Studies on the Germination of Cereals: 2. Factors Determining the Germination Behaviour of Wheat Grains during Maturation

A comparative study has been made of the germination behaviourof a white-grained and a red-grained wheat variety throughoutthe course of maturation. Although freshly harvested wheat grains do not germinate duringthe early stages of ripening, the embryo will develop undergermination conditions when it is completely exposed, or theepidermis of the pericarp only is removed. At the harvest-ripestage the white grains germinate rapidly, but the emergenceof the embryo is considerably delayed in the red grains. Thevarietal difference is due to the presence of the covering layersand becomes increasingly apparent in grains which have beenprematurely harvested, between 3 and 6 weeks after anthesis,and then subjected to desiccation. This effect of the coveringlayers on embryo development does not appear to be related totheir permeability to water or oxygen. The behaviour of thetwo wheat varieties is thus determined by the mechanical propertiesof the covering layers, and especially the pericarp. These delaygermination in the red grains until there has been an increasein the water absorbing power of the imbibed embryo.     

Effects of crop thinning and reduced grain numbers per ear on grain size in two winter wheat varieties given different amounts of nitrogen

To study the importance for final grain size in wheat (Triticum aestivum, L.) of assimilate supply and the storage capacity of the grain, two field experiments were done. In 1976 nitrogen was applied in the range from none to 180 kg ha^-1, part of the crop was thinned, and the top halves of some ears of the short variety Hobbit and of the tall variety Maris Huntsman were removed soon after anthesis. In 1977 ears of Maris Huntsman were halved 5 days after anthesis or at 30 days after anthesis when grain volume was maximum. Thinning the crop from 360 to 180 ear-bearing shoots m^-2 30 days before anthesis increased the number of grains per ear, except in the absence of nitrogen fertiliser, but did not increase grain size, grain dry weight per ear or total dry weight per culm. Removing the upper half of ears of Hobbit 5 days after anthesis increased dry weight per grain, but when this treatment was applied to Maris Huntsman either 5 days after anthesis in 1976 and 1977, or when grain volume was maximal in 1977, the grains failed to increase in dry weight. Non-grain dry weight of both varieties was increased by halving the ear. In both varieties the maximum volume of grains in halved ears was larger than in intact ears. Grain dry weight increased relatively less than volume after halving the ear of Hobbit, and the decrease in volume up to maturity was greater in halved than intact ears of both varieties. The larger grain volume in halved ears of Maris Huntsman in 1977 was associated with more endosperm cells.     

The competence to acquire cellular desiccation tolerance is independent of seed morphological development.

Acquisition of desiccation tolerance and the related changes at the cellular level in wheat (Triticum aestivum cv. Priokskaya) kernels during normal development and premature drying on the ear were studied using a spin probe technique and low temperature scanning electron microscopy. During normal development, the ability of embryos to germinate after rapid drying and rehydration was acquired after completion of morphological development, which is a few days before mass maturity. The acquisition of desiccation tolerance, as assessed by germination, was associated with an upsurge in cytoplasmic viscosity, the onset of accumulation of protein and oil bodies, and the retention of membrane integrity upon dehydration/rehydration. These features were also used to assess cellular desiccation tolerance in the cases when germination could not occur. Slow premature drying was used to decouple the acquisition of cellular desiccation tolerance from morphogenesis. Upon premature drying of kernels on the ears of plants cut at 5 d after anthesis, desiccation-tolerant dwarf embryos were formed that were able to germinate. When plants were cut at earlier stages poorly developed embryos were formed that were unable to germinate, but cellular desiccation tolerance was nevertheless acquired. In such prematurely dried kernels, peripheral meristematic endosperm cells had already passed through similar physiological and ultrastructural changes associated with the acquisition of cellular desiccation tolerance. It is concluded that despite the apparent strong integration in seed development, desiccation tolerance can be acquired by the meristematic cells in the developing embryo and cambial layer of endosperm, independently of morphological development.     

