Leaf expansion and carbon assimilation in cotton leaves grown at two photosynthetic photon flux densities

Increasing photosynthetic photon flux density (PPFD) received during development from 5.5 to 31.2 mol m^-2 d^-1 resulted in greater leaf and mesophyll cell surface areas in cotton (Gossypium hirsutum L.). The relationships between the amounts of these surface areas and potential CO₂ assimilation by these leaves were evaluated. Leaf area (epidermal surface area of one side of a leaf), mesophyll cell surface area, and net rate of CO₂ uptake (P_n) were measured from the time leaves first unfolded until P., was substantially reduced. At the higher PPFD, leaf and mesophyll surface areas increased more rapidly during expansion, and P_n per unit leaf area was greater than at the lower PPFD. Although leaves at the higher PPFD reached the maximum P., per unit mesophyll cell surface area 4 to 5 days earlier than leaves at the lower PPFD, the maxima for these P., were similar. Leaves grown at the higher PPFD had the potential to assimilate 2.2, 3.5, or 5.8 times the amount of CO₂ as leaves from the lower PPFD when P., was expressed per unit mesophyll surface, per unit leaf surface, or per whole leaf, respectively. Greater and earlier development of both P., and mesophyll cell surface area at higher PPFD apparently had a compounding effect on the potential for carbon assimilation by a leaf.     

Stress-dependent redistribution of abscisic acid (ABA) in Hordeum vulgare L leaves: the role of epidermal ABA metabolism, tonoplastic transport and the cuticle

When ¹⁴C-labelled abscisic acid ([¹⁴C]ABA) was supplied to isolated protoplasts of the barley leaf at pH 6, initial rates of metabolism were about five times higher in epidermal cell protoplasts than in mesophyll cell protoplasts if equal cytosolic volumes were considered. In spite of the fact that epidermal cells make up only about 35% of the total water space in barley leaves, and despite the small cytosolic volume of these cells, in intact leaves all epidermal cells would thus metabolize half as much ABA per unit time as the mesophyll cells (0–27 and 0–51 mmol h^?1 m^?3 leaf water). Therefore, under these conditions epidermal cells seem to be a stronger sink than mesophyll cells for ABA that arrives via the transpiration stream. However, at an apoplastic pH of 7–25, which occurs in stressed leaves, the proportion of total metabolized ABA would be much smaller in epidermal than in mesophyll cells (0–029 and 0–204 mmolh^?l m^?3 leaf water). Our results indicate that under conditions of slightly alkaline apoplastic pH the epidermis may serve as the main source for fast stress-dependent ABA redistribution into the guard cell apoplast. This is partly the result of ABA transport across the epidermal tonoplast, which is dependent on the apoplastic pH and possibly on the cytosolic calcium concentration. The cuticle seems to be of no particular importance in stress-induced apoplastic ABA shifts and cannot be regarded as a significant sink for high ABA concentrations under stress.     

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.     

PHOTOSYNTHETIC RATE AND MESOPHYLL SURFACE AREA IN EXPANDING LEAVES OF ALTERNANTHERA PHILOXEROIDES GROWN AT TWO LIGHT LEVELS

Leaves of Alternanthera philoxeroides, alligator weed, developed at a photosynthetic photon flux density (PPFD, light energy at wavelengths of 400 to 700 nm) of 790 μmol sec⁻¹ m⁻² (High Light) had less surface area, were thicker, had a higher maximum P_n (net rate of CO₂ uptake), and required a higher PPFD for saturation of P_n, than leaves developed at 160 μmol sec⁻¹ m⁻² (Low Light). Mesophyll thickness at Low Light was within 19% of maximum 2 days after emergence but at High Light, thickness increased 79% between 2 and 16 days after leaf emergence. The ratio of mesophyll surface area to leaf surface area decreased during development in both light treatments; the ratio, however, was over 70% greater in fully expanded High Light leaves than in Low Light leaves. Maximum P_n expressed on a leaf surface area basis was 158% greater in High Light leaves than in Low Light leaves, but P_n was only 58% greater when expressed on a mesophyll surface area basis. It was estimated that fully expanded High Light leaves fixed 72% more CO₂ per leaf (P_n expressed per unit surface area times the total surface area per leaf) than fully expanded Low Light leaves when P_n was measured at the PPFD leaves expanded under. Both High and Low Light leaves would fix about the same amount of CO₂ per leaf when P_n was measured at 160 μmol sec⁻¹ m⁻² because the larger surface area of the Low Light leaves offset small differences in P_n.     

