Duration Of Leaf Expansion Research Articles

Component traits of plant water use are modulated by vapour pressure deficit in pearl millet (Pennisetum glaucum (L.) R.Br.).

Traits influencing plant water use eventually define the fitness of genotypes for specific rainfall environments. We assessed the response of several water use traits to vapour pressure deficit (VPD) in pearl millet (Pennisetum glaucum (L.) R.Br.) genotypes known to differ in drought adaptation mechanisms: PRLT 2/89-33 (terminal drought-adapted parent), H 77/833-2 (terminal drought-sensitive parent) and four near-isogenic lines introgressed with a terminal drought tolerance quantitative trait locus (QTL) from PRLT 2/89-33 (ICMR01029, ICMR01031, ICMR02042, and ICMR02044). Plant water use traits at various levels of plant organisation were evaluated in seven experiments in plants exposed either transiently or over the long term to different VPD regimes: biomass components, transpiration (water usage per time unit) and transpiration rate (TR) upon transient VPD increase (gH2Ocm-2h-1)), transpiration efficiency (g dry biomass per kg H2O transpired), leaf expansion rate (cm per thermal time unit) and root anatomy (endodermis dimensions)). High VPD decreased biomass accumulation by reducing tillering, the leaf expansion rate and the duration of leaf expansion; decreased root endodermis cell size; and increased TR and the rate of TR increase upon gradual short-term VPD increases. Such changes may allow plants to increase their water transport capacity in a high VPD environment and are genotype-specific. Some variation in water use components was associated with terminal drought adaptation QTL. Knowledge of water use traits' plasticity in growth environments that varied in evaporative demand, and on their genetic determinacy, is necessary to develop trait-based breeding approaches to complex constraints.

Rewatering plants after a long water-deficit treatment reveals that leaf epidermal cells retain their ability to expand after the leaf has apparently reached its final size.

Leaves expand during a given period of time until they reach their final size and form, which is called determinate growth. Duration of leaf expansion is stable when expressed in thermal-time and in the absence of stress, and consequently it is often proposed that it is controlled by a robust programme at the plant scale. The usual hypothesis is that growth cessation occurs when cell expansion becomes limited by an irreversible tightening of cell wall, and that leaf size is fixed once cell expansion ceases. The objective of this paper was to test whether leaf expansion could be restored by rewatering plants after a long soil water-deficit period. Four experiments were performed on two different species (Arabidopsis thaliana and Helianthus annuus) in which the area of leaves that had apparently reached their final size was measured upon reversal of water stresses of different intensities and durations. Re-growth of leaves that had apparently reached their final size occurred in both species, and its magnitude depended only on the time elapsed from growth cessation to rewatering. Leaf area increased up to 186% in A. thaliana and up to 88% in H. annuus after rewatering, with respect to the leaves of plants that remained under water deficit. Re-growth was accounted for by cell expansion. Increase in leaf area represented actual growth and not only a reversible change due to increased turgor. After the leaf has ceased to grow, leaf cells retain their ability to expand for several days before leaf size becomes fixed. A response window was identified in both species, during which the extent of leaf area recovery decreased with time after the 'initial' leaf growth cessation. These results suggest that re-growth after rewatering of leaves having apparently attained their final size could be a generalized phenomenon, at least in dicotyledonous plants.

Simulations of virtual plants reveal a role for SERRATE in the response of leaf development to light in Arabidopsis thaliana

The SERRATE gene (SE) was shown to determine leaf organogenesis and morphogenesis patterning in Arabidopsis thaliana. The se-1 mutant was used here to investigate the role of SE in leaf development in response to incident light. Virtual plants were modelled to analyse the phenotypes induced by this mutation. Plants were grown under various levels of incident light. The amount of light absorbed by the plant was estimated by combining detailed characterizations of the radiative environment and virtual plant simulations. Four major changes in leaf development were induced by the se-1 mutation. Two constitutive leaf growth variables were modified, with a lower initial expansion rate and a higher duration of expansion. Two original responses to a reduced incident light were identified, concerning the leaf-initiation rate and the duration of leaf expansion. The se-1 mutation dramatically affects both changes in the leaf development pattern and the response to reduced incident light. Virtual plants helped to reveal the combined effects of the multiple changes induced by this mutation.

