Individual leaf development in Arabidopsis thaliana: a stable thermal-time-based programme

In crop species, the impact of temperature on plant development is classically modelled using thermal time. We examined whether this method could be used in a non-crop species, Arabidopsis thaliana, to analyse the response to temperature of leaf initiation rate and of the development of two leaves of the rosette. The results confirmed the large plant-to-plant variability in the studied isogenic line of the Columbia ecotype: 100-fold differences in leaf area among plants sown on the same date were commonly observed at a given date. These differences disappeared in mature leaves, suggesting that they were due to a variability in plant developmental stage. The whole population could therefore be represented by any group of synchronous plants labelled at the two-leaf stage and followed during their development. Leaf initiation rate, duration of leaf expansion and maximal relative leaf expansion rate varied considerably among experiments performed at different temperatures (from 6 to 26 degrees C) but they were linearly related to temperature in the range 6-26 degrees C, with a common x-intercept of 3 degrees C. Expressing time in thermal time with a threshold temperature of 3 degrees C unified the time courses of leaf initiation and of individual leaf development for plants grown at different temperatures and experimental conditions. The two leaves studied (leaf 2 and leaf 6) had a two-phase development, with an exponential phase followed by a phase with decreasing relative elongation rate. Both phases had constant durations for a given leaf position if expressed in thermal time. Changes in temperature caused changes in both the rate of development and in the expansion rate which mutually compensated such that they had no consequence on leaf area at a given thermal time. The resulting model of leaf development was applied to ten experiments carried out in a glasshouse or in a growth chamber, with plants grown in soil or hydroponically. Because it predicts accurately the stage of development and the relative expansion rate of any leaf of the rosette, this model facilitates precise planning of sampling procedures and the comparison of treatments in growth analyses.     

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.     

The ligules of green leaves of cock's-foot grass, Dactylis glomerata L., are a micro-habitat for bacteria and fungi

Microbial populations on the ligule and leaf blade of cock's-foot grass, Dactylis glomerata L., were examined by three different methods. The ligule was identified as an important microbial niche for both bacteria and fungi. Microbial populations on the ligule increased after emergence and declined again as it approached senescence. The area of leaf blade adjacent to the ligule also had a rich microbial flora. This contrasts with the remainder of the leaf blade which is sparsely populated. However, the tip can also support a rich microbial population.     

The prediction of leaf canopy expansion in the leek from a simple model dependent on primordial development

In an analysis of leaf development of leek plants grown in the field in 1988, successive leaves initiated, appeared (tip and ligule) and senesced at equal intervals of accumulated temperature/thermal time. These intervals corresponded to a plastochron of 92°C days and phyllochrons of 135 (tip) and 233 (ligule) °C days. The rate of appearance of ligules was exactly equal to the rate of leaf senescence, with the result that the number of fully-expanded leaves per plant remained constant at 1.4. These data, which were compatible with results from previous seasons, were used to develop a model of the interrelationships between primordium initiation at the shoot apex and subsequent events in the development of individual leaves. Primordium initiation is considered to be the primary controlling event in the life of a leaf, and the processes of tip appearance, ligule appearance and death can be predicted from knowledge of the number of primordia which have been initiated, without reference to the environment. A model of canopy expansion, based on the central role of the shoot apex, was developed using the temperature relations of primordium initiation and additional data on leaf expansion and leaf dimensions. Leaf area indices computed in this way provided a satisfactory simulation of the thermal-time course of leaf area index observed in a previous season, 1985.     

Response of Fructan to Water Deficit in Growing Leaves of Tall Fescue

Changes in dry matter and water-soluble carbohydrate components, especially fructan, were examined in the basal 25 mm of expanding leaf blades of tall fescue (Festuca arundinacea Schreb.) to assess their roles in plant response to water deficit. Water was withheld from vegetative plants grown in soil in controlled-environment chambers. As stress progressed, leaf elongation rate decreased sooner in the light period than it did in the dark period. The decrease in growth rate in the dark period was associated with a decrease in local relative elongation rates and a shortening of the elongation zone from about 25 mm (control) to 15 mm. Dry matter content of the leaf base increased 23% during stress, due mainly to increased water-soluble carbohydrate near the ligule and to increased water-soluble, carbohydrate-free dry matter at distal positions. Sucrose content increased 258% in the leaf base, but especially (over 4-fold) within 10 mm of the ligule. Hexose content increased 187% in the leaf base. Content of total fructan decreased to 69% of control, mostly in regions farther from the ligule. Fructan hydrolysis could account for the hexose accumulated. Stress caused the osmotic potential to decrease throughout the leaf base, but more toward the ligule. With stress there was 70% less direct contribution of low-degree-of-polymerization fructan to osmotic potential in the leaf base, but that for sucrose and hexose increased 96 and 67%, respectively. Thus, fructan metabolism is involved but fructan itself contributes only indirectly to osmotic adjustment.     

