Rapid Stem Elongation Research Articles

Stem and pseudostem growth play a key role in biomass accumulation of guineagrass in long regrowth cycles

Context Tall bunch-type tropical forage grasses are known for their rapid (true) stem elongation late in the regrowth cycle, even during the vegetative phase. Aims This study aimed to evaluate the stem and pseudostem growth pattern of ‘Mombaça’ guineagrass [Megathyrsus maximus (Jacq.) BK Simon & SWL Jacobs] grown in field conditions and how this relates to its high biomass accumulation during long regrowth cycles. Methods A guineagrass field was managed from December 2017 to January 2019, in three regrowth cycles of 10, 14 and 12 weeks duration. Individual tillers were assessed weekly to measure stem, pseudostem and leaf elongation, leaf appearance; and angle of insertion of the leaves. Aboveground biomass samples were taken weekly for biomass accumulation and leaf area index assessment. Key results At the beginning of each regrowth cycle, the pseudostem elongated while the stem length remained constant. Subsequently, the pseudostem length reached a plateau, while the stem length increased at a constant rate. Because of a positive relationship between pseudostem and leaf laminae lengths, the long pseudostem ensured the formation of long leaves in tall tillers and positive net leaf elongation in long regrowth cycles. Conclusions The high biomass accumulation reflected the continuous positive net leaf elongation by the tillers and was underpinned by the younger leaves being more erect than the older ones, allowing for lower self-shading of the older leaves and greater leaf tissue retention. Implications The high production of tropical forage grasses in late regrowth should be utilised with caution, as it is underpinned by stem elongation and meristem elevation.

Fine Mapping of qd1, a Dominant Gene that Regulates Stem Elongation in Bread Wheat.

Stem elongation is a critical phase for yield determination and, as a major trait, is targeted for manipulation for improvement in bread wheat (Triticum aestivum L.). In a previous study, we characterized a mutant showing rapid stem elongation but with no effect on plant height at maturity. The present study aimed to finely map the underlying mutated gene, qd1, in this mutant. By analyzing an F2 segregating population consisting of 606 individuals, we found that the qd1 gene behaved in a dominant manner. Moreover, by using the bulked segregant RNA sequencing (BSR-seq)-based linkage analysis method, we initially mapped the qd1 gene to a 13.55 Mb region on chromosome 4B (from 15.41 to 28.96 Mb). This result was further confirmed in F2 and BC3F2 segregating populations. Furthermore, by using transcriptome sequencing data, we developed 14 Kompetitive Allele-Specific PCR (KASP) markers and then mapped the qd1 gene to a smaller and more precise 5.08 Mb interval from 26.80 to 31.88 Mb. To develop additional markers to finely map the qd1 gene, a total of 4,481 single-nucleotide polymorphisms (SNPs) within the 5.08 Mb interval were screened, and 25 KASP markers were developed based on 10x-depth genome resequencing data from both wild-type (WT) and mutant plants. The qd1 gene was finally mapped to a 1.33 Mb interval from 28.86 to 30.19 Mb on chromosome 4B. Four candidate genes were identified in this region. Among them, the expression pattern of only TraesCS4B02G042300 in the stems was concurrent with the stem development of the mutant and WT. The qd1 gene could be used in conjunction with molecular markers to manipulate stem development in the future.

Can ‘On-Farm’ Seed Priming and Chitosan Seed Treatments Induce Host Defences in Winter Barley (Hordeum vulgare L.) under Field Conditions?

Enhancing host defences through induced resistance, disease tolerance, and/or escape, in combination with current disease management regimes may be a valuable strategy to reduce pesticide use. Since both ‘on-farm’ seed priming (OSP) and chitosan priming (CHP) have been reported to confer varying levels of host defence, this study sought to investigate their potential to deliver disease control as a strategy for sustainable management of foliar pathogens in winter barley. Field experiments were conducted to determine the effects of OSP and CHP at two different field sites using three different cultivars under fungicide/non-fungicide regimes. Overall, no evidence was found to suggest that CHP or OSP can induce effective resistance in temperate field conditions. However, these field trials enabled the identification of candidate traits to deliver disease tolerance (and escape) for the primary and secondary spread of powdery mildew, i.e., large canopies and rapid stem elongation respectively. Thus, these seed treatments may deliver disease tolerance and escape traits, but these benefits are dependent upon successful establishment and vigour first. The integration of seed treatments into sustainable crop protection may be better undertaken with spring crops or in semi-arid agriculture where the added vigour at emergence can help compensate for negative environmental interactions.

