Photoinhibition in differently coloured juvenile leaves of Syzygium species

Photoinhibition, as measured by the dark-adapted chlorophyll a fluorescence ratio Fv/Fm, was assessed in Syzygium moorei, a species with dark green juvenile leaves, Syzygium corynanthum, which has light green juvenile leaves, and two species with pink-red juvenile leaves (Syzygium wilsonii and Syzygium luehmannii). All plants were glasshouse-grown (maximum PPFD 1500 mol m^-2 s^-1) under optimum nutrition and water.When measured at midday, dark-adapted Fv/Fm ratios of juvenile leaves gradually increased in all species as percentage of full leaf expansion (% FLE) increased. Fluorescence measurement 3h after sunset or pre-dawn also showed a developmental effect on Fv/Fm, with juvenile leaves of S. luehmannii and S. wilsonii showing much lower Fv/Fm at all stages of development. Dark-adapted Fv/Fm, values in both juvenile and mature leaves generally never exceeded 0.8 at any stage in any of the species.Courses of Fv/Fm on sunny days showed greater diurnal photoinhibition in green juvenile (c. 50% FLE) leaves of S. moorei (24%) and S. corynanthum (36%) than in mature leaves of the previous flush in these species (<10%). Diurnal photoinhibition was statistically similar (18-24%) in pink-red juvenile and green mature leaves of S. luehmannii and S. wilsonii. Re-positioning juvenile leaves of S. wilsonii horizontally increased diurnal photoinhibition.Exposure of leaves to a standard mild photoinhibitory light treatment (30 min at 1000 mol m^-2s^-1) showed that juvenile leaves of all species had a lower percentage of high energy state quenching (qE) and a higher percentage of photoinhibitory quenching (qI) than mature leaves.     

Leaf development and photosynthetic properties of three tropical tree species with delayed greening

Leaf developmental patterns were characterized for three tropical tree species with delayed greening. Changes in the pigment contents, photosynthetic capacity, stomata development, photosystem 2 efficiency, rate of energy dissipation, and the activity of partial protective enzymes were followed in developing leaves in an attempt to elucidate the relative importance of various photoprotective mechanisms during leaf ontogeny. Big leaves of Anthocephalus chinensis, a fast-growing light demanding species, expanded following an exponential pattern, while relatively small leaves of two shade-tolerant species Litsea pierrei and Litsea dilleniifolia followed a sigmoidal pattern. The juvenile leaves of A. chinensis and L. pierrei contained anthocyanin located below the upper epidermis, while L. dilleniifolia did not contain anthocyanin. Leaves of A. chinensis required about 12 d for full leaf expansion (FLE) and photosynthetic development was delayed 4 d, while L. pierrei and L. dilleniifolia required 18 or 25 d for FLE and photosynthetic development was delayed 10 or 15 d, respectively. During the leaf development the increase in maximum net photosynthetic rate was significantly related to changes in stomatal conductance and the leaf maturation period was positively related to the steady-state leaf dry mass per area for the three studied species. Dark respiration rate of leaves at developing stages was greater, and pre-dawn initial photochemical efficiency was lower than that of mature leaves. Young leaves displayed greater energy dissipation than mature leaves, but nevertheless, the diurnal photoinhibition of young L. dilleniifolia leaves was higher than that of mature leaves. The young red leaves of A. chinensis and L. pierrei with high anthocyanin contents and similar diurnal photoinhibition contained more protective enzymes (superoxide dismutase, ascorbate peroxidase) than mature leaves. Consequently, red leaves may have higher antioxidant ability.     

Short communication. Do anthocyanins play a role in UV protection of the red juvenile leaves of Syzygium?

The biological function of juvenile leaves pigmented with anthocyanin is poorly understood. The role anthocyanins play in UV protection was assessed in juvenile leaves of two Syzygium species (S. luehmannii and S. wilsonii) which contain high anthocyanin concentrations. HPLC was used to separate UV-absorbing anthocyanins from other soluble UV-absorbing phenolic compounds. The isolated anthocyanins (predominantly malvidin-3,5-diglucoside) contributed little to the total absorbance of UV-A and UV-8 radiation. This was because the non-acylated anthocyanins only effectively absorbed shortwave UV-B radiation and the strong absorbance by other compounds. These results suggest that the UV protection hypothesis is not valid for anthocyanins in juvenile Syzygium leaves.     

