Apical Development Research Articles

The role of apical development around the time of leaf initiation in determining leaf width at maturity in wheat seedlings (Triticum aestivumL.) with impeded roots

High soil resistance to root penetration (measured as penetrometer resistance, R s ) slows down leaf growth and reduces mature leaf size in wheat seedlings (Triticum aestivum L.). Underlying changes in the kinetics of cell partitioning and expansion and in the size and organization of mature cells were reported in companion papers (Beemster and Masle, 1996 ; Beemster et al., 1996). In the present study, the relationships between apex growth, primordium initiation and expansion were analysed for plants grown at contrasting R s , focusing on a leaf whose whole development proceeded after the onset of root impedance (leaf 5). High R s reduced the rates of apex and leaf development, but did not appear to have immediate effects on the pattern of development of the newly initiated phytomers. During an initial short period, the rate of development of a leaf primordium and associated node were related to plastochronic age, according to similar relationships (slopes) at the two R s . Effects on developmental patterns were first detected on phytomer radial expansion during plastochron 2. The ontogenetic pattern of leaf elongation was affected later, during the next few plastochrons preceding leaf emergence ('post-primordial stage'). It is concluded that a reduction in the number of formative divisions and in the number of proliferative cells along the intercalary meristem reported earlier (Beemster and Masle, 1996 ; Beemster et al., 1996) is not related to the size of the apical dome at leaf initiation nor to the size and number of meristematic cells initially recruited to the leaf primordium, which were all unaffected by R s . Rather they are generated at the primordial and post-primordial stages.

The role ofEMF1in regulating the vegetative and reproductive transition inArabidopsis thaliana (Brassicaceae)

To understand the genetic regulation of vegetative to reproductive transition in higher plants, further characterization of the Arabidopsis mutant embryonic flower1, emf1, was conducted. Using three flowering symptoms, we showed that emf1 mutants could only grow reproductive and not rosette shoots under five different growth conditions. The mutant embryos did not produce the typical tunica–corpus shoot apical structures at the heart‐, torpedo‐, and mature stages. The divergent shoot apical development during mutant and wild‐type embryogenesis indicated that the wild‐type EMF1 gene was expressed in early embryogenesis. Mutations in the EMF1 gene affected the embryonic shoot apical development and caused the germinating embryo and regenerating callus to grow inflorescence, instead of rosette, shoots. Our results support the hypothesis that the EMF1 gene regulates the switch between vegetative and reproductive growth in Arabidopsis.

Grain Yield and Quality of Wheat as Affected by Nitrogen Fertilizer Application Timed According to Apical Development

Optimal yield and quality of wheat (Triticum aestivum L.) depend to a large extent on the rate of nitrogen (N) fertilizer application, which may often be inadequate. Uptake and utilization of N by ...

Apical Development and Growth of Barley under Different CO2 and Nitrogen Regimes

AbstractIncreases in atmospheric carbon dioxide (CO2) concentration have stimulated interest in the response of agricultural crops to elevated levels of CO2. Several studies have addressed the response of C3 cereals to CO2, but the interactive effect of nutrient supply and CO2 on apical development and spikelet set and survival has not been investigated thoroughly. Hence, an experiment was conducted in the greenhouse to evaluate the effect of high (700 μmol CO2mol−1 air) and low (400 μmol mol−1) levels of atmospheric CO2 on apical development, spikelet set and abortion, and pre‐ and post‐anthesis growth in spring barley (Hordeum vulgare L.) grown under high N (0.3 g N pot−1 before sowing −1–0.11 g N pot−1 week−1) and low N (0.3 g N pot−1) regimes. The plants were grown in 5 L pots. Development of spike was hastened due to CO2 enrichment, and the C+ plants pollinated few days earlier than the C— plants. Carbon dioxide enrichment had no effect on date of ripening. Development of spike slowed following application of extra N, and plants pollinated 10 days later and matured 2 weeks later when compared with plants under low N. Carbon dioxide enrichment did not affect the number of spikelets at anthesis. Excess N decreased spikelet abortion and the increased maximum number of spikelets under both [CO2]. Barley plants did not tiller when grown in low [CO2] and low N. Increased endogenous IAA concentration in those plants, recorded three days before tillers appeared in other treatments, may have contributed to this. Carbon dioxide enrichment increased the C concentration of plants, but decreased the N concentration under high N regime. Both the C and N concentration of plants were increased under high N regime. Carbon dioxide enrichment increased the total dry matter of mature plants by 9 % under high N regime and by 21 % under low N regime. Under high [CO2] increased kernel number on tiller spikes, and increased kernel weight both on main stem and on tiller spikes resulted in a 23 % increase in kernel yield under low N regime and 76 % increase in kernel yield under high N regime. The rate of N application influenced growth and yield components to a greater extent than CO2 enrichment. At maturity, plant dry matter, kernel weight, the number of kernels per spike, and the number of spikes per plant were higher under high N regime than under low N regime.Long days (16 h), low light intensity (280 μmol m−2s−1), and at constant temperature of 20 °C high [CO2] increased kernel weight and the number of kernels on tiller spikes under high and low N application rate, but did not increase the number of kernels on main stem spike, or the number of tillers or tiller spikes per plant.