CHARACTERIZATION OF DEVELOPMENT IN MAIZE THROUGH THE USE OF MUTANTS. II. THE ABNORMAL GROWTH CONDITIONED BY THE KNOTTED MUTANT

The maize mutant Knotted (Kn) is characterized by hollow, finger-like outgrowths (knots) occurring mainly in the leaf blade. Portions of the ligule are displaced from the normal position to more distal locations within the blade. Knots apparently result from continued meristematic activity of isolated patches of cells surrounded by maturing tissue. Small knots appear to be centers of cell division. Epidermal cells overlying a small knot have been observed to undergo periclinal divisions. In addition to cell division, a reorientation of the axis of cell elongation is associated with knot formation. The pattern of knot distribution varies at different levels on the plant axis and within a leaf blade. From leaf 4 to leaf 10 or 11 the number of knots per leaf increases progressively, then declines in leaves initiated later. Knots always occur in association with lateral veins. The greatest number per vein occurs on the 3rd or 4th vein from the midrib. One plant developing from an X-rayed heterozygous seed possessed a sector of normal tissue bisecting the plant in a vertical plane passing through the midrib of each leaf except the top two. The normal sector was knot-free and had the ligule restored to the normal position. These observations suggest that cells with the characteristics of those from intercalary meristems occur throughout the blade in Knotted plants.     

Secondary cell wall deposition causes radial growth of fibre cells in the maturation zone of elongating tall fescue leaf blades

A gradient of development consisting of successive zones of cell division, cell elongation and cell maturation occurs along the longitudinal axis of elongating leaf blades of tall fescue (Festuca arundinacea Schreb.), a C3 grass. An increase in specific leaf weight (SLW; dry weight per unit leaf area) in the maturation region has been hypothesized to result from deposition of secondary cell walls in structural tissues. Our objective was to measure the transverse cell wall area (CWA) associated with the increase in SLW, which occurs following the cessation of leaf blade elongation at about 25 mm distal to the ligule. Digital image analysis of transverse sections at 5, 15, 45, 75 and 105 mm distal to the ligule was used to determine cell number, cell area and protoplast area of structural tissues, namely fibre bundles, mestome sheaths and xylem vessel elements, along the developmental gradient. Cell diameter, protoplast diameter and area, and cell wall thickness and area of fibre bundle cells were calculated from these data. CWA of structural tissues increased in sections up to 75 mm distal to the ligule, confirming the role of cell wall deposition in the increase in SLW (r2 = 0.924; P < or = 0.01). However, protoplast diameter of fibre cells did not decrease significantly as CWA increased, although mean thickness of fibre cell walls increased by 95 % between 15 and 105 mm distal to the ligule. Therefore, secondary cell wall deposition in fibre bundles of tall fescue leaf blades resulted in continued radial expansion of fibre cells rather than in a decrease in protoplast diameter.     

Effect of the Duration of the Vegetative Phase on Shoot Growth, Development and Yield in Pearl Millet (Pennisetum americanum (L.) Leeke)

Craufurd, P. Q. and Bidinger, F. R. 1988. Effect of the durationof the vegetative phase on shoot growth, development and yieldin pearl millet (Pennisetum americanum (L.) Leeke).–J.exp. Bot. 38: 124–139 The duration of the vegetative phase (DVP) in millet, whichis the major cause of variation in the crop duration, has markedeffects on the number of productive tillers per plant and onmainshoot (MS) and tiller grain yield. Daylength extensionswere used to vary the DVP and the effect on factors affectingpanicle (tiller) number per plant and panicle yield examinedin millet hybrid 841A x J104, grown in the field at Hyderabad,India. Tiller appearance, shoot leaf appearance and leaf area,and stem and panicle growth, in both MS and primary tillers(PTs), were monitored at frequent intervals over the season.At maturity grain yield per shoot was measured The concept of thermal time was used to describe shoot development.The rates of tiller appearance and shoot leaf appearance werelinearly related to thermal time and were not affected by DVPtreatments. The duration of the growth phase from panicle initiationto flowering (GS2) and from flowering to maturity (GS3) was320 and 390 degree days (°Cd), respectively. There was nodifference in rates of leaf or tiller appearance or developmentbetween MS and PTs. Tiller appearance, tiller leaf appearanceand tiller apical development all ceased at the same time inthe later initiated PTs, approximately 550 °Cd from sowing,shortly after rapid stem growth had begun. Tillers that didnot survive were all vegetative or in the early stages of reproductivedevelopment at this time The rate of accumulation of dry matter per plant was similarin all DVP treatments, but in the longer DVP treatments a greaterproportion of the dry matter was partitioned to the MS. Mainshootstem and panicle growth rates were increased by a longer DVP,as was grain yield on the MS, and these were related to increasedMS leaf area. Concurrently, growth rates and yields in laterinitiated tillers were reduced in relation to their leaf areas.Stem growth rate was proportionately increased more than paniclegrowth rate in the longer DVP treatments and this, combinedwith a longer duration of stem growth, resulted in greater stemdry matter at maturity and, therefore, in reduced harvest index.     