Day length affects the dynamics of leaf expansion and cellular development in Arabidopsis thaliana partially through floral transition timing.

Plant aerial development is well known to be affected by day length in terms of the timing and developmental stage of floral transition. Arabidopsis thaliana is a 'long day' plant in which the time to flower is delayed by short days and leaf number is increased. The aim of the work presented here was to determine the effects of different day lengths on individual leaf area expansion. The effect of flower emergence per se on the regulation of leaf expansion was also tested in this study. Care was taken to ensure that day length was the only source of micro-meteorological variation. The dynamics of individual leaf expansion were analysed in Ler and Col-0 plants grown under five day lengths in five independent experiments. Responses at cellular level were analysed in Ler plants grown under various day lengths and treatments to alter the onset of flowering. When the same leaf position was compared, the final leaf area and both the relative and absolute rates of leaf expansion were decreased by short days, whereas the duration of leaf expansion was increased. Epidermal cell number and cell area were also altered by day-length treatments and some of these responses could be mimicked by manipulating the date of flowering. Both the dynamics and cellular bases of leaf development are altered by differences in day length even when visible phenotypes are absent. To some extent, cell area and its response to day length are controlled by whole plant control mechanisms associated with the onset of flowering.

Plasticity to soil water deficit in Arabidopsis thaliana: dissection of leaf development into underlying growth dynamic and cellular variables reveals invisible phenotypes

Genetic variability in the plasticity of leaf area expansion in response to water deficit has been reported in Arabidopsis thaliana. Here, the objective was to identify the underlying dynamic and cellular processes involved in this variability. Twenty-five accessions were subjected to identical soil water deficit treatments. In all accessions, the plasticity of leaf production was low compared with that of individual leaf expansion. A subset of accessions was selected for further dissection of individual leaf expansion into its underlying variables: the rate and duration of leaf expansion and epidermal cell number and area. In all accessions, water deficit had opposite effects on the rate and duration of leaf expansion. The accumulation of these effects was reflected in changes in final leaf area. At the cellular level, moderate water deficits had opposite effects on cell number and cell size, but more severe ones reduced both variables. The importance of these opposing effects is highlighted by the behaviour of the accession An-1, for which the compensation between the decrease in leaf expansion rate and the increase in the duration of expansion is total. This dynamic plasticity in response to water deficit is not detectable when only final measurements are done.

Cell and leaf size plasticity in Arabidopsis: what is the role of endoreduplication?

Leaf area expansion is affected by environmental conditions because of differences in cell number and/or cell size. Increases in the DNA content (ploidy) of a cell by endoreduplication are related to its size. The aim of this work was to determine how cell ploidy interacts with the regulation of cell size and with leaf area expansion. The approach used was to grow Arabidopsis thaliana plants performing increased or decreased rounds of endoreduplication under shading and water deficit. The shading and water deficit treatments reduced final leaf area and cell number; however, cell area was increased and decreased, respectively. These differences in cell size were unrelated to alterations of the endocycle, which was reduced by these treatments. The genetic modification of the extent of endoreduplication altered leaf growth responses to shading and water deficit. An increase in the extent of endoreduplication in a leaf rendered it more sensitive to the shade treatment but less sensitive to water deficit conditions. The link between the control of whole organ and individual cell expansion under different environmental conditions was demonstrated by the correlation between the plasticity of cell size and the changes in the duration of leaf expansion.