Phytochrome A elevates plant circadian-clock components to suppress shade avoidance in deep-canopy shade

Shade-avoiding plants can detect the presence of neighboring vegetation and evoke escape responses before canopy cover limits photosynthesis. Rapid stem elongation facilitates light foraging and enables plants to overtop competitors. A major regulator of this response is the phytochrome B photoreceptor, which becomes inactivated in light environments with a low ratio of red to far-red light (low R:FR), characteristic of vegetational shade. Although shade avoidance can provide plants with a competitive advantage in fast-growing stands, excessive stem elongation can be detrimental to plant survival. As such, plants have evolved multiple feedback mechanisms to attenuate shade-avoidance signaling. The very low R:FR and reduced levels of photosynthetically active radiation (PAR) present in deep canopy shade can, together, trigger phytochrome A (phyA) signaling, inhibiting shade avoidance and promoting plant survival when resources are severely limited. The molecular mechanisms underlying this response have not been fully elucidated. Here, we show that Arabidopsis thaliana phyA elevates early-evening expression of the central circadian-clock components TIMING OF CAB EXPRESSION 1 (TOC1), PSEUDO RESPONSE REGULATOR 7 (PRR7), EARLY FLOWERING 3 (ELF3), and ELF4 in photocycles of low R:FR and low PAR. These collectively suppress stem elongation, antagonizing shade avoidance in deep canopy shade.

Physiological and transcriptomic responses of Mikania micrantha stem to shading yield novel insights into its invasiveness

The rapid stem elongation of the invasive weed Mikania micrantha in the forest understory is of vital significance for its successful invasion. To understand the physiological and molecular mechanisms for this process, here we comparatively investigated the physiological characteristics and transcriptome patterns of M. micrantha stem under low light (30%) and full light (100%) conditions. The results showed that M. micrantha stem had photosynthetic capacity, which was highly plastic to light intensities, constituting an indispensable part of the plastic response of M. micrantha to shading. M. micrantha had longer internodes, epidermal cells, and consequently longer stems under low light than full light conditions, which could proximally be attributed to phytohormone production and reduction of photoprotective substances as potential metabolic trade-off, as observed here under shading treatment. The transcriptome sequencing and qPCR verified the results from physiological investigation, and showed that under low light condition the expression levels of genes involved in photosynthesis (e.g. MmPsaA, MmPsbO1 and MmFd3) were generally down-regulated in comparison to full light condition, as were genes related to the photoprotective substances synthesis (e.g. MmCHS, and MmF3H1) and the negative regulators of phytohormone (e.g. MmAUX1, MmRR1 and MmGAI). It was concluded that the regulation of phytohormones and photoprotective substances are important material basis for the rapid elongation of M. micrantha stems with high plasticity, which facilitates the vine's invasiveness in the forest understory.

Comparative Study of the Developmental Traits and Yield Components of Bread Wheat under Field Conditions in Several Years of Multi‐Sowing Time Experiments

