callus types developed on leaf explant tissue cultured on modified MS medium, but only Type 3 (embryogenic) and Type 4 (organogenic) were capable of plant regeneration. Cell suspension cultures were initiated from embryogenic callus incubated in a liquid medium. In stage one the callus adapted to the liquid medium. In stage two a heterogeneous cell suspension culture formed in 14 cultivars after five to eight weeks of culture. In stage three a homogeneous cell suspension culture was developed in six cultivars after 10 to 14 weeks by selective subculturing to increase the proportion of actively dividing cells from the heterogeneous cell suspension culture. Plants were regenerated from cell aggregates in heterogeneous cell suspension cultures for up to 148 days of culture but plants could not be regenerated from homogeneous cell suspension cultures. High yields of protoplasts were obtained from homogeneous cell suspension cultures (3.4 to 5.2 × 106 protoplasts per gram fresh weight of cells [gfwt-~]) compared to heterogeneous cell suspension cultures (0.1 × 106 protoplasts gfwt -~). Higher yields of protoplasts were obtained from homogeneous cell suspension cultures for cuitivars Q63 and Q96 after regenerating callus from the cell suspension cultures, then recycling this callus to liquid medium (S-cell suspension cultures). This process increased protoplast yield to 9.4 × 106 protoplasts gfwt -~. Protoplasts isolated from S-cell suspension cultures were regenerated to callus and recycled to produce SP-cell suspension cultures yielding 6.4 to 13.2 × 106 protoplasts gfwt-~. This recycling of callus to produce S-cell suspension cultures allowed protoplasts to be isolated for the first time from cell lines of cultivars Qll0 and Q138.
Sodium nitrate and nitrite (50–100 mM) induced germination in three out of four genetically pure dormant lines of Avena fatua L. The sensitivity to these treatments was low immediately ater harvest and increased markedly after six months of dry after‐ripening. The observation that a fourth dormant line failed to respond suggests at least two metabolic blocks may be involved in expression of dormancy. An inhibitor of gibberellin biosynthesis, 2‐chloroethyl trimethylammonium chloride, completely inhibited the dormancy‐breaking effect by nitrate and nitrite, indicating a requirement for gibberellin biosynthesis. Among reduced nitrogenous compounds, ammonium chloride and urea failed to break dormancy in all partly after‐ripened lines, suggesting that nitrate and nitrite may induce germination through their ability to act as electron acceptors. The sensitivity to all nitrogenous compounds tested increased with the length of after‐ripening indicating that the depth of the second dormancy block amy decrease with the time of after‐ripening. Other reduced nitrogenous compounds, thiourea and hydroxylamine hydrochloride, promoted some germination in the least dormant, partially after‐ripened lines. The function of these compounds as electron acceptors and their similarity in activity to the cytochrome oxidase inhibitor, sodium azide, is discussed with reference to dormancy and the possible involvement of the alternative pathway of respiration.
The germination of caryopses on water or GA3 solutions was used to characterize the degree of primary dormancy present in the seed progeny of 10 genetically pure lines of wild oats (Avena fatua L. # AVEFA). These 10 lines represented a range of types from lines exhibiting no dormancy to those with a high degree of dormancy. Repeated propagation of these pure lines under constant environmental conditions identified several genetically inherited characters that were associated with the inherited differences in degree of primary dormancy. Correlation of the seed development period, number of primary caryopses produced, and caryopsis moisture content to the degree of primary dormancy shows: a) Lines characterized by low degrees of primary dormancy were, in general, those that had a short seed development time and produced few heavy seed low in percent water; and b) lines characterized by high degrees of primary dormancy were, in general, those that had a long seed development time and produced many light seed high in percent water. The adaptive advantage of the coupling of physiological and morphological characters with degree of dormancy is discussed and the possibility that some characters are determined by others is indicated.
Native grasses have become increasingly important in the post-mining landscape where land rehabilitators try to reconstruct vegetation communities similar to those present before land clearing. So as to include native grasses in these communities, there is a requirement to understand their germination biology, because in the past, many grasses have typically been hard to establish in the final community. The present study found that poor germination of 13 native grass species was due to (1) low percentage of seed fill, (2) low seed viability of filled seeds and/or (3) seed dormancy. Eight species had dormancy treatments investigated. Most were found to exhibit at least one form of dormancy that was either located in the hull structures immediately external to the caryopsis (i.e. the lemma, palea and glumes), within the seed coat (i.e. testa and pericarp, tissues that are found inside the hull, but external to the embryo and endosperm) and/or within the embryo. Seven of the grass species tested were found to have a dormancy mechanism present in two or more locations of their dispersal unit. Germination of the selected native grass species could be improved by (1) processing seeds to enrich the percentage of seeds that are filled, (2) testing viability to ensure a high proportion of the seeds are likely to germinate or (3) using methods to help overcome dormancy and promote germination.
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