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Plants respond to environmental stressors, such as an oligotrophic environments, by altering the morphological and physiological functions of their leaves. Sex affects these functions because of the asymmetric cost of reproduction in dioecious plants. We compared the leaf mass per leaf area (LMA), ratio of intercellular air space in leaf mesophyll tissue (mesophyll porosity), palisade thickness, and carbon isotope ratio (δ 13 C) of leaves of the dioecious shrub Myrica gale based on sex and gradients of soil water chemistry across habitats in the field. The PCA showed that the first three principal components accounted for 84.5% of the variation. PC1 to PC3 were associated with the origin of soil water, nitrogen status of habitats, and sea–salt contributions, respectively. LMA varied from 5.22 to 7.13 μg/cm 2 , and it was positively related to PC2 and negatively related to PC3, but not to PC1 or sex, suggesting that LMA was low under poor nitrogen conditions and varied with salinity. Mesophyll porosity values were over 50% for all habitats. Mesophyll porosity was positively affected by PC3 and smaller in females than in males. This suggests that M . gale exhibits differences in mesophyll anatomy according to sex. Palisade thickness ranged from 0.466 to 0.559 mm/mm. The leaves of females had thinner palisade layers per mesophyll layer than those of males; however, the habitat did not affect the thickness of the palisade layer per mesophyll layer. The δ 13 C values of leaves varied from −32.14 to −30.51 ‰. We found that δ 13 C values were positively related to PC2 but not to PC1, PC3, and sex. Under poor nitrogen conditions, the δ 13 C of M . gale leaves decreased, suggesting that nutrient deficiency would decrease more under the long-term averaged ratio of photosynthesis than stomatal conductance, leading to low water use efficiency.

Key messageIn urban shrub trees, the species-specific photosynthetic response and water-use properties are related to the xylem anatomy of the petiole.It is becoming essential to select urban tree species based on drought response in warm temperate regions, because water limitation is prone to occur in an urban environment, and furthermore, urban warming along with global warming intensifies drought stress even in relatively humid regions. We focused on leaf photosynthesis, water use efficiency, and leaf water relations as key factors for the evaluation of drought response in urban trees, and compared their responses to drought stress and re-watering (recovery) in five major urban shrub tree species planted in Japan. In addition, species-specific xylem anatomical traits in the leaf petiole were evaluated. The five species showed diverse responses to drought and recovery. Rhaphiolepis umbellata possessed both the highest photosynthesis (A) and highest intrinsic water use efficiency (A/gs) under drought, as well as full recovery in the midday leaf water potential (Ψmid). These results suggest that R. umbellata is the most favorable species as an urban tree among the five species. In contrast, A and A/gs in Rhododendron obtusum were only 19% and 55%, respectively, of those in R. umbellata under drought, along with incomplete recovery in Ψmid. The responses of A, A/gs, and Ψmid for the other three species were intermediate between R. umbellata and R. obtusum. We found that during recovery, the species-specific coordination between photosynthesis and leaf hydraulic traits was mediated by stomatal regulation. The species with large stomatal conductance had both high photosynthesis and high leaf hydraulic conductance, along with a large vessel area in the leaf petiole. The selection of trees with consideration of the drought response, along with appropriate watering management, will improve the photosynthetic ability, and thus, will enhance CO2 absorption by urban trees.

Phytotoxic air pollutants such as atmospheric nitrogen dioxide (NO 2 ) are among the major stresses affecting tree photosynthesis in urban areas. We clarified the relationship between NO 2 concentrations and photosynthetic function for three major urban trees, Prunus  ×  yedoensis , Rhododendron pulchrum, and Ginkgo biloba , planted in Kyoto and surrounding cities, combining our published data and new data collected from 2020 to 2023. High NO 2 increased long-term water use efficiency for all species. High NO 2 decreased photosynthesis in P. yedoensis and R. pulchrum , while for G. biloba , NO 2 imposed little effect on photosynthesis. We then focused on the decrease in NO 2 due to (1) air pollution control measures from 2005 to 2023 and (2) the economic recession caused by the COVID-19 pandemic, and examined whether these factors improved photosynthesis in urban trees. The historic decrease in NO 2 improved leaf photosynthesis for P. yedoensis and R. pulchrum , while the COVID-19 pandemic reduced NO 2 by only 0.3 ppb and did not further improve photosynthesis in these tree species. This report shows that air pollution control measures improved photosynthesis in urban trees over several years in Japan, and is valuable because it demonstrates that air pollution control measures can increase CO 2 uptake by urban trees.

Although the use of as a roadside tree has been slightly declined in Japan, the number of planted in Europe and other countries has been increasing in recent years because of its high adaptability to diverse environmental stresses. To re-evaluate the value of as an urban tree, we focused on three environmental stress factors that can be notable in urban environments in Japan: (1) air pollution, (2) soil salinity, and (3) excess humidity. We evaluated the leaf photosynthetic functions of in response to the above three types of environmental stresses. We compared the responses of to air pollution (Experiment 1) and to soil salinity (Experiment 2) with those of × , the most commonly used roadside shrub in Japan. For experiment 1, we collected branches of and , which were planted as roadside trees in Kyoto city, in 2014 and 2017 to measure their photosynthetic functions. For experiments 2, we conducted a growth experiment with and seedlings, supplying 50 mM NaCl for three weeks. Experiment 3, an excess humidity experiment, was conducted only for seedlings from 2020 to 2022. It involved a two- to three-week growth experiment under excess humidity and recovery conditions. exhibited a smaller decrease in photosynthetic function in response to air pollution and soil salinity stress than did, confirming its robustness to diverse environmental stresses. The low stomatal density, sunken stomata, and thick mesophyll of contributed to its high tolerance of photosynthetic function to air pollution stress. The low stomatal density, and likely low proportion of xylem conduit, caused photosynthetic function of to be less sensitive to soil salinity stress. Conversely, plants grown under excess humidity exhibited reduced leaf mesophyll development, negatively impacting photosynthesis. This suggests that does not possess high tolerance to excess humidity.