Temporal and Spatial Development of the Cells of the Expanding First Leaf of Arabidopsis thaliana (L.) Heynh

The three-dimensional quantitative leaf anatomy in developingyoung (9–22 d) first leaves of wild type Arabidopsis thalianacv. Landsberg erecta from mitosis through cell and leaf expansionto the cessation of lamina growth has been studied. The domainsof cell division, the relative proportion of the cell typespresent during development and the production of intercellularspace in the developing leaf have been determined by image analysisof entire leaves sectioned in three planes. Mitotic activityoccurs throughout the youngest leaves prior to unfolding andcell expansion is initiated firstly at the leaf tip with a persistentzone of mitotic cells at the leaf base resulting in a gradientof development along the leaf axis, which persists in the olderleaves. Major anatomical changes which occur during the developmentare, a rapid increase in mesophyll volume, an increase in thevein network, and expansion of the intercellular spaces. Thepattern of cell expansion results in a 10-fold variation inmesophyll cell size in mature leaves. In the youngest leavesthe plan area of mesophyll cells varies between 100 µm²and 400 µm² whereas in mature leaves mesophyll cells rangein plan area from 800 µm² to 9500 µm². The volumesof mesophyll tissue and airspace under unit leaf area increase3-fold and 35-fold, respectively, during leaf expansion. Thevolume proportions of tissue types mesophyll:airspace:epiderrnal:vascularin the mature leaf are 61:26:12:1, respectively. This studyprovides comparative information for future identification andanalysis of leaf development mutants of Arabidopsis thaliana. Key words: Arabidopsis, quantitative leaf anatomy, leaf expansion, image analysis     

Effects of weather during leaf development on photosynthetic characteristics of soybean leaves

Net photosynthetic rates and mesophyll conductances at 25 °C at light saturation and air levels of carbon dioxide and oxygen were measured on recently fully expanded leaflets of second trifoliolate leaves of soybeans (Glycine max cv. Kent). Plants were grown outdoors in pots at Beltsville, Maryland with 14 planting times from May through August, 1983. Air temperature and humidity, and photosynthetically active radiation (PAR) were measured for the expansion periods of the second trifoliolate leaves. Rates of net photosynthesis ranged from 24 to 33 mol m^–2 s^–1, and mesophyll conductances from 0.24 to 0.35 cm s^–1 for the different planting dates. Mean 24-h air temperatures ranged from 20.6 to 29.0 °C, and mean daily PAR ranged from 29.4 to 58.4 mol m^–2 d^–1 for the leaf expansion periods. There was a positive relationship between photosynthetic characteristics and PAR during leaf expansion, and a negative relationship between photosynthetic characteristics and leaf expansion rates, with 96% of the variation in photosynthetic characteristics accounted for by these two variables. Leaf expansion rates were highly correlated with air temperature.     

