The Effects of Increased Illumination and Shading on the Low-Light-Induced Decline in Photosynthesis in Cotton Leaves

An experiment was carried out to study whether low-light-induced damage to the photosynthetic system in leaves of cotton (Gossypium hirsutum cv. Deltapine) which are below the compensation point in the canopy can be arrested and reversed by increased illumination. In addition it was intended to find out whether the photosynthetic system in leaves of shade plants show a greater resistance to low-light-induced damage than leaves of plants from more exposed habitats. The plants were grown at high density, and increased illumination to the shade leaves in the canopy was achieved by thinning the stand. Thinning was carried out at two stages and its effects on the decline in the photosynthetic capacity of the 4th leaf were followed. An early thinning was carried out shortly after the 4th leaf dropped below the compensation point and a late thinning 2 weeks later. Comparison was also made between the low-light-induced damage to the photosynthetic capacity of the 4th leaf in plants grown under two light regimes during the progressive increase in self-shading of the 4th leaf within the canopy. It was observed that both types of thinning arrested the low-light-induced damage to the photosynthetic system in shade leaves. The decline in photosynthetic capacity of the 4th leaf was stopped after both early and late thinning. The dry weight of the shoot system in the early and late thinned plants was not significantly different. It was double that of the control plants. The plants thinned early did not have higher shoot weight than the late thinned plants since there was a rapid shedding of flowers and fruits after early thinning. The 4th leaf in the early thinned plants showed a 30% increase in chlorophyll content and dry weight per unit leaf area. It is suggested that shedding of flowers and fruits, and increases in chlorophyll and dry weight per unit leaf area in the early thinned plants were caused by a change in the hormonal balance of the plants. The photosynthetic system in leaves of shade plants showed a greater resistance to damage by low light intensity than the photosynthetic system in leaves of plants grown at higher light intensities.     

Photosynthesis and resource distribution through plant canopies

Plant canopies are characterized by dramatic gradients of light between canopy top and bottom, and interactions between light, temperature and water vapour deficits. This review summarizes current knowledge of potentials and limitations of acclimation of foliage photosynthetic capacity (A(max)) and light-harvesting efficiency to complex environmental gradients within the canopies. Acclimation of A(max) to high light availability involves accumulation of rate-limiting photosynthetic proteins per unit leaf area as the result of increases in leaf thickness in broad-leaved species and volume: total area ratio and mesophyll thickness in species with complex geometry of leaf cross-section. Enhancement of light-harvesting efficiency in low light occurs through increased chlorophyll production per unit dry mass, greater leaf area per unit dry mass investment in leaves and shoot architectural modifications that improve leaf exposure and reduce within-shoot shading. All these acclimation responses vary among species, resulting in species-specific use efficiencies of low and high light. In fast-growing canopies and in evergreen species, where foliage developed and acclimated to a certain light environment becomes shaded by newly developing foliage, leaf senescence, age-dependent changes in cell wall characteristics and limited foliage re-acclimation capacity can constrain adjustment of older leaves to modified light availabilities. The review further demonstrates that leaves in different canopy positions respond differently to dynamic fluctuations in light availability and to multiple environmental stresses. Foliage acclimated to high irradiance respond more plastically to rapid changes in leaf light environment, and is more resistant to co-occurring heat and water stress. However, in higher light, co-occurring stresses can more strongly curb the efficiency of foliage photosynthetic machinery through reductions in internal diffusion conductance to CO(2). This review demonstrates strong foliage potential for acclimation to within-canopy environmental gradients, but also highlights complex constraints on acclimation and foliage functioning resulting from light x foliage age interactions, multiple environmental stresses, dynamic light fluctuations and species-specific leaf and shoot structural constraints.     

