Genome-Dependent Factors in the Development of Leaf Phototrophic Tissues in Diploid and Alloploid Wheat Species

Growth and mesostructure of the photosynthetic apparatus were studied in leaves of ten Triticum L. species. Plants with the A^u genome were shown to develop larger leaf assimilation areas due to expanding areas of individual leaves and an increase in the absolute growth rate. Leaf and mesophyll thickness and mesophyll cell size decreased in the G-genome species. Leaf compactness, which depended on cell size and number per unit leaf area and leaf folding, determined the specific patterns of internal leaf organization in wheat species with diverse genotypes. These patterns did not affect cell plastid-to-cytoplasm ratio as shown by the stable indices of cell surface area/cell volume, cell surface area per chloroplast, and cell volume per chloroplast. The structural indices of leaf phototrophic tissues, mesophyll density, and mesophyll CO₂ conductance in alloploids, as compared to diploid species, depended on both ploidy and genome constitution.     

Stimulation of Cell Elongation by Tetraploidy in Hypocotyls of Dark-Grown Arabidopsis Seedlings

Plant size is largely determined by the size of individual cells. A number of studies showed a link between ploidy and cell size in land plants, but this link remains controversial. In this study, post-germination growth, which occurs entirely by cell elongation, was examined in diploid and autotetraploid hypocotyls of Arabidopsis thaliana (L.) Heynh. Final hypocotyl length was longer in tetraploid plants than in diploid plants, particularly when seedlings were grown in the dark. The longer hypocotyl in the tetraploid seedlings developed as a result of enhanced cell elongation rather than by an increase in cell number. DNA microarray analysis showed that genes involved in the transport of cuticle precursors were downregulated in a defined region of the tetraploid hypocotyl when compared to the diploid hypocotyl. Cuticle permeability, as assessed by toluidine-blue staining, and cuticular structure, as visualized by electron microscopy, were altered in tetraploid plants. Taken together, these data indicate that promotion of cell elongation is responsible for ploidy-dependent size determination in the Arabidopsis hypocotyl, and that this process is directly or indirectly related to cuticular function.     

The decrease in growth of phosphorus-deficient maize leaves is related to a lower cell production

The spatial distribution of leaf elongation and adaxial epidermal cell production in leaf 6 of maize (Zea mays L. cv. Cecilia) plants grown in a growth chamber under two contrasting availabilities of P in the soil was investigated. Lower displacement velocities from 32.5 mm from leaf base and a shorter growth zone were found in low P (LP) leaves compared with control leaves. P deficiency significantly diminished maximum relative elemental growth rate and shifted its location closer to the leaf base. Cells were significantly longer in LP than in control leaves for all positions from the leaf base except at the end of the growth zone. For both treatments it took a similar time for a cell situated at the leaf base to reach the limit of the growth zone. The average length of the cell division zone was decreased by 21% in LP leaves. Significant differences were found in cell production and cell division rates from 12.5 mm from the leaf base although maximum values were similar between P treatments. A shorter zone of cell division with lower cell production rates along most of its length was the regulatory event that decreased cell production, and ultimately leaf elongation rates, in P‐deficient maize plants.     

Variation in Mesophyll Cell Number and Size in Wheat Leaves

The numbers of mesophyll cells in wheat leaves were determinedin a variety of wheat species differing in ploidy level andin leaves from different positions on the wheat plant. Leafsize and mesophyll cell number are linearly related in bothcases but differences were observed in mesophyll cell numberper unit leaf area with changing leaf size. Where changes incell size are caused either by nuclear ploidy or leaf position,differences in mesophyll cell number per unit leaf are negativelycorrelated with mesophyll cell plan area. The decrease in cellsize with increasing leaf position also results in a greaternumber of chloroplasts per unit leaf area. These results arediscussed in relation to anatomical variation of the wheat leaf. Mesophyll cell, cell numbers, leaf size, Triticum     

Effects of nitrogen on mesophyll cell division and epidermal cell elongation in tall fescue leaf blades

