Main Stem Apex Research Articles

Family-Wide Evaluation of Multiple C2 Domain and Transmembrane Region Protein in Gossypium hirsutum.

Multiple C2 domain and transmembrane region proteins (MCTPs) are a group of evolutionarily conserved proteins and show emerging roles in mediating protein trafficking and signaling transduction. Although, several studies showed that MCTPs play important roles during plant growth and development, their biological functions in cotton remain largely unknown. Here, we identify and characterize 33 GhMCTP genes from upland cotton (Gossypium hirsutum) and reveal the diverse expression patterns of GhMCTPs in various tissues. We also find that GhMCTP7, GhMCTP12, and GhMCTP17 are highly expressed in the main stem apex, suggesting their possible roles in shoot development. Through analyzing different cotton species, we discover plant heights are closely related to the expression levels of GhMCTP7, GhMCTP12, and GhMCTP17. Furthermore, we silence the expression of GhMCTP genes using virus-induced gene silencing (VIGS) system in cotton and find that GhMCTP7, GhMCTP12, and GhMCTP17 play an essential role in shoot meristem development. GhMCTPs interact with GhKNAT1 and GhKNAT2 and regulate meristem development through integrating multiple signal pathways. Taken together, our results demonstrate functional redundancy of GhMCTPs in cotton shoot meristem development and provide a valuable resource to further study various functions of GhMCTPs in plant growth and development.

Transcriptome profiling reveals the occurrence mechanism of bisexual flowers in melon (Cucumis melo L.)

Most cultivated melons are andromonoecies in which male flowers arose both in main stem and lateral branches but bisexual flowers only emerged from the leaf axils of lateral branches. However, bisexual flowers emerged in leaf axils of main stem after ethephon treatment. Therefore, the mechanism regulating the occurrence of bisexual flowers were investigated by performing transcriptome analysis in two comparison sets: shoot apex of main stem (MA) versus that of lateral branches (LA), and shoot apex of main stem after ethephon treatment (Eth) versus control (Cont). KEGG results showed that genes involved in “plant hormone signal transduction”, “MAPK signaling pathway” and “carbon metabolism” were significantly upregulated both in LA and Eth. Further, details of DEGs involved in ethylene signaling pathway were surveyed and six genes were co-upregulated in two comparison sets. Among these, CmERF1, downstream in ethylene signaling pathway, showed the most significantly difference and expressed higher in bisexual buds than that in male buds. Furthermore, fifteen DEGs were found to contain GCC box or CRT/DRE cis-element for CmERF1 in their putative promoter region, and these DEGs involved in several plant hormones signaling pathway, camalexin synthesis, carbon metabolism and plant pathogen interaction.

Genotypic Variation in Potassium Uptake in Dryland Cotton

ABSTRACT The variation in potassium (K) uptake was studied by the analysis of leaf K concentration in cotton varieties grown under conditions of terminal drought. Youngest fully-expanded leaves (3rd- 4th from apex of main stem) were used for the analysis of leaf K concentration and physiological parameters (photosynthesis, stomatal conductance, leaf water content, etc). Sampling was performed several years at two different times during the cycle in two different experimental sites. Significant variation in leaf K concentration was recorded among genotypes sampled at flowering and fruiting stages. Leaf K concentration decreased from flowering to boll opening ranging from 2.3–1.36% to 1.29–0.85% and critical leaf K concentrations (below 1.7%K) were recorded at times of high nutrient requirement. Terminal drought imposed a severe restriction over seedcotton yield which varied from 574 up to 1612 kg ha−1 depending on years and location. Boll production and seedcotton yield was positively correlated with leaf K concentration only in 1998. Stomatal conductance and specific leaf weight were the physiological traits more consistently related to leaf K concentration. The strong effect of water stress plant growth and yield components might have shaded any K effect. Environmental factors (drought intensity, soil properties) and genotypic intrinsic traits (variation in K use efficiency) may have interacted to provide a complex response. Genetic correlation for leaf K concentration was found when analyzing sampling dates in each site but it did not occur among sites. Under controlled conditions, differences in root K uptake were found in cultivars and the number of secondary roots was associated with greater shoot K contents. Conditions of diminished K availability and root uptake in drying soils might have affected plant nutrition at times of high requirements. Our results demonstrate that variation in K uptake may be found among dryland-grown cotton genotypes. This feature might be exploited to improve plant K status and yield when cropping in water-limited environments. Search on specific traits related to K use efficiency and further selection of genotypes might contribute to improve cotton K nutrition and productivity under dryland conditions.