Effects on dry weight and nitrogen content of grains of semi-dwarf and tall varieties of winter wheat caused by decreasing the number of grains per ear

The number of grains in ears of six varieties of winter wheat growing in the field was decreased by removing the top half of the ear 6 days after anthesis (halving). In the semi-dwarf varieties Hustler, Sentry and Hobbit, and in the taller varieties Armada and Flanders, the mean dry weight per grain in the lower six spikelets of halved ears was about 23% greater than the dry weight of grains in the comparable part of intact ears. In Maris Huntsman the increase in grain size amounted to only 13%. Halving increased the number of grains in the semi-dwarf varieties but not in the others. Consequently, the increases caused by halving in the total weight of grain in the lower six spikelets ranged from 41% in Hustler and Hobbit to 7% in Maris Huntsman. Halving increased the amount of nitrogen in the grain of the lower half of the ear similarly in all varieties, and relatively more than it increased dry weight. So nitrogen per cent dry weight of grain was increased, especially in Armada, Flanders and Maris Huntsman which responded least in dry weight. The uptake of nitrogen into the stem plus ear after anthesis was unaffected by halving. Most of the nitrogen that normally moved to the upper grains accumulated in the lower grains of halved ears. More nitrogen moved into the grain of intact ears of tall than of semi-dwarf varieties after anthesis, because there was greater uptake of nitrogen into the shoot from the roots.     

Studies on the Germination of Cereals: 4. The Oxygen Requirement for Germination of Wheat Grains during Maturation

The oxygen required by the wheat embryo, for germination atdifferent stages of maturation, has been determined by the numberof grains with the embryo exposed which germinated in differentoxygen concentrations. Some embryos required only 0.5 per cent.,and the majority required less than 5 per cent., but there wasalso a proportion which required a concentration approachingthe level in air. There were more embryos in the last categoryin the grains of a red variety than in those of a white varietybut this did not account for the varietal difference in germinationwhen the grains were intact. The effect of the covering layers was also determined in eachatmosphere by the difference in germination between intact grainsand grains with the embryo exposed. The covering layers weresufficiently impermeable to prevent germination in both varietieswhen the external oxygen concentration was very low; when theconcentration was increased from 5 to 60 per cent., however,the inhibiting effect of the covering layers was reduced inthe white variety but not in the red. The delayed germinationof the red grains was therefore attributed to some factorotherthan the impermeability of the covering layers to oxygen.     

Effect of abscisic acid on the transport of assimilates in barley

The effect of abscisic acid (ABA) on assimilate transport in barley was investigated in two parallel experiments. First, the effect upon [¹⁴C]sucrose transport from the flag leaf to the ear of a single ABA application made at different stages of growth of the fruits was investigated; the effect was measured 24 h after treatment. Second, the effect of a single application of ABA made at the same stages of growth as above on grain weight of the mature plant was investigated. In both types of experiments ABA was applied once to the ear of different plants as an aqueous solution (10^-3–10^-5 M), one to five weeks after anthesis. [¹⁴C] sucrose was applied by means of agar blocks. Parallel to these experiments, the endogenous content of ABA was investigated in the developing grains. When ears were treated with ABA two or four weeks after anthesis, an increase of up to 70% in the ¹⁴C-transport from the flag leaf to the ear was observed within a 24-h period after treatment (short duration experiments). At these growth stages the endogenous concentrations of ABA were low. In sharp contrast, ABA, especially in a concentration of 10^-3 M, decreased ¹⁴C-import from the flag leaf when applied three weeks after anthesis. At this stage the endogenous ABA content had reached its maximum. Long duration experiments with a single application of ABA to the car two weeks after anthesis resulted in a marked increase of weight per thousand kernels. ABA applications made earlier or later than two weeks after anthesis either reduced the grain weight or had no effect. It is concluded that ABA is involved in the regulation of assimilate transport from the leaves to the grains, possibly by influencing the unloading of sieve tubes in the ears. Promotion or inhibition of assimilate import by exogenously applied ABA may depend on the developmental stage of the grains and on the endogenous ABA level.Abbreviations ABA abscisic acid - TKW weight per thousand kernels     

