Varietal Differences in Photosynthesis of Ears and Leaves of Barley

Rates of apparent photosynthesis of ears and of the combinedflag leaf and sheath and peduncle of Proctor barley grown inpots or in the field were similar to those of Plumage Archer,or slightly smaller when the dimensions of the ear and leafarea of Proctor were less than those of Plumage Archer. Thephotosynthesis rate of the ear—about 1.0 mg. CO₂ per earper hour—was similar or slightly less than the rate ofthe flag leaf and sheath and peduncle. These rates of photosynthesisindicated that 40-50 per cent, of the carbohydrate in the grainwas provided by photosynthesis in the shoot and about 40 percent, by photosynthesis in the ear. The total CO₂ fixed by theear was equivalent to about 60 per cent, of the grain weight,20 per cent, being lost by respiration. Shading the ear underestimatedthe total amount of CO₂ fixed by the ear and decreased dry weightof grain per ear of both Proctor and Plumage Archer by 26 percent., as in pots. The contribution of ear photosynthesis toyield of grain per acre was greater for Proctor than for PlumageArcher because Proctor had more ears. The rate of apparent photosynthesis per dm.² of leaves of Proctorwas similar to that of Plumage Archer both before and afterear emergence. Before ear emergence, the photosynthesis rateof a particular leaf decreased linearly with time and was slowerfor lower than for higher leaves on the shoot. Respiration ratesper g. dry weight of ears of Proctor and Plumage Archer weresimilar; in one experiment the leaves of Proctor respired slightlyfaster than those of Plumage Archer.     

Dark Respiration of Several Varieties of Winter Wheat Given Different Amounts of Nitrogen Fertilizer

Carbon dioxide production in the dark by ears and by the restof the shoot of winter wheat grown in the field was measuredin 2 years during grain growth. The respiration rate per g d.wt of the ears was increased by nitrogen fertilizer. Ears ofthe semi-dwarf varieties Maris Fundin and Hobbit respired moreslowly than ears of Maris Huntsman and Cappelle-Desprez. Respirationrates of the rest of the shoot were unaffected by nitrogen orvariety. The amount of carbohydrate required to provide the CO₂ respiredduring the whole period of grain growth varied from 163 to 443g m^–2, or 42 to 76 per cent of the dry weight of the grain.More than half the CO₂ lost was respired by the ear. The additionof 180 kg N ha^–1, which increased grain yield by 78 percent in 1975, almost trebled the amount of CO₂ lost by the ears.The semi-dwarf varieties lost less CO₂ from ears and shootsthan did the taller ones, and had larger yields of grain. Respiration was also estimated from the difference between the¹⁴C contents of shoots sampled immediately after a 30 s exposureto ¹⁴CO₂ and at maturity. When 14C was supplied 10 days afteranthesis, the loss by maturity amounted to 16–28 per centof that initially absorbed by flag leaves and 40 per cent ofthat absorbed by the leaf below the flag leaf. Most of the lossoccurred in the first day. The loss of 14C by maturity was significantlyincreased by nitrogen fertilizer in 1975. Triticum aestivum L., wheat, respiration, nitrogen supply, fertilizer treatment     

Effects of shading on inflorescence size and development in temperate perennial grasses

Single plants of S24 perennial ryegrass (Lolium perenne L.) and S215 meadow fescue (Festuca pratensis Huds.) were transferred between, or exposed continuously to, contrasting light intensities obtained by decreasing the natural light in a glasshouse with Tygan shades. Inflorescence development in main shoots was studied by dissections of shoot apices, and by counts of branches and florets when ears emerged. Apical growth was slower, and spikelet initiation and inflorescence development were delayed or inhibited, in decreased light intensities. The number of main branches in the ear depended on the rate of apical growth before and after spikelet initiation, and on the time of spikelet initiation. In meadow fescue these processes were influenced by light intensity. Floret numbers per inflorescence branch were generally decreased by decreased light intensity. Most of the effects of light intensity on inflorescence development were smaller in ryegrass than in meadow fescue.     