Variation in development of contrasting rice cultivars

SummaryExperiments were conducted in glasshouses at IRRI, Philippines, during 1987 and 1988 to identify varietal differences in apical development using six rice cultivars having extreme characters; Jirasar 280 (early senescence), Neo-Dunghan 1 (late senescence), Macunting (small seeds), ABB (large seeds), C12474–1 (long culm), and B581–A6–458 (short culm); and five cultivars, IR20, IR22, IR34, IR54, IR64, of the same growth duration.The length of time from physiological panicle initiation (PI) to flowering varied from 25 to 50 days. The onset of physiological PI occurred when the fourth penultimate leaf was fully exserted. The visual panicle initiation stage (VPI) coincided with spikelet differentiation in all cultivars. Nitrogen top-dressing at 55–57 days before the anticipated maturity date (the present N management recommendation) did not coincide with the actual panicle initiation stage in any of the cultivars and the major yield determinants (i.e. the primary and secondary branches) therefore differentiated in conditions of internal nitrogen shortage. Spikelet abortion took place both before and after flowering. Reduction of pre-flowering abortion is important in increasing rice grain yields and could be achieved by improved N management. It is important to recognise cultivar differences and periods of loss of potential yield in developing efficient N management protocols.

Leaf Emergence of Spring Wheat Receiving Varying Nitrogen Supply at Different Stages of Development

We examined effects of nitrogen (N) supply on leaf emergence of spring wheat (Triticum aestivum L.) grown in sand with nutrient solution containing different N concentrations (9NO3: 1NH4). In expt 1, the cultivar 'Gamenya' received nutrient solution twice weekly containing a constant N supply ranging from 50 to 2400 microM N. In expts 2 and 3, cultivars 'Aroona' and 'Gamenya' were irrigated hourly with nutrient solution containing either low (L = 50 micrroM N) or high (H = 2000 microM N) N supply. In expt 2, the N supply to half of the plants receiving L and H was changed at the double ridge stage of apical development, producing plants receiving LL, LH, HL and HH. In expt 3, N supply was changed firstly when the main stem apex was vegetative (one to two leaves) and secondly when the main stem apex was at double ridge stage (four to five leaves), producing plants receiving LLL, LHL, HLH and HHH. Leaves on the main stem and primary tillers were counted. Rate of leaf emergence was estimated from regression of number of leaves against thermal time; the phyllochron was calculated as 1/ rate of emergence. Severely N-deficient plants (which had at least a 60 % reduction in shoot dry weight) had slower rates of leaf emergence (expt 1). Fluctuating N supply sometimes, but not always, changed the rate of leaf emergence (expts 2 and 3). The N supply before double ridge stage had bigger effects on the phyllochron than that afterwards (expt 3). The phyllocrons of the main stems were generally lower than those of tillers, with a greater difference between stems in N-deficient plants. Low N supply at the vegetative apex stage decreased the total number of leaves on the main stem, while low N supply after double ridge did not.