The phenology of leaf quality and its within‐canopy variation is essential for accurate modeling of photosynthesis in tropical evergreen forests

Leaf quantity (i.e., canopy leaf area index, LAI), quality (i.e., per‐area photosynthetic capacity), and longevity all influence the photosynthetic seasonality of tropical evergreen forests. However, these components of tropical leaf phenology are poorly represented in most terrestrial biosphere models (TBMs). Here, we explored alternative options for the representation of leaf phenology effects in TBMs that employ the Farquahar, von Caemmerer & Berry (FvCB) representation of CO₂ assimilation. We developed a two‐fraction leaf (sun and shade), two‐layer canopy (upper and lower) photosynthesis model to evaluate different modeling approaches and assessed three components of phenological variations (i.e., leaf quantity, quality, and within‐canopy variation in leaf longevity). Our model was driven by the prescribed seasonality of leaf quantity and quality derived from ground‐based measurements within an Amazonian evergreen forest. Modeled photosynthetic seasonality was not sensitive to leaf quantity, but was highly sensitive to leaf quality and its vertical distribution within the canopy, with markedly more sensitivity to upper canopy leaf quality. This is because light absorption in tropical canopies is near maximal for the entire year, implying that seasonal changes in LAI have little impact on total canopy light absorption; and because leaf quality has a greater effect on photosynthesis of sunlit leaves than light limited, shade leaves and sunlit foliage are more abundant in the upper canopy. Our two‐fraction leaf, two‐layer canopy model, which accounted for all three phenological components, was able to simulate photosynthetic seasonality, explaining ~90% of the average seasonal variation in eddy covariance‐derived CO₂ assimilation. This work identifies a parsimonious approach for representing tropical evergreen forest photosynthetic seasonality in TBMs that utilize the FvCB model of CO₂ assimilation and highlights the importance of incorporating more realistic phenological mechanisms in models that seek to improve the projection of future carbon dynamics in tropical evergreen forests.     

Comparative ecophysiology of leaf and canopy photosynthesis

Leaves and herbaceous leaf canopies photosynthesize efficiently although the distribution of light, the ultimate resource of photosynthesis, is very biased in these systems. As has been suggested in theoretical studies, if a photosynthetic system is organized such that every photosynthetic apparatus photosynthesizes in concert, the system as a whole has the sharpest light response curve and is most adaptive. This condition can be approached by (i) homogenization of the light environment and (ii) acclimation of the photosynthetic properties of leaves or chloroplasts to their local light environments. This review examines these two factors in the herbaceous leaf canopy and in the leaf. Changes in the inclination of leaves in the canopy and differentiation of mesophyll into palisade and spongy tissue contribute to the moderation of the light gradient. Leaf and chloroplast movements in the upper parts of these systems under high irradiances also moderate light gradients. Moreover, acclimation of leaves and chloroplasts to the local light environment is substantial. These factors increase the efficiency of photosynthesis considerably. However, the systems appear to be less efficient than the theoretical optimum. When the systems are optically dense, the light gradients may be too great for leaves or chloroplasts to acclimate. The loss of photosynthetic production attributed to the imperfect adjustment of photosynthetic apparatus to the local light environment is most apparent when the photosynthesis of the system is in the transition between the light-limited and light-saturated phases. Although acclimation of the photosynthetic apparatus and moderation of light gradients are imperfect, these markedly raise the efficiency of photosynthesis. Thus more mechanistic studies on these adaptive attributes are needed. The causes and consequences of imperfect adjustment should also be investigated.     

The geometry of light interception by shoots of Heteromeles arbutifolia: morphological and physiological consequences for individual leaves