Response to red and blue lights by electrical currents on the surface of intact leaves

Exposure to red and blue lights caused an increase in electrical currents (0.14 μA cm^-2 for red and 0.05 μA cm^-2 for blue, respectively) flowing on the lower surface of leaves fromCommelina communis. However, no changes were measured in currents from isolated epidermal cells. To determine the influence of the mesophyll on such electrical changes, those cells were infiltrated with photosynthesis inhibitors. Both DCCD treated and control leaf discs showed the same level of response to red light. Epidermal strips were also removed to measure the currents above partially exposed mesophyll cells in order to elucidate the relationship between intact leaves and those mesophyll cells. Changes in current were smaller in the latter type. The partially exposed mesophyll cells of a leaf also showed electrical current changes, but smaller than those of the intact leaf. In DCMU-infiltrated leaf discs, the electrical currents of intact leaves were increased to 0.05 μA cm^-2 in response to red light. For sodium azide-infiltrated leaf discs, however, intact leaves showed no response. Likewise, a measure of photosynthetic efficiency, the Fv/Fm ratio, was reduced to that measured in the control, thereby indicating that photosynthetic activity significantly altered the electrical current for intact leaves. Therefore, these results demonstrate that the current observed from the lower side of intact leaves is related to photosynthetic activity in the mesophyll cells.     

Ability of leaf mesophyll to retain potassium correlates with salinity tolerance in wheat and barley

This work investigated the importance of the ability of leaf mesophyll cells to control K⁺ flux across the plasma membrane as a trait conferring tissue tolerance mechanism in plants grown under saline conditions. Four wheat (Triticum aestivum and Triticum turgidum) and four barley (Hordeum vulgare) genotypes contrasting in their salinity tolerance were grown under glasshouse conditions. Seven to 10‐day‐old leaves were excised, and net K⁺ and H⁺ fluxes were measured from either epidermal or mesophyll cells upon acute 100 mM treatment (mimicking plant failure to restrict Na⁺ delivery to the shoot) using non‐invasive microelectrode ion flux estimation (the MIFE) system. To enable net ion flux measurements from leaf epidermal cells, removal of epicuticular waxes was trialed with organic solvents. A series of methodological experiments was conducted to test the efficiency of different methods of wax removal, and the impact of experimental procedures on cell viability, in order to optimize the method. A strong positive correlation was found between plants' ability to retain K⁺ in salt‐treated leaves and their salinity tolerance, in both wheat and especially barley. The observed effects were related to the ionic but not osmotic component of salt stress. Pharmacological experiments have suggested that voltage‐gated K⁺‐permeable channels mediate K⁺ retention in leaf mesophyll upon elevated NaCl levels in the apoplast. It is concluded that MIFE measurements of NaCl‐induced K⁺ fluxes from leaf mesophyll may be used as an efficient screening tool for breeding in cereals for salinity tissue tolerance.     

Time-course of programmed cell death during leaf senescence in <Emphasis Type="Italic">Eucommia ulmoides</Emphasis>

Leaves of Eucommia ulmoides Oliv. harvested between April to November were examined for programmed cell death (PCD) during growth and senescence. Leaves developed in April, becoming fully expanded in late May, remaining unchanged until November when they started to dehisce. Falling leaves retained a green color. Our results showed that (1) mesophyll cells gradually reduced their nuclei from September to November, (2) positive TUNEL signals appeared on the nuclei from August, (3) ladder-like DNA fragmentation occurred in September and October, and (4) a 20-kDa Ca²⁺-dependent DNase appeared in these same months. In fallen leaves, intact mesophyll cell nuclei could not be detected, but a few cells around the vascular bundle had nuclei. Therefore, (1) programmed cell death (PCD) of leaf cells occurred in the leaves of E. ulmoides, (2) the progress of mesophyll cell PCD lasted for more than 2 months, and (3) PCD of leaf cells was asynchronous in natural senescing leaves. Electronic Publication     