Leaf Emergence of Spring Wheat Receiving Varying Nitrogen Supply at Different Stages of Development

We examined effects of nitrogen (N) supply on leaf emergence of spring wheat (Triticum aestivum L.) grown in sand with nutrient solution containing different N concentrations (9NO3: 1NH4). In expt 1, the cultivar 'Gamenya' received nutrient solution twice weekly containing a constant N supply ranging from 50 to 2400 μM N. In expts 2 and 3, cultivars 'Aroona' and 'Gamenya' were irrigated hourly with nutrient solution containing either low (L = 50 μM N) or high (H = 2000 μM N) N supply. In expt 2, the N supply to half of the plants receiving L and H was changed at the double ridge stage of apical development, producing plants receiving LL, LH, HL and HH. In expt 3, N supply was changed firstly when the main stem apex was vegetative (one to two leaves) and secondly when the main stem apex was at double ridge stage (four to five leaves), producing plants receiving LLL, LHL, HLH and HHH. Leaves on the main stem and primary tillers were counted. Rate of leaf emergence was estimated from regression of number of leaves against thermal time; the phyllochron was calculated as 1/ rate of emergence.Severely N-deficient plants (which had at least a 60% reduction in shoot dry weight) had slower rates of leaf emergence (expt 1). Fluctuating N supply sometimes, but not always, changed the rate of leaf emergence (expts 2 and 3). The N supply before double ridge stage had bigger effects on the phyllochron than that afterwards (expt 3). The phyllocrons of the main stems were generally lower than those of tillers, with a greater difference between stems in N-deficient plants. Low N supply at the vegetative apex stage decreased the total number of leaves on the main stem, while low N supply after double ridge did not.

EFFECT OF TRANSPLANTING ON APICAL DEVELOPMENT OF RICE VARIETY AT 85-2

Experiments were conducted in the greenho~e at the Regional Agricultural Research Station, Angunukolapalessa, to study the physiological and ~norphological changes in apical development of rice under broadcast sown and transplanted conditions. Results show that transplanting shock increases the differentiation of leafprimordia, but not the physiological development stage. Thus increased rice yields under transplanted conditions can be attributed to increased leaf area which enhances assimilation with a favourable sink-source relationship. Howeverpre-and post-spikelet abortion was observed under both transplanted and broadcast sown conditions even though the rate is higher in broadcast sowing.

Effect of timing and severity of water deficit on four diverse rice cultivars III. Phenological development, crop growth and grain yield

Water deficit is known to retard phenological development and reduce growth of rice. We investigated whether the ability of a cultivar to extract soil water and to tolerate water deficit affected its growth during and folloing water deficit, and consequently its grain yield. Four contrasting rice (Oryza sativa L.) cultivars (CPIC8, Lemont, Rikuto-Norin 12, and Todoroki-Wase) were subjected to water deficit during either vegetative or reproductive growth stages. We examined the contribution of genotypic differences in water-extraction ability and drought tolerance to phenological development, crop growth rate and grain yield. Biomass production during water deficit was less than 56% of that in irrigated conditions, dependeing on severity of stress and cultivar. Following the relief of water stress, crop growth rate depended on growth stage and severity of stress imposed. The cultivars differed in rate of recovery from water deficit and this was partly attributed to differences in dehydration tolerance. Severe water deficit suspended apical development until rewatering occurred, while mild water deficit reduced the rate of apical development. Water deficit during vegetative growth did not significantly reduce grain yield, whereas water deficit imposed during the reproductive period reduced grain yield to 20-70% of the irrigated control. A small growth rate during panicle development reduced grain number and potential grain size, while cultivars which recovered quickly after water deficit had a relatively larger grain yield. Greater ability to extract water and greater dehydration tolerance during water deficit, increased growth rate during recovery. Selecting for greater water extraction capacity and dehydration tolerance may be useful in improving adaptation of cultivars to water-limited environments.

Response to Daylength in Oats: Pre‐Anthesis Development and Set of Spikelets and Florets

AbstractDaylength ranges between 15 h and 22 h when oats are sown in Finland. This study evaluated the effect of 15‐h, 18‐h and 24‐h daylengths on apical development, pre‐anthesis growth, and spikelet and floret set in two oat cultivars (Avena sativa L.) adapted to northern growing conditions. Experiments were done in a growth chamber at 18°C.Increases in daylength shortened the period of apical development. The oat cultivars were especially responsive to increase in daylength from 15 h to 18 h. The increase in daylength resulted in a 45 % reduction in number of fertile florets and a slight increase in the proportion of incompletely development and stunted florets. Fertility of florets was most reduced on the lower primary and secondary branches of the panicle. Total number of leaves and phyllochron decreased as daylength increased. Moreover, the longer the day, the lower the number of green leaves per main stem until developmental stage 11. However, at pollination, oats exposed to 15‐h days had the lowest number of green leaves. Results of study indicated that delayed sowing, when associated with longer days at pre‐anthesis, hastened apical development and decreased yield potential in oats.