中华水韭叶舌和缘膜的发生及其发育进程研究

采用石蜡切片技术,以人工培养的中华水韭幼苗的最初几枚叶至成熟植株的叶为实验材料,连续解剖观察其叶舌和缘膜的发生、发育进程,并分析其发育进程与孢子囊和叶片的关系.结果显示:(1)中华水韭叶舌与叶片在其个体发育早期来自于同一原基,但叶舌最初的发育速度快于叶片.(2)中华水韭的苗龄达到15枚叶时开始有孢子囊发生,此时的叶舌下方有明显的缘膜结构.(3)当中华水韭的孢子体达到30枚叶片以上时,早期产生于植株外围的孢子囊已经发育成熟,可以清楚地区分出大、小孢子囊,其中在已经成熟的大孢子叶上,叶舌相对于孢子囊的长度变短,下唇萎缩,缘膜消失;成熟小孢子叶的叶舌比大孢子叶的叶舌长,上翻程度大,下唇萎缩程度不如大孢子叶明显,缘膜也退化消失.研究认为,缘膜是水韭系统发育早期的普遍结构,而演化后期一些地区的缘膜则显著退化甚至消失;对于系统发育初期的中华水韭,其叶舌与叶片的差异并不像现代水韭那么明显.     

Co-ordination between leaf initiation and leaf appearance in field-grown maize (Zea mays): genotypic differences in response of rates to temperature

BACKGROUND AND AIMS: In maize (Zea mays), early flowering date, which is a valuable trait for several cropping systems, is associated with the number of leaves per plant and the leaf appearance rate. Final leaf number depends upon the rate and duration of leaf initiation. The aims of this study were to analyse the genotypic variation in the response to temperature of leaf appearance rate and leaf initiation rate, and to investigate the co-ordination between these processes under field conditions. METHODS: Sixteen hybrids of different origins were grown under six contrasting environmental conditions. The number of appeared leaves was measured twice a week to estimate leaf appearance rate (leaves d(-1)). Plants were dissected at four sampling dates to determine the number of initiated leaves and estimate leaf initiation rate (leaves d(-1)). A co-ordination model was fitted between the number of initiated leaves and the number of appeared leaves. This model was validated using two independent data sets. KEY RESULTS: Significant (P < 0.05) differences were found among hybrids in the response to temperature of leaf initiation rate (plastochron) and leaf appearance rate (phyllochron). Plastochron ranged between 24.3 and 36.4 degree days (degrees Cd), with a base temperature (Tb) between 4.0 and 8.2 degrees C. Phyllochron ranged between 48.6 and 65.5 degrees Cd, with a Tb between 2.9 and 5.0 degrees C. A single co-ordination model was fitted between the two processes for all hybrids and environments (r2= 0.96, P < 0.0001), and was successfully validated (coefficient of variation < 9 %). CONCLUSIONS: This work has established the existence of genotypic variability in leaf initiation rate and leaf appearance rate in response to temperature, which is a promising result for maize breeding; and the interdependence between these processes from seedling emergence up to floral initiation.     

EFFECTS OF RED AND FAR-RED RADIATION ON CELL DIVISION AND ELONGATION IN THE STEM OF PINTO BEAN SEEDLINGS

Nine-day-old Pinto bean seedlings, Phaseolus vulgaris L., were treated daily for 12 days with supplementary red or far-red radiation following daily exposure to 8 hr of daylight. Stem elongation was nearly exponential from the 4th to the 12th day of treatment and was about 2.9 times greater under far-red radiation. Cell division and cell elongation were promoted essentially equally by far-red radiation. There was virtually no difference in either the rate of leaf initiation or the duration of growth of internodes under the supplementary radiation.     