DIFFERENCES IN LEAF STRUCTURE,CHLOROPHYLL, AND NUTRIENTS FOR THE UNDERSTORY TREE ASIMINA TRILOBA

To investigate differences in leaf structure, chlorophyll and nutrients on terminal branches of the understory tree Asimina triloba, the first (proximal) and the last (distal) leaves to develop in the spring were compared. Proximal leaf expansion was completed before the overstory canopy was fully closed but distal leaf expansion occurred during and after the development of the overstory canopy. Fully expanded proximal leaves were 76% smaller in area, were 18% thicker and had 36% more stomates per m of leaf area when compared to distal leaves. In addition, maximum stomatal conductance to water vapor was greater (150 vs. 120 mmol m^−-2s^−-1) and the minimum PPFD required for maximum conductance was higher (200 vs. 150 μmol m^−-2s^−-1) for the proximal leaves. Chlorophyll content was also greater for proximal leaves, but nitrogen and phosphorus contents were lower throughout the entire summer. Seasonal measurements indicated an increase in chlorophyll a content and reductions in nitrogen content throughout the summer growth period for leaves from both positions. The results suggest that distal and proximal leaves differed physiologically and that the measured differences were related to the changing irradiance environment during leaf development. The time of leaf expansion, as indicated by leaf position on the branch, may be an important consideration when examining the water and photosynthetic relations of understory trees.     

Leaf Structure of Tobacco In Vitro Grown Plantlets as Affected by Saccharose and Irradiance

Tobacco plantlets were cultured in vitro under high (200 µmol m^–2 s^–1) or low (60 µmol m^–2 s^–1) irradiance with or without saccharose in the medium. Light microscopy and image analysis were used to evaluate the effect of these culture conditions on leaf anatomy. Addition of saccharose resulted in thicker leaves (all leaf layers) and larger mesophyll cells under both growth irradiances. Various irradiance affected leaf anatomy differently when plantlets had been cultivated in presence or absence of saccharose in the medium. While under high irradiance in presence of saccharose leaf thickness and number of chloroplasts per cell section were increased, plantlets grown under high irradiance in absence of saccharose had thinner leaves and less chloroplasts per cell section. The changes were more pronounced in palisade parenchyma layer.     

CO2 transfer conductance, leaf structure and carbon isotope composition of Polygonum cuspidatum leaves from low and high altitudes

Anatomy and some physiological characteristics of the leaves in Polygonum cuspidatum Sieb. et Zucc., a dioecious clonal herb, were compared between two populations, one from a lowland in Shizuoka City (10 m above sea level), and another from a highland on Mt. Fuji (2500 m above sea level). Leaf mass per area (LMA) of the highland plants was about twice that of the lowland plants. The greater leaf thickness, thicker mesophyll cell walls and higher mesophyll cell density in the highland leaves contributed to the larger LMA. Although mesophyll area exposed to intercellular airspaces was greater in the highland leaves than in the lowland leaves by 30%, the surface area of chloroplasts facing intercellular airspaces was similar between these leaves. CO₂ transfer conductance inside the leaf (g_i) of the highland leaves (0·75 μmol m^?2 s^?1 Pa^?1) is the lowest recorded for herbaceous plants and was only 40% of that in the lowland leaves. On the other hand, the difference in stomatal conductance was small. δ¹³C values in the leaf dry matter were greater in the highland leaves by 4‰. These data and the estimation of CO₂ partial pressures in the intercellular air spaces and in the chloroplast suggested that the greater dry matter δ¹³C in the highland leaves, indicative of lower long‐term ratio of the chloroplast stroma to the ambient CO₂ partial pressures, would be mainly attributed to their lower g_i.     

Structure of the Photosynthetic Apparatus in Leaves of Freshwater Hydrophytes: 1. General Characteristics of the Leaf Mesophyll and a Comparison with Terrestrial Plants