Growth, Development, and Yield of Spring Wheat in Artificial Climates

Spring wheat was grown to maturity in three growth rooms providing:(a) 18 h of light at 20° C and 6 h of darkness at 15°C (hot long days, HL); (b) 18 h of light at 15° C and 6h of darkness at 15° C (cold long days, CL); (c) 14 h lightat 20° C and 10 h of darkness at 15° C (hot short days,HS). Plants were moved between environments at spikelet initiationand anthesis, so dividing the growth period into three. Meanlengths in days of these periods in the different environmentswere: Period 1: HL 16, CL 18, HS 25; Period 2: HL 42, CL andHS 61; Period3: HL 53, CL 83, HS 63. The length of periods 2and 3 also depended on previous treatments. Grain dry weight was affected by environmental differences inall periods and effects in successive periods were additive.Compared with HL, CL or HS in period I before initiation increasedgrain yield by 6 per cent by increasing grain number per ear,HS in period 2 between initiation and anthesis decreased itby 24 per cent by decreasing the number of grains per spikeletand the proportion of spikelets that contained grain; CL inperiod 2 increased it by 21 per cent by increasing the numberof ears; CL in period 3 after anthesis increased it by 16 percent because leaves died later; HS in period 3 decreased itby 14 per cent because there was less radiation and hence lessphotosynthesis. Dry weight of shoot and root at maturity wasincreased by CL or HS in periods 1 or 2, and increased by CLand decreased by HS in period 3. The effects on final yieldof treatment during periods 1 and 2 were the consequence ofsimilar effects already produced at anthesis, and shoot androot dry weight changed little during period 3. The effects of environmental differences on grain dry weightcould not be explained by differences in leaf-area durationafter anthesis (D₃), except that CL in period 3 increased bothyield and D₃ but not proportionately, so that, as with HS inthe same period, grain: leaf ratio was decreased. Environmentaldifferences in periods 1 and 2 appeared to affect grain weightby altering the capacity of the ear to accumulate carbohydrates,determined by the number of grains per ear, rather than by alteringthe supply of carbohydrates, determined by D₃. There were some interactions between environments in differentperiods which were usually small compared with the main effects.     

Physiological Factors Limiting Grain Size in Wheat

The effects on grain size of changing the supply of assimilates,by thinning before anthesis or by shading the plants or by halvingthe ears either early or late in grain growth, were studiedin two glasshouse experiments with Kleiber spring wheat (Triticumaestivum L.), in 1976 and 1977. Late treatments had no effect,presumably because little grain growth occurred thereafter.Thinning the plants before anthesis increased, and shading theplants soon after anthesis decreased grain size. Halving theears soon after anthesis increased the size of the remaininggrains, but grain weight per ear decreased. The effect on grainsize of halving the ear tended to be smaller under conditionsmore favourable for photosynthesis, except when the plants werethinned before anthesis. Shading decreased the total amountof nitrogen per culm and the proportion of total nitrogen recoveredin the ear. Halving increased the retention of nitrogen in thestem of unshaded shoots and had no effect on nitrogen distributionwithin shaded shoots. In 1977 halving the ear increased the rate of dry matter accumulationin the grain throughout the grain filling period, but in 1976the increase in dry weight was faster in the grains of halvedears only during early grain growth. Later the grains in halvedand intact ears increased in dry weight at the same rate, eventhough the supply of photosynthate and the capacity of the grains(as measured by volume) were greater in the halved ears. Theseresults are discussed in relation to the influence on finalgrain weight of assimilate supply and the storage capacity ofthe grain.     