The main aim of the experiments was to demonstrate the possible correlation between developmental and morphological traits and yield components under variable climatic conditions. For this purpose, a collection of 188 wheat (Triticum aestivum L.) genotypes with a heterogeneous gene pool was included in a 3‐yr field experiment applying normal and late sowing dates each year. Under these conditions, almost all the traits were significantly influenced by the genotype, which had the greatest effect on the morphological traits and yield components (explaining 20–58 and 50–60% of the phenotypic variance for the two trait groups, respectively). In the case of plant development, however, the year effect, particularly in the late sowing treatment, was more significant than that of the genotype. Sowing date had the strongest effect on the early developmental phases, explaining 50% of the phenotypic variance, whereas the year had a significant influence on the late developmental phases, being responsible for 37 to 53% of the phenotypic variance. The turning point between the two factors was during the first phase of rapid stem elongation. The environmental driven variation in developmental patterns led to significant variation in yield‐related traits, which ranged from 4.6 (average thousand‐kernel weight) to 16.3% (average seed number) with normal sowing, and from 2.9 (average seed weight) to 29.4% (average thousand‐kernel weight) under late sowing conditions. The significance of the two intervals, from sowing to the start of rapid (intensive) stem elongation and from the start of stem elongation to the boot stage (Z49), was most apparent in the case of yield‐related traits.

Gene Expression Profiling Provides Insight into the Escape Behavior of Deepwater Rice During Submergence

Submergence harms plants by limiting gas exchange and decreasing incoming light. Upon prolonged flooding, deepwater rice (Oryza sativa) escapes complete submergence by rapid stem growth, but the mechanism of stem elongation in the early stages following submergence remains unclear. To elucidate changes in gene expression during early submergence-induced stem elongation, we performed transcriptome analysis using microarray system in deepwater rice. We compared gene expression between the control (0 h) and 1, 4 hour after submergence. We found 586 and 1,353 genes for Sub 1 h and 4 h treatments each that were up-regulated by at least two-fold, compared to the control (P < 0.05), submergence up-regulated gene expression related to at least 10 independent biological processes. Expression of deepwater rice genes related to energy production through fermentation increased with time after submergence, which likely ensures sufficient energy for rapid stem elongation. We provide the first report of initial and overall expression of deepwater rice genes related to the response specific to submergence escape.

A new outlook on sporadic flowering of bamboo

ABSTRACTMoso bamboo (Phyllostachys edulis) is a temperate grass species with a tree-like habitus and an unusual reproduction strategy. While flowering is irregular and infrequent, new clonal bamboo shoots are established from an underground rhizome network during the spring season. In our previous study, we performed transcriptome analyses using bamboo shoot buds to understand the initiation of bamboo stem elongation. Interestingly, the expression profile in the shoot apical meristem (SAM) region of young bamboo shoots is similar to that of other plants. Specifically, some of the genes that control the timing of flowering and floral development are active in the SAM region. This data raises the question of how bamboo shoots start to elongate, and why they do not proceed to a seasonal cycle of flowering. Our analyses of the activation of shoot buds and subsequent rapid stem elongation provide new hints to unravel the unpredictable flowering pattern of bamboo. In this short communication, we discuss how bamboo might coordinate and integrate the vegetative and reproductive phases in relation to shoot emergence and stem elongation.

Genetics, Physiological Mechanisms and Breeding of Flood-Tolerant Rice (Oryza sativa L.).

Flooding of rice fields is a serious problem in the river basins of South and South-East Asia where about 15 Mha of lowland rice cultivation is regularly affected. Flooding creates hypoxic conditions resulting in poor germination and seedling establishment. Flash flooding, where rice plants are completely submerged for 10-15 d during their vegetative stage, causes huge losses. Water stagnation for weeks to months also leads to substantial yield losses when large parts of rice aerial tissues are inundated. The low-yielding traditional varieties and landraces of rice adapted to these flooding conditions have been replaced by flood-sensitive high-yielding rice varieties. The 'FR13A' rice variety and the Submergence 1A (SUB1A) gene were identified for flash flooding and subsequently introgressed to high-yielding rice varieties. The challenge is to find superior alleles of the SUB1A gene, or even new genes that may confer greater tolerance to submergence. Similarly, genes have been identified in tolerant landraces of rice for their ability to survive by rapid stem elongation (SNORKEL1 and SNORKEL2) during deep-water flooding, and for anaerobic germination ability (TPP7). Research on rice genotypes and novel genes that are tolerant to prolonged water stagnation is in progress. These studies will greatly assist in devising more efficient and precise molecular breeding strategies for developing climate-resilient high-yielding rice varieties for flood-prone regions. Here we review the state of our knowledge of flooding tolerance in rice and its application in varietal improvement.