The structure of photosynthetic elements was investigated in leaves of 42 boreal plant species featuring different degrees of submergence (helophytes, neustophytes, and hydatophytes). The mesophyll structure types were identified for all these species. Chlorenchyma tissues and phototrophic cells were quantitatively described by such characteristics as the sizes of cells and chloroplasts in the mesophyll and epidermis, the abundance of cells and chloroplasts in these tissues, the total surface area of cells and chloroplasts per unit leaf area, the number of plastids per cell, etc. The hydrophytes typically had thick leaves (200–350 m) with a well-developed aerenchyma; their specific density per unit area (100–200 mg/dm²) was lower than in terrestrial plants. Mesophyll cells in aquatic plants occupied a larger volume (5–20 × 10³m³) than epidermal cells (1–15 × 10³m³). The number of mesophyll cells per unit leaf area was nearly 1.5 times higher than that of epidermal cells. Chloroplasts were present in the epidermis of almost all species, including emergent leaves, but the ratio of the chloroplast total number to the number of all plastids varied depending on the degree of leaf submergence. The total number of plastids per unit leaf area (2–6 × 10⁶/cm²) and the surface of chloroplasts per unit leaf area (2–6 cm²/cm²) were lower in hydrophytes than in terrestrial plants from climatically similar habitats. The functional relations between mesophyll parameters were similar for hydrophytes and terrestrial plants (a positive correlation between the leaf weight per unit area, leaf thickness, and the number of mesophyll cells per unit leaf area), although no correlation was found in hydrophytes between the volume of mesophyll cells and the leaf thickness. Phototrophic tissues in aquatic plants contributed a larger fraction to the leaf weight than in terrestrial plants, because the mechanical tissues were less developed in hydrophytes. The CO₂assimilation rates by leaves were lower in hydrophytes than in terrestrial plants, because the total surface area of chloroplasts per unit leaf area is comparatively small in hydrophytes, which reduces the conductivity for carbon dioxide diffusion towards the carboxylation sites.     

Tropospheric ozone-induced structural changes in leaf mesophyll cell walls in grapevine plants

The structural changes in leaves of grapevine plants (Vitis vinifera L.) exposed to different ozone concentrations were investigated. Ozone fumigations were performed in open-top chambers at four different ozone levels (charcoal-filtered air (F), ambient air (N), ambient air + 25 mm³m⁻³ ozone (O-25) and ambient air + 50 mm³m⁻³ ozone (O-50)). The leaves of plants from chambers with increased ozone concentrations (O-25 and O-50) were significantly thicker than the controls (F), owing to increased thickness of the mesophyll layer. Observing O-50 leaves, it was found that the mesophyll cell wall displayed structural changes. In some places cell wall thickness increased up to 1 μm. We found callose deposits on the inner side of the cell walls of mesophyll cells. These data are in accord with the concept that the mesophyll cell wall acts as a barrier against the penetration of tropospheric ozone into the cells.     

Contributions of photosynthetic and non‐photosynthetic cell types to leaf respiration in Vicia faba L. and their responses to growth temperature

In intact leaves, mitochondrial populations are highly heterogeneous among contrasting cell types; how such contrasting populations respond to sustained changes in the environment remains, however, unclear. Here, we examined respiratory rates, mitochondrial protein composition and response to growth temperature in photosynthetic (mesophyll) and non‐photosynthetic (epidermal) cells from fully expanded leaves of warm‐developed (WD) and cold‐developed (CD) broad bean (Vicia faba L.). Rates of respiration were significantly higher in mesophyll cell protoplasts (MCPs) than epidermal cell protoplasts (ECPs), with both protoplast types exhibiting capacity for cytochrome and alternative oxidase activity. Compared with ECPs, MCPs contained greater relative quantities of porin, suggesting higher mitochondrial surface area in mesophyll cells. Nevertheless, the relative quantities of respiratory proteins (normalized to porin) were similar in MCPs and ECPs, suggesting that ECPs have lower numbers of mitochondria yet similar protein complement to MCP mitochondria (albeit with lower abundance serine hydroxymethyltransferase). Several mitochondrial proteins (both non‐photorespiratory and photorespiratory) exhibited an increased abundance in response to cold in both protoplast types. Based on estimates of individual protoplast respiration rates, combined with leaf cell abundance data, epidermal cells make a small but significant (2%) contribution to overall leaf respiration which increases twofold in the cold. Taken together, our data highlight the heterogeneous nature of mitochondrial populations in leaves, both among contrasting cell types and in how those populations respond to growth temperature.     

