Committee for the study of the Scottish Flora

Summary

Three different oxygen-containing germination environments demonstrate the profound influence exerted by environmental oxygen (0₂) on growth and plastogenesis in coleoptiles of light-germinated rice seedlings. Coleoptile greening is extensive in low numbers of seedlings germinated in a sealed, initially air-saturated, static water environment and in large numbers of seedlings germinated under unagitated water underambient gaseous exchange conditions. In seedlings germinated in air (?21% 0₂), coleoptile greening is sparse and extension growth is much reduced compared with coleoptile extension growth of the submerged seedlings. Coleoptile greening and shoot and root growth are completely inhibited under hypoxia resulting from large numbers of germinating seedlings competing for the limited 0₂ supply in the sealed, initially air-saturated, static water environment. Coleoptile extension growth is highest under hypoxia and lowest under ?21% 0₂. The observations presented here demonstrate that 0₂ stress and non- stress conditions serve as environmental signals which influence growth behaviour and plastogenesis in coleoptiles of light-germinated rice seedlings.     

Environmental Anoxia is Unnecessary for Inhibiting Chloroplast Photomorphogenesis in Rice Coleoptiles (Oryza sativa L.)

High population densities of germinating rice seedlings in initiallyair-saturated sealed aquatic environments exhibited d^–seedling growth consisting solely of coleoptile emergence inlight and dark environments. Residual oxygen tensions of 17–23%of the initially air-saturated water containing the d^–seedlings were evident after 15 d in both the light and dark.Coleoptiles of all d^– seedlings were stark white in appearance,lacked protochlorophyllide, and contained proplastids and amyloplasts,there being no evidence of normal etioplast development in thelight or dark and no chloroplast development in the light. Thus,complete environmental anoxia was observed to be unnecessaryfor inhibiting normal chloroplast photomorphogenesis in coleoptilesof light-germinated rice seedlings. Increasing the oxygen tensionsof the 15-d-old aquatic environments of light- and dark-germinatedd^– seedlings placed in the light resulted in normal chloroplastphotomorphogenesis in coleoptiles, shoots, and roots. Key words: Oryza sativa, environmental anoxia, chloroplast photomorphogenesis, rice coleoptiles     

Mitotic activity in rice seedlings germinating under oxygen deficiency.

Rice seedlings germinating in oxygen-deficient environments do not manifest visual evidence of seminal or adventitious root growth or normal leaf growth, the coleoptile being the only seedling structure to emerge under these environmental conditions. Microscopic evidence, however, shows that mitotic activity occurs in the different tissues comprising the shoots of the seedlings germinating under oxygen deficiency. Thus evidence is presented here which shows that mitotic activity in rice seedlings germinating in oxygen-deficient environments is not confined solely to the emerging coleoptiles of the germinating seedlings.     

Ethylene and the growth of rice seedlings

Etiolated whole rice seedlings enclosed in sealed vials produced ethylene at a rate of 0.9 picomole per hour per seedling. When 2-centimeter-long shoots were subdivided into 5-millimeter-long sections, the sections containing the tip of the shoot evolved 37% of the total ethylene with the remaining 63% being produced along a gradient decreasing to the base of the shoot. The tip of the coleoptile also had the highest level of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid and of the ethylene-forming enzyme activity. Ethylene is one of the factors controlling coleoptile elongation. Decapitation of the seedling reduced ethylene evolution to one-third its original level and inhibited coleoptile growth. In short-term experiments, the growth rate of decapitated seedlings was restored to almost that of intact seedlings by application of ethylene at a concentration of 10 microliters per liter. Apart from ethylene, O₂ also participates in the control of coleoptile growth. When rice seedlings were grown in a gas mixture of N₂ and O₂, the length of the coleoptiles reached a maximum at a concentration of 2.5% O₂. Lower and higher concentrations of O₂ reduced coleoptile growth. The effect of exogenous ethylene on coleoptile growth was also O₂ dependent.     