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.     

Field photosynthesis,microclimate and water relations of an exotic temperate liana,Pueraria lobata,kudzu

Summary Kudzu occurs in a variety of habitats in the southeastern United States. It is most common in exposed, forest edge sites and road cuts where it forms an extensive ground canopy as well as a canopy overtopping nearby trees, but it can also be found in completely open fields and deeply shaded sites within a forest. Microclimate, stomatal conductance, leaf water potential and photosynthetic responses to light, temperature and humidity were measured in two contrasting microhabitats on Pueraria lobata, kudzu. Midsummer leaf temperatures and leaf-to-air water vapor deficits for plants growing in an exposed site were significantly greater than for those in a shaded site, exceeding 35° C and 50 mmol mol^-1, respectively. Maximum stomatal conductance exceeded 400 mmol m^-2 s^-1 in exposed leaves during peak vegetative growth. Stomatal conductance in shaded leaves was approximately half the value measured in exposed leaves on any particular dya. Maximum photosynthetic carbon uptake was also higher in leaves growing in exposed sites compared to leaves in shaded sites, exceeding 18.7 and 14.0 mol m^-2 s^-1, respectively. Photosynthesis, stomatal conductance and intercellular CO₂ concentration decreased dramatically in response to increasing water vapor deficit for leaves from both sites. However, transpiration showed an initial increase at intermediate water vapor deficits, leveling off or even decreasing at higher values. Leaf water potential demonstrated marked diurnal variation, but remained constant over a wide range of transpirational water fluxes. This latter feature, combined with microenvironmental modification through rapid leaf orientation and pronounced stomatal responses to water vapor deficits may represent important adaptive responses in the exploitation of a diverse array of habitats by kudzu.     

Leaf chloroplast ultrastructure and photosynthetic properties of a chlorophyll-deficient mutant of rice

Leaf chloroplast ultrastructure and photosynthetic properties of a natural, yellow-green leaf mutant (ygl1) of rice were characterized. Our results showed that chloroplast development was significantly delayed in the mutant leaves compared with the wild-type rice (WT). As leaves matured, more grana stacks formed concurrently with increasing leaf chlorophyll (Chl) content. Except for the lower intercellular CO₂ concentration, the ygl1 plants had a higher leaf net photosynthetic rate, stomatal conductance, and transpiration rate than those of the WT plants. Under equal amounts of Chl, the excitation energy of PSI and PSII was much stronger in the mutant than that in the WT. The ygl1 plants showed higher nonphotochemical quenching and lower photochemical quenching. They also exhibited higher actual photochemical efficiency of PSII with a higher electron transport rate. Under the light of 200 μmol(photon) m^?2 s^?1, the ygl1 mutant showed lesser deepoxidation of violaxanthin in the xanthophyll cycle than WT, but it increased substantially under strong light conditions. In conclusion, the photosynthetic machinery of the ygl1 remained stable during leaf development. The plants were less sensitive to photoinhibition compared with WT due to the active xanthophyll cycle. The ygl1 plants were efficient in both light harvesting and conversion of solar energy.     

Diurnal courses and factorial dependencies of leaf conductance and transpiration of differently potassium fertilized and watered field grown barley plants