Survival of Tillers and Distribution of Dry Matter between Ear and Shoot of Barley Varieties

Plants of Plumage Archer barley grown in pots produced moreshoots than did Proctor, but had fewer ears at maturity, becausesome shoots died about 7 days before ear emergence. The numberand position on the plant of the shoots that died were consistentfor particular growing conditions. Shoots that died were notalways the last to be produced. There were no consistent differencesin dry weight, leaf area, nitrogen content, or apex developmentbetween shoots that did and did not survive; nor did removingthree developing ears on older shoots affect the survival ofother shoots. Growth of Plumage Archer plants was not checkedby death of shoots; dry matter, nitrogen, and leaf area wereredistributed so that increase in surviving shoots compensatedfor losses in the dead shoots. The dry-weight ratio of ear to shoot was smaller for Proctorthan for Plumage Archer at ear emergence and anthesis, but atmaturity it was greater for Proctor, both when ears were shadedto prevent them photosynthesizing and when they were unshaded.The varietal difference in ear: shoot dry-weight ratio was reversedbecause the relative growth-rate of ears of Proctor was greaterand because more dry matter was lost from its shoots. Both changesmay have been caused by translocation to the ear of a greaterproportion of the assimilate from the shoots of Proctor thanof Plumage Archer. Leaves of Proctor appeared to be more efficientin producing dry matter for the ear than those of Plumage Archer.     

Effect of awns and drought on the supply of photosynthate and its distribution within wheat ears

The presence of awns doubled the net photosynthetic rate of wheat ears and also increased the proportion of ¹⁴CO₂ assimilated by the ear that moved to the grain. The effect of water supply on photosynthesis and movement of assimilates was greater for leaves than ears, so that drought increased the proportion of assimilate contributed by ear photosynthesis to grain filling from 13% to 24% in the awnless ears, and from 34% to 43% in the awned ears. ¹⁴C assimilated by the ears was most important to the economy of the upper spikelets and to the distal florets in each spikelet, whereas flag leaf assimilate went mainly to the spikelets in the lower half of the ear, and to the proximal florets. Awns increased grain yield in the dry but not in the irrigated treatment, despite the large contribution of awned ears to grain filling. Either the supply of assimilate did not limit grain yield when water supply was not limiting, or there were compensating disadvantages to awns. However, they did not seem to have any adverse effect on the development of the upper florets, nor did they reduce grain number per ear.     

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.     

Effect of plant population on ear differentiation and growth in maize

Maize was grown at 2.47, 3.70 and 4.94 plants m^-2. Ears were apparently initiated about 2 days later in the densest than in the least dense population, but rate of formation and final number of female floret primordia per row of the ear were similar in the two populations. In the densest population fewer primordia produced fully developed florets than in the least dense population and of those that were produced fewer extruded silks before pollen shedding ceased. Relative growth rates of dry mass and of length of the ear before flowering were unaffected by density, but with the apparent later start of ear growth in the densest population ears were lighter and shorter than in the least dense population. Number of florets per row of the ear at the time of flowering decreased slightly with increase in population. Number of kernels per row decreased with time after flowering, especially in the densest population. At final harvest size and mass per kernel, number of kernels per ear, length and circumference of the rachis, and space per kernel on the rachis all decreased with increase in population. Thinning the intermediate population at various times from shortly before flowering until near final harvest increased mass of grain and shoot, and number of kernels of the remaining plants at final harvest. The extent of the increases decreased progressively the later the thinning. Mass and volume per kernel were increased by thinnings done up to the time of flowering, but were unaffected by thinning from c. 16 days after flowering.     

Physiological Causes of Differences in Grain Yield between Varieties of Barley

In a field experiment on barley at Rothamsted with the highmean yield of 49 cwt. of grain per acre, the varieties Proctorand Herta produced 10—15 per cent, more grain than Plumage-Archeron plots that received no nitrogenous fertilizer. When nitrogenwas applied the difference was increased to about 30 per cent.,because the higher nitrogen supply caused the Plumage-Archercrop to lodge and did not increase its yield, while Proctorand Herta remained standing. The three varieties did not differ in leaf-area index nor innet assimilation rate before ear emergence, so that all hadthe same total dry weight. After ear emergence, the leaf-areaindices of Proctor and Plumage-Archer were nearly equal, butthat of Herta was smaller. Assuming that the photosyntheticefficiency of the leaves continued to be the same in all varieties,the higher grain yields of Proctor and Herta cannot be attributedto greater production of dry matter by the leaves, either beforeor after ear emergence. A pot experiment on plants with shadedears confirmed that the dry matter contributed to grain yieldby unit leaf area was nearly equal in all the varieties. The higher grain yield of Proctor and Herta than of Plumage-Archermust therefore have come from additional photosynthesis in partsof the plant other than the leaves, i.e. in the ears themselves.An attempt to demonstrate this directly in a pot experiment,by comparing the grain yields of plants with shaded or withunshaded ears, was unsuccessful because the varieties behaveddifferently in pots; Proctor and Herta produced only about 6per cent, more grain yield than Plumage-Archer, and though thedecrease in grain yield by shading the ears was slightly greaterfor Proctor and Herta, the differences were not significant. The sum of ear sizes (estimated from length and breadth measurements)per m.1 in the field experiment was greater for Proctor andHerta than for Plumage-Archer. Also the distribution of drymatter between developing ears and shoots apparently differedwith variety, so that at ear emergence the dry weight of earsper m.² was greater in the two higher yielding varieties. Theincreased amount of photosynthetic tissue in the ears of Proctorand Herta, as measured by size or weight, may not wholly explaintheir greater dry-matter production; ears of Herta may alsohave a higher photosynthetic efficiency. No differences in nutrient uptake that could account for thevarietal differences in grain yield were found. Plumage-Archerabsorbed more potassium, and Herta less phosphorus than theother varieties. About a quarter of the final content of nitrogen,and a third of the phosphorus, was absorbed after ear emergence,but the potassium content was nearly maximal at ear emergenceand later decreased. The pot experiment showed that, on the average of all varieties,26 per cent. of the dry matter in the grain at harvest originatedfrom photosynthesis in the ears, including 10 per cent, fromthe awns; 59 per cent, came from photosynthesis in the flag-leaflamina and sheath and peduncle, and 15 per cent, from partsof the shoot below the flag leaf.     