Effects of Age and Environment on Net Assimilation Rate of Barley

To distinguish between the effects of age and environment onnet assimilation rate (E) of barley (var. Brant) grown in theopen in pots at Ottawa, E was measured outdoors (treatment N)and on similar plants transferred to a constant environmentfor the 2 weeks during which E was determined (treatment T).During June and July, E of treatment N decreased by 77 per cent.;for treatment T this fall increased to 90 per cent, of the initialvalue. E and average day and night temperatures were greaterin the constant environment than outdoors in early June, andin July they were greater outdoors. Incident light energy wasalways greater outdoors. Thus, the fall in E with age was partiallymasked outdoors by an increase caused by the improvement inenvironmental conditions during the experiment. Plants grown continuously in the constant environment had lowerE, greater leaf area and dry weight, and ears emerged earlierthan plants of similar age that had been transferred for 2 weeksto the constant environment from outdoors. E of plants growncontinuously in the constant environment decreased with time,both when the pots were moved away from the light panel to maintaina constant light intensity at the base of the youngest leafand when the pots remained stationary so that the plants grewtowards the lights. After ears emerged, E for treatments N and T was measured onplants whose ears were shaded to prevent them photosynthesizing.Photosynthesis in the ear accounted for 19 per cent, of thefinal ear dry weight during the first 2 weeks after emergence,12 per cent, during the 3rd and 4th, and 3 per cent, duringthe 5th and 6th weeks. Shading had no effect on weight of plantparts other than ears and had similar effects outdoors and inthe constant environment.     

Effects of decreasing the number of grains in ears of cvs Hobbit and Maris Huntsman winter wheat

At 6 days after anthesis, grain numbers in ears of Maris Huntsman and Hobbit winter wheat growing in the field were decreased either by removing the two lower grains in each spikelet (degraining) or by removing the top half of the ear (halving). At maturity, degraining increased the dry weight of the third grains in each spikelet of Maris Huntsman by 11% and of Hobbit by 40%, compared with third grains in intact ears. Halving increased the mean dry weight of all the grains in the lower six spikelets of the ear slightly less; it increased the number of grains in Hobbit but not in Maris Huntsman. The responses to halving in Hobbit were greater with additional nitrogen fertiliser. At 28 days after anthesis in both varieties, degraining increased grain dry weight and the amount of water, reducing sugar, amino acids and total nitrogen in third grains. Effects of halving on these properties of the two lower grains of each spikelet were much less or nil. The increases in nitrogen content of grains at 28 days and at maturity caused by degraining or halving were relatively greater than the increases in dry weight and were similar in the two varieties.     

Effects of Radiation and Temperature on Tiller Survival, Grain Number and Grain Yield in Winter Wheat

Effects of the environment on shoot survival were studied inwinter wheat cv. Avalon grown in microplots at a density of247 plants m^–2. The incident radiation and mean temperaturewere altered during one of three periods of between 14 and 29d duration, the first (P1) starting at the end of tiller productionand the last (P3) finishing near the end of the tiller deathphase, about three weeks before anthesis. Plants were giventemperature and radiation treatments in growth rooms in twoexperiments and extra light outdoors in a third experiment:they were at other times grown outdoors. Increasing radiation by between 60 and 100 per cent during P1had negligible effects on shoot number; during P2 it alwaysdelayed tiller death but increased final ear number in onlyone experiment; during P3 it consistently increased ear numberby up to 100 m^–2. Increased radiation always increasedcrop dry weight immediately after treatment but only sometimesdid this increase persist to maturity. Grain dry weight wasincreased by treatment during P3 of one experiment. Increasingthe temperature by 4 C decreased shoot number, usually onlytemporarily, by hastening death of some tillers. Warmer temperaturesdecreased crop growth after, but not during, treatment and decreasedgrain dry weight. Radiation and temperature rarely interacted. Variation in grain yield within and between experiments relatedwell to variation in number of grains m^–2, which in turnrelated to variation in ear dry weight at anthesis. Triticum aestivum L., wheat, radiation, temperature, tillers, grain yield, grain number     