Leaf elongation rate (LER) in grasses is dependent on epidermal cell supply (number) and on rate and duration of epidermal cell elongation. Nitrogen (N) fertilization increases LER. Longitudinal sections from two genotypes of tall fescue (Festuca arundinacea Schreb.), which differ by 50% in LER, were used to quantify the effects of N on the components of epidermal cell elongation and on mesophyll cell division. Rate and duration of epidermal cell elongation were determined by using a relationship between cell length and displacement velocity derived from the continuity equation. Rate of epidermal cell elongation was exponential. Relative rates of epidermal cell elongation increased by 9% with high N, even though high N increased LER by 89%. Duration of cell elongation was approximately 20 h longer in the high- than in the low-LER genotype regardless of N treatment. The percentage of mesophyll cells in division was greater in the high- than in the low-LER genotype. This increased with high N in both genotypes, indicating that LER increased with cell supply. Division of mesophyll cells adjacent to abaxial epidermal cells continued after epidermal cell division stopped, until epidermal cells had elongated to a mean length of 40 micrometers in the high-LER and a mean length of 50 micrometers in the low-LER genotype. The cell cycle length for mesophyll cells was calculated to be 12 to 13 hours. Nitrogen increased mesophyll cell number more than epidermal cell number: in both genotypes, the final number of mesophyll cells adjacent to each abaxial epidermal cell was 10 with low N and 14 with high N. A spatial model is used to describe three cell development processes relevant to leaf growth. It illustrates the overlap of mesophyll cell division and epidermal cell elongation, and the transition from epidermal cell elongation to secondary cell wall deposition.     

Ploidy Effects in Isogenic Populations of Alfalfa : II. Photosynthesis, Chloroplast Number, Ribulose-1,5-Bisphosphate Carboxylase, Chlorophyll, and DNA in Protoplasts

Photosynthetically-active protoplasts isolated from isogenic sets of diploid-tetraploid and tetraploid-octoploid alfalfa (Medicago sativa L.) leaves were used to investigate the consequences of polyploidization on several aspects related to photosynthesis at the cellular level. Protoplasts from the tetraploid population contained twice the amount of DNA, ribulose-1,5-bisphosphate carboxylase (RuBPCase), chlorophyll (Chl), and chloroplasts per cell compared to protoplasts from the diploid population. Although protoplasts from the octoploid population contained nearly twice the number of chloroplasts and amount of Chl per cell as tetraploid protoplasts, the amount of DNA and RuBPCase per octoploid cell was only 50% higher than in protoplasts from the tetraploid population. The rate of CO₂-dependent O₂ evolution in protoplasts nearly doubled with an increase in ploidy from the diploid to tetraploid level, but increased only 67% with an increase in ploidy from the tetraploid to octoploid level. Whereas leaves and protoplasts had similar increases in RuBPCase, DNA, and Chl with increase in ploidy level, it was concluded that increased cell volume rather than increased cell number per leaf is responsible for the increase in leaf size with ploidy.     

Spatial distributions of expansion rate, cell division rate and cell size in maize leaves: a synthesis of the effects of soil water status, evaporative demand and temperature

The spatial distributions of leaf expansion rate, cell division rate and cell size was examined under contrasting soil water conditions, evaporative demands and temperatures in a series of experiments carried out in either constant or naturally fluctuating conditions. They were examined in the epidermis and all leaf tissues. (1) Meristem temperature affected relative elongation rate by a constant ratio at all positions in the leaf. If expressed per unit thermal time, the distribution of relative expansion rate was independent of temperature and was similar in all experiments with low evaporative demand and no water deficit. This provides a reference distribution, characteristic of the studied genotype, to which any distribution in stressed plants can be compared. (2) Evaporative demand and soil water deficit affected independently the distribution of relative elongation rate and had near-additive effects. For a given stress, a nearly constant difference was observed, at all positions of the leaf, between the relative elongation rates of stressed plants and those of control plants. This caused a reduction in the length of the zone with tissue elongation. (3) Methods for calculating cell division rate in the epidermis and in all leaf tissues are proposed and discussed. In control plants, the zone with cell division was 30 mm and 60 mm long in the epidermis and in whole tissues, respectively. Both this length and relative division rate were reduced by soil water deficit. The size of epidermal and of mesophyll cells was nearly unaffected in the leaf zone with both cell division and tissue expansion, suggesting that water deficit affects tissue expansion rate and cell division rate to the same extent. Conversely, cell size of epidermis and mesophyll were reduced by water deficit in mature parts of the leaf.     