A Dynamic Analysis of the Shade-induced Plasticity in Arabidopsis thaliana Rosette Leaf Development Reveals New Components of the Shade-adaptative Response

It is well known that plant aerial development is affected by light intensity in terms of the date of flowering, the length of stems and petioles, and the final individual leaf area. The aim of the work presented here was to analyse how shade-induced changes in leaf development occur on a dynamic basis from the whole rosette level to that of the cells. Care was taken to ensure that light intensity was the only source of micro-meteorological variation in the study. The dynamics of leaf production, rosette expansion, individual leaf area expansion and epidermal cell expansion were analysed in Arabidopsis thaliana plants grown under two light intensities in three independent experiments. The total area of rosette leaves was reduced by the shading treatment. Both the number of leaves produced and their individual leaf areas were reduced. The reduction in leaf number was associated with a reduction in leaf initiation rate and the duration of the phase of leaf production. The reduction in individual leaf area was associated with a reduction in leaf expansion rate and an increase in the duration of leaf expansion. The changes in leaf expansion dynamics were accompanied by a decrease in epidermal cell number which was partly compensated for by an increase in epidermal cell area. Overall, the whole rosette leaf expansion rate was reduced by shading, whereas the total duration of rosette leaf expansion was unaffected. This was mainly due to the accumulation of the increases in the durations of expansion of each individual leaf which was associated with an increase in cell expansion. The dynamic analysis presented here reveals a new shade-adaptative response mediated via the control of area expansion at the cell, organ and whole plant levels.

Effect of nitrogen supply on leaf appearance, leaf growth, leaf nitrogen economy and photosynthetic capacity in maize ( Zea mays L.)

Leaf area growth and nitrogen concentration per unit leaf area, N a (g m −2 N) are two options plants can use to adapt to nitrogen limitation. Previous work indicated that potato ( Solanum tuberosum L.) adapts the size of leaves to maintain N a and photosynthetic capacity per unit leaf area. This paper reports on the effect of N limitation on leaf area production and photosynthetic capacity in maize, a C4 cereal. Maize was grown in two experiments in pots in glasshouses with three (0.84–6.0 g N pot −1) and five rates (0.5–6.0 g pot −1) of N. Leaf tip and ligule appearance were monitored and final individual leaf area was determined. Changes with leaf age in leaf area, leaf N content and light-saturated photosynthetic capacity, P max, were measured on two leaves per plant in each experiment. The final area of the largest leaf and total plant leaf area differed by 16 and 29% from the lowest to highest N supply, but leaf appearance rate and the duration of leaf expansion were unaffected. The N concentration of expanding leaves (N a or %N in dry matter) differed by at least a factor 2 from the lowest to highest N supply. A hyperbolic function described the relation between P max and N a. The results confirm the ‘maize strategy’: leaf N content, photosynthetic capacity, and ultimately radiation use efficiency is more sensitive to nitrogen limitation than are leaf area expansion and light interception. The generality of the findings is discussed and it is suggested that at canopy level species showing the ‘potato strategy’ can be recognized from little effect of nitrogen supply on radiation use efficiency, while the reverse is true for species showing the ‘maize strategy’ for adaptation to N limitation.

Impact of successive drought and re-watering cycles on growth and specific leaf area of two Populus x canadensis (Moench) clones, 'Dorskamp' and 'Luisa_Avanzo'.