The influence of leaf orientation and position within shoots on individual leaf light environments, carbon gain, and susceptibility to photoinhibition was studied in the California chaparral shrub Heteromeles arbutifolia with measurements of gas exchange and chlorophyll fluorescence, and by application of a three-dimensional canopy architecture model. Simulations of light absorption and photosynthesis revealed a complex pattern of leaf light environments and resulting leaf carbon gain within the shoots. Upper, south-facing leaves were potentially the most productive because they intercepted greater daily photon flux density (PFD) than leaves of any other orientation. North-facing leaves intercepted less PFD but of this, more was received on the abaxial surface because of the steep leaf angles. Leaves differed in their response to abaxial versus adaxial illumination depending on their orientation. While most had lower photosynthetic rates when illuminated on their abaxial as compared to adaxial surface, the photosynthetic rates of north-facing leaves were independent of the surface of illumination. Because of the increasing self-shading, there were strong decreases in absorbed PFD and daily carbon gain in the basipetal direction. Leaf nitrogen per unit mass also decreased in the basipetal direction but on a per unit area basis was nearly constant along the shoot. The decrease in leaf N per unit mass was accounted for by an increase in leaf mass per unit area (LMA) rather than by movement of N from older to younger leaves during shoot growth. The increased LMA of older lower leaves may have contributed directly to their lower photosynthetic capacities by increasing the limitations to diffusion of CO₂ within the leaf to the sites of carboxylation. There was no evidence for sun/shade acclimation along the shoot. Upper leaves and especially south-facing upper leaves had a potential risk for photoinhibition as demonstrated by the high PFDs received and the diurnal decreases in the fluorescence ratio F _v/F _m. Predawn F _v/F _m ratios remained high (>0.8) indicating that when in their normal orientations leaves sustained no photoinhibition. Reorientation of the leaves to horizontal induced a strong sustained decrease in F _v/F _m and CO₂ exchange that slowly recovered over the next 10–15?days. If leaves were also inverted so that the abaxial surface received the increased PFDs, then the reduction in F _v/F _m and CO₂ assimilation was much greater with no evidence for recovery. The heterogeneity of responses was due to a combination of differences between leaves of different orientation, differences between responses on their abaxial versus adaxial surfaces, and differences along the shoot due to leaf age and self-shading effects.     

Leaf traits and leaf life spans of two xeric-adapted palmettos

Plants of nutrient-poor, arid environments often have leaf traits that include small size, sclerophylly, long life span, low nutrient concentration, and low photosynthetic rate. Hence, the success of two large-leaved palmettos in peninsular Florida's seasonally xeric, nutrient-impoverished uplands seems anomalous, given that their leaves are orders of magnitude larger than the leaves of sympatric species. An examination of a 16-yr data set of leaf traits and leaf life spans across four vegetative associations differing in available light showed that Serenoa repens and Sabal etonia had low rates of leaf production coupled with long leaf life spans reaching 3.5 yr in heavily shaded plants. The adaptation of these palmettos to xeric, nutrient-poor habitats has generated dwarf statures, diminished leaf sizes and numbers, increased leaf life spans, and reduced rates of leaf production relative to other palms and congeners of more mesic sites. Leaf and petiole size, plant leaf canopy area, and leaf life span increased in both palmettos with decreasing available light, helping to compensate for reduced photosynthetic rates under shaded conditions and for the high leaf construction costs of the large, thick palmetto leaves. Large leaf size in these palmettos, likely due to phylogenetic conservatism, is compensated by other leaf traits (e.g., heavily cutinized epidermises, thick laminas) that increase survival in seasonally xeric, nutrient-impoverished environments.     

Nitrogen reallocation and photosynthetic acclimation in response to partial shading in soybean plants

The first trifoliate of soybean was shaded when fully expanded, while the plant remained in high light; a situation representative for plants growing in a closed crop. Leaf mass and respiration rate per unit area declined sharply in the first few days upon shading and remained rather constant during the further 12 days of the shading treatment. Leaf nitrogen per unit area decreased gradually until the leaves were shed. Leaf senescence was enhanced by the shading treatment in contrast to control plants growing in low light. Shaded leaves on plants grown at low nutrient availability senesced earlier than shaded leaves on plants grown at high nutrient availability. The light saturated rate of photosynthesis decreased also gradually during the shading treatment, but somewhat faster than leaf N, whereas chlorophyll contents declined somewhat slower than leaf N.
Partitioning of N in the leaf over main photosynthetic functions was estimated from parameters derived from the response of photosynthesis to CO₂. It appeared that the N exported from the leaf was more at the expense of compounds that make up photosynthetic capacity than of those involved in photon absorption, resulting in a change in partitioning of N within the photosynthetic apparatus. Photosynthetic nitrogen use efficiency increased during the shading treatment, which was for the largest part due to the decrease in leaf N content, to some extent to the decrease in respiration rate and only for a small part to change in partitioning of N within the photosynthetic apparatus.     