Control of Leaf Growth and its Role in Determining Variation in Plant Growth Rate from an Ecological Perspective

Abstract: Plants vary widely in their relative growth rate (RGR), be it dependent on environmental conditions or due to their genetic background. In a comparison of the RGR of grasses growing under different environmental conditions, variation in RGR tends to correlate with that in the leaf elongation rate (LER). When different species or genotypes thereof are compared under identical growing conditions, variation in LER may or may not correlate with that in RGR, depending on the comparison. However, since RGR is described by an exponential equation, whereas LER is mainly a linear process, we conclude that any correlation between RGR and LER must be fortuitous. That is, exponential growth must be due to increases with time in plant traits such as 1) leaf dry mass per unit leaf length invested per unit time, and/or 2), i.e., the total LER of all the growing leaves at one point in time. The latter can be achieved as follows: 1) each subsequent leaf has a higher LER than the preceding one; 2) leaves appear at an increasing rate; 3) the duration of the process of leaf elongation increases for subsequent leaves. In this review, we only explore possible factors that account for changes in with time, in different genotypes and under different environmental conditions. Inherent variation in LER of individual leaves and variation due to environmental factors may reflect variation in the rate of cell division and/or in cell elongation.     

Regulation of leaf growth of grass by blue light

Changes in light quality occur naturally within a canopy when a plant grows from unshaded to shaded conditions, and the reverse occurs after a cut that reduces shading. These changes in light quality could be responsible for the variation in leaf elongation and appearance rates of grasses. The role of blue light in leaf growth was investigated in tall fescue (Festuca arundinacea Schreb.) and perennial ryegrass (Lolium perenne L.). Leaf length was measured daily following a decrease or an increase in blue light to evaluate effects on duration of leaf growth, leaf elongation and the rate of leaf appearance rate. A reduction in blue light increased sheath length by 8 to 14% and lamina length by 6 to 12% for both species. These increases could be reversed by enrichment of blue light. With low blue light treatment, final leaf length was increased due to a greater leaf elongation rate. In tall fescue, but not in perennial ryegrass, this effect was coupled with a greater phyllochron and a longer duration of leaf elongation. Development of successive leaves on a tall fescue tiller were co-ordinated. A decrease in blue light increased the duration of elongation in the oldest growing leaf and also delayed the appearance of a new leaf, maintaining this co-ordination. We conclude that final leaf size and phyllochron for tall fescue can be significantly modified by blue light. Perennial ryegrass appeared less responsive, except for displaying longer sheaths and laminae in low blue light, as also occurred for tall fescue. We hypothesize that leaf length could be regulated by the quality of the light reaching the growing region itself.     

A comparative analysis of the temperature response of leaf elongation in Bromus stamineus and Lolium perenne plants in the field: intrinsic and size-mediated effects

BACKGROUND AND AIMS: Growth of grass species in temperate-humid regions is restricted by low temperatures. This study analyses the origin (intrinsic or size-mediated) and mechanisms (activity of individual meristems vs. number of active meristems) of differences between Bromus stamineus and Lolium perenne in the response of leaf elongation to moderately low temperatures. METHODS: Field experiments were conducted at Balcarce, Argentina over 2 years (2003 and 2004) using four cultivars, two of B. stamineus and two of L. perenne. Leaf elongation rate (LER) per tiller and of each growing leaf, number of growing leaves and total leaf length per tiller were measured on 15-20 tillers per cultivar, for 12 (2003) or 10 weeks (2004) during autumn and winter. KEY RESULTS: LER was faster in B. stamineus than in L. perenne. In part, this was related to size-mediated effects, as total leaf length per tiller correlated with LER and B. stamineus tillers were 71% larger than L. perenne tillers. However, accounting for size effects revealed intrinsic differences between species in their temperature response. These were based on the number of leaf meristems simultaneously active and not on the (maximum) rate at which individual leaves elongated. Species differences were greater at higher temperatures, being barely notable below 5 degrees C (air temperature). CONCLUSIONS: Bromus stamineus can sustain a higher LER per tiller than L. perenne at air temperatures > 6 degrees C. In the field, this effect would be compounded with time as higher elongation rates lead to greater tiller sizes.     