Carbohydrate reserve and aerenchyma formation enhance submergence tolerance in rice

An attempt was made to evaluate the submergence tolerance in rice using four Aman rice varieties viz. FR13A (local flood resistant, check), BRRI dhan 51 (HYV flood tolerant, check), BR 5 (HYV, susceptible) and a local aromatic rice,Ukunimadhu (local susceptible). Twenty days old seedlings of the rice varieties raised in earthen pots were submerged under 90 cm water depth in a submergence tank for 12 days and data were recorded on plant height, tiller number, carbohydrate reserve and internal anatomical structure of roots of the submerged and control (ambient) plants. Under submergence treatment Ukunimadhu showed rapid stem elongation with taller plants than the HYVs which showed shorter plant height. Tillering pattern was almost similar both in submergence treated and control plants in all the varieties. Variation in carbohydrate reserve was negligible among the varieties at ambient condition but the submergence treatment brought significant variation in carbohydrate content in them. Submergence treated FR13A and BRRI dhan 51 had the highest sugar and starch conservation ability while BR 5 and Ukunimadhu had the lowest. The higher reserved carbohydrate in the tolerant varieties might assist them to regenerate their normal growth rapidly after desubmergence. The cellular morphology of root tissue showed that intensity of aerenchyma development in submergence treated FR13A and BRRI dhan 51 was more pronounced compared to the local aromatic rice Ukunimadhu and perhaps this cellular large aerenchymatous development might be associated with the submergence tolerance of FR13A and BRRI dhan 51 than the local variety, Ukunimadhu.Progressive Agriculture 27 (3): 256-264, 2016

Use in animal feed forage sorghum

The sorghum crop plays a great importance in the production system of the Brazilian cattle industry, because it has resistance to drought, rapid growth and issuance of tillers. It is a crop with high photosynthetic rates and rapid stem elongation, and is used for making silage and rotational grazing. This culture has a high capacity to produce forage for use in periods of low production or grazing of native pastures. Sorghum silage has a high quality capable of meeting the nutritional needs of animals.

Molecular Events Underlying Coordinated Hormone Action in Submergence Escape Response of Deepwater Rice

Recent studies revealed that some rice varieties adopt opposite strategies to overcome flooding stress. While certain varieties hold metabolism and stay stunted until floodwater recedes, deepwater rice varieties undergo rapid stem elongation and do not suffer drowning problems. Both varieties use the same signaling agents, the ethylene response factors, as key factors even though they display opposite submergence responses. In deepwater rice, ethylene response factor genes SNORKEL1 and SNORKEL2 are believed to play a major role in submergence escape by mediating ethylene signaling, which leads to rapid stem elongation. These genes connect hormone signaling cascades from ethylene to ABA and gibberellins (GAs). Submergence increases ethylene levels in the internodal space, ethylene upregulates an ABA inactivating enzyme gene, OsCYP707A5 or OsABA8ox1, and some GA metabolism genes such as OsGA20ox genes and OsGA3ox genes. As a result of gene regulation by ethylene, internodal ABA levels decrease while GA levels increase, finally upregulating growth-related genes like expansin genes (OsEXPs). Along with the ethylene signaling in submergence, it is necessary to consider an alternative signaling pathway induced by hypoxia. Taken together, study on the submergence responses of rice plants will lead to improvement of crop production and contribution to basic research on plant growth.

Topophysic effects differ between node and organogenic cultures of the eucalypt Corymbia torelliana × C. citriodora