Photosynthetic stimulation under long-term CO2 enrichment and fertilization is sustained across a closed Populus canopy profile (EUROFACE)

The long-term response of leaf photosynthesis to rising CO2 concentrations [CO2] depends on biochemical and morphological feedbacks. Additionally, responses to elevated [CO2] might depend on the nutrient availability and the light environment, affecting the net carbon uptake of a forest stand. After 6 yr of exposure to free-air CO2 enrichment (EUROFACE) during two rotation cycles (with fertilization during the second cycle), profiles of light, leaf characteristics and photosynthetic parameters were measured in the closed canopy of a poplar (Populus) short-rotation coppice. Net photosynthetic rate (A(growth)) was 49% higher in poplars grown in elevated [CO2], independently of the canopy position. Jmax significantly increased (15%), whereas leaf carboxylation capacity (Vcmax), leaf nitrogen (N(a)) and chlorophyll (Chl(a)) were unaffected in elevated [CO2]. Leaf mass per unit area (LMA) increased in the upper canopy. Fertilization created more leaves in the top of the crown. These results suggest that the photosynthetic stimulation by elevated [CO2] in a closed-canopy poplar coppice might be sustained in the long term. The absence of any down-regulation, given a sufficient sink capacity and nutrient availability, provides more carbon for growth and storage in this bioenergy plantation.     

Vertical gradients in photosynthetic physiology diverge at the latitudinal range extremes of white spruce

Light availability drives vertical canopy gradients in photosynthetic functioning and carbon (C) balance, yet patterns of variability in these gradients remain unclear. We measured light availability, photosynthetic CO₂ and light response curves, foliar C, nitrogen (N) and pigment concentrations, and the photochemical reflectance index (PRI) on upper and lower canopy needles of white spruce trees (Picea glauca) at the species' northern and southern range extremes. We combined our photosynthetic data with previously published respiratory data to compare and contrast canopy C balance between latitudinal extremes. We found steep canopy gradients in irradiance, photosynthesis and leaf traits at the southern range limit, but a lack of variation across canopy positions at the northern range limit. Thus, unlike many tree species from tropical to mid-latitude forests, high latitude trees may not require vertical gradients of metabolic activity to optimize photosynthetic C gain. Consequently, accounting for self-shading is less critical for predicting gross primary productivity at northern relative to southern latitudes. Northern trees also had a significantly smaller net positive leaf C balance than southern trees suggesting that, regardless of canopy position, low photosynthetic rates coupled with high respiratory costs may ultimately constrain the northern range limit of this widely distributed boreal species.     

Photosynthetic acclimation to simultaneous and interacting environmental stresses along natural light gradients: optimality and constraints

There is a strong natural light gradient from the top to the bottom in plant canopies and along gap-understorey continua. Leaf structure and photosynthetic capacities change close to proportionally along these gradients, leading to maximisation of whole canopy photosynthesis. However, other environmental factors also vary within the light gradients in a correlative manner. Specifically, the leaves exposed to higher irradiance suffer from more severe heat, water, and photoinhibition stresses. Research in tree canopies and across gap-understorey gradients demonstrates that plants have a large potential to acclimate to interacting environmental limitations. The optimum temperature for photosynthetic electron transport increases with increasing growth irradiance in the canopy, improving the resistance of photosynthetic apparatus to heat stress. Stomatal constraints on photosynthesis are also larger at higher irradiance because the leaves at greater evaporative demands regulate water use more efficiently. Furthermore, upper canopy leaves are more rigid and have lower leaf osmotic potentials to improve water extraction from drying soil. The current review highlights that such an array of complex interactions significantly modifies the potential and realized whole canopy photosynthetic productivity, but also that the interactive effects cannot be simply predicted as composites of additive partial environmental stresses. We hypothesize that plant photosynthetic capacities deviate from the theoretical optimum values because of the interacting stresses in plant canopies and evolutionary trade-offs between leaf- and canopy-level plastic adjustments in light capture and use.     