Effects of the deficiens allele on ear development and yield in barley

Lines partially-isogenic for the deficiens (V^tV^t and two-rowed (VV) barley ear types were developed in the genetic background of cv. Kym, for comparisons of ear development and yield components. The suppression of lateral floret growth by the V^t allele during pre-anthesis ear development occurred at a relatively late stage and only occasionally was median floret survival enhanced in the deficiens line. The final size of lateral florets in both deficiens and normal ear types appeared to be largely governed by assimilate supply. At anthesis, the lateral florets comprised about 10% of the dry matter of normal ears, whereas in the deficiens ears the median florets were correspondingly heavier. After anthesis, grain growth in the deficiens line was slower than in the normal line, although final grain weight was not affected. In another experiment the V^t allele occasionally increased the number of grains per ear, but consistently decreased the weight per grain. In drilled plot trials, the mean yield of the deficiens line was 8% lower than the normal. Excision of lateral florets at anthesis showed that photosynthesis in these organs did not contribute to the yield advantage of the normal ear type. It is suggested that the inferior performance of the deficiens type was attributable to the greater physical constriction of endosperm expansion imposed by the development of thicker, more rigid, palea and lemma tissues.     

Changes in growth-substance contents during growth of wheat grains

Samples of developing wheat grains were extracted at weekly intervals from ear emergence until maturity and the growth substances estimated by bioassay. Immature grains contained two cytokinins; one was similar to zeatin and another, with more cytokinin activity, had different properties. Ovules contained only small amounts of growth substances but at the end of anthesis the grains had a maximum content of cytokinin. The gibberellin content increased until 3 wk after anthesis then decreased; their auxin content increased until 4 wk after anthesis but decreased as the grains ripened and lost fresh weight. The husks contained smaller amounts of growth substances than the grains they surrounded. Exudates from young stems contained cytokinins and these may originate in the roots and move to the ears through the stems. The auxin in the grains was identified as indole-3yl-acetic acid and may be derived from the phenols present reacting with tryptophan.     

黄皮种子发育过程中脱水敏感性与细胞膜透性的关系

黄皮（Ｃｌａｕｓｅｎａ ｌａｎｓｉｕｍ （Ｌｏｕｒ．） Ｓｋｅｅｌｓ）胚轴与完整种子的发育模式以及发育中电解质渗漏率变化有些不同． 种子生理成熟前、后的胚轴对脱水的反应也不同,前者经轻微脱水可提高萌发率和活力指数,后者不耐任何程度的脱水．活力指数的急剧下降伴随着电解质渗漏率的迅速上升．实验表明,黄皮种子在发育过程中没有形成耐脱水性． 细胞膜透性变化可反映脱水对种子的伤害程度     

Effects of Nitrogen Fertilizer on the Distribution of Photosynthate during Grain Growth of Spring Wheat

The distribution of photosynthate labelled with ¹⁴C was studiedin spring wheat grown with different amounts of nitrogen fertilizerin the three years 1972–4, after exposing the flag leafor the leaf below the flag leaf to ¹⁴CO₂ at 6–10 or 19–26days after anthesis. The movement of ¹⁴C to ears was unaffectedby nitrogen fertilizer except after early exposure in 1973,when nitrogen increased the retention of ¹⁴C in stems at maturity The concentration of sugar in the top part of the shoot at theend of the day was unaffected by nitrogen in 1973, but at 22days after anthesis in 1974 the concentration of sucrose inthe glumes and rachis, and in the flag leaf lamina was increasedby nitrogen. Loss of sugar by translocation and respirationduring the night may explain why this increase in concentrationwas not reflected in the ¹⁴C distribution 24 h after supplying¹⁴C. The proportion of the total ¹⁴C content of the shoot that wasin the ear at maturity ranged from 68 to 95 per cent dependingon when and to which leaf the ¹⁴CO₂ was supplied. Less than5 per cent remained in the leaf exposed to ¹⁴CO₂. The proportionof the final ear weight contributed by the leaf below the flagleaf was about half that contributed by the flag leaf. In 1974 about 24 per cent of the ¹⁴C absorbed by the flag leaf,and 56 per cent of that absorbed by the second leaf, was lostby maturity, presumably by respiration. Most loss occurred inthe first 24 h.     