Spatial distribution of growth rates and of epidermal cell lengths in the elongation zone during leaf development in Lolium perenne L.

Relative elemental growth rates (REGR) and lengths of epidermal cells along the elongation zone of Lolium perenne L. leaves were determined at four developmental stages ranging from shortly after emergence of the leaf tip to shortly before cessation of leaf growth. Plants were grown at constant light and temperature. At all developmental stages the length of epidermal cells in the elongation zone of both the blade and sheath increased from 12 m at the leaf base to about 550 m at the distal end of the elongation zone, whereas the length of epidermal cells within the joint region only increased from 12 to 40 m. Throughout the developmental stages elongation was confined to the basal 20 to 30 mm of the leaf with maximum REGR occurring near the center of the elongation zone. Leaf elongation rate (LER) and the spatial distributions of REGR and epidermal cell lengths were steady to a first approximation between emergence of the leaf tip and transition from blade to sheath growth. Elongation of epidermal cells in the sheath started immediately after the onset of elongation of the most proximal blade epidermal cells. During transition from blade to sheath growth the length of the blade and sheath portion of the elongation zone decreased and increased, respectively, with the total length of the elongation zone and the spatial distribution of REGR staying near constant, with exception of the joint region which elongated little during displacement through the elongation zone. Leaf elongation rate decreased rapidly during the phase when only the sheath was growing. This was associated with decreasing REGR and only a small decrease in the length of the elongation zone. Data on the spatial distributions of growth rates and of epidermal cell lengths during blade elongation were used to derive the temporal pattern of epidermal cell elongation. These data demonstrate that the elongation rate of an epidermal cell increased for days and that cessation of epidermal cell elongation was an abrupt event with cell elongation rate declining from maximum to zero within less than 10 h.Abbreviations LER leaf elongation rate - REGR relative elemental growth rates     

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.     

Leaf area establishment of a maize (Zea Mays L.) field crop under potassium deficiency

The effect of K deficiency on leaf area index (LAI) establishment of a maize field crop (Zea Mays L.) was studied. The experimental work was carried out in 2000 and 2001 on a long-term K fertilization trial. Three K fertilization regimes (K0, K1 and K4) have been applied since 1995, thus leading to contrasted levels of available K in soils (14, 23 and 44 µg exchangeable K per g of dry soil for the three fertilization regimes, respectively). The rate of leaf appearance, the leaf elongation rate (LER), the leaf elongation duration (LED), their final length and width and the number of senescent leaves were investigated. K concentrations in shoot tissue water were lower in K0 plants, whereas concentrations of Ca and Mg were higher. The LAI was reduced in the K0 treatment, mainly because of a slower rate of leaf appearance and a reduced final size of individual leaves. The reduced final length of individual leaves was almost entirely accounted for by a reduced LER during the quasi linear elongation phase. The LED was only slightly affected. A rough parallelism was observed between the relative reduction of leaf length and the relative reduction of plant water content during leaf elongation. Conversely, there was no evidence that leaf elongation was limited by carbohydrate availability in leaf growing zones. This suggests that K deficiency reduced LER probably because of altered plant-water relationships. On the whole, these results strengthen the idea that leaf growth is a key variable for analyzing, and later on modeling, crop growth under K deficiency.     

Leaf Senescence Induced by Mild Water Deficit Follows the Same Sequence of Macroscopic, Biochemical, and Molecular Events as Monocarpic Senescence in Pea

We have compared the time course of leaf senescence in pea (Pisum sativum L. cv Messire) plants subjected to a mild water deficit to that of monocarpic senescence in leaves of three different ages in well-watered plants and to that of plants in which leaf senescence was delayed by flower excision. The mild water deficit (with photosynthesis rate maintained at appreciable levels) sped up senescence by 15 d (200 degrees Cd), whereas flower excision delayed it by 17 d (270 degrees Cd) compared with leaves of the same age in well-watered plants. The range of life spans in leaves of different ages in control plants was 25 d (340 degrees Cd). In all cases, the first detected event was an increase in the mRNA encoding a cysteine-proteinase homologous to Arabidopsis SAG2. This happened while the photosynthesis rate and the chlorophyll and protein contents were still high. The 2-fold variability in life span of the studied leaves was closely linked to the duration from leaf unfolding to the beginning of accumulation of this mRNA. In contrast, the duration of the subsequent phases was essentially conserved in all studied cases, except in plants with excised flowers, where the degradation processes were slower. These results suggest that senescence in water-deficient plants was triggered by an early signal occurring while leaf photosynthesis was still active, followed by a program similar to that of monocarpic senescence. They also suggest that reproductive development plays a crucial role in the triggering of senescence.     