Leaf canopy as a dynamic system: ecophysiology and optimality in leaf turnover

BACKGROUND AND AIMS: In a leaf canopy, there is a turnover of leaves; i.e. they are produced, senesce and fall. These processes determine the amount of leaf area in the canopy, which in turn determines canopy photosynthesis. The turnover rate of leaves is affected by environmental factors and is different among species. This mini-review discusses factors responsible for leaf dynamics in plant canopies, focusing on the role of nitrogen. SCOPE: Leaf production is supported by canopy photosynthesis that is determined by distribution of light and leaf nitrogen. Leaf nitrogen determines photosynthetic capacity. Nitrogen taken up from roots is allocated to new leaves. When leaves age or their light availability is lowered, part of the leaf nitrogen is resorbed. Resorbed nitrogen is re-utilized in new organs and the rest is lost with dead leaves. The sink-source balance is important in the regulation of leaf senescence. Several models have been proposed to predict response to environmental changes. A mathematical model that incorporated nitrogen use for photosynthesis explained well the variations in leaf lifespan within and between species. CONCLUSION: When leaf turnover is at a steady state, the ratio of biomass production to nitrogen uptake is equal to the ratio of litter fall to nitrogen loss, which is an inverse of the nitrogen concentration in dead leaves. Thus nitrogen concentration in dead leaves (nitrogen resorption proficiency) and nitrogen availability in the soil determine the rate of photosynthesis in the canopy. Dynamics of leaves are regulated so as to maximize carbon gain and resource-use efficiency of the plant.     

Optimal photosynthetic use of light by tropical tree crowns achieved by adjustment of individual leaf angles and nitrogen content

Background and Aims

Theory for optimal allocation of foliar nitrogen (ONA) predicts that both nitrogen concentration and photosynthetic capacity will scale linearly with gradients of insolation within plant canopies. ONA is expected to allow plants to efficiently use both light and nitrogen. However, empirical data generally do not exhibit perfect ONA, and light-use optimization per se is little explored. The aim was to examine to what degree partitioning of nitrogen or light is optimized in the crowns of three tropical canopy tree species.

Methods

Instantaneous photosynthetic photon flux density (PPFD) incident on the adaxial surface of individual leaves was measured along vertical PPFD gradients in tree canopies at a frequency of 0·5 Hz over 9–17 d, and summed to obtain the average daily integral of PPFD for each leaf to characterize its insolation regime. Also measured were leaf N per area (N_area), leaf mass per area (LMA), the cosine of leaf inclination and the parameters of the photosynthetic light response curve [photosynthetic capacity (A_max), dark respiration (R_d), apparent quantum yield (ϕ) and curvature (θ)]. The instantaneous PPFD measurements and light response curves were used to estimate leaf daily photosynthesis (A_daily) for each leaf.

Key Results

Leaf N_area and A_max changed as a hyperbolic asymptotic function of the PPFD regime, not the linear relationship predicted by ONA. Despite this suboptimal nitrogen partitioning among leaves, A_daily did increase linearly with PPFD regime through co-ordinated adjustments in both leaf angle and physiology along canopy gradients in insolation, exhibiting a strong convergence among the three species.

Conclusions

The results suggest that canopy tree leaves in this tropical forest optimize photosynthetic use of PPFD rather than N per se. Tropical tree canopies then can be considered simple ‘big-leaves’ in which all constituent ‘small leaves’ use PPFD with the same photosynthetic efficiency.Key words: Optimal resource allocation, nitrogen, photosynthetic capacity, leaf mass per area, tropical trees, radiation use efficiency, scaling, leaf angle, canopy architecture, big leaf model     