During the grain filling period we followed diurnal courses in leaf water potential (ψ₁), leaf osmotic potential (ψ_π), transpiration (E), leaf conductance to water vapour transfer (g) and microclimatic parameters in field-grown spring barley (Hordeum distichum L. cv. Gunnar). The barley crop was grown on a coarse textured sandy soil at low (50 kg ha⁻¹) or high (200 kg ha⁻¹) levels of potassium applied as KCl. The investigation was undertaken at full irrigation or under drought. Drought was imposed at the beginning of the grain filling period. Leaf conductance and rate of transpiration were higher in the flag leaf than in the leaves of lower insertion. The rate of transpiration of the awns on a dry weight basis was of similar magnitude to that of the flag leaves. On clear days the rate of transpiration of fully watered barley plants was at a high level during most part of the day. The transpiration only decreased at low light intensities. The rate of transpiration was high despite leaf water potentials falling to rather low values due to high evaporative demands. In water stressed plants transpiration decreased and midday depression of transpiration occurred. Normally, daily accumulated transpirational water loss was lower in high K leaves than in low K leaves and generally the bulk water relations of the leaves were more favourable in high K plants than in low K plants. The factorial dependency of the flag leaf conductances on leaf water potential, light intensity, leaf temperature, and leaf-to-air water vapour concentration difference (ΔW) was analysed from a set of field data. From these data, similar sets of microclimatic conditions were classified, and dependencies of leaf conductance on the various environmental parameters were ascertained. The resulting mathematical functions were combined in an empirical simulation model. The results of the model were tested against other sets of measured data. Deviations between measured and predicted leaf conductance occurred at low light intensities. In the flag leaf, water potentials below-1.6 MPa reduced the stomatal apertures and determined the upper limit of leaf conductance. In leaves of lower insertion level conductances were reduced already at higher leaf water potentials. Leaf conductance was increased hyperbolically as photosynthetic active radiation (PAR) increased from darkness to full light. Leaf conductance as a function of leaf temperature followed an optimum curve which in the model was replaced by two linear regression lines intersecting at the optimum temperature of 23.4°C. Increasing leaf-to-air water vapour concentration difference caused a linear decrease in leaf conductance. Leaf conductances became slightly more reduced by lowered water potentials in the low K plants. Stomatal closure in response to a temperature change away from the optimum was more sensitive in high K plants, and also the decrease in leaf conductance under the influence of lowered ambient humidity proceeded with a higher sensitivity in high K plants. Thus, under conditions which favoured high conductances increase of evaporative demand caused an about 10% larger decrease in leaf conductance in the high K plants than in the low K plants. Stomatal sizes and density in the flag leaves differed between low and high K plants. In plants with partially open stomata, leaf conductance, calculated from stomatal pore dimensions, was up to 10% lower in the high K plants than in the low K plants. A similar reduction in leaf conductance in high K plants was measured porometrically. It was concluded that the beneficial effect of K supply on water use efficiency reported in former studies primarily resulted from altered stomatal sizes and densities.     

Photosynthetic characterization of Jerusalem artichoke during leaf expansion

Gas exchange, chlorophyll a fluorescence and modulated 820 nm reflection were investigated to explore the development of photosynthesis in Jerusalem artichoke (Helianthus tuberosus L.) leaves from initiation to full expansion. During leaf expansion, photosynthetic rate (Pn) increased and reached the maximal level when leaves were fully expanded. The same change pattern was also found in the stomatal conductance and chlorophyll content. Lower Pn could not be ascribed to the higher stomatal resistance in developing leaves, as intercellular CO₂ concentration was not significantly lower in these leaves. Lower Pn partly resulted from the lower actual photochemical efficiency of PSII in developing leaves, as more excited energy was dissipated through non-photochemical quenching. The development of primary photochemical reaction and electron transport in the donor side of PSII was completed in the initiating leaves. However, the development of electron transport in the acceptor side of PSII was not accomplished until leaves were fully expanded, indicated by the change in probability that an electron moves further than primary quinone (ψo). PSI activity changed in parallel with ψo suggesting that PSI cooperated well with PSII during leaf expansion. It should be stressed that the development of carbon fixation process was later than primary photochemical reaction but earlier than photosynthetic electron transport during leaf expansion. The later development of photosynthetic electron transport may reduce the production of reactive oxygen species from Mehler reaction, particularly under low carbon fixation.     