Germination and seedling growth under anaerobic conditions in Echinochloa crus-galli (barnyard grass)*

Abstract Although rice has long been recognized to be uniquely adapted for growth in low oxygen environments of flooded rice fields, rice weeds of the Echinochloa crus-galli complex appear to be at least as well specialized for germination and growth under such unusual biological conditions. Seeds of two varieties of E. crus-galli germinate and grow for prolonged periods in a totally oxygen-free environment. E. crus-galli germinates as well as rice (Oryza sativa) under a total nitrogen atmosphere and produces as large a seedling in spite of its much smaller seed size. Like rice, the seedlings of E. crus-galli are unpigmented, the primary leaves do not emerge from the coleoptile and no root growth occurs without oxygen. Of particular interest is the ultrastructure of mitochondria from anaerobically-grown seedlings. Mitochondrial profiles from the primary leaf of seedlings grown continuously in nitrogen are very similar to those grown aerobically. The size and shape of the mitochondria are similar and the cristae are numerous and normal in appearance. This is in sharp contrast to previous studies of other species which have reported that mitochondria were vesiculate and tended to lose their normal fine-structure after similar periods without oxygen. Finally, based on ultrastructure and ¹⁴C labeling studies, anaerobically-grown seedlings are highly active metabolically, which may explain, at least for E. crus-galli var. oryzicola, its ability to germinate and emerge from flooded rice fields.     

Regulation of growth in rice seedlings

Etiolated rice seedlings (Oryza sativa L.) exhibited marked morphological differences when grown in sealed containers or in containers through which air was passed continuously. Enhancement of coleoptile and mesocotyl growth and inhibition of leaf and root growth in the sealed containers (“enclosure syndrome”) were accompanied by accumulation of CO₂ and C₂H₄ in and depletion of O₂ from the atmosphere. Ethylene (1 μl 1^?1), high levels of CO₂, and reduced levels of O₂ contributed equally to the increase in coleoptile and mesocotyl growth. The effect of enclosure could be mimicked by passing a gas mixture of 3% O₂, 82% N₂, 15% CO₂ (all v/v), and 1 μl l^?1) C₂H₄ through the vials containing the etiolated seedlings. The effects of high CO₂ and low O₂ concentrations were not mediated through increased C₂H₄ production. The enclosure syndrome was also observed in rice seedlings grown under water either in darkness or in light. The length of the rice coleoptile was positively correlated with the depth of planting in water-saturated vermiculite. The length of coleoptiles of wheat, barley, and oats was not affected by the depth of planting. In rice, the length of coleoptile was determined by the levels of O₂, CO₂, and ethylene, rather than by light. This regulatory mechanism allows rice seedlings to grow out of shallow water in which the concentration of O₂ is limiting.     

Mechanisms associated with tolerance to flooding during germination and early seedling growth in rice (Oryza sativa)

Background and Aims

Flooding slows seed germination, imposes fatalities and delays seedling establishment in direct-seeded rice. This study describes responses of contrasting rice genotypes subjected to flooding or low oxygen stress during germination and discusses the basis of tolerance shown by certain cultivars.

Methods

In one set of experiments, dry seeds were sown in soil and either watered normally or flooded with 10 cm of water. Seedling survival and shoot and root growth were assessed and seed portions of germinating seedlings were assayed for soluble sugars and starch concentrations. The whole germinating seedlings were assayed for amylase and peroxidase activities and for ethylene production. Activities of enzymes associated with anaerobic respiration were examined and gene expression was analysed separately with seeds germinating under different amounts of dissolved oxygen in dilute agar.

Key Results

Flooding during germination reduced survival but to a lesser extent in tolerant genotypes. Starch concentration in germinating seeds decreased while sugar concentration increased under flooding, but more so in tolerant genotypes. Amylase activity correlated positively with elongation (r = 0·85 for shoot and 0·83 for root length) and with plant survival (r = 0·92). Tolerant genotypes had higher amylase activity and higher RAmy3D gene expression. Ethylene was not detected in seeds within 2 d after sowing, but increased thereafter, with a greater increase in tolerant genotypes starting 3 d after sowing. Peroxidase activity was higher in germinating seeds of sensitive genotypes and correlated negatively with survival.

Conclusions

Under low oxygen stress, tolerant genotypes germinate, grow faster and more seedlings survive. They maintain their ability to use stored starch reserves through higher amylase activity and anaerobic respiration, have higher rates of ethylene production and lower peroxidase activity as germinating seeds and as seedlings. Relevance of these traits to tolerance of flooding during germination and early growth is discussed.Key words: Amylase, anoxia, crop establishment, direct-seeded rice, ethylene, flooding, germination, hypoxia, Oryza sativa     

Site of Oxygen Absorption for Downward Transport in Seedlings of Rice and Other Plants