Photosynthetic acclimation to elevated CO2 in a sunflower canopy

Sunflower canopies were grown in mesocosom gas exchange chambers at ambient and elevated CO2 concentrations (360 and 700 ppm) and leaf photosynthetic capacities measured at several depths within each canopy. Elevated [CO2] had little effect on whole-canopy photosynthetic capacity and total leaf area, but had marked effects on the distribution of photosynthetic capacity and leaf area within the canopy. Elevated [CO2] did not significantly reduce the photosynthetic capacities per unit leaf area of young leaves at the top of the canopy, but it did reduce the photosynthetic capacities of older leaves by as much as 40%. This effect was not dependent on the canopy light environment since elevated [CO2] also reduced the photosynthetic capacities of older leaves exposed to full sun on the south edge of the canopy. In addition to the effects on leaf photosynthetic capacity, elevated [CO2] shifted the distribution of leaf area within the canopy so that more leaf area was concentrated near the top of the canopy. This change resulted in as much as a 50% reduction in photon flux density in the upper portions of the elevated [CO2] canopy relative to the ambient [CO2] canopy, even though there was no significant difference in the total canopy leaf area. This reduction in PFD appeared to account for leaf carbohydrate contents that were actually lower for many of the shaded leaves in the elevated as opposed to the ambient [CO2] canopy. Photosynthetic capacities were not significantly correlated with any of the individual leaf carbohydrate contents. However, there was a strong negative correlation between photosynthetic capacity and the ratio of hexose sugars to sucrose, consistent with the hypothesis that sucrose cycling is a component of the biochemical signalling pathway controlling photosynthetic acclimation to elevated [CO2].     

Is N‐resorption efficiency related to secondary compounds and leaf longevity in coexisting plant species of the arid Patagonian Monte,Argentina?

Leaf longevity and nutrient resorption efficiency are important strategies to conserve plant nutrients. Theory suggests a negative relationship between them and also proposes that high concentration of phenolics in long‐lived leaves may reduce nutrient resorption. In order to provide new evidence on these relationships, we explored whether N‐resorption efficiency is related to leaf longevity, secondary compounds and other leaf traits in coexisting plant species of different life forms in the arid Patagonian Monte, Argentina. We assessed N‐resorption efficiency, green leaf traits (leaf mass per area (LMA), leaf longevity and lignin, total soluble phenolics and N concentrations) and N concentration in senescent leaves of 12 species of different life forms (evergreen shrubs, deciduous shrubs and perennial grasses) with contrasting leaf traits. We found that leaf longevity was positively correlated to LMA and lignin, and negatively correlated to N concentration in green leaves. N concentrations both in green and senescent leaves were positively related. N‐resorption efficiency was not associated with the concentration of secondary compounds (total soluble phenolics and lignin) but it was negatively related to LMA and leaf longevity and positively related to N concentration in green leaves. Furthermore, leaf traits overlapped among life forms highlighting that life forms are not a good indicator of the functional properties (at least in relation to nutrient conservation) of species. In conclusion, our findings indicated that differences in N‐resorption efficiency among coexisting species were more related to N concentration in green leaves, leaf lifespan and LMA than to the presence of secondary compounds at least those assessed in our study (soluble phenolics and lignin). Accordingly, N‐resorption efficiency seems to be modulated, at least in part, by the productivity–persistence trade‐off.     

Acclimation of plants to light gradients in leaf canopies: evidence for a possible role for cytokinins transported in the transpiration stream.

The mechanism of response of plants to vertical light intensity gradients in leaf canopies was investigated. Since shaded leaves transpire less than leaves in high light, it was hypothesized that cytokinins (CKs) carried by mass transport in the transpiration stream would be distributed over the leaf area of partially shaded plants parallel to the gradient in light intensity. It was also hypothesized that this causes the distribution of leaf growth, leaf N and photosynthetic capacity, and possibly chloroplast acclimation as observed in plants growing in leaf canopies. In a field experiment, the distribution of Ca, N and CKs in a bean leaf canopy of a dense and an open stand supported the concept of a role for CKs in the response of N allocation to the light gradient when a decreasing sensitivity for CKs with increasing leaf age is assumed. Both shading of one leaf of the pair of primary bean leaves and independent reduction of its transpiration rate in a growth cabinet experiment caused lower dry mass, N and Ca per unit leaf area in comparison to the opposite not treated leaf. Shading caused a parallel reduction in CK concentration, which supports the hypothesis, but independent reduction of transpiration rate failed to do the same. Application of benzylaminopurine (BA) counteracted the reduction caused by shade of leaf N, photosynthetic capacity and leaf area growth. The experiments show an important role for the transpiration stream in the response of plants to light gradients. Evidence is presented here that CKs carried in the transpiration stream may be important mediators for the acclimation of plants to leaf canopy density.     