Effects of variation in ear temperature on growth and yield of spring wheat

Wheat plants were kept in a growth room at 15 ^oC from 7 days after an-thesis until maturity and their ears were warmed to 20 or 25 ^oC for various periods. Continuous warming initially increased the growth rate of the ear and decreased that of the stem, but hastened senescence of the ear and decreased final grain yield by decreasing dry weight per grain. Warming the ears increased the movement of ¹⁴C and nitrogen to them from the leaves and stem during the early stages of grain growth but decreased it later. Warming the ears for only the first 10 or 20 days also hastened ear senescence; grain yield decreased progressively with increase in duration of the warm period. All effects were greater at 25 than at 20 ^oC. The effects of changing ear temperature from 15 to 20 ^oC were independent of the temperature of the rest of the plant. Altering the humidity of the air around the ears by 4–7 mb at constant temperature had no effect on ear growth or senescence. Warming all of the plant except the ears from 15 to 20 ^oC increased ear growth slightly during early grain growth and decreased it later, irrespective of ear temperature: stem dry weight, leaf area and net photosynthetic rate of flag leaves and green stems were decreased and dark respiration rate of stems was increased.     

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.     

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.     

Carbon dioxide exchange in relation to sink demand in wheat

Summary In this paper, experiments are described which examine the effect of requirement for assimilates by the ear on the rate of net photosynthesis in leaves of wheat (Triticum aestivum L.). Different levels of requirement were achieved by various levels of sterilization of florets just before anthesis, which resulted in a range of grain numbers per ear, and by inhibiting photosynthesis of the intact ear by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Only the ear and two uppermost leaves of the main shoot were considered, all the lower leaves and tiller leaves being excised when the experimental treatments were imposed. In two experiments, tiller regrowth was permitted during the experimental period, while in a third, new tillers were defoliated regularly.The response of leaf photosynthesis to the level of assimilate requirement by the ear was influenced by the treatment of the vegetative tillers. Thus, the net photosynthesis rate of the flag leaf was decreased by a reduction in grain number, or increased by inhibition of photosynthesis in the ear, only when the vegetative tillers were kept defoliated; when these tillers were allowed to re-grow normally, there was no influence of ear treatment on leaf photosynthesis. Temporal changes in leaf photosynthesis were consistent with this response pattern, i.e., when tillers were defoliated, the initial high rates of photosynthesis persisted for much longer.In the experiment where photosynthesis was influenced by the requirement for assimilate in the ear, the variation occurred through change in stomatal conductance on the abaxial surface of the leaf. This surface has a lesser conductance to CO₂ exchange than the adaxial surface. The implication of this finding to rapid methods of plant screening is discussed.     

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.     

Effect of variation in ear temperature on gibberellin content of wheat ears

When the ears of wheat were grown after anthesis at temperatures different from the rest of the plant the gibberellin content of the ears increased more rapidly at higher ear temperatures, whilst the plant temperature had little effect. This greater rate of gibberellin accumulation was associated with a greater rate of ear dry weight increase. Wheat ears detached from the plant increased their gibberellin content over 4 days when provided with sucrose and ewfkaurene.     

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.     