Photosynthesis of Ears and Flag Leaves of Wheat and Barley

Immediately after anthesis ears of spring wheat absorbed lessthan 0.5 mg CO₂, per hour in daylight and later evolved CO₂,in the light and in the dark. The rate of apparent photosynthesisof the combined flag-leaf lamina and sheath and peduncle (collectivelycalled flag leaf) of two spring wheat varieties, Atle and JufyI, was 3–4 mg per hour; the rates of the flag leaf andthe ear of two spring barleys, Plumage Archer and Proctor, wereeach about 1 mg per hour. The gas exchange of ears and flag leaves between ear emergenceand maturity accounted for most of the final grain dry weight.The CO₂, fixed by the wheat ear was equivalent to between 17and 30 per cent of the grain weight, but more than this waslost by respiration, so assimilation in the flag leaf was equivalentto 110–20 per cent of the final grain weight. In barley,photosynthesis in the flag leaf and the net CO₂ uptake by theear each provided about half of the carbohydrate in the grain. Barley ears photosynthesized more than wheat ears because oftheir greater surface, and flag leaves of wheat photosynthesizedmore than those of barley because they had more surface anda slightly greater rate of photosynthesis per dm².     

Relation Between Desiccation Sensitivity and Membrane Permeability of Developing Clausena lansium Seeds

Desiccation sensitivity and its relation to membrane permeability of the embryonic axes of the developing wampee (Clausena lansium (Lour.) Skeels) seeds were studied by measuring the changes in electrolyte leakage, germination and vigor index after the embryonic axes were rapidly air-dried to various water contents. During development, the fresh and dry weight per seed reached nearly maximum value at 72 d after anthesis, but the dry weight per embryonic axis continuously increased until 85 d after anthesis. The embryonic axes acquired the full capacity for germination at 58 d after anthesis and their vigor index continuously rose up from 51 to 92 d after anthesis. The electrolyte leakage of the developing the embryonic axes linearly declined to the minimum value at 72 d after anthesis and then went up again. The electrolyte leakage of the embryonic axes was higher than that of the whole seeds at the same time. The immature embryonic axes did not germinate completely, while mild desiccation could improve their viability. Any degree of desiccation decreased the vigor index of the embryonic axes which have reached physiological maturation and the decline of vigor index was corresponded to the increase of electrolyte leakage. According to this experiment, the authors concluded that wampee seeds did not gain desiccation-tolerance which was a characteristic of orthodox seeds during development. High water content was essential for maintaining membrane integrity and stabiligy of matured wampee seeds. The injury of seed viability during dehydration could be estimated by using the electro-conductivity method.     

Effect of Water Loss on Germination Ability of Maize (Zea mays L.) Pollen

The correlation between water content and viability of maizepollen grains was studied on the basis of the germination abilityof pollen from a single cross hybrid. There was found to beclose correlation between viability of the grains and theirtolerance to desiccation. Most of the pollen grains in the hybridexamined survived a reduction by almost 50 per cent of the originalwater content without loss of normal function. With water lossgreater than this, less vigorous pollen grains died or losttheir ability to form pollen tubes. Consequently, when pollinationwas carried out using pollen with a water content reduced bymore than 50 per cent, only the most tolerant pollen grainssurvived to take part in the competition which precedes fertilization.Dry pollen grains required a longer period to establish adhesionto the stigma surface and to initiate pollen tubes than pollengrains with higher water content, but otherwise their behaviourwas normal. If more than 80 per cent of the original water contentwas lost, disturbances occurred in the physiology of the grainssurviving the treatment. This was exhibited as death or a reductionin rate of pollen tube growth. Drying of pollen by an amount which does not irreversibly damagethe more tolerant grains could possibly be used as a means ofpollen selection. Zea mays L., maize, pollen viability, pollen treatment, dehydration, pollen tube     