Leaf life spans in wild perennial herbaceous plants: a survey and attempts at a functional interpretation

Summary Leaf longevity in 29 herbaceous plant species of Central Europe was studied by inspecting tagged leaves at weekly intervals. About half of the species are elements of the lowland meadow flora, the other half comprises a representative sample of species from the highest sites where vascular plants grow in the Alps. Shaded and water-stressed sites were avoided. Overall mean leaf longevity did not differ significantly between sites and amounted to 71±5 days at low and 68±4 days at high altitude. Leaf life spans ranged (with no clear altitudinal trend) from 41 to 95 days. Low-altitude forbs and grasses produced several leaf cohorts during their growth period, while most alpine species produced only one. Correlations were found between leaf duration and percent nitrogen content and carbon-cost/carbon-gain ratios, but not with leaf dry mass per unit leaf area and photosynthetic capacity alone. As leaf life spans increase, more C tends to be invested per unit CO₂ uptake and less N is invested per unit invested C. Thus, mass relationships rather than area relationships seem to be linked to leaf life span in these species, suggesting that leaf duration is associated with properties other than the efficiency of light utilization (e.g. mechanical strength, herbivory or pathogen resistance). It seems that the explanations of leaf duration that have been developed for evergreen/deciduous plants and for plants along steep light gradients do not apply to the variable life spans in leaves of perennial herbaceous plants of open habitats.     

间伐改形对陇东高原密闭富士苹果园冠层微域环境及叶片生理特性的影响

为了研究间伐改形对成龄乔化密闭红富士苹果园冠层微域环境、叶片显微结构、叶片生理特性和光合能力的影响,以16年生密闭红富士苹果园为研究对象,对果园冠层相对光照强度、温度、相对湿度、叶片叶绿素含量、显微结构、叶片光合和荧光等参数进行了测定。结果表明: 间伐改形后树体冠层相对光照强度、温度得到显著改善,分布更均衡,>30%的有效光强是对照(未间伐改形,CK)的1.57倍,温度比CK平均高1.1 ℃;间伐树体叶片叶绿素、叶片厚度、栅栏组织厚度显著提高,分别比CK提高了8.7%、5.4%、9.2%;叶片净光合速率、蒸腾速率、气孔导度也显著提高,分别比CK提高了12.6%、17.1%和7.3%。间伐果园和密闭果园叶片光合作用均受非气孔因素限制,间伐树体叶片的PSⅡ最大荧光产量和非光化学猝灭系数比CK提高了1.5%和2.1%。间伐改形后,叶片并未发生强光抑制,叶片单位反应中心吸收的光能、捕获的用于电子传递的能量和用于还原Q_A的能量得到显著提高。叶片生理特性与所处的光照、温度环境密切相关,密闭果园间伐改形后,果园冠层光照、温度得到改善,促进了叶片生长发育,改善了叶片显微结构,提高了叶片光合效能,是适宜陇东高原苹果产区密闭红富士果园调整和优化的关键措施。     