Responses to successive drought and re-watering cycles (1-3 cycles) were compared in greenhouse-grown cuttings of Populus x canadensis (Moench) clones, 'Luisa_ Avanzo' and 'Dorskamp.' Total leaf number increment rate, duration of leaf expansion, total and individual leaf area expansion rates and stomatal conductance were recorded periodically during the experiment. Soil water content (SWC) and predawn leaf water potential (Psi(WP)) were measured four times during each drought cycle. In parallel, relative leaf water content (RWC) and specific leaf area (SLA) were estimated on leaves collected from the top to bottom of each cutting. Under well-watered conditions, 'Luisa_Avanzo' and 'Dorskamp' differed in their patterns of leaf area expansion. Although duration of leaf expansion was similar between clones, 'Luisa_ Avanzo' exhibited higher total leaf number increment rates and individual leaf area increases than 'Dorskamp.' As a result, 'Luisa_Avanzo' cuttings reached larger individual and total leaf areas than 'Dorskamp.' 'Dorskamp' leaves had lower SLA than 'Luisa_Avanzo' leaves. In response to successive drought cycles, both clones underwent decreases in total leaf number increment rates and in total leaf area expansion rates, but both whole-plant and individual leaf areas were drastically reduced only in 'Luisa_Avanzo.' 'Dorskamp' maintained a constant leaf area as a result of an increase in the duration of leaf expansion during drought and a significant stimulation of individual leaf area expansion rate and total leaf number increment rate in response to re-watering. Drought caused a greater decrease in SLA in 'Luisa_Avanzo' than in 'Dorskamp.' Expanded leaves of 'Dorskamp' were constitutively dense or thick, or both, whereas leaves of 'Luisa_Avanzo' became dense or thick, or both, only in response to drought. In both clones, re-watering caused partial recovery of SLA to control values. Our data confirm previous field and greenhouse observations that 'Dorskamp' is more tolerant to successive drought and re-watering cycles than 'Luisa_Avanzo.' We conclude that this difference is mainly attributable to the plasticity of 'Dorskamp' after re-watering, rather than to clonal differences in drought responses.

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.

Effects of nitrogen on development and growth of the leaves of vegetables. 2. Appearance, expansion growth and life span of leaves of leek plants

In greenhouse pot experiments and field trials, leek cv. Albana plants were supplied with different amounts of N fertilizer at various growth stages. Leaf emergence, expansion, size and senescence were monitored. The rate of leaf appearance was not affected by N treatments and almost constant across experiments at 0.15/day. The rate of leaf expansion and the mature leaf area increased with leaf number, reaching maximum values between leaf numbers 11 and 14 and decreasing with higher leaf numbers. Both variables increased with increasing N application rate. The duration of leaf expansion was more or less constant across leaf numbers and not influenced by N treatments; the leaf expansion rate was the main factor determining mature leaf area. The rate of leaf senescence was not influenced by N treatments. Differences in total green leaf area per plant were caused by differences in the area of individual mature leaves and not by differences in the number of leaves. The specific leaf area of all leaves was more or less constant at 100 cmsuperscript 2/g.

Temperature Effects on Pima Cotton Leaf Growth

AbstractTemperature is a primary environmental factor affecting cotton growth. Information on temperature effects on Pima cotton (Gossypium barbadense L.) growth is limited. The objective of this paper is to provide quantitative estimates of temperature effects on leaf emergence rates, the duration of leaf expansion, and rate of expansion. Pima cotton plants were grown from seed in naturally lit controlled environments with unlimited water and nutrients under five day/night temperature regimes ranging from 20/12 to 40/32 °C. Weighted average daily temperature was calculated from 900‐s average temperatures. Carbon dioxide was maintained at 350 µL L−1 during the daylight hours. Leaf area expansion and biomass accumulation were monitored from emergence to 64 d after emergence. Leaf initiation took about 2 d longer for prefruiting nodes at near optimum temperatures than for leaves formed after square initiation. Fruiting leaf initiation required about 4.5 d leaf−1 at 20 °C and about 2.5 d leaf−1 at 30 °C. Leaves expanded in excess of three times more rapidly at 31.3 °C than at 18.9 °C. The optimum temperature for leaf expansion was 31.3 °C. Leaf growth at 35.5 °C was only approximately 60% of that at 31.3 °C. Leaves required about 40% longer to expand at 18.9 °C than at 31.3 °C. All leaves required the same length of time to expand when grown at the same temperature, regardless of position on the mainstem. Since leaves varied greatly in mature leaf size, depending on leaf position, rates of expansion also varied similarly. Maximum leaf growth rates occurred during the 28‐to‐64‐days‐after‐emergence period compared with earlier growth periods, apparently reflecting greater light interception during that period. Biomass accumulation and growth rates were very temperature sensitive and closely related to leaf growth.