Leaf area and photosynthesis of newly emerged trifoliolate leaves are regulated by mature leaves in soybean

Leaf anatomy and the stomatal development of developing leaves of plants have been shown to be regulated by the same light environment as that of mature leaves, but no report has yet been written on whether such a long-distance signal from mature leaves regulates the total leaf area of newly emerged leaves. To explore this question, we created an investigation in which we collected data on the leaf area, leaf mass per area (LMA), leaf anatomy, cell size, cell number, gas exchange and soluble sugar content of leaves from three soybean varieties grown under full sunlight (NS), shaded mature leaves (MS) or whole plants grown in shade (WS). Our results show that MS or WS cause a marked decline both in leaf area and LMA in newly developing leaves. Leaf anatomy also showed characteristics of shade leaves with decreased leaf thickness, palisade tissue thickness, sponge tissue thickness, cell size and cell numbers. In addition, in the MS and WS treatments, newly developed leaves exhibited lower net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (E), but higher carbon dioxide (CO ₂ ) concentration in the intercellular space (Ci) than plants grown in full sunlight. Moreover, soluble sugar content was significantly decreased in newly developed leaves in MS and WS treatments. These results clearly indicate that (1) leaf area, leaf anatomical structure, and photosynthetic function of newly developing leaves are regulated by a systemic irradiance signal from mature leaves; (2) decreased cell size and cell number are the major cause of smaller and thinner leaves in shade; and (3) sugars could possibly act as candidate signal substances to regulate leaf area systemically.     

Strategies of leaf expansion in Ficus carica under semiarid conditions

Leaf area expansion, thickness and inclination, gas exchange parameters and relative chlorophyll content were analysed in field‐grown fig (Ficus carica L.) leaves over time, from emergence until after full leaf expansion (FLE). Ficus carica leaves showed a subtle change in shape during the early stages of development, and FLE was reached within ca. 30 days after emergence. Changes in leaf thickness and inclination after FLE demonstrated good adaptation to environmental conditions during summer in areas with a Mediterranean climate. Changes in gas exchange parameters and relative chlorophyll content showed that F. carica is a delayed‐greening species, reaching maximum values 20 days after FLE. Correlation analysis of datasets collected during leaf expansion, confirmed dependence among structural and functional traits in F. carica. Pn was directly correlated with stomatal conductance (Gs), transpiration (E), leaf area (LA) and relative chlorophyll content up to FLE. The effect of pruning on leaf expansion, a cultural technique commonly applied in this fruit tree, was also evaluated. Although leaf development in pruned branches gave a significantly higher relative leaf area growth rate (RGR_l) and higher LA than non‐pruned branches, no significant differences were found in other morphological and physiological traits, indicating no pruning effect on leaf development. All studied morphological and physiological characteristics indicate that F. carica is well adapted to semiarid conditions. The delayed greening strategy of this species is discussed.     

镉胁迫下硒对罗汉果组培苗光合特性的影响

实验以罗汉果组培苗为材料,室内栽培在内装市售营养土的塑料盆中,以0、10、50、100、200mg·kg-1浓度镉离子和1mg·kg-1浓度硒处理,培养20d后分析罗汉果幼苗的相关光合生理指标。结果表明:低浓度Cd2+对叶片叶绿素含量、光合速率(Pn)、蒸腾速率(Tr)、气孔导度(Gs)影响不大或稍有上升,但高浓度镉离子处理植株叶片的叶绿素含量、光合速率(Pn)、蒸腾速率(Tr)、气孔导度(Gs)明显下降;随Cd2+处理浓度的增加,叶片胞间CO2浓度(Ci)呈现上升趋势;加硒则延缓叶绿素下降,促进光合速率(Pn)、蒸腾速率(Tr)、气孔导度(Gs)上升,降低叶片胞间CO2浓度(Ci)。表明高浓度镉离子的毒害导致罗汉果组培苗叶片光合性能受到伤害,从而影响罗汉果幼苗生长。镉硒混合处理反映出硒对镉的毒害有缓解作用。     

Soil salinity effects on transpiration and net photosynthetic rates,stomatal conductance and Na+ and Cl− contents in durum wheat