Our earlier reports have shown that appreciable portions (ranging from 20% to 70%) of the total amount of oxygen absorbed by the aerial part can be transported downwards to roots in water cultured intact seedlings of rice, barnyard grass, wheat, pea, etc. By interrupting the alternative paths of transport, it has been demonstrated that oxygen moves downwards mainly through gaseous diffusion along the intercellularspaces in the cortex. The aim of the present investigation is to ascertain the site of oxygen absorption for downward transport in the aerial part and to show that such a transport does not necessarily involve active participation of the absorbing organ. The results are summarized below: 1. Provided that a small upper portion of the leaf is left exposed in air, flooding of the aerial part of the rice seedling does not reduce the amount of total oxygen absorption to any appreciable extent (Fig. 1). In agreement with field observation, the unflooded tip is capable of furnishing the submerged part with enough oxygen to keep it alive. 2. Nor does the complete or partial removal of leaves by cutting in seedlings of rice and pea affect downward oxygen transport appreciably, provided that the stem segment or a leaf sheath is left exposed in air. 3. The following common notion has been confirmed by actual measurement: The abnormal excessive elongation of the coleoptile in rice seedling germinated under water, which may easily extend itself above the water surface, is an adaptive device to furnish the seedling with the oxygen required for root development. 4. The "floating" roots developed at the later stage in rice culture have been demonstrated to be a possible site of oxygen absorption for downward transport. 5. When a rice seedling is held up side down, with its roots exposed in air and the shoot submerged, downward oxygen transport still takes place, although to a lesser extent than in its normal position.     

Regulation of growth in rice seedlings

Etiolated rice seedlings (Oryza sativa L.) exhibited marked morphological differences when grown in sealed containers or in containers through which air was passed continuously. Enhancement of coleoptile and mesocotyl growth and inhibition of leaf and root growth in the sealed containers (enclosure syndrome) were accompanied by accumulation of CO₂ and C₂H₄ in and depletion of O₂ from the atmosphere. Ethylene (1 l 1^–1), high levels of CO₂, and reduced levels of O₂ contributed equally to the increase in coleoptile and mesocotyl growth. The effect of enclosure could be mimicked by passing a gas mixture of 3% O₂, 82% N₂, 15% CO₂ (all v/v), and 1 l l^–1) C₂H₄ through the vials containing the etiolated seedlings. The effects of high CO₂ and low O₂ concentrations were not mediated through increased C₂H₄ production. The enclosure syndrome was also observed in rice seedlings grown under water either in darkness or in light. The length of the rice coleoptile was positively correlated with the depth of planting in water-saturated vermiculite. The length of coleoptiles of wheat, barley, and oats was not affected by the depth of planting. In rice, the length of coleoptile was determined by the levels of O₂, CO₂, and ethylene, rather than by light. This regulatory mechanism allows rice seedlings to grow out of shallow water in which the concentration of O₂ is limiting.     

Anaerobiosis-Induced Etiolation in Light-germinated Rice Seedlings

Rice seedlings germinating in the light in anaerobic environmentsshow the same growth manifestations associated with etiolationof rice seedlings germinating in the dark under stagnant water,namely the failure to synthesize chlorophyll and the failureto manifest normal root and leaf development. This shows thatetiolation in this organism can be induced in the light by anaerobiosis.Exposure of the etiolated light-germinated seedlings to normalenvironmental oxygen concentrations brings about normal greeningof the plants as well as normal root and shoot development,thus showing that this phenomenon is reversible.     

Morphological Observation on “Polyembryonic Seedling” of Rice

The formation of the so-called polyembryonic seedling and its morphology were observed. The main conclusions are as follows: 1. All caryopses contain only one embryo; 2, “The polyembryonic seedling” reported is actually single seedling with 1 or 2 lateral shoots sprouted from the main shoot. The first lateral shoot arises from the axil of coleoptile and the second one from the axil of lower leaf in the first lateral shoot. These lateral shoots are not independent seedlings. The formation of lateral shoot in wheat is the same as that of rice as mentioned above. The authors had dissected 1500 rice caryopses, 1600 young seedlings from the field and 1102 seedlings germinated under artificial conditions. Thus, polyembryonic seedlings as preriously reported is not present, at least, in rice C1001B line.     