The causes of over-winter plant losses of autumn-sown white lupins (Lupinus albus) in different regions of the UK over three seasons

A series of experiments were carried out at 14 sites in the major arable areas of the UK, in the years 1993–94, 1994–95 and 1995–96 to determine the causes and extent of over-winter plant losses of two autumn-sown cultivars of white lupin (Lupinus albus L.).Over the three seasons frost was the major cause of plant losses. Two mechanisms (one known and one not previously reported in the UK) of frost tolerance under field conditions were identified: (i) lignification of the root parenchyma early in the life of the seedling, and (ii) a large vernalization requirement of the main-stem apex, which delays stem elongation in older plants. A model was developed that could be used to predict the susceptibility of these lupin cultivars to the first severe frost of the winter using accumulated thermal time (above a base temperature of 3 °C).The effect of sowing at the beginning or end of a sowing window, calculated to optimize plant architecture the following summer, therefore varied with the weather during the autumn/winter period. A combination of cool autumn weather and late sowing (outside of the sowing window) resulted in plant losses due to a lack of lignification of the root parenchyma. In unusually warm autumn conditions (1994–95 and 1995–96) plants sown at the beginning of the sowing window were well developed before the first frosts occurred, with consequent stem elongation and plant losses.Although losses due to pests were not severe (c. 3 to 5 plants/m2), plants were attacked during the autumn by grey field slugs (Deroceras reticulatum Muller), bean seed flies (Delia platura Meigen), and probably thrips (Thrips angusticeps Uzel, Noctuidae), although this has not been confirmed. The fungal pathogens Fusarium spp. and Botrytis cinerea Pers. caused losses in conjunction with frost damage.

Relationship Between Apex Developmental Stage, Tiller Density and Yield In Durum Wheat (Triticum durum Desf.)

In the semiarid Pampas of Argentina, durum wheat yields are highly dependent on rain distribution and N availability, and consequently very variable. The aim of this study was to determine if simple phenotypic parameters, evaluated at the spike differentiation stage, which in term are influenced by prevailing environmental conditions, can be used to predict grain and protein yield, with a certain precision at an early enough stage of the crop cycle to allow interventive decisions. In order to assess the influence of the developmental stage of the main stem apex (APEX), the number of tillers per square meter (TILL) and the dry matter production (DMW), upon grain yield, thirteen trials were conducted in the Semiarid Pampas over two years, with two durum wheat cultivars (Buck Cristal and Buck Candisur), two fertilization treatments: unfertilized (YUNF) and 138 kg N ha-1 (YFER) at the end of tillering. Multiple linear regression analysis was performed. Kernel weight, protein content and protein production per unit area were also regressed with APEX, TELL and DMW. For each model variables were selected with the “forward” selection STEPWISE procedure in the BMDP New System computer program package. In the best model, APEX and TILL significantly predicted grain yield of unfertilized plots at the spike differentiation period (R2 = 0.596; p ≤ 0.01). These variables also correlated significantly with kernel weight and protein production, although these results must be considered as preliminary ones.

Branch development in Lupinus angustifolius L. II. Relationship with endogenous ABA, IAA and cytokinins in axillary and main stem buds