Leaf gas exchange, plant growth and leaf ion content were measured in wheat (Triticum durum L. cv. HD 4502) exposed to steady- state salinities (1.6, 12.0 and 16.0 dS nr⁻¹) for 8 weeks. Salinity reduced leaf area and number of tillers, and increased Na⁺ and Cl⁻ concentrations in leaves. Leaf- to- leaf gradients of these ions were observed. The oldest leaf contained 6 to 8 times more Na⁺ and Cl⁻ than the flag leaf. Net photosynthetic rate (P_N), transpiration rate (E) and stomatal conductance (g_S) were the highest in flag leaf, declined in the middle and fully expanded leaves, and were minimum in the oldest leaves. These processes were reduced by salinity with similar leaf- to- leaf gradients. Intercellular CO₂ concentrations in the older leaves were higher than in the flag leaf in non-saline plants, and increased similarly with salinity. Leaf age was the major factor in reducing P_N, and senescence processes were promoted by salinity.     

The Heterogeneity and Spatial Patterning of Structure and Physiology across the Leaf Surface in Giant Leaves of Alocasia macrorrhiza

Leaf physiology determines the carbon acquisition of the whole plant, but there can be considerable variation in physiology and carbon acquisition within individual leaves. Alocasia macrorrhiza (L.) Schott is an herbaceous species that can develop very large leaves of up to 1 m in length. However, little is known about the hydraulic and photosynthetic design of such giant leaves. Based on previous studies of smaller leaves, and on the greater surface area for trait variation in large leaves, we hypothesized that A. macrorrhiza leaves would exhibit significant heterogeneity in structure and function. We found evidence of reduced hydraulic supply and demand in the outer leaf regions; leaf mass per area, chlorophyll concentration, and guard cell length decreased, as did stomatal conductance, net photosynthetic rate and quantum efficiency of photosystem II. This heterogeneity in physiology was opposite to that expected from a thinner boundary layer at the leaf edge, which would have led to greater rates of gas exchange. Leaf temperature was 8.8°C higher in the outer than in the central region in the afternoon, consistent with reduced stomatal conductance and transpiration caused by a hydraulic limitation to the outer lamina. The reduced stomatal conductance in the outer regions would explain the observed homogeneous distribution of leaf water potential across the leaf surface. These findings indicate substantial heterogeneity in gas exchange across the leaf surface in large leaves, greater than that reported for smaller-leafed species, though the observed structural differences across the lamina were within the range reported for smaller-leafed species. Future work will determine whether the challenge of transporting water to the outer regions can limit leaf size for plants experiencing drought, and whether the heterogeneity of function across the leaf surface represents a particular disadvantage for large simple leaves that might explain their global rarity, even in resource-rich environments.     

高粱、紫苏叶脉密度与光合特性的关系

叶脉是植物叶片光合作用水分输送的重要结构。为阐述叶脉与光合特性之间的关系,以C4植物高粱(Sorghum bicolor)、C3植物紫苏(Perilla frutescens)为实验材料研究了叶脉密度和光合特性之间的关系。结果表明,与紫苏相比,高粱叶片叶脉密度大,导水能力强,蒸腾速率高,但气孔密度小。进一步分析表明,高粱叶片近轴侧气孔密度占总气孔的比例明显高于紫苏。叶脉密度大的高粱具有较高的净光合速率;而紫苏叶脉密度小,净光合速率也较低。由此表明,较高的叶脉密度有利于支持较高的光合速率,但研究表明叶脉密度和气孔密度可能不存在严格的协同变异关系。研究结果对理解植物光合作用适应有重要意义。     

Photosynthesis and stomatal conductance of potato leaves—effects of leaf age,irradiance, and leaf water potential