Nitrogen deficiency inhibits leaf blade growth in Lolium perenne by increasing cell cycle duration and decreasing mitotic and post-mitotic growth rates

Nitrogen deficiency severely inhibits leaf growth. This response was analysed at the cellular level by growing Lolium perenne L. under 7.5 mM (high) or 1 mM (low) nitrate supply, and performing a kinematic analysis to assess the effect of nitrogen status on cell proliferation and cell growth in the leaf blade epidermis. Low nitrogen supply reduced leaf elongation rate (LER) by 43% through a similar decrease in the cell production rate and final cell length. The former was entirely because of a decreased average cell division rate (0.023 versus 0.032 h(-1)) and thus longer cell cycle duration (30 versus 22 h). Nitrogen status did not affect the number of division cycles of the initial cell's progeny (5.7), and accordingly the meristematic cell number (53). Meristematic cell length was unaffected by nitrogen deficiency, implying that the division and mitotic growth rates were equally impaired. The shorter mature cell length arose from a considerably reduced post-mitotic growth rate (0.033 versus 0.049 h(-1)). But, nitrogen stress did not affect the position where elongation stopped, and increased cell elongation duration. In conclusion, nitrogen deficiency limited leaf growth by increasing the cell cycle duration and decreasing mitotic and post-mitotic elongation rates, delaying cell maturation.     

The CURLY LEAF gene controls both division and elongation of cells during the expansion of the leaf blade in Arabidopsis thaliana

The CURLY LEAF (CLF ) gene in Arabidopsis thaliana (L.) Heynh. is required for stable repression of a floral homeotic gene, AGAMOUS in leaves and stems To clarify the function of CLF in organ development, we characterized clf mutants using an anatomical and genetic approach. The clf mutants had normal roots, hypocotyls, and cotyledons, but the foliage leaves and the stems had reduced dimensions. A decrease both in the extent of cell elongation and in the number of cells was evident in the clf mutant leaves, suggesting that the CLF gene might be involved in the division and elongation of cells during leaf morphogenesis. An analysis of the development of clf mutant leaves revealed that the period during which cell division or cell elongation occurred was of normal duration, while the rates of both cell production and cell elongation were lower than in the wild type. Two phases in the elongation of cells were also recognized from this analysis. From analysis of an angustifolia clf double mutant, we found that the two phases of elongation of leaf cells were regulated independently by each gene. Thus, the CLF gene appears to affect cell division at an earlier stage and cell elongation throughout the development of leaf primordia. Received: 19 February 1998 / Accepted: 24 March 1998     

Optimization of the Leaf Mesophyll Structure in Alloploid and Diploid Wheat Species

Comparative analysis of the indices of plant growth and mesostructure of the photosynthetic apparatus was carried out with ten wheat (Triticum L.) species of various origins. Wheat alloploid forms (tetra- and hexaploids with the chromosome numbers of 28 and 42) exceeded the diploid forms (the chromosome number of 14) 2.3–2.4-fold by their absolute growth rate (AGR). As a result, the alloploid species developed a larger assimilation area; this change involved the internal reorganization of leaf phototrophic tissues and an increase in the cumulative internal assimilation area. In addition, the alloploid species manifested a higher correlation between the surface areas of cell and chloroplast membranes caused by a decrease in the cell number per the unit leaf area, a relative increase of the number of composite multifaveolate cells, a considerable expansion (in volume and surface area) of mesophyll cells, and an increase in chloroplast size and numbers. The decreased ratio between the characteristics of the cell membrane and chloroplast envelope presumes that CO₂ diffusion via cell and chloroplast membranes in the leaves was better balanced in the alloploid wheat species than in the diploid forms. All wheat species did not notably differ in their plastid–cytoplasm ratio (cell volume corresponding to one chloroplast and cell surface area per one chloroplast) and the ratio of surface area of cells to cell volume. The discriminant analysis revealed the indices of leaf growth and mesophyll structure instrumental in distinguishing between the diploid and alloploid species: leaf area, AGR, and cell size and number. The change in the latter indices optimized the structure of leaf phototrophic tissues in tetraploid and hexaploid species; as a result, the internal assimilation area was expanded and, consequently, leaf CO₂ conductance was increased.     