level appeared to play a secondary role, as a growth inhibitor. The concentrations of indole-3-acetic acid (IAA), cytokinins (CK) and abscisic acid (ABA) were measured in Key words: Apical dominance, cytokinin, auxin, abscisic buds of different regions (main stem and lateral acid, Lupinus angustifolius. branches) of Lupinus angustifolius L. (cv. Merrit) and at different stages in the development of branches. In lupin, branching patterns are the result of discrete Introduction regions of axillary branches (upper, middle and basal) which elongate at much different rates. Early in devel- A strongly growing main shoot apex represses the opment only the main shoot elongates, followed usu- growth of lateral buds by exerting ‘apical dominance’. ally by basal branch growth and then rapid upper Sachs and Thimann (1967) proposed that apical dominbranch growth. Branches in the middle of the main ance was hormonally controlled and that auxin played a stem grow only weakly or fail to develop. Levels of IAA dominant role. The model envisages auxin being transwere generally high in the apical buds of slowly grow- ported from its principal site of synthesis, the dominant ing branches and low in buds from strongly growing apical bud, towards the lateral buds where it inhibits branches, whereas CK levels showed the opposite growth either directly or indirectly. Support for this relationship. CK5IAA ratio showed a closer rela- model relies on the fact that exogenous auxin applicationship with the rate of growth of a particular branch tion, either re-establishes apical dominance when applied better than the levels of either CK or IAA alone. During to the stump of decapitated plants, or suppresses lateral early stages of growth ABA concentration did not growth when applied to lateral buds of intact plants follow the rate of branch growth. However, later in (Cline, 1991; Tamas, 1995). development, where growth did not closely match the In such a scenario, quiescent axillary buds are regarded ratio of CK5IAA, ABA level showed a strong negative as ‘replacement apices’; being called upon following loss relationship with growth. A significant decrease in ABA of the strongly growing shoot apex. While this scenario was associated with continued strong growth of the may apply to many plant species, the hypothesis has main stem apex following a decline in CK5IAA ratio. limitations, and cannot readily explain branching patOverall, the best relationship between the level of terns in plants with weak apical dominance, such as growth factors in apical buds and branching pattern Coleus blumei Benth. and Arabidopsis thaliana L., or in in lupin was the ratio of CK5IAA, implying that high those for which decapitation leads to outgrowth of buds which are furthest away from the apex (Cline, 1996).

Branch development in Lupinus angustifolius L. I. Not all branches have the same potential growth rate

Although the basal and uppermost lateral branches of Lupinus angustifolius L. frequently grow and contribute to yield, buds formed in the axils of leaves 6–12 (referred to as middle buds) rarely grow. This may be due to an inherent limitation of these buds, or some form of apical dominance or competition imposed by the plant. The hypothesis that middle buds have the full capacity to grow, but remain suppressed on intact plants was tested. The main stem apex and buds from the axils of leaves 1 and 8 (bud 1 and bud 8) were excised and cultured on sterile agar. The buds were removed from culture and weighed every 2–3 d for 21 d. The growth rate of apices from the main stem was approximately 5.8 mg d−1, compared to 2.4 mg d−1 for bud 1 and 0.9 mg d−1 for bud 8. Buds in the axils of leaves 6–10 on intact plants were painted six times with a synthetic cytokinin, benzylaminopurine, from 40 d after sowing. This promoted rapid elongation and thickening of these buds, visible as early as 5 d after painting began. The rapid growth of these branches was associated with a reduction in the length of the remaining branches on the plant. However, excision of lower branches did not increase the growth of the middle buds. It is concluded that buds 6–12 of Lupinus angustifolius L. have a partial potential to grow. This potential appears to be limited by innate factors in the bud, and may be structural and/or hormonal. The limitation appears to develop very early in the plant, and potential growth is not modified by subsequent nutrition of the plant.

Making sense of wheat development: a critique of methodology

Development in wheat is traditionally described in terms of durations of phases between observable events on the apex. The most commonly used events are seedling emergence, the formation of double ridges, and the terminal spikelet on the main stem, and the onset of anthesis. Responses of development to environmental variation is then described in terms of changes in the durations of the phases. We claim that the description itself causes confusion in describing plant responses. This is particularly because the double ridges event is not well coupled to the timing of floral development, and although the formation of the terminal spikelet is an event important for the early development of the ear, it is not a prime determinant of anthesis time. An alternative framework based on the rate of production and numbers of leaves and leaf primordia on the main stem apex, and the coordination of leaf and leaf primordium appearance, provides a simpler and more mechanistic description. We use simulations to show that such a framework explains simply the apparently complex responses of phenophase durations to the environment, and provides a basis for explaining such concepts as ‘basic development rate’ and ‘intrinsic earliness’.

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.

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.

Shoot Meristem Activity during Floral Transition in Glycine max (L.) Merr.