The eucalypt Corymbia torelliana × C. citriodora is planted widely in India, Brazil and Australia although plantation establishment has been limited by inadequate seed supply and low amenability to propagation via cuttings. This study optimised node culture and organogenic culture methods for in vitro propagation of Corymbia hybrids by identifying explant position (topophysic) effects on rooting, shoot elongation and shoot proliferation. Strong, negative morphogenic gradients in shoot elongation and proliferation capacity were evident from the cotyledonary node to the fourth or fifth node of seedlings when their nodes were transferred to node culture (without benzyladenine). These topophysic effects were related to differences in rooting capacity of individual nodes. Root formation in node culture was associated with formation of long multi-nodal axillary shoots, and so higher rooting of shoots from the cotyledonary node or first true-leaf node was associated with higher shoot proliferation. However, all nodes were equally capable of shoot proliferation in organogenic culture (with 2.2 μM benzyladenine), where rooting and rapid stem elongation did not occur. Most shoots (61–100%) from both node culture and organogenic culture were converted to plantlets, with plantlet conversion and primary root number not differing significantly among explant node positions. The strong topophysic effect in node culture, combined with the lack of a topophysic effect in organogenic culture, provides for an optimised clonal propagation system based on segregation of nodes from the same seedling into separate node and organogenic culture pathways.

Maize grain yield responses to plant height variability resulting from crop rotation and tillage system in a long-term experiment

Research emphasizing slower plant growth and delayed maturity in continuous maize ( Zea mays L.), no-till (MM–NT) systems has often led to the conclusion that lower grain yields in this environment are associated with reduced plant heights. Yet prior research has shown that early-season and mature plants are not always shorter in MM–NT systems, suggesting that overall plant height may not be an accurate morphometric indicator of decreased yield in MM–NT environments. Given that plant-to-plant morpho-physiological uniformity is strongly associated with higher yield in maize, we hypothesized that greater plant height variability would provide a better agronomic explanation for yield loss in MM–NT environments than overall plant height reductions. This 14-year study primarily examined the effects of crop rotation {maize–soybean [ Glycine max (L.) Merr.] and continuous maize} and tillage system (no-till and moldboard plow) on the yield, 4-week plant population, and 4- and 8-week plant height and plant height variability of a single maize cultivar. Due to sizeable year-to-year variation, actual crop response means for the MM–NT; maize–soybean, no-till (MB–NT); and continuous maize, moldboard plow (MM–PL) treatment combinations were expressed relative to the accompanying means for the maize–soybean, moldboard plow (MB–PL) treatment. In numerous years, the MM–NT system exhibited reduced actual and relative yields and lower 4- and 8-week plant heights compared to the other treatment combinations. Both actual and relative 4- and 8-week plant height variability were rarely greatest for the MM–NT treatment, and in only a few years were actual and/or relative plant density lowest for this system. However, single-factor regression analyses between relative yield and the aforementioned relative agronomic measures revealed that a decline in relative MM–NT yield was most strongly associated with an increase in relative 4-week plant height variability. Multi-factor regression analyses between relative yield, relative 4-week plant height variability, and various weather parameters suggested that this strong inverse relationship was potentially a manifestation of (i) non-uniform germination, emergence, and early seedling growth and (ii) later-season intra-specific competition. Regression analyses between relative 4-week plant height variability and various weather parameters suggested that phenomenon (i) was potentially promoted by cool and moist or warm and dry pre-plant weather conditions while phenomenon (ii) was possibly encouraged by low precipitation and/or high temperatures during rapid stem elongation. While MM–NT systems should be managed to limit plant density reductions and minimize growth and developmental delays, increased focus should be placed on minimizing the occurrence of plant-to-plant variability in these environments.

Stem and leaf growth of alpine sun and prairie shade ecotypes of Stellaria longipes under different photoperiods: role of ethylene

Plant ecotypes of Stellaria longipes Goldie from competitive, shade-adapted prairie habitat and less competitive, nonshaded alpine habitat were subjected to shortened or extended photoperiod conditions. Increasing daylength was positively correlated to increased stem elongation in both ecotypes. Leaf length and area for shade (prairie) plants was significantly altered with increased photoperiods, whereas sun (alpine) plants exhibited minimal leaf expansion in response to increased photoperiod. Increased ethylene evolution in the alpine genotype during rapid stem elongation and extended photoperiods suggests that ethylene plays a growth regulatory role in this sun-adapted genotype. The prairie genotype evolved less ethylene during these same periods, indicating either a diminished requirement for elevated ethylene to effect elongation and leaf expansion responses or possibly increased ethylene sensitivity because of interactions with other hormones, such as gibberellin or auxin. The sun genotype consistently produced more ethylene than the shade genotype under all photoperiod treatments. We conclude that photoperiod alters stem elongation and leaf expansion responses; similar trends were observed for extended photoperiods as were observed for shaded conditions, specifically low light intensity (photosynthetically active radiation) conditions. Further, ethylene levels altered during these responses, especially in sun-adapted alpine plants, which suggests that ethylene is involved in these growth responses.