Dark Respiration of Winter Wheat Crops in Relation to Temperature and Simulated Photosynthesis

The dark respiration of shoots (measured between March and anthesisin mid-June) and of ears (measured between anthesis and maturityat end of July) of winter wheat crops, grown in 1982 and 1985under different nitrogen application and irrigation conditions,was determined in the field. The respiration rate of 126 averagesof four samples was measured hourly for a 12–14-h darkperiod including the night. Respiration (expressed per unitdry mass) generally declined through the season for both shootsand ears. The average rate of respiration obtained on the samenight was greater for fertilized and irrigated crops, comparedwith unfertilized and droughted crops. The relationship betweenthe measured respiration and photosynthesis, simulated usinga modified version of the model developed by Spitters (1986),was analysed. This revealed that: (a) Shoot respiration was less well correlated with photosynthesisfrom the day preceding measurement than with the average ofthe photosynthesis from the two days preceding measurement. (b) The constants relating shoot respiration to total crop photosynthesisper unit crop mass and ear respiration to total crop photosynthesisper unit ear mass had similar values. This suggests that allgrowth respiration takes place in the ears at the end of theseason. (c) Crop growth respiration consumes about 35% of assimilatebefore anthesis, and that growth respiration of the ear consumesabout 40% of assimilate at the end of the season. (d) No significant effect of treatment on the relationship betweenrespiration and photosynthesis was detected, suggesting thatthe observed effect of treatment on respiration is due entirelyto differences in photosynthesis. Triticwn aestivum var. Avalon, winter wheat, dark respiration, growth coefficient, photosynthesis model, nitrogen nutrition, irrigation     

Distribution of Dry Matter between Ear and Shoot of Plumage Archer and Proctor Barley grown in the Field

In a field experiment where Proctor barley yielded slightlymore grain than Plumage Archer but had similar total dry weight,the dry-weight ratio of ear to shoot was greater for PlumageArcher than for Proctor at ear emergence and greater for Proctorat maturity, as in a previous pot experiment. When ears wereshaded, the ear: shoot dry-weight ratio at maturity of Proctorwas not significantly greater than that of Plumage Archer. Thevarietal difference in ear: shoot ratio was reversed becauseears of Proctor had a greater relative growth-rate than earsof Plumage Archer, apparently because they photosynthesizedmore. Proctor shoots decreased in weight slightly less thanshoots of Plumage Archer.     

The effect of growth retardant application on floret site utilisation and assimilate distribution in ears of perennial ryegrass cv. S.24

Application of the growth retardant paclobutrazol (PP333), at 2 kg a.i. ha^-1 at spikelet initiation to plots of perennial ryegrass cv. S.24 in 1981 and 1982 significantly increased the number of seeds per spikelet present at final harvest by reducing the number of seeds aborted during seed development. Distribution of florets and seeds per spikelet was altered by PP333, as both basal and penultimate spikelets contained more florets and seeds than did those of untreated plants. Seed weight and germination were increased in florets of penultimate spikelets, although PP333 application delayed maturity by 3–5 days. In untreated plants, assimilate recovery was significantly lower from the terminal section of the ear, whereas in PP333 treated plants, no differences were found between basal, intermediate or terminal sections of the ear. PP333 increased assimilate demand at all sections of the ear when the ear and leaves were fed. The implications of this are discussed.     

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.     

Effect of increased temperature in apical regions of maize ears on starch-synthesis enzymes and accumulation of sugars and starch

Apical florets of maize (Zea mays L.) ears differentiate later than basal florets and form kernels which have lower dry matter accumulation rates. The purpose of this study was to determine whether increasing the temperature of apical kernels during the dry matter accumulation period would alter the difference in growth rate between apical and basal kernels. Apical regions of field-grown maize (cultivar Cornell 175) ears were heated to 25 ± 3°C from 7 days after pollination to maturity (tip-heated ears) and compared with unheated ears (control). In controls, apical-kernel endosperm had 24% smaller dry weight at maturity, lower concentration of sucrose, and lower activity of ADP-Glc starch synthase than basal-kernel endosperm, whereas ADP-Glc-pyrophosphorylase (ADPG-PPase) activities were similar. In tip-heated ears apical-kernel endosperm had the same growth rate and final weight as basal-kernel endosperm and apical kernels had higher sucrose concentrations, higher ADP-Glc starch synthase activity, and similar ADPG-PPase activity. Total grain weight per ear was not increased by tip-heating because the increase in size of apical kernels was partially offset by a slight decrease in size of the basal- and middle-position kernels. Tip-heating hastened some of the developmental events in apical kernels. ADPG-PPase and ADP-Glc starch synthase activities reached peak levels and starch concentration began rising earlier in apical kernels. However, tip-heating did not shorten the period of starch accumulation in apical kernels. The results indicate that the lower growth rate and smaller size of apical kernels are not solely determined by differences in prepollination floret development.