The soybean (Glycine max [L.] Merr.) is a quantitative short-day (SD) plant requiring two inductive cycles for floral initiation, which occurs first in the most undifferentiated meristem in an axil of a main stem leaf. Floral initiation at the main stem apex, however, requires additional SD inductive cycles. Under continuous SD the transition to flowering in the main stem apex is completed after 8 SD cycles. Differentiation and organogenesis of the first flower in the terminal raceme is apparent after 10 SD cycles. The changes in apical size and geometry, nuclear DNA, and rate of leaf initiation were followed daily during this 10-d period and compared with apices from plants kept under noninductive long days (LD). At emergence all plants had initiated three trifoliolate leaf primordia and during the vegetative stage of development maintained a plastochron of 2.0 d/leaf. The plastochron was shortened to 1.0 d/leaf in SD plants on day 7, just prior to the end of the transition. Apical size and geometry remained unchanged until after 6 SD cycles when height of the dome decreased and there was less elongation of the rib meristem. Earlier events included significantly lower amounts of nuclear DNA in cells of SD apices after 1 and 3 SD cycles. Later, the amount of nuclear DNA increased in cells of SD apices beginning after 5 SD and peaking after 6 SD before decreasing back to control levels. Shifts in increasing proportions of the population of nuclei from the 4C to 2C condition occurred after 1 SD and 3 SD. As in other species, both of these shifts are apparently essential components for the floral transition at the shoot apex in soybean. The first shift, or "mitotic" stimulus, signals that the process of the floral transition has begun, while the second shift, or "floral" stimulus, is required for completion of the process.

Primordium initiation at the stem apex as the primary event controlling plant development: preliminary evidence from wheat for the regulation of leaf development

Abstract. The results from field studies of 12 crops of winter wheat (two seasons, four sites in the U.K., four cultivars), in which leaf appearance and events at the mainstem apex were fully documented, were used to investigate the inter‐relationship between leaf initiation and appearance. It was shown that the progress of leaf appearance could be divided into two phases around collar initiation. During the first phase, which was common to all crops examined, leaf appearance was linearly related to leaf initiation, indicating that leaf appearance was not independently regulated by the environment but dependent on initiation. The slope of the second phase relationship varied between crops, indicating the intervention of an additional controlling factor. This shows that, for the first phase at least, the time course of leaf appearance was predetermined by the rate of leaf initiation at the stem apex.

The effect of leaf size on mutual shading and cultivar differences in soybean leaf photosynthetic capacity

This study investigated the basis of the negative relationship between leaf size and photosynthetic rate per unit of area among five cultivars of soybeans. Exposure of developing mainstem leaves to light, and sizes and light saturated photosynthesis rates of those leaves at maturity were compared in cultivars grown in field plots for two years at Beltsville, Maryland, USA. Plants were grown both in stands at 2.5 cm by 1 m spacing and as isolated plants. While cultivar differences in leaf size were large and consistent in both planting arrangements, significant cultivar differences in light saturated photosynthetic rates were found only in plants grown in stands. Similarly, leaf size was significantly correlated with specific leaf weight only for plants grown in stands. The mainstem apex and developing mainstem leaves experienced more severe shading in large-leaved cultivars than in small-leaved cultivars when plants were grown in stands. Thus, cultivar differences in photosynthetic capacity were probably a consequence of differences in the exposure of developing leaves to light.

Vegetative Activity of the Main Stem Terminal Bud Under Photoperiod and Flower Removal Treatments in Soybean

Effects of photoperiod and flower removal on vegetative activity of the main stem apical bud were examined for an indeterminate ('Williams') and a determinate ('Bragg') soybean cultivar. Plants grew under long day conditions until the V2 stage. Then, they were subjected to three photoperiodic treatments: (1) short days of 9 h of solar radiation during all the experiment (SD); (2) 10 short days followed by long days until the end of the experiment (SD + LD); and (3) long days during all the experiment (LD). From the moment anthesis was reached, half of the plants of each photoperiodic treatment were periodically deflowered. Flower removal induced an additional but limited vegetative growth of the main stem apex, due to the elongation of the youngest internodes. This effect was only seen under SD because long day applications (SD+LD and LD) induced a high flower abortion. On the contrary, long days stimulated internode elongation, leaf expansion and, under LD, delayed anthesis which resulted in enhanced vegetative activity of apical buds and a greater production of nodes and branches. Thus, a close but inverse relation was observed between flower induction and vegetative structure differentiation by apical buds. As in Bragg, Williams may stop vegetative activity of buds by their simple transition to a terminal raceme hence, only posterior differentiated internode elongation will be either limited or stimulated depending on SD or LD conditions, respectively.