Stilt Root Structure in the Neotropical Palms Iriartea deltoidea and Socratea exorrhiza1

ABSTRACTMany arborescent palms develop a stilt root cone that provides increased mechanical support on steep hillsides, better root aeration under waterlogged conditions, resprouting after mechanical damage, and rapid stem elongation. However, for most species the role of stilt roots is not well understood. We determined how palm size and slope inclination affected the structure of stilt roots in the neotropical palms Iriartea deltoidea and Socratea exorrhiza. We expected palm height to be lower on steep slopes because the effectiveness of root support could decrease as slope inclination increases, and thus, the structure of the root cone would vary mostly with slope. Alternatively, if stilt root development is determined by palm size, their production should match stem height, with taller palms having larger cones. We measured the stilt root cone of 31 Iriartea and 36 Socratea palms in San Ramón Biological Reserve and Golfito Wildlife Refuge, Costa Rica. We divided the cone into five variables (horizontal projections of the cone base and stilt root height up and down the slope, and width of the cone base), from which we extracted the first two principal components and used them to measure the effects of slope and palm size on stilt root structure. We found stilt roots to be determined by palm size, not by slope conditions. Stilt roots matched palm size, with larger palms having taller and larger cones, maintaining stability under diverse slope conditions.

Latitudinal Adaptation of Switchgrass Populations

Switchgrass (Panicum virgatum L.) is a widely adapted warm‐season perennial that has considerable potential as a biofuel crop. Evolutionary processes and environmental factors have combined to create considerable ecotypic differentiation in switchgrass. The objective of this study was to determine the nature of population × location interaction for switchgrass, quantifying potential differences in latitudinal adaptation of switchgrass populations. Twenty populations were evaluated for biofuel and agronomic traits for 2 yr at five locations ranging from 36 to 46° N lat. Biomass yield, survival, and plant height had considerable population × location interaction, much of which (53–65%) could be attributed to the linear effect of latitude and to germplasm groups (Northern Upland, Southern Upland, Northern Lowland, and Southern Lowland). Differences among populations were consistent across locations for maturity, dry matter, and lodging. Increasingly later maturity and the more rapid stem elongation rate of more southern‐origin ecotypes (mainly lowland cytotypes) resulted in high biomass yield potential, reduced dry matter concentration, and longer retention of photosynthetically active tissue at more southern locations. Conversely, increasing cold tolerance of more northern‐origin ecotypes (mainly upland cytotypes) resulted in higher survival, stand longevity, and sustained biomass yields at more northern locations, allowing switchgrass to thrive at cold, northern latitudes. Although cytotype explained much of the variation among populations and the population × location interaction, ecotypic differentiation within cytotypes accounted for considerable variation in adaption of switchgrass populations.

The effect of nitrogen fertilization on nitrogen use efficiency of irrigated and non-irrigated tobacco (Nicotiana tabacum L.).