The effect of temperature on leaf appearance and canopy establishment in fibre hemp (Cannabis sativa L.)

The effects of temperature on the development and growth of hemp (Cannabis sativa L.) have never been quantified. Therefore, to establish the effect of temperature on leaf appearance and canopy establishment of fibre hemp under controlled and field conditions, plants were grown in growth chambers at 11 regimes with average temperatures between 10°C and 28°C, and three cultivars were sown in the field in March, April and May in 1990, 1991 and 1992. In the field, thermal time (base 0°C) between sowing and emergence ranged from 68°Cd to 109.5°Cd (average 88.3°Cd). Rates of leaf appearance and stem elongation increased linearly with temperature between 10°C and 28°C. The base temperature for leaf appearance was 5.7°C from the growth chamber experiments and 1°C from the field experiments. In the field, the base temperature for the relationship between light interception by the canopy and thermal time was 2.5°C, and thermal time, calculated at the appropriate base temperature, accounted for about 98% of the variance in the number of leaves and for 98.6% of the variance in the proportion of light intercepted by the canopy. Days from emergence accounted for less of the variance in both parameters than thermal time. Interception of 90% of light was attained on average at 465°Cd (base 0°C) after emergence. It is concluded that thermal time is a simple and accurate tool to describe leaf appearance and light interception in fibre hemp.     

The influence of temperature on leaf development and growth in potatoes in controlled environments

Three controlled environment experiments were conducted at different temperatures to determine the relation between temperature and leaf development and growth in the potato (cv. Maris Piper). Developmental stages are defined for the appearance and duration of leaf extension in the potato and comparisons made with other temperate zone crops. The rate of leaf appearance was linear over the temperature range (9–25°C) and above 25°C there was no further increase in the rate. The temperature coefficient for the rate of appearance of leaves was 0.032 leaves (degree days)^-1 using a base temperature of 0°C. The duration of extension of an individual leaf decreased with increase in temperature up to 25°C such that the thermal duration was constant at 170 degree days using a base temperature of 0°C for leaf positions 4–10 on the main stem. At higher leaf positions the thermal duration was similar or greater. The advantages and limitations of controlled environment work as a parallel to field experimentation are discussed.     

Dynamics of the Elongation of Internodes in Maize (Zea mays L.). Effects of Shade Treatment on Elongation Patterns

The dynamics of elongation of individual maize internodes havepreviously been characterized under standard agronomic conditions.In this paper these dynamics are compared with those under ashading treatment. Shading was applied after the tip appearanceof leaf 6, using a black net that transmitted 20% of light,without altering spectral composition. Internode length wasdetermined as a function of thermal time by measuring the verticaldisplacement of individual leaf collars. Shading slowed plantdevelopment and caused significant reductions in internode length.The onset of the linear phase of elongation was delayed by shading,but its duration was not affected. The reduction in the linearelongation rate was almost totally responsible for the reductionin the final length of phytomers in the shade treatment. Theco-ordination between collar emergence and the onset of linearelongation remained. These results support the contention thatthe kinetics of internode elongation are controlled by the emergenceof leaf collars. Copyright 2000 Annals of Botany Company Zea mays L., internode, elongation, dynamics, model, shade, light, phytomer, stem, thermal time, photomorphogenesis     

Mutant characters of knotted maize leaves are determined in the innermost tissue layers

Knotted (Kn1), a dominant mutation in maize, perturbs normal leaf development. Mutant leaves have localized regions of extra growth called knots and, in addition to the normal ligule, ectopic fringes of ligule are found on the leaf blade. Previous clonal analysis showed that the epidermal genotype was immaterial in knot formation. To establish which inner leaf layer was required for formation of knots and ectopic ligule we used a closely linked albino mutation to mark X-ray-induced clonal sectors of wild type (kn) tissue in Kn1 plants. The sectors examined frequently changed in composition of layers in the leaf both transversely and longitudinally. We present results that show that both mutant characters are determined in the middle mesophyll-bundle sheath (MMBS) layer. We show that a lateral vein can produce a knot when only half the MMBS layer around the lateral vein contains the mutant gene. We also show that the ectopic ligule in Kn1 has contributions from both the adaxial epidermal and adaxial mesophyll layer.