Growth and yield of indeterminate soybeans. 2. Effect of removal of the mainstem apex

The mainstem apex was removed from indeterminate soybeans (Glycine max, varieties Chaffey and Farrer) in the field over 3 years to estimate their ability to recover from damage to terminal buds by foliage feeding and stem boring pests. In years 1 and 2, the growing tip was plucked off the main stem of 50 or 100% of plants (variety Chaffey) early in vegetative growth (V2), at the beginning of flowering (Fl), at full flowering (F100) or at the end of flowering (EF100) as single treatments. In year 3, tips were removed from 20, 33 or 50% ofplants (variety Farrer) at V3, F1 or F100 as single treatments or as repeated treatments starting at those times and repeated twice at weekly intervals thereafter. Tip removal at V2 or V3 delayed flowering (F50) and pod set (R3) by up to 10 days but did not delay maturity (P95), whereas tip removal at EF100 hastened P95 by 5-8 days. Damage during flowering did not affect phenological development. Indeterminate soybeans have the ability to compensate vegetatively for damage to the mainstem apex. Tip removal at V2, F1 or F100 significantly reduced the number of nodes on the main stem, increased branching and did not reduce the total number of nodes per plant at maturity. Tip removal at V2 increased the number of nodes per plant by increasing the number of nodes per branch. Repeated tip removal prior to flowering and single or repeated treatments during flowering reduced final plant height. Tip removal from all plants reduced yield by around 10% irrespective of the time of damage. Fifty per cent damage prior to flowering or after full flowering did not affect yield but, at the beginning of flowering, removal of tips from as few as 20% of plants reduced yield by at least 10%. The most severe yield reduction was 24% following repeated tip removal from 50% of plants starting at V3. Repeated damage to 50% of plants starting at F1 reduced yield by 18%. Seed size was not affected by any treatment. We conclude that pests in indeterminate soybean crops should be controlled if they have removed the growing tip from the main stem of 50% of plants before flowering or 20% of plants at the start of flowering and are continuing to cause damage.

Sowing date and the relationships between plant and apex development in winter cereals

Abstract The plant and mainstem apex development of a range of sowings of winter wheat, barley and rye (early September to late April) was monitored at weekly intervals at a single site in the West of Scotland throughout the growing season 1982–1983. For each species there was no relationship between mainstem leaf number and the stage of apex development. However, in each case, three developmental events — the terminal spikelet (or awn primordium) stage, the beginning of mainstem extension and the death of later-formed tillers — were clearly associated. In wheat and rye, stem internode extension began shortly before the terminal spikelet or awn primordium stage, but the interval between the two events was progressively reduced by delay in sowing such that they were virtually synchronous for the spring sowings. In contrast, the awn primordium stage in barley occurred before the onset of stem extension. The start of tiller death, which is difficult to record objectively, appeared to occur at approximately the same time as stem internode extension in each species. For each sowing of the three species the length of the sheath of mainstem leaves increased exponentially with leaf number; furthermore, the sheath length of a given leaf increased progressively with delay in sowing so that successive leaves had further to extend before they reached the ligule of the previous leaf and were classed as having appeared. The implications of this effect for the rate of leaf appearance are discussed. The crop measurements also provide further evidence for a relationship between pseudostem height and the achievement of the double ridge stage not only in wheat but also in rye and barley. Overall, the findings for each species confirm the usefulness of field measurements of pseudostem height and “ear at 1 cm” as reliable guides to the development of the mainstem apex in wheat, but much more field evidence is required for winter barley.