Leaf anatomy during leaf development of photoautotrophically <Emphasis Type="Italic">in vitro</Emphasis>-grown tobacco plants as affected by growth irradiance

Tobacco (Nicotiana tabacum L.) plants were cultured in vitro photoautotrophically at three levels of irradiance (PAR 400–700 nm): low (LI, 60 μmol m⁻² s⁻¹), middle (MI, 180 μmol m⁻² s⁻¹) and high (HI, 270 μmol m⁻² s⁻¹). Anatomy of the fourth leaf from bottom was followed during leaf development. In HI and MI plants, leaf area expansion started earlier as compared to LI plants, and both HI and MI plants developed some adaptations of sun species: leaves were thicker with higher proportion of palisade parenchyma to spongy parenchyma tissue. Furthermore, in HI and MI plants palisade and spongy parenchyma cells were larger and relative abundance of chloroplasts in parenchyma cells measured as chloroplasts cross-sectional area in the cell was lower than in LI plants. During leaf growth, chloroplasts crosssectional area in both palisade and spongy parenchyma cells in all treatments considerably decreased and finally it occupied only about 5 to 8 % of the cell cross-sectional area. Thus, leaf anatomy of photoautotrophically in vitro cultured plants showed a similar response to growth irradiance as in vivo grown plants, however, the formation of chloroplasts and therefore of photosynthetic apparatus was strongly impaired.     

Cytokinin-Regulated Mesophyll Cell Division and Expansion during Development of Cucurbita pepo Leaves

The role of cytokinins in the development of mesophyll structure was studied in developing pumpkin Cucurbita pepo L. leaves. Leaves were treated with cytokinins at different stages of growth: when they reached 25 or 50% of their final size (S _max), immediately after leaf growth ceased, and during senescence. At the early stages of leaf development, treatment with exogenous benzyladenine accelerated division of mesophyll cells. At the later stages of development, BA treatment activated expansion of growing cells and those, which have just accomplished their growth. The exogenous cytokinin did not affect the senescent leaf cells. The content of endogenous cytokinins changed during mesophyll development. The juvenile leaves (25% of S _max) were characterized by low level of these phytohormones. In the expanding leaves (50% of S _max), the content of phytohormones increased and decreased when leaf growth ceased. In the senescent leaves, the cytokinin content decreased markedly. It was concluded that the response of mesophyll cells to cytokinin depended on the cell growth phase at the moment of hormone action. Furthermore, in the young leaves, lower cytokinin concentrations were required for division of mesophyll cells in vivo than for cell expansion at the final stage of leaf development.     

Concentrations of inorganic and organic solutes in extracts from individual epidermal,mesophyll and bundle-sheath cells of barley leaves

The distribution of solutes between epidermal, mesophyll and bundle-sheath cells in barley (Hordeum vulgare L. cv. Klaxon) leaves was studied by analysing extracts obtained from single cells with a modified pressure probe. Activity of the cytoplasmic marker enzyme, malate dehydrogenase, revealed that epidermal cell extracts were completely vacuolar in origin, but extracts from mesophyll cells also contained cytoplasmic constituents. The extracts were analysed for osmolality and the concentrations of K, Na, Ca, Cl, P, S, NO ₃ ^– , sugars and total amino acids. Epidermal and mesophyll cell extracts had similar osmolalities but these varied between 420 and 565 mosmol, kg ¹ depending on the leaf developmental stage; the osmolality of bundle-sheath extracts was approximately 100 mosmol, kg^–1 lower. Under the growth conditions used, K and NO ₃ ^– were found in all three cell types and their concentrations generally ranged between 180 and 230 mM. In contrast, Ca was almost restricted to epidermal cells, where it increased to 70 mM during leaf ageing. Phosphorus was only detectable ( 5 mM) in extracts from mesophyll and bundle-sheath cells, while Cl concentrations were highest in epidermal and lowest in mesophyll cell extracts. The concentrations of sugars and amino acids were close to the detection limit (approx. 2 mM) in epidermal cells but mesophyll cells contained total sugar (glucose, fructose and sucrose) of up to 78 mM and total amino-acid concentrations of up to 13.5 mM. Concentrations in bundle-sheath cells were intermediate between those in the epidermis and mesophyll.Abbreviations EDX analysis energy dispersive X-ray analysis - MDH malate dehydrogenase We wish to thank Paul Richardson, Jeremy Pritchard, Peter Hinde and Andrew Davies (Banger) for their helpfull discussion and technical advice. This work was financed by a grant (LR5/521) from the Agricultural and Food Research Council.     