Changes in apical morphology during floral initiation and development in pyrethrum (Tanacetum cinerariaefoliumL.)

SummaryChanges in apical morphology during floral initiation and development in pyrethrum (Tanacetum cinerariaefolium L.) were investigated by scanning electron microscopy. The sequence of events may be divided into eleven stages of apical development which are distinctive in both morphology and size. The environmental stimulus promoting rapid flower initiation was demonstrated to be vernalization. Flower initiation occurred after four months under non-vernalizing conditions through an autonomous flower induction process. Devernalization was observed under short day (10 h daylength), low photon flux density (200 µmol m2 s−1) conditions. Apices were never observed to revert to vegetative growth after the initiation of the first involucral bract and therefore this was considered to be the developmental stage at which the apex was committed to generative development.

Leaf Emergence, Tiller Growth, and Apical Development of Nitrogen‐Dificient Spring Wheat

Conflicting reports exist about the effect of N supply on the rate of leaf emergence. We examined effects of N deficiency on leaf and tiller emergence, tiller initiation and apical development in ‘Aroona’ and ‘Gamenya’ spring wheat (Triticum aestivum L.). Four levels of N (e.g., 50 μM N = N50) were supplied by hourly irrigation with complete nutrient solution of plants growing in sand. The control plants in Exp. 1 (N1600) had 64 g kg‐1 N intheshoots at the two‐leaf stag e, compared with 33 in N50, 50 in N300, and 58 in N800. Compared with control plants, dry matter of N50 plants was 10%, N300 50%, and N800 80%. Results in Exp. 2 were similar. The rate of leaf emergence was decreased in all N50‐treated and some N200‐treated plants, but not or N300‐treated plants. Tiller bud initiation was decreased in the treatment. The number of tiller buds was correlated with total number of leaves; if a leaf emerged, N deficiency did not affect tiller initiation. Nitrogen treatment did not alter the sequence of tiller emergence, but tiller emergence was delayed or did not occur in N50, N200, and N300 plants. Nitrogen treatment had little effect on the rate of apical development. The double‐ridge stage of development was delayed ≈2 d for both cultivars at the two lowest N treatments. Terminal spikelet production was also delayed by ≈2 d at these N treatments in Aroona, but not in Gamenya spring wheat. The rate of primordia initiation was decreased in N50 and N300 plants, resulting in fewer spikelet primordia. The level of N deficiency affected plant response to the stress.

The effect of late season damage on the apical bud development of Scots pine seedlings

The effect of Lygus rugulipennis Popp. feeding and artificial damage on the apical bud of Scots pine (Pinus sylvestris L.) late in the growing season was studied on two-year-old seedlings. Lygus feeding in late July and August caused inhibition of bud formation in the subsequent year's shoots, which led to loss of apical dominance and formation of interfascicular buds. Mechanical damage caused by piercing the apical bud with a needle in July, August and October produced scars and malformations on subsequent year's shoots and buds, but did not inhibit bud formation. Damage to the apical meristem could not be detected visually on the dormant bud before shoot elongation.

The effects of the duration and timing of drought plus heat plus gibberellin A4/7 on apical meristem development and coning in Sitka spruce [Picea sitchensis (Bong.) Carr