Burley tobacco (Nicotiana tabacum L.) plants were grown in the field with or without irrigation and fertilized with 0, 120, 240 or 360 kg N ha−1 over two growing seasons to assess nitrogen use under Mediterranean climate conditions. Kjeldahl-N and NO3-N in leaves and stems and NO3-N and NH4-N in the soil at two depths (0–0.3 and 0.3–0.6 m) were determined. The effect of N fertilization on total N accumulated in the canopy biomass was markedly different between irrigated and non-irrigated plants. Under non-irrigated conditions N accumulated in the plant did not depend on the amount of N applied. In both years, the amount of N in irrigated plants increased in response to the amount of N applied, starting from 49 to 56 days after transplanting (DAT). The average amount of total N in the canopy of irrigated plants, measured across all sampling dates of both years, ranged from 30 kg ha−1 of the unfertilized control to 88 kg ha−1 of the 360 kg ha−1 of N applied. The average amount of plant NO3-N was 2.6 and 4.4 kg ha−1 for non-irrigated and irrigated plots across all N treatments (means of 1996 and 1997). Nitrogen uptake rate (NUR) of non-irrigated plants was high between seedling establishment and the period of rapid stem elongation in 1996 (from 36 to 50 DAT) and until flowering in 1997 (from 42 to 71 DAT), but much less or negligible at later stages of plant development. Irrigation increased NUR dramatically in the second part of the growing season. Maximum NUR was estimated for plants receiving 240 or 360 kg N ha−1 in both years. The year of study did not affect the recovery fraction (RF), physiological efficiency (PE) or agronomic efficiency (AE). Irrigation and N fertilization had significant effects on both RF and AE, but not on PE. Maximum values of RF were 45 and 22% for irrigated and non-irrigated treatments, respectively. In irrigated plots there was a negative relationship between RF and increasing N levels at all sampling dates.

Effect of light quality and 5‐azacytidine on genomic methylation and stem elongation in two ecotypes of Stellaria longipes

Changes in cytosine methylation are known to occur in response to various environmental stimuli, therefore, we looked at methylation changes in relation to stem elongation. More specifically, we investigated the response of genomic cytosine methylation to irradiance‐mediated plasticity of stem elongation in two ecotypes of Stellaria longipes. Ramets of S. longipes were grown under high and low ratios of red/far‐red light (F/FR; 3.7 and 0.7, respectively). Stem elongation and methylated cytosine content were measured over a period of 7 days. Ramets of S. longipes demonstrated the highest level of demethylation after 4 days of long‐day warm (LDW) treatment, which coincides with the first day of rapid stem elongation initiation. The extent of demethylation associated with day 4 depended upon the relative ratio of R/FR light. In particular, those plants treated with low R/FR light ratios showed a lower level of methylation, and were taller than the high R/FR light grown counterparts. In addition, prairie ecotype plants demonstrated lower day 4 methylation levels, as well as longer day 7 stem lengths, than the alpine ecotype plants within the same R/FR light treatments. To investigate if the degree of methylation was a crucial factor in controlling the stem elongation response, ramets of both alpine and prairie plants were grown in MS media supplemented with 5‐azacytidine (5‐AzaC), and grown for 14 days under a R/FR ratio of 3.7 and two different PAR values. 5‐AzaC treatments demonstrated that the prairie ecotype plants required greater doses of 5‐AzaC, and thus lower methylation levels, than the alpine ecotype plants in order to promote maximal stem elongation. These observations suggest that DNA demethylation is involved in the shade‐avoidance response.

Responsiveness of Three Tayota (Sechium edule) Types to Gibberellic Acid Treatment

Container experiments were conducted in the Dominican Republic to determine the effects of gibberellic acid applications on the `White', `Green', and `Spiny Green' tayota (Sechium edule) Dominican types. Young plants (25 cm in length) of the three types were sprayed with aqueous solutions of gibberellic acid at rates 0, 50, 100, 150, and 200 parts per million. A completely randomized design with seven replications was utilized. Plant height and shoot dry weight were determined 30 days after treatment and resulting data was subjected to analysis of variance and regression analysis. Gibberellic acid treatment significantly increased tayota shoot dry weight and stem elongation. Both variables responded with similar patterns to gibberellic acid application. A linear regression (Y = 39.82 + 0.214 X, r2 0.95) described stem elongation as a function of gibberellic acid rates. Results show that gibberellic acid can be effectively used in either tayota type to promote early stem elongation. From a practical standpoint, due to the climbing nature of Sechium edulis, rapid stem elongation could be instrumental in a more rapid establishment of the crop vines in the trellis system, as well as to provide the crop with an advantage on weeds competing for light during the usually critical early growth stages.