Role of the soil Seed Bank in the restoration of a native pinewood at Glen Garry,Inverness-shire

Summary

The potential recovery of ground vegetation in a pinewood at Glen Garry, from which introduced tree species had been removed, was studied by sampling the upper soil seed bank and comparing the species composition of germinating seedlings with extant vegetation. Mean numbers and species of seedlings emerging differed between a control site (not under-planted) and two formerly under-planted sites cleared at different times. Germinating seedling numbers also differed with depth (0–10 cm). Calluna vulgaris and Juncus spp. dominated fifteen species germinating from the seed bank, while some other pinewood species were not found. Although seedling pine occurred, their survival will be affected by competition from Betula spp. and Deschampsia flexuosa. Other elements of the vegetation will recover from the seed bank or by migration, their distribution being influenced by topographic variations and the nature of the developing tree stand. Timing of clearance of exotics in relation to pine seed production and soil scarification might accelerate recovery of the pinewood flora generally.     

Incorporation of nitrate nitrogen in rice seedlings transferred to anaerobic conditions

Summary Incorporation of¹⁵NO₃- into amino acids was studied in 3-day-old aerobic rice seedlings (with coleoptile and root) subjected for 24h to anaerobic conditions. The incorporation of¹⁵N into glutamate, glutamine and alanine accounted for 89% and 84% of total incorporation in coleoptile and root, respectively. These findings indicate that, after the primary incorporation of¹⁵N into glutamate and glutamine, the main fate of nitrate nitrogen in rice seedlings subjected to anoxia is alanine.     

亚热带中山常绿阔叶林木本植物幼苗数量动态及其与生境的相关性

以百山祖自然保护区5 hm2永久样地150个幼苗监测站木本植物幼苗为研究对象,2008—2011年定期调查样方中幼苗的种类、数量、萌发、死亡等,探究亚热带中山常绿阔叶林幼苗种类组成、数量动态及其与生境的相关性。结果表明:1)百山祖样地在2008年至2011年出现的幼苗属于26科,40属,共53个物种,不同物种萌发时段有异;2)2009年样地幼苗存活比率为7.7%,2010年为-20.8%,2011年则是-0.3%,幼苗存活比率不高,种类和数量呈减少趋势;3)存活幼苗中有明显的优势物种,分别为光亮山矾(Symplocos lucida)、尖连蕊茶(Camellia cuspidata)、浙闽新木姜子(Neolitsea aurata var.undulatula)、尖叶菝葜(Smilax arisanensis)和短尾柯(Lithocarpus brevicaudatus),5个物种之和占幼苗总数比例50%;4)种子的萌发与生境有极显著的相关性,且与生境因子中水分关联最大;5)存活幼苗数与样站坡位、水分、落叶层厚度呈现显著相关性,水和光照是影响幼苗存活的主要因素。     

Effect of Barbiturates on Rice Seedling Germination and Developmental Behaviour

Rice seedling germination and developmental behaviour were examinedin the presence of six different barbituric acid derivatives.The relationship between liquid/aqueous partition coefficientsof barbiturates and their interference with, but not completeinhibition of, normal mitochondria1 oxidative metabolism observedin animal mitochondria in vitro also appear to be factors influencingthe effects of these compounds on the germination and developmentalbehaviour of the rice seedlings described in this investigation.Evidence is presented here which points to normal oxidativemetabolism as being a necessary requisite for normal light responseand normal morphogenetic development in germinating rice seedlings.     

Indole-3-acetic acid and rice coleoptile elongation under anoxia

To investigate the presence of a possible synergistic effect of IAA and anaerobiosis on rice coleoptile elongation, excised coleoptiles grown in aerobic and anaerobic conditions were tested and compared with intact seedling aerial parts for response to exogenous IAA and for levels of endogenous IAA. Excised coleoptiles were fed with³H-IAA to study aerobic and anaerobic IAA metabolism. Our results can be summarized as follows. (1) IAA and anaerobiosis have no synergistic effect on rice coleoptile elongation. (2) This behavior is due not to an inhibition of IAA uptake but probably to a reduced and different IAA metabolism in coleoptile grown in the absence of oxygen. (3) In anaerobic rice coleoptiles, the conversion to inactive conjugate (IAA-Asp) could be adopted as means of detoxification in the case of abnormally high and unutilized IAA levels. (4) The increase in IAA level found in coleoptiles of intact seedlings during anaerobic treatment could be due, as in the roots, to a translocation from the endosperm, in which the hormone is contained in a great quantity.     

Counteraction of exogenous L-proline with NaCl in salt-sensitive cultivar of rice

The counteraction of exogenous L-proline at different concentrations with salinity (100 mM NaCl at sublethal concentration) were observed on germinating rice. Supplemented 30 mM L-proline was shown to be the most effective; at higher concentration it reduced seedling growth and K⁺/Na⁺ ratio. Thus 30 mM L-proline can alleviate the salinity stress in rice seedlings.