Coning was induced on container-grown grafts of Picea sitchensis (Bong.) Carr. by stem injection of 2 mg per month or part of a month of gibberellin A4/7 in combination with drought and high temperatures. The treatments were applied in a polyethylene-covered greenhouse at five different times or durations during spring and summer. Coning response was determined the following spring. Shoots were sampled every fortnight from early May until late August and buds were dissected and fixed for anatomical study of apical development. The best coning resulted from the May to June, and June treatments with lesser coning occurring in the May, July, and May (2 wk only) treatments (all received heat + drought + gibberellin A4/7 ). Both pollen cones and seed cones were induced. Apices from control trees began to initiate bud scales in early May and leaf primordia about mid-July. Apices from May-treated trees began to differentiate into vegetative, pollen-cone or seed-cone buds in mid-June. Apices from the June-treated trees showed the earliest stages of vegetative, seed-cone and pollen-cone bud differentiation in mid-July, about 2 wk after treatment stopped. Apices from the May to June, and the July-treated trees began to differentiate into vegetative, pollen-cone and seed-cone buds in mid-June and mid-July respectively. The 2 wk May treatment induced too few cones to warrant anatomical study. The long period (May-July), during which apices can be induced to differentiate into cone buds, may be explained partly by variation in the stage of bud development and partly because treatments alter the time of differentiation and synchronize apical development. Thus the earliest treatment (May) produced earlier differentiation and promoted more buds to develop in a uniform and vigorous manner to the stage where differentiation occurs. Other treatments (May-June, and June) which preceded the natural time of bud differentiation also promoted early and uniform bud development and the longer treatment stimulated many apices, which would otherwise have slowed or stopped their development, to continue developing. Later treatment, in July, delayed apical differentiation and allowed lagging and retarded apices to catch up and thus be susceptible to the inductive treatment.

Ear Developmental Stage Used for Timing Supplemental Nitrogen Application to Spring Wheat

Nitrogen fertilization at sowing is not always optimal or sufficient for stable yield and quality formation of wbeat (Triticum aestivum L.). Precise timing of supplementary N application for wheat requires detailed knowledge of the physiology of the reproductive phase. Our objective was to compare developmental stages of the wheat ear, for timing supplemental N application to ensure stable wheat yield and quality. The experiments were carried out at two locations in 1989 and 1990, with four spring wheat cultivars. Soil types were sandy loam (Mollie Cryoborolls). Foliar N (20 kg ha−1) as an aqueous solution of urea (144 g L−1) was applied at f our stages of ear development. A greenhouse experiment was designed to clarify the effect of N on the formation of spikelet and floret numbers in the apex during reproductive period. Maximum effect of supplementary N was achieved at the double‐ridge stage, and at the stage when the stigmatic branches of the carpel had formed. At these developmental stages, supplementary N increased the number of grains per ear and grains per square meter. Nitrogen added at pollination mainly increased the kernel weight and bread‐making quality (rated according to protein concentration in the flour, wet gluten content, Zeleny sedimentation value, farinograph water absorption, dough stability, and Panasonic loaf volume). Timing N application to apical development may provide a useful method for the producer to stabilize wheat grain yield and protein concentration.

Apical Development of Spring Barley under Field Conditions in Northeastern North America

Although the apical development of barley (Hordeum vulgare L.) has been extensively researched, a complete description of spring barley apical development stages has not been published for continental North American field conditions. This study determined the timing and duration of the apical development stages, and the pattern and dynamics of spikelet primordium initiation. Data were collected for two barley cultivars for three successive seasons in eastern Canada. From the seedling stage to anthesis, apical development and the total number of primordia on the mainstem were monitored, and both were related to days after seeding (DAS) and to accumulated growing degree‐days (AGDD). The mainstem apex reached the double ridge stage 22 to 24 DAS (320–340 AGDD), awn initiation 40 to 42 DAS (≈650 AGDD), and anthesis 50 to 56 DAS (860–940 AGDD), with some variations among cultivars and years. Nine mainstem leaf primordia were initiated by 16 DAS. Spikelet primordium initiation was completed by awn initiation when 30 to 35 primordia had been formed. The data provide a description of barley apical development stages and indicate that spikelet primoridum production (i) occurs at a variable rate, (ii) is largely a function of thermal time, although genotype and climatic factors have some effect, and (iii) is related to time or thermal time in a curvilinear, rather than a linear fashion.

Effects of manganese deficiency on the pattern of tillering and development of barley ( Hordeum vulgare c.v. Galleon)