Potatoes (Solanum tuberosum L., cv. Bintje) were grown in a naturally lit glasshouse. Laboratory measurements on leaves at three insertion levels showed a decline with leaf age in photosynthetic capacity and in stomatal conductance at near saturating irradiance. Conductance declined somewhat more with age than photosynthesis, resulting in a smaller internal CO₂ concentration in older relative to younger leaves. Leaves with different insertion number behaved similarly. The changes in photosynthesis rate and in nitrogen content with leaf age were closely correlated. When PAR exceeded circa 100 W m^–2 the rate of photosynthesis and stomatal conductance changed proportionally as indicated by a constant internal CO₂ concentration. The photosynthesis-irradiance data were fitted to an asymptotic exponential model. The parameters of the model are AMAX, the rate of photosynthesis at infinite irradiance, and EFF, the slope at low light levels. AMAX declined strongly with leaf age, as did EFF, but to a smaller extent. During drought stress photosynthetic capacity declined directly with decreasing water potential (range –0.6 to –1.1 MPa). Initially, stomatal conductance declined faster than photosynthetic capacity.Abbreviations LNx leaf number x, counted in acropetal direction - DAP days after planting - DALA days after leaf appearance - C_i CO₂ concentration in the leaf - C_a CO₂ concentration in ambient air - LWP leaf water potential - OP osmotic potential - PAR photosynthetically active radiation     

Analysis of salinity effects on basil leaf surface area, photosynthetic activity, and growth

Basil (Ocimum basilicum L.) seedlings were cultured on liquid medium in controlled conditions. Two varieties differing in leaf size were compared. When plants were 30?days old, the medium was supplemented with 50?mM NaCl. After 15?days of treatment, root, stem and leaf biomass, leaf number, and leaf surface area were measured. Ion accumulation was determined in roots, stems, and leaves. Photosynthetic parameters (CO₂ fixation rate, internal CO₂ concentration, stomatal conductance) as well as transpiration rate were determined on separate leaves. Electrolyte leakage and malondialdehyde content were used to estimate damage to membranes and lipid peroxidation, respectively. Several antioxidant enzymatic activities were used as proxies of oxidative stress. High Na⁺ concentration was reached in leaf tissues. Salt restricted whole plant biomass deposition rate by diminishing leaf number and leaf expansion, as well as photosynthetic activity were estimated from whole plant biomass production per unit leaf surface area. Diminished stomatal conductance restricted CO₂ fixation rate, and decrease in chlorophyll content presumably limited photosynthetic activity. Lipid peroxidation revealed damages to membranes. The magnitude of these responses differed between the two varieties, indicating that an intraspecific variability in salt response exists in basil.     

Systemic regulation of leaf anatomical structure, photosynthetic performance, and high-light tolerance in sorghum

Leaf anatomy of C3 plants is mainly regulated by a systemic irradiance signal. Since the anatomical features of C4 plants are different from that of C3 plants, we investigated whether the systemic irradiance signal regulates leaf anatomical structure and photosynthetic performance in sorghum (Sorghum bicolor), a C4 plant. Compared with growth under ambient conditions (A), no significant changes in anatomical structure were observed in newly developed leaves by shading young leaves alone (YS). Shading mature leaves (MS) or whole plants (S), on the other hand, caused shade-leaf anatomy in newly developed leaves. By contrast, chloroplast ultrastructure in developing leaves depended only on their local light conditions. Functionally, shading young leaves alone had little effect on their net photosynthetic capacity and stomatal conductance, but shading mature leaves or whole plants significantly decreased these two parameters in newly developed leaves. Specifically, the net photosynthetic rate in newly developed leaves exhibited a positive linear correlation with that of mature leaves, as did stomatal conductance. In MS and S treatments, newly developed leaves exhibited severe photoinhibition under high light. By contrast, newly developed leaves in A and YS treatments were more resistant to high light relative to those in MS- and S-treated seedlings. We suggest that (1) leaf anatomical structure, photosynthetic capacity, and high-light tolerance in newly developed sorghum leaves were regulated by a systemic irradiance signal from mature leaves; and (2) chloroplast ultrastructure only weakly influenced the development of photosynthetic capacity and high-light tolerance. The potential significance of the regulation by a systemic irradiance signal is discussed.     