SHADING EFFECTS ON GROWTH AND PARTITIONING OF PLANT BIOMASS IN PASPALUM CONJUGATUM BERG

Glasshouse studies were carried out to determine the effect of shading on the growth and partitioning of plant biomass in Paspalum conjugatum Berg. The invidual leaf rate expansion, final leaf are, specific leaf area, and the whole plant vegetative growth pattern, dry-matter production, leaf area as well as biomass partitioning were significantly influenced by shading. At the 75% level of shading, P. conjugatum produced the highest values of leaf weight ratio, specific leaf area and leaf area ratio. Individual leaf assessment revealed that shading significantly increased the final leaf area, duration of leaf expansion and specific leaf area.

Gibberellins and leaf expansion in near-isogenic wheat lines containing Rht1 and Rht3 dwarfing alleles

In near-isogenic lines of winter wheat (Triticum aestivum L. cv. Maris Huntsman) grown at 20° C under long days the reduced-height genes, Rht1 (semi-dwarf) and Rht3 (dwarf) reduced the rate of extension of leaf 2 by 12% and 52%, respectively, compared with corresponding rht (tall) lines. Lowering the growing temperature from 20° to 10° C reduced the rate of linear extension of leaf 2 by 2.5-fold (60% reduction) in the rht3 line but by only 1.6-fold (36% reduction) in the Rht3 line. For both genotypes, the duration of leaf expansion was greater at the lower temperature so that final leaf length was reduced by only 35% in the rht3 line and was similar in the Rht3 line at both temperatures. Seedlings of the rht3 (tall) line growing at 20° C responded positively to root-applied gibberellin A1 (GA1) in the range 1-10 μM GA1; there was a linear increase in sheath length of leaf 1 whereas the Rht3 (dwarf) line remained unresponsive. Gibberellins A1, 3, 4, 8, 19, 20, 29, 34, 44 and 53 were identified by full-scan gas chromatography-mass spectrometry in aseptically grown 4-d-old shoots of the Rht3 line. In 12-d-old seedlings grown at 20° C, there were fourfold and 24-fold increases in the concentration of GA1 in the leaf expansion zone of Rht1 and Rht3 lines, respectively, compared with corresponding rht lines. Although GA3 was present at a similar level to GA1 in the rht3 (tall) line it accumulated only fivefold in the Rht3 (dwarf) line. The steady-state pool sizes of endogenous GAs were GA19 ≫ GA20 = GA1 in the GA-responsive rht3 line whereas in the GA non-responsive Rht3 line the content of GA19≈ GA20 ⋘ GA1. It is proposed that one of the consequences of GA1 action is suppression of GA19-oxidase activity such that the conversion of GA19 to GA20 becomes a rate-limiting step on the pathway to GA1 in GA-responsive lines. In the GA-non-responsive Rht lines it is suggested that GA19 oxidase is not downregulated to the same extent and GA1 accumulates before the next rate-limiting step on the pathway, its 2β-hydroxylation to GA8. The steady-state pool sizes of GA19, 20, 1, 3 and 8 were similar in developmentally equivalent tissues of the rht3 (tall) line growing at 10° C and 20° C despite a 2.5-fold difference in the rate of leaf expansion. In contrast, in the Rht3 (dwarf) line, the extent of accumulation of GA1 reflected the severity of the phenotype at the two temperatures with slower growing tissues accumulating less, not more, GA1. These results are interpreted as supporting the proposed model of regulation of the GA-biosynthetic pathway rather than previous suggestions that GA1 accumulates in GA-insensitive dwarfs as a consequence of reduced growth rates.

