Studies on the Germination of the Seeds of Striga hermonthica: III. On the Nature of Pretreatment and after-ripening

An attempt has been made to elucidate some of the changes whichtake place in seeds of Striga hermonthica (immature, ripe andoverripe), (a) during moisture-treatment at three temperatures(15°C., 22°C., and 32°C.), and (b) during after-ripeningin dry storage. Observations on the drift of respiration of immature seed throughoutpretreatment at 15°C. show that an inverse relationshipexists between germination and respirational activity. Thissuggests that germination depends on the accumulation of somemetabolite during pretreatment. It would seem that this metabolitemay also be used as a substrate for respiration during pretreatment. After-ripening of the immature seeds has been shown to changethe nature of the seeds in some way so that their peak respirationon exposure to moisture is at first progressively reduced. Thisreduction has been ascribed to the presence of some barriercapable of restricting gaseous diffusion. The data presented, togetherwith those of the previous paperof this series, suggest that the specific metabolite requiredfor germination might be proteinaceous. Certain other data appearto rule out the suggestion that germination capacity is conditionedby the sugar content of the seeds. Reasons are given which supportthe view that one of the main effects of the stimulating solutionon the seed is to increase the permeability of the seeds togaseous diffusion.     

Observations on the Gaseous Exchanges which take place between Menyanthes Trifotiata L. and its Environment

The exchange of gases between whole plants of Menyanthes andtheir environment has been investigated. When a 45 cm. rhizomeis surrounded by an almost oxygen-free environment, leavingthe leafy shoot exposed to air, some oxygen passes right throughthe rhizome, and the Q_O2 of the plant is significantly reduced. Analyses of gases from the air-space system show that oxygenconcentrations fall and CO concentrations rise when plants aretransferred to, and maintained under conditions like these.Oxygen concentrations show a linear gradient from top to bottomof the plant, whilst the reverse is true for CO₂. The main barrier to gas movement is shown to lie within theair-space system of the rhizome, and the number of leaves presenton the plant has little effect upon the oxygen content of theinternal atmosphere. It is shown that as the concentration of CO₂ increases in theair space system of a rhizome, so the consumption of oxygenfalls off, thus allowing more oxygen to pass through the rhizometo more distal regions. Variation in the seasonal activity of the plant is discussedin relation to these find ings and to reported studies on therespiration of rhizome-tissue slices. Deter minations of respirationon tissues such as these are held to be less meaningful if carriedout according to the ‘direct’ method. The biological significance of these features is discussed.     

The Composition of the Gas contained in the Vesicles of Fucus vesiculosus

Analysis of the gases contained in the vesicles of Fucus vesiculosusshowed that their oxygen content may be markedly enhanced byphotosynthesis. Under conditions of exposure to the air it was found that theoxygen content rarely fell much below 20 per cent. even afterprolonged periods in the dark. The carbon dioxide content of the vesicular gas varied from0 to 2·5 per cent. and appeared to bear no relationshipto metabolic activity. The results as a whole suggested that vesicle formation maypossibly be conditioned by the intensity of photosynthetic activity.     

The Germination of the Seeds of Rhinanthus Crista-galli

The germination of the seeds of Rhinanthus Crista-galli hasbeen induced by exposing them to moisture at 2° C. for periodsof from 17 weeks to over a year, depending on the amount ofdry storage to which the seeds had previously been subjected.Germination could not be brought about by moisture-treatmentat 20° C. During moisture-treatment at 2° C. the respirationrate falls significantly after an initial increase, and thengradually rises, after which germination takes place. Generally,respiration is significantly lower during moisture-treatmentat 20° C. Analyses of the treated seeds suggest that therespiratory substrate might be protein. It was found by paperchromatography that during moisture-treatment at 2° C. thenumber of amino-acids in the alcohol extract first diminishesand then increases, reaching a maximum in the ungerminated seedjust before germination becomes apparent. Differences were detectedin the amino-acids found in the alcohol extracts of seeds treatedat 2° C. compared with those moisture-treated at 20°C. Reasons are given which suggest that the limiting factorin the germination process may be the nature and rate of thehydrolysis of the reserve proteins of the seeds.     

Observations on the Gaseous Exchanges which take place between Menyanthes trifoliata L. and its Environment: I. THE COMPOSITION OF THE INTERNAL GAS OF THE PLANT

1. As part of a general investigation into the exchange of gasesbetween the bogbean, Menyanthes trifoliata, and its environment,determinations have been made of the composition of the internalatmosphere of the plant at five different levels, viz. leafyshoot, three successive portions of stem, and roots.
2. Procedureis described for the analysis of small gas samplesusing a modificationof the Bonnier and Mangin apparatus.
3. In darkness with thelower part of the plant surrounded by anatmosphere of nitrogen,it has not been possible to show thepresence of a regular downwardsgradient of oxygen, such aswould be expected on a basis ofnormal gaseous diffusion.
4. Under these Air Top/Nitrogen Bottomconditions the roots areshown to contain between 12?5 and 17?5percent, of oxygen intheir intercellular gas system. The portionof stem immediatelyadjacent to the roots showed an oxygen levelof from 14?0 to18?0 per cent.
5. In plants kept under NitrogenTop/Air Bottom conditions in thedark for 48 hours, the concentrationof oxygen in the rootsfell to a level of 3?1 per cent, or belowin three plants andto 6?o per cent, in a fourth. The oxygenconcentration in theadjacent lowermost part of the stem wasas high as 18?0 percent., depending on the degree of aerationof the solution bathingthe stem. It is suggested that thisdistribution of oxygen canbe related to the relative impermeabilityof roots and of stemendodermis to gaseous diffusion from without,and it is thoughtthat the major part of the oxygen supply tothe roots is transportedto them through the stelar air passages.
6. Some evidence is presented to show that oxygen passes fromthelower parts of the plant into the medium surrounding it.

     

Studies on the Germination of the Seeds of Striga hermonthica: II. THE EFFECT OF THE STIMULATING SOLUTION ON SEED RESPIRATION

1. Data are presented which show the effect of the natural stimulatingsolution on the respiration of the seeds of Striga hermonthica.
2. The stimulating solution has been shown to enhance the aerobicrespiration of seeds exposed to it, compared with that of seedsto distilled water at the same temperature. This effect on therespiration of the seeds is quite independent of any previousmoisture-treatment of the seeds, and thus of germination.
3. Themaximum anaerobic rate of carbon-dioxide output of air-dryseedsafter treatment with the stimulating solution may be upto fifteentimes greater than that of seeds moistened with distilledwater.The anaerobic respiration of air-dry seeds after exposuretothe stimulating solution is greatly reduced by 0?025 M. sodiumfluoride and 0?001 M. sodium monoiodoacetate.
4. The stimulatingsolution has been found to have little effecton the anaerobicoutput of carbon dioxide from seeds which hadbeen moisture-treatedfor 6 days at 22? C. before treatmentwith the stimulating solution.
5. No correlation could be established between the effects ofthestimulating solution on germination and respiration.