The effects severe, moderate or slight manganese (Mn) deficiency on tillering and development of Galleon barley were examined in field and growth-cabinet experiments. Manganese deficiency delayed maturity in both field and growth-cabinet experiments. In the field, Mn-deficient plants were still tillering when the plants in plots receiving Mn fertilizer had ceased tillering and their stems were elongating. In growth-cabinet experiments, plants with moderate or severe Mn deficiency did not reach the stem-elongation stage of development, and continued to tiller until they died. The pattern of tillering was altered by Mn deficiency. Moderate deficiency decreased the rate of tiller emergence without affecting the rate of leaf emergence on the main culm. Therefore, at early tillering, deficient plants had fewer tillers. Manganese deficiency extended the period of tillering, and after control plants ceased tillering and their stems began elongating, deficient plants continued to tiller, eventually reaching the number of tillers of the control plants. Moderate and severe Mn deficiency prevented stem elongation and reproduction and plants continued to tiller. Eventually, moderate Mn-deficient plants had produced significantly more tillers than control plants or slightly deficient plants. Severe Mn deficiency resulted in death of plants before any increased tiller production. Manganese status affected apical development. When moderately Mn-deficient plants had one tiller, the main stem apex was at the double-ridge stage, compared with four tillers on the control plants which were at the late double-ridge or triple-mound stage. When the number of tillers of the Mn-deficient plants had increased to that of the control plants, the apex of the main stem was dead (shrivelled and dry) in the deficient plants, while that of the controls was at the awn primordium stage. There was no difference in grain-yield of slightly Mn-deficient and control plants, even though early tillering had been depressed and shoot biomass at maturity was lower in the former. Although moderately Mn-deficient plants produced greater number of tillers than control plants, these tillers did not produce any heads.

Lowland Rice Apical Development: Stages and Cultivar Differences Detected by Electron Microscopy

AbstractIdentification of growth stages and cultivar differences are necessary to develop efficient N fertilizer management for lowland rice (Oryza sativa L.). To help characterize growth stages an electron microscope was used to identify different panicle development stages, cultivar growth differences, and potential yield loss periods. Varieties IR50 (90‐d maturity) and IR42 (120‐d maturity) were grown at the International Rice Research Institute during the 1987 wet season. Thirteen morphologically different development stages up to floral organ development were identified. Between cultivars, duration of each development stage varied. Necknode differentiation was defined as physiological panicle initiation (PI) and observed at growth stage (GS) 1.8 occurring 35 days after seeding (DAS) in IR50 and at GS 1.10 (49 DAS) in IR42. Visual panicle initiation (VPI) was observed at GS 1.9 (42 DAS) in IR50 and at GS 1.11 (64 DAS) in IR42. Panicle development took 15 to 18 d (4–7 d for panicle branches) in IR50 and 24 to 31 d (11‐15 d for panicle branches) in IR42. Therefore the present N management practice of N topdressing 5 to 7 d before VPI coincided with PI for short duration IR50 but not for later maturing IR42. Preflowering and post flowering spikelet abortion reduced the spikelet number in IR50 and more so in IR42 where preflowering spikelet abortion started even before spikelet development was completed. Comprehensive N timing studies based on rice apical development stages are required for improving N efficiency and increasing rice yields.

Apex and ear development in relation to the number of grains on the main-stem ears in spring barley (Hordeum distichon)

SUMMARYApex and ear development in nine spring barley genotypes were studied at two sites in southern Spain in 1988. The genotypes were grouped into medium and early types, depending on the time from sowing to anthesis. Medium genotypes had significantly more spikelet primordia than early ones at both sites. Spikelet mortality was similar between genotypes and sites, resulting in a higher final number of grains/ear in the medium than the early genotypes.There was a positive correlation between the maximum number of spikelet primordia and the duration of three stages of apical development: lemma-, stamen- and awn-primordia. Negative correlations were observed between spikelet mortality and the duration of certain developmental phases of the ear.

Prediction of flag-leaf emergence in wheat crops

A variety of models describing the timing of flag-leaf emergence in crops were evaluated, with the aim of establishing a reliable method of predicting this event for crops grown in northern European latitudes. Observations of development from some 130 crops grown in Great Britain, consisting of nine winter varieties and one spring variety of wheat, were compiled. The majority of these were used to optimize the parameters of the models, and the predictions of the optimised models were tested on independent data from 35 crops grown in the 1988/1989 season. The unusually high temperatures of this season led models which assumed a constant thermal or photothermal time interval between flag-leaf emergence and other developmental events, to predict much earlier flag-leaf emergence than was observed. The most successful model was one in which the thermal time from the double-ridges event of apical development to flag-leaf emergence had a negative linear dependence on the Julian day on which double-ridges occurred. This trend suggests that the crop compensates for a large number of leaf primordia which have not yet emerged, and/or a slow rate of leaf emergence, by ending leaf initiation earlier, to ensure a relatively constant date of flag-leaf emergence.