Nitrate availability and shoot area development in small willow (Salix viminalis)

Abstract. The effect of nitrate supply upon leaf area development in willow (Salix viminalis) was investigated. Rooted cuttings were grown in culture solution at two different rates of exponentially‐increasing nitrate supply and with free access to other nutrients. During the lag‐phase in attaining stable nutrition, the rate of production of unfolded leaves, in the treatment with poorer N‐supply, decreased. After acclimation to the different N‐availabilities, the rate of dry matter increase per plant nitrogen was constant and the same in both treatments. The ratio of total leaf area to total amount of nitrogen in the plant was slightly higher in the treatment with poorer nitrogen supply. During the exponential (stable) phases of growth, nearly all the increase in total leaf area was attributable to main‐stem leaves; large numbers of leaves on axillary shoots in the treatment with better N‐supply contributed little to the total area. During the stable growth phase, the rate of production of unfolded leaves was the same in both treatments. Higher maximum values of relative rate of increase in area of single leaves were found with a better N‐supply and at higher leaf positions on the main stem. The duration of leaf expansion was not much affected by N‐supply. Final leaf size was greater at higher positions on the main‐stem. Within a treatment (investigated for poorer N‐supply), differences in final leaf size were proportional to differences in numbers of epidermal cells. Final leaf size was greater, at the same node, in the treatment with better N‐supply. Better N‐availability had increased the final size but not the number of epidermal cells. It was concluded that the higher exponential rate of total area increase with a better N‐supply was largely associated with higher rates of expansion in epidermal cells.

Field studies of leaf development and expansion in the leek (Allium porrum)

SUMMARYAn analysis of leek leaf development and expansion was carried out over three seasons using field‐grown plants of three varieties which were directly sown at different dates or transplanted from controlled conditions. In all cases, successive leaves appeared (tip visible) at equal intervals of accumulated temperature. Detailed analysis of a single sowing in 1985 showed that the regularity of leaf appearance was a consequence of the coordinated response to accumulated temperature of leaf initiation (plastochron 100°C days > 0°C) and leaf blade and sheath extension. For each successive leaf, an additional 32°C days were required between initiation and appearance to allow for the linear increase in ‘sheath’ height, giving a phyllochron of 132°C days. Direct measurement of leaf extension before and after leaf appearance, and of the length of the leaf extension zone, confirmed that the rate of leaf extension, in terms of accumulated tempeature, was constant, and independent of leaf number. However, there were differences between seasons and between varieties in the responses of leaf appearance, leaf extension and ‘sheath’ length to accumulated temperature. It was concluded that the simple ontogenetic increase in leaf dimensions, which was a feature of all the crops studied, was a consequence of the progressive increase in the duration of leaf expansion.

Effects of Temperature on Leaf Expansion in Sunflower

Five commercial cultivars of sunflower were grown in cabinets at three temperature regimes, 32/22, 27/17 and 22/12°C, and with 15-h and 11-h photoperiods, and expansion of leaves 5-15 was followed. Leaves appeared faster with increasing temperature (0.022 leaves day-1 °C-1) and with increasing daylength. Areas of individual leaves increased linearly up the plant profile and, although final area per leaf (Amax) decreased with increasing temperature, the relative change was similar for each leaf position. Cultivars maintained their ranking for Amax across temperatures, and these rankings agreed with those in previous field studies. Within each temperature regime, both the expansion rate of leaves and the duration of expansion increased with leaf position. As temperature increased, leaves grew for shorter periods with a change of 1.04 days °C-1, but under the photon flux density used (500 �mol m-2 s-1, or about 25% full sunlight) expansion rates were greatest at the lowest temperature. Expansion rates were only one-third of those in field studies at comparable temperatures, but durations were similar. Cultivars that achieved the largest Amax did so via faster rates of expansion and not via longer durations: only one cultivar differed from the mean (20 days) duration of leaf expansion. All cultivars reached floral initiation progressively earlier with extension of photoperiod from 10 to 15 h, with the change for the most sensitive cultivars being 8 days and for the least sensitive 5 days. Rates of leaf emergence were linked with this sensitivity.