The nutritional status of plants from high altitudes

Summary Are plants at high altitudes short in nutrients? In order to answer this question the mineral nutrient content of leaves from over 150 plant species from 9 different mountain areas of all major climatic zones were analyzed (Kjeldahl nitrogen in all, phosphate in half of the samples, K, Mg, Mn, Ca in the Alps only). The majority of data are from herbaceous perennials, but shrubs and trees were studied as well. N-partitioning was studied in 45 herbaceous species from contrasting altitudes in the Alps. The survey falls into three categories: (1) comparisons of whole communities of species from contrasting altitudes, (2) analysis of altitudinal gradients, and (3) additional collections from high altitude sites alone. Unlike the other mineral nutrients, nitrogen content follows consistent altitudinal and latitudinal trends. The higher altitude sample always had higher N content per unit leaf area, irrespective of life form, wherever comparable plants (the same or related species) were investigated at contrasting altitudes. N content per unit dry weight (%) increased with altitude in herbaceous plants (in some species >4%), but was remarkably stable in evergreen woody plants (around 1%). The mean fraction of total plant N allocated to leaves of herbaceous plants in the Alps was the same at low and high altitude (1/3 of total). Leaf N (%) from the regional upper limits of higher plant life reveals a latitudinal decrease from subarctic to equatorial mountains, which may be related to the duration of annual leaf activity. Since mean N content per leaf area hardly differs between the uppermost sites, life span expectation (sink-duration) seems to control carbon investments rather than N input per leaf area. The growth of leaves at high altitude seems to be controlled in a way that leads to comparatively high nutrient contents, which in turn support high metabolic activity. Inherent developmental growth constraints inhibit nutrient dilution in the plant body and thus defy the application of classical concepts of plant-nutrient versus soil-nutrient relations developed for lowlands and in particular for cultivated plants. The results re-emphasize the global significance of links between nitrogen content, leaf sclerophylly, leaf longevity and photosynthetic capacity.Dedicated to Prof. Walter Larcher on the occasion of his 60th birthday, with thanks for leading me to comparative plant ecology     

The role of crown architecture, leaf phenology and photosynthetic activity in promoting complementary use of light among coexisting species in temperate forests

Mixed forests comprising multiple tree species with contrasting crown architectures, leaf phenologies, and photosynthetic activity, tend to have high ecosystem productivity. We propose that in such forests, differentiation among coexisting species in their spatial and temporal strategies for light interception, results in complementary use of light. Spatial differentiation among coexisting tree species occurs as a result of adaptation of crown architecture and shoot/leaf morphology to the spatially variable light conditions of the canopy, sub-canopy, and understory. Temporal differentiation occurs as a result of variation in leaf phenology and photosynthetic activity. The arrangement of leaves in both space and time is an important aspect of plant strategies for light interception and determines photosynthetic carbon gain of the plant canopy. For example, at the shoot level, morphological and phenological differentiation between long and short shoots reflects their respective shoot functions, indicating that spatial and temporal strategies for light interception are linked. Complementary use of light is a consequence of the spatiotemporal differentiation in light interception among coexisting species. Because coexisting species may show differentiation in strategies for resource acquisition (functional diversification) or convergence with respect to some limiting resource (functional convergence), the relative importance of various crown functions and their contribution to growth and survival of individuals need to be evaluated quantitatively and compared among coexisting species.     

Development of a nutrient pollution indicator using the seagrass,Zostera marina,along nutrient gradients in three New England estuaries

Worldwide, seagrasses provide important habitats in coastal ecosystems, but seagrass meadows are often degraded or destroyed by cultural eutrophication. Presently, there are no available tools for early assessment of nutrient over-enrichment; direct measurements of water column nutrients are ineffective since the nutrients typical of early enrichment are rapidly taken up by plants within the ecosystem. We investigated whether, in a gradient of nutrient availability but prior to actual habitat loss, eelgrass (Zostera marina L.) plant morphology and tissue nutrients might reflect environmental nutrient availability. Eelgrass responses to nitrogen along estuarine gradients were assessed; two of these plant responses were combined to create an early indicator of nutrient over-enrichment. Eelgrass plant morphology and leaf tissue nitrogen (N) were measured along nutrient gradients in three New England estuaries: Great Bay Estuary (NH), Narragansett Bay (RI) and Waquoit Bay (MA). Eelgrass leaf N was significantly higher in up-estuary sampling stations than stations down-estuary, reflecting environmental nitrogen gradients. Leaf N content showed high variance, however, limiting its ability to discriminate the early stages of eutrophication. To find a stronger indicator, plant morphological characteristics such as number of leaves per shoot, blade width, and leaf and sheath length were examined, but they only weakly correlated with leaf tissue N. Area normalized leaf mass (mg dry weight cm⁻²), however, exhibited a strong and consistently negative relationship with leaf tissue N and a significant response to the estuarine nutrient gradients. We found the ratio of leaf N to leaf mass to be a more sensitive and consistent indicator of early eutrophication than either characteristic alone. We suggest the use of this ratio as a nutrient pollution indicator (NPI).