Effect of leaf surface wetness and wettability on photosynthesis in bean and pea

Leaf surface wetness that occurs frequently in natural environments has a significant impact on leaf photosynthesis. However, the physiological mechanisms for the photosynthetic responses to wetness are not well understood. The responses of leaf CO₂ assimilation rate (A) to 72 h of artificial mist of a wettable (bean; Phaseolus vulgaris) and a non‐wettable species (pea; Pisum sativum) were compared. Stomatal and non‐stomatal limitations to A were investigated. A 28% inhibition of A was observed in the bean leaves as a result of a 16% decrease in stomatal conductance and a 55% reduction in the amount of Rubisco. The decrease of Rubisco was mainly due to its partial degradation. In contrast to the bean leaves, a 22% stimulation of A was obtained in the 72 h mist‐treated pea leaves. Mist treatment increased stomatal conductance by 12.5% and had no effect on the amount of Rubisco. These results indicated that a positive photosynthetic response to wetness occurred only in non‐wettable species and is due to the change in stomatal regulation.     

The consequences of delayed greening during leaf development for light absorption and light use efficiency

Many species of rainforest plants have an unusual form of leaf development such that leaves delay greening until after full leaf expansion. Chlorophyll accumulation was measured during leaf development in five woody rainforest species, three with white young leaves, and two with ‘normal’ greening. In the three species with white leaves, the chlorophyll content of the expanding leaves was about 0.4mg dm^?2, whereas in the two species with green young leaves, chlorophyll content was about 2.1 mg dm^?2. Chlorophyll accumulation in greenhouse and field experiments was independent of light level. During leaf expansion, species with delayed chloroplast development only absorb 18–25% of the maximum possible light, compared with 80% for species with normal greening. Furthermore, species with delayed greening have low chlorophyll contents and reduced absorption for at least 30 d after full expansion. At a PPFD typical of the forest under story, the photosynthetic light use efficiency based upon incident radiation was 0.030–0.036 for species with delayed chloroplast development and 0.068–0.085 for the two species with normal greening. The lower light use efficiency of white species was primarily due to decreased light absorption. However, they also had a slightly lower light use efficiency based upon absorbed radiation, suggesting that development of other components of the photo-synthetic apparatus also may be delayed. Despite the fact that delayed greening decreases light absorption and light use efficiency during leaf development, it is extremely common in shade-tolerant species. We suggest that an advantage of delayed greening is that resources are not invested in the leaf until it is fully expanded and better defended from herbivores.     

SEASONAL PATTERNS OF LEAF PHOTOSYNTHETIC CAPACITY IN SUCCESSIONAL NORTHERN HARDWOOD TREE SPECIES

Seasonal patterns of leaf photosynthetic capacity and conductance were determined for deciduous hardwood tree species in natural habitats in northern lower Michigan. Leaves of bigtooth aspen and red oak at the top of the canopy had higher maximum CO₂ Exchange Rate (CER) (10–15 μmol m ² s ¹) than leaves of sugar maple, red maple, red oak, and beech growing in the understory (4–5 μmol m ² s ¹). In all leaves, CER measured at light-saturation increased to a maximum near the completion of leaf expansion in early June, was constant until mid-September, and then rapidly declined until leaf death. A similar pattern was seen for CER measured in low light (1.5% full sun). Respiration rate in the dark was highest in young leaves and decreased during leaf expansion; a relatively constant rate was then maintained for the rest of leaf lifespan. The seasonal pattern of the initial slope of the light response of CER paralleled the pattern of light-saturated CER. The initial slope in midsummer ranged from values of 37 to 44 μmol/mol for species in the understory to 51 and 56 μmol/mol for red oak and bigtooth aspen, respectively, at the top of the canopy. Leaf conductance was constant throughout most of leaf lifespan, with some decline occurring in autumn. Leaves at the top of the canopy had higher conductances for water vapor (2–5 mm/s) than leaves in the understory (1–2 mm/s). All species maintained leaf intercellular CO, mole fractions (c,) near 200 uML/L until autumn, when c, increased during leaf senescence.