Leaf expansion of four sunflower (Helianthus annuus L) cultivars in relation to water deficits. I. Patterns during plant development

Abstract. The influence of a slow stress and recovery cycle on the pattern of leaf expansion in four diverse sunflower cultivars (Helianthus annuus L. cvs. Hysun 31, Havasupai, Hopi and Seneca) was studied in a glasshouse. Stress had no significant effect on the time of flower bud emergence and anthesis, or on final leaf number, but delayed the appearance of leaves at high insertions in all cultivars except Hysun 31.Leaf expansion was markedly reduced as the predawn leaf water potential decreased from −0.35 to −0.60 MPa, and the predawn turgor pressure decreased from 0.3 to 0.2 MPa, and expansion ceased at a predawn leaf water potential of about −1.0 MPa, i.e. when the predawn turgor pressure reached zero.The leaves most reduced in final size when water was withheld were those at the insertions which grew the most rapidly in unstressed plants. The maximum reduction in final leaf size of 25–35% was similar in all cultivars and was due to retardation of the rate of leaf expansion: the duration of leaf expansion was actually increased by stress. However, leaves that were initiated during stress, but emerged after rewatering, had final leaf areas at least equal to those in the unstressed plants: in the cultivar Seneca, the final size of leaves of high insertion was significantly greater in stressed than unstressed plants, whereas in the three other cultivars the final leaf sizes were similar in both treatments. All four cultivars examined adjusted osmotically to the same degree, but leaf water potentials in one, Seneca, increased more rapidly after rewatering than in the other three, and this may have contributed to the greater relative leaf size in the leaves of high insertion in this cultivar.

EFFECTS OF ULTRAVIOLET‐B IRRADIANCES ON SOYBEAN. IV. LEAF ONTOGENY AS A FACTOR IN EVALUATING ULTRAVIOLET‐B IRRADIANCE EFFECTS ON NET PHOTOSYNTHESIS

There has recently been concern that some anthropogenic atmospheric pollutants, such as chlorofluoromethanes, may result in a global reduction in stratospheric ozone. This would undoubtedly increase the level of ultraviolet radiation reaching the surface of the earth. Such an increase in solar ultraviolet irradiation might have important biological consequences.Unifoliate leaves of decapitated soybean plants were exposed to 0, 20, 50, and 90 effective mW m2 UV‐BBE (weighted ultraviolet‐B irradiance) from special lamps in a greenhouse. The effects of UV‐B radiation were examined in leaves at three different ontogenetic stages: bud; one‐third expanded, and fully expanded leaves. Leaf area expansion, net photosynthesis, specific leaf weight, chlorophyll concentrations, and acidified methanol extract absorbance were measured during the course of the study. These data revealed several generalizations concerning the relationship between leaf ontogeny and photosynthetic response to UV‐B radiation: 1) The rate and duration of leaf expansion are affected by low levels of UV‐B irradiation. 2) There appears to be a shift in the ontogenetic sequence of photosynthetic capacity as a function of leaf age. 3) This results in leaves of similar chronological ages being in different physiological stages of maturity.

The effects of temperature on leaf growth of sugar beet varieties

SUMMARYLeaf growth of nine varieties of sugar beet (Beta vulgaris L.) was studied at constant temperatures of 7, 11, 15 and 20·C, using generalised logistic curves fitted to the data to estimate the parameters of growth. The rate of leaf appearance increased linearly with temperature and was the same in all varieties. There were differences between varieties in the weighted mean rates of expansion of leaf area per plant (Ā), the temperature coefficient of Ā and the leaf area duration (D); these differences were caused more by differences in rates of expansion and final sizes of individual leaves than by differences in rates of leaf production. The growth of the first six leaves produced by each plant was examined in detail. The greater size of successive leaves of plants and genotypic differences between comparable leaves were more attributable to differences in the rate than differences in the duration of leaf expansion. Increasing temperatures increased leaf size because they accelerated the rate of expansion more than they shortened the duration of the expansion phase. It is inferred that all effects arose through differences in the initial sizes of leaves before they unrolled from the shoot apex. Dry matter production was proportional to D but was partitioned more to the storage root at the colder temperatures. This may have been related to the differential effects of temperature on cell division and expansion and the relative contribution of these two processes to the final sizes of the leaves and storage root.