Ear photosynthesis, carbon isotope discrimination and the contribution of respiratory CO2 to differences in grain mass in durum wheat

The role of ear photosynthesis in grain filling was studied in a number of durum wheat (Triticum turgidum var durum L.) landraces and varieties from the Middle East, North Africa, and from the collections of ‘Institut National de la Recherche Agronomique’ (INRA, France) and ‘Centro International de Mejora de Maiz y Trigo’ (CIMMYT, Mexico). Plants were grown in the field in a Mediterranean climate. Flag leaves (blade plus sheath) and ears were kept in the dark from 1 week after anthesis to maturity which reduced grain weight by 22.4% and 59.0%, respectively. In a further experiment, the carbon isotope discrimination ratio (Δ) of ear bracts, awns and flag leaves was measured on samples taken at anthesis and on mature kernels. The mean value of Δ for the water soluble fraction of bracts (17.0‰) and awns (17.7‰) were lower than those of leaves (19.5‰) and fairly similar to those of kernels (17.4‰) averaged across all genotypes. Data indicate that most of the photosynthates in the grain come from ear parts and not from flag leaves. In addition, a higher water use efficiency (WUE) of ear parts than of the flag leaf is suggested by their lower Δ values. Gas exchange in ears and flag leaves was measured during grain filling. Averaged over all genotypes, CO₂ diffusive conductance was about five times higher in the flag leaf than in the spike (with distal portions of awns outside the photosynthetic chamber) 2 weeks after anthesis. In absolute terms, the dark respiration rate (R_d) was greater than the net photosynthesis rate (P_n) by a factor of 1.74 in the spike, whereas R_d was much smaller, only 22.1, 65.7 and 24.8% of P_n in blade, sheath and awns, respectively. Data indicate that photosynthesis, and hence the water use efficiency (photosynthesis/transpiration), is greatly underestimated in ears because of the high rates of respiration which diminish the measured rates of net CO₂ exchange. Results of ¹³C discrimination and gas exchange show that genotypes from North Africa have higher WUE than those from the Middle East. The high R_d values of ears as well as their low diffusive conductance suggest that CO₂ from respiration may be used as source of carbon for ear photosynthesis. In the same way, the anatomy of glumes, for example, supports the role of bracts using internal CO₂ as source of photosynthesis. In the first experiment, the Δ in mature grains from culms with darkened ears compared with control culms provided further evidence in support of this hypothesis. Thus, the Δ from kernels of control plants was 0.40 higher than that from ear-darkened plants, probably because of some degree of refixation (recycling) of respired CO₂ in the grains.     

Acquisition of desiccation tolerance in developing wheat embryos correlates with appearance of a fluid phase in membranes

Membrane behaviour in developing wheat (Triticum aestivum cv Priokskaya) embryos was studied in relation to the acquisition of desiccation tolerance, using spin probe techniques. Fresh embryos were able to develop into seedlings at day 15 after anthesis, but it took 18 d before fast‐dried, isolated embryos could germinate. On the basis of membrane integrity measurements it was estimated that between 14 and 18 d after anthesis the proportion of embryonic cells surviving fast drying increased and the critical moisture content, to which embryonic cells could be dehydrated, decreased. Apparently, embryonic cells do not acquire the same level of desiccation tolerance simultaneously. Only when all cells had become desiccation tolerant was germination of air‐dried embryos possible. Using 5‐doxylstearic acid as the probe molecule, an approximately similar lipid–water interface ordering of membranes was observed in all hydrated embryos, irrespective of age. Dehydration had a dual effect on the lipid interface: further ordering of the major part of the interface and the appearance of additional, disturbed regions. The proportion of these regions correlated with the proportion of desiccation‐tolerant cells. We propose that the membrane surface disturbance be caused by endogenous amphiphiles that partition from the cytoplasm into membranes during drying. The absence of such disturbed regions in dried, desiccation‐sensitive embryos might reflect a lack of sufficient amphiphiles. The relevance of membrane surface disturbance for desiccation tolerance is discussed.