Spatial and temporal analysis of non-steady elongation of rice leaves

A precise knowledge of the temporal and spatial distributions of cell division and tissue expansion is essential for appropriate leaf sampling in omics studies and for analyses of plant–environment relations. Elongating leaves of rice were studied during their whole development for elongation rate, distribution of cell length, cell production rate and spatial distribution of growth in the leaf. In seven genotypes, the pattern of leaf elongation rate followed three phases: (1) an exponential increase before leaf appearance; (2) a short phase (2–4 d at 20 °C) with a stable leaf elongation rate around leaf appearance; and (3) a phase of 8–10 d with a progressive decrease in elongation rate. The profile of cell length along the leaf changed with time during the first and last phases, but was time invariant around appearance. We propose a method adapted to non-steady elongation based on anatomical measurements, which was successfully tested by comparing it with the pricking method. It allowed analysis of the change with time in the spatial distribution of growth from initiation to end of leaf growth. The length of leaf zones with cell division and tissue elongation varied with time, with maximums of 21 and 60 mm respectively around leaf appearance.     

不同倍性大青杨的光合特性及叶片解剖结构比较

通过对二年生二倍体、三倍体和四倍体大青杨叶片结构和光合特性的研究,探讨不同倍性大青杨生长性状差异的原因。结果表明：不同倍性大青杨的净光合速率（Pn）、光饱和点（LSP）、光补偿点（LCP）等光合指标差异显著。三倍体的最大净光合速率（Pmax）高出二倍体21%。叶片厚度、表皮细胞厚度、栅栏组织厚度、栅栏组织与海绵组织厚度的比值,都是三倍体和四倍体大青杨比二倍体高,且均呈现增加的趋势。其中,四倍体大青杨表皮细胞最厚,上下表皮分别高出二倍体64%和17%。三倍体大青杨的栅栏组织厚度及栅栏组织与海绵组织厚度的比值最大,比值高出二倍体50%。通过叶片光合特性和解剖结构比较证明,三倍体和四倍体大青杨对光的适应性和光合作用能力强于二倍体。     

Effect of N deficiency on leaf growth and cell wall peroxidase activity in contrasting maize genotypes

Two maize genotypes differing in leaf elongation rate (high-LER and low-LER) were used for the investigation of the effects of nitrogen deficiency on leaf growth and development and activity of enzyme cell wall peroxidase in the leaf growth zone. Plants were grown in a growth cabinet in perlite as a substrate and watered with complete N-NO₃ solution (+N) and N-NO₃ deficient solution (–N). Comparison between the investigated genotypes showed that final leaf length in both N treatments was related with LER, but not with the duration of leaf elongation. Faster leaf elongation rate in high-LER compared with low-LER genotype, was associated with longer growth zone, a bigger number of cells in it, and higher cell flux rate, although cell elongation rate was similar in both genotypes. These lines of evidence indirectly indicated that leaves of the faster growing genotype were characterized by higher meristematic activity. Nitrogen deficiency reduced the flux of cells and cell elongation rate, length of cell division zone and the number of cells in whole zone, significantly for both genotypes, although duration of cell elongation was increased and final epidermal cell length was unchanged. These results showed that N deficiency reduced both cell division and cell elongation, which in turn resulted in decreased leaf length and prolonged time for leaf development. Nitrogen deficiency significantly increased both bulk and segmental cell wall peroxidase activity in the growth zone of both investigated genotypes, thus showing an interaction between leaf growth cessation and enzyme activity.     

Observation of Apparently Unchanging Mesophyll Cell Diameters Throughout Leaf Ontogeny in Woody Species

The expansion of plant leaves usually lasts 3–6 weeks and it is widely believed that most cell types (epidermal and mesophyll) continue to expand in unison over a similar time period. The evidence supporting this account was derived from studies of herb leaves. We observed in woody species, however, that the diameter of mesophyll cells (spongy and palisade) changed little during leaf expansion from about 5 to 100 % maximum size. To keep pace with epidermal cell enlargement and leaf area expansion, mesophyll cells divided but palisade cell length expanded as leaves grew thicker. The prolonged division of mesophyll and apparently unchanging mesophyll cell diameters constitute a novel pattern of leaf cell development, different from that previously described for herbs. Possible mechanisms that attribute the varied expansion direction and speed to the different cellulose distributions in woody and herbaceous species are suggested. This finding could contribute to an enhanced understanding of the overall mechanism of leaf development.     