Photosynthate and Dry Matter Distribution in Effectively and Ineffectively Nodulated Alfalfa1

Ineffectively nodulated alfalfa (Medicago sativa L.) grown with combined N [In(+N)] was compared with symbiotically‐grown, effectively‐nodulated alfalfa (Sar) to determine whether ineffective nodulation had correlated effects on photosynthate partitioning into dry matter. Ineffectively nodulated alfalfa grown with [In( + N)] and without [In( − N)] combined N were compared to determine the effects of N stress on photosynthate partitioning. Either the topmost or lowest fully‐expanded, intact green leaf on the main stem was labeled with 14CO2 in pulse‐chase experiments. After a 24 h chase period, the plants were divided into source leaf, main stem, unexpanded leaves and shoot apex on the main stem, other fully‐expanded leaves on the main stem, shoots growing from axillary buds on the main stem, shoots growing from crown buds, crown, nodules, and root. The 14C partitioning was measured as relative specific activity (RSA) and percent of total plant recovered radioactivity (%TPR). The dry mass, %TPR, and RSA of aerial organs were similar in In( + N) and Sar. However, the dry mass and %TPR of the belowground organs (roots + crown + nodules) were 46% and 25% less, respectively, in In(+N) than in Sar. This differential partitioning of dry matter and photosynthate to roots and crowns between the populations indicates that the single recessive gene for ineffective nodulation may have correlated effects on growth or development. The allocation of photosynthate from the topmost and lowest fully‐expanded leaf was significantly (P<0.05) different to all organs except nodules and roots. The topmost fullyexpanded leaf was the primary supplier of photosynthate to the unexpanded leaves and shoot apex on the main stem, the main stem, and fully‐expanded leaves on the main stem. The lowest fully‐expanded leaf was the primary supplier to the shoots growing from axillary buds on the main stem, the shoots growing from crown buds, and the crown. The dry mass, RSA, and %TPR of the main stem, shoots growing from axillary buds on the main stem, fully‐expanded leaves on the main stem, and shoots growing from the crown were less in the In(−N) than in the In(+N). The converse was true for the unexpanded leaves and shoot apex on the main stem, crown, roots, and nodules. The carbon exchange rate of the topmost fully‐expanded leaf was reduced 45%, but the export of photosynthate significantly increased, under N stress. The dry mass, %TPR, and RSA of the nodules were greater by 19‐, 36‐, and 6‐fold, respectively, in In(−N) than in In( + N). These results indicate that the nitrate inhibition of nodulation and photosynthate partitioning to alfalfa nodules is unrelated to rhizobial effectiveness or to the functioning of the bacteroids.

Cultivar, Nitrogen, and Soil Water Effects on Apex Development in Spring Wheat1

AbstractA better understanding of the growth and yield determining traits of spring wheat (Triticum aestivum L.) is important to development of improved cultivars, management practices, and growth simulation models. The objectives of this research were to evaluate cultivars, soil water content, and fertilizer N with respect to development of the main stem apex under field conditions. The experiment was conducted over 4 years on a Williams loam soil (fine‐loamy, mixed Typic Argiborolls). Cultivars of hard red spring wheat and durum wheat (T. turgidum L.) averaged 17.6 and 20.2 days, respectively, from emergence to apex double‐ridge formation. Cultivars of hard red spring wheat also differed significantly in accumulated growing degree days (GDD) to double‐ridge formation. Plant growth stage at apex double ridge as determined by the Haun scale, differed among hard red spring wheat cultivars. In general, cultivars that had a longer vegetative apex development stage had a higher Haun score at double‐ridge formation and also higher grain yields. Apex development of the hard red spring wheat cvs. Olaf, James, and Alex grown with two fertilizer N levels and three soil water levels was affected more by cultivar than by soil water content or fertilizer N level. The seven‐leaf cv., James, compared with eight‐leaf cvs., Olaf and Alex, generally had a shorter duration apex vegetative growth stage and spikelet development stage, required fewer GDD to reach double‐ridge and terminal spikelet stages, and developed double‐ridges and terminal spikelets at a lower Haun score. Stepwise regression analyses indicated that differences in grain yield and spikes/plant at harvest could be partly explained by duration of apex development to double ridges and the terminal spikelet expressed in days, accumulted GDD, and Haun score. Yield and spikes per plant were also associated with the number of spikelets/spike.