Function of leaf hamamelitol as a compatible solute during water stress treatment of Hedera helix L.

Hamamelitol is an unusual branched-chain sugar alcohol previously suggested to function as a leaf compatible solute. In this study, we have examined the leaf metabolism and intracelluiar compartmentalization of hamamelitol and other soluble sugars during long-term water stress treatment of Hedera helix (English ivy). Total leaf hamamelitol content was relatively low in greenhouse control plants, but increased 2-fold during water stress treatment to levels approaching those observed in field-grown plants (6–7 μmol g^?1 fresh weight). Using density gradient fractionation with non-aqueous solvents, we showed that hamamelitol occurs primarily in the cytoplasm and vacuoles of leaf mesophyll cells. During water stress treatment most of the increase in leaf hamamelitol occurred in the mesophyll cytoplasm, compensating osmotically for a decrease in cytoplasmic sucrose concentration. The maximum concentration of cytoplasmic hamamelitol was 155 mol m^?3 and occurred in field-grown plants. Labelling experiments showed that hamamelitol is slowly synthesized from ¹⁴CO₂ in leaves of H. helix, but is very long-lived (estimated t_1/2 of 4 years). Together, these data indicate that hamamelitol probably functions during long-term stress conditions as an osmotically active, compatible solute in plant leaves. We suggest that the signal for enhanced accumulation of hamamelitol during the water stress treatment was initiated by decreased plant growth and increased leaf sucrose hydrolysis.     

High-light effects on CO2 fixation gradients across leaves

Chlorophyll fluorescence and internal patterns of ¹⁴CO₂ fixation were measured in sun and shade leaves of spinach after treatment with various light intensities. When sun leaves were irradiated with 2000μmol m^?2 s^?1 for 2h, F_V/F_M decreased by about 15%, but ¹⁴CO₂ fixation was unaffected, whereas shade leaves exhibited a 21% decrease in F_v/F_M and a 25% decrease in ¹⁴CO₂ fixation. Irradiation of sun and shade leaves with 4000μmol m^?1 for 4 h decreased F_V/F_M by 30% in sun leaves and 40% in shade leaves, while total ¹⁴CO₂ fixation decreased by 41% in sun leaves and 55% in shade leaves. After light treatment, gradients of CO₂ fixation across leaves were determined by measuring ¹⁴CO₂ fixed in paradermal leaf sections after a 10s pulse of ¹⁴CO₂. Gradients of ¹⁴CO₂ fixation in control sun and shade leaves were identified when expressed on a relative basis and normalized for leaf depth. Treatment of leaves with 2000 μmol PAR m^?2 s^?1 for 2h did not after patterns of carbon fixation across sun leaves, but slightly altered the pattern in shade leaves. In contrast, treatment of sun and shade leaves with 4000μmol m^?2 s^?1 for 4h decreased carbon fixation more in the palisade mesophyll cells than in the spongy mesophyll cells of sun and shade leaves, and fixation in medial tissue of shade leaves was dramatically decreased compared to the adaxial and abaxial tissue. The interaction between leaf anatomy and biochemical parameters involved in tolerance to photoinhibition in spinach is discussed.