Cellular Levels of Ribulose 1,5 Bisphosphate Carboxylase and Chloroplast Compartment Size in Wheat Mesophyll Cells

Pyke, K. A. and Leech, R. M. 1987. Cellular levels of ribulose1,5 bisphosphate carboxylase and chloroplast compartment sizein wheat mesophyll cells.—J. exp. Bot. 38: 1949–1956. The amount of the photosynthetic enzyme ribulose 1,5 bisphosphatecarboxylase (RUBISCO),as determined in mesophyll cells in primarywheat leaves was related to the size of the chloroplast compartmentwithin the cell for wheat species of three ploidy levels. Asimilar comparison was made for several genotypes of the hexaploidbreadwheat Triticum aestivum. Estimation of total chloroplastvolume per mesophyll cell was made assuming chloroplasts tobe oblate spheroid in shape. A significant correlation was found between the amount of RUBISCOper cell and the total chloroplast volume per cell for diploid,tetraploid and hexaploid wheat species. A significant correlationbetween cellular RUBISCO level and total chloroplast volumeper cell was also observed for a range of genotypes of the hexaploidT. aestivum but these genotypes of T. aestivutn accumulate agreater amount of RUBISCO per unit chloroplast volume than doany other wheat species. For these genotypes of T. aestivumthe stromal concentration of RUBISCO was estimated at 0·5mol m^–3 with a ribulose Msphosphate binding site concentrationof 4·0 mol m^–3. These results are discussed with respect to a gene dosage hypothesisto explain the accumulation of RUBISCO in leaf mesophyll cells. Key words: Ribulose, bisphosphate carboxylase, wheat chloroplasts, mesophyll cells     

A role for leaf epidermis in the control of leaf size and the rate and extent of mesophyll cell division

Little is known about the control of leaf size in plants, yet there must be mechanisms by which organ size is measured. Because the control of leaf size extends beyond the action of individual genes or cells, an understanding of the role of leaf cell layers in the determination of leaf size is warranted. Following the construction of graft chimeras composed of small- and large-leaf genotypes of Nicotiana, bilateral leaf blade asymmetry was observed on leaves possessing either a genetically larger or smaller epidermis on one side of the midrib. Although cell size was unaffected by the genotype of the epidermis, the rate and extent of cell division in leaf epidermis altered the rate and extent of cell division in mesophyll and affected leaf size. The data presented neither prove nor disprove whether the mesophyll impacts epidermal cell division but provide the first unequivocal evidence that the extent of cell division in the leaf epidermis alters the extent of cell division in the mesophyll and is a factor regulating blade expansion and ultimate leaf size.     

From individual leaf elongation to whole shoot leaf area expansion: a comparison of three Aegilops and two Triticum species

BACKGROUND AND AIMS: Rapid leaf area expansion is a desirable trait in the early growth stages of cereal crops grown in low-rainfall areas. In this study, the traits associated with inherent variation in early leaf area expansion rates have been investigated in two wheat species (Triticum aestivum and T. durum) and three of its wild relatives (Aegilops umbellulata, A. caudata and A. tauschii) to find out whether the Aegilops species have a faster leaf area expansion in their early developmental stage than some of the current wheat species. METHODS: Growth of individual leaves, biomass allocation, and gas exchange were measured on hydroponically grown plants for 4 weeks. KEY RESULTS: Leaf elongation rate (LER) was strongly and positively correlated with leaf width but not with leaf elongation duration (LED). The species with more rapidly elongating leaves showed a faster increase with leaf position in LER, leaf width and leaf area, higher relative leaf area expansion rates, and more biomass allocation to leaf sheaths and less to roots. No differences in leaf appearance rate were found amongst the species. CONCLUSIONS: Aegilops tauschii was the only wild species with rapid leaf expansion rates similar to those of wheat, and it achieved the highest photosynthetic rates, making it an interesting species for further study.