Alteration in the leaf mesophyll anatomy by genetic modification is potentially a promising tool for improving the physiological functions of trees by improving leaf photosynthesis. Homeodomain leucine zipper (HD-Zip) transcription factors are candidates for anatomical alterations of leaves through modification of cell multiplication, differentiation, and expansion. Full-length cDNA encoding a Eucalyptus camaldulensis HD-Zip class II transcription factor (EcHB1) was over-expressed in vivo in the hybrid Eucalyptus GUT5 generated from Eucalyptus grandis and Eucalyptus urophylla . Overexpression of EcHB1 induced significant modification in the mesophyll anatomy of Eucalyptus with enhancements in the number of cells and chloroplasts on a leaf-area basis. The leaf-area-based photosynthesis of Eucalyptus was improved in the EcHB1 -overexpression lines, which was due to both enhanced CO 2 diffusion into chloroplasts and increased photosynthetic biochemical functions through increased number of chloroplasts per unit leaf area. Additionally, overexpression of EcHB1 suppressed defoliation and thus improved the growth of Eucalyptus trees under drought stress, which was a result of reduced water loss from trees due to the reduction in leaf area with no changes in stomatal morphology. These results gave us new insights into the role of the HD-Zip II gene.

The subject of this paper, sun leaves are thicker and show higher photosynthetic rates than the shade leaves, is approached in two ways. The first seeks to answer the question: why are sun leaves thicker than shade leaves? To do this, CO₂ diffusion within a leaf is examined first. Because affinity of Rubisco for CO₂ is low, the carboxylation of ribulose 1,5-bisphosphate is competitively inhibited by O₂, and the oxygenation of ribulose 1,5-bisphosphate leads to energy-consuming photorespiration, it is essential for C₃ plants to maintain the CO₂ concentration in the chloroplast as high as possible. Since the internal conductance for CO₂ diffusion from the intercellular space to the chloroplast stroma is finite and relatively small, C₃ leaves should have sufficient mesophyll surfaces occupied by chloroplasts to secure the area for CO₂ dissolution and transport. This explains why sun leaves are thicker. The second approach is mechanistic or 'how-oriented'. Mechanisms are discussed as to how sun leaves become thicker than shade leaves, in particular, the long-distance signal transduction from mature leaves to leaf primordia inducing the periclinal division of the palisade tissue cells. To increase the mesophyll surface area, the leaf can either be thicker or have smaller cells. Issues of cell size are discussed to understand plasticity in leaf thickness.

Ferns and fern allies have low photosynthetic rates compared with seed plants. Their photosynthesis is thought to be limited principally by physical CO2 diffusion from the atmosphere to chloroplasts. The aim of this study was to understand the reasons for low photosynthesis in species of ferns and fern allies (Lycopodiopsida and Polypodiopsida). We performed a comprehensive assessment of the foliar gas-exchange and mesophyll structural traits involved in photosynthetic function for 35 species of ferns and fern allies. Additionally, the leaf economics spectrum (the interrelationships between photosynthetic capacity and leaf/frond traits such as leaf dry mass per unit area or nitrogen content) was tested. Low mesophyll conductance to CO2 was the main cause for low photosynthesis in ferns and fern allies, which, in turn, was associated with thick cell walls and reduced chloroplast distribution towards intercellular mesophyll air spaces. Generally, the leaf economics spectrum in ferns follows a trend similar to that in seed plants. Nevertheless, ferns and allies had less nitrogen per unit DW than seed plants (i.e. the same slope but a different intercept) and lower photosynthesis rates per leaf mass area and per unit of nitrogen.

Insect-induced galls are microhabitats distinct from the outer environment that support inhabitants by providing improved nutrients, defence against enemies, and other unique features. It is intriguing as to how insects reprogram and modify plant morphogenesis. Because most of the gall systems are formed on trees, it is difficult to maintain them in laboratories and to comprehend the mechanisms operative in them through experimental manipulations. Herein, we propose a new model insect, Smicronyx madaranus , for studying the mechanisms of gall formation. This weevil forms spherical galls on the shoots of Cuscuta campestris , an obligate parasitic plant. We established a stable system for breeding and maintaining this ecologically intriguing insect in the laboratory, and succeeded in detailed analyses of the gall-forming behaviour, gall formation process, and histochemical and physiological features. Parasitic C. campestris depends on host plants for its nutrients, and usually shows low chlorophyll content and photosynthetic activity. We demonstrate that S. madaranus- induced galls have significantly increased CO 2 absorbance. Moreover, chloroplasts and starch accumulated in gall tissues at locations inhabited by the weevil larvae. These results suggest that the gall-inducing weevils enhance the photosynthetic activity in C. campestris , and modify the plant tissue to a nutrient-rich shelter for them.

Key messageSeasonal variations in the leaf photosynthetic traits of an urban tree, Ginkgo biloba, were almost synchronized with the photoperiod. Non-stomatal limitations were a cue for photosynthesis in Ginkgo biloba.Photosynthetic functions, which are key traits in determining the carbon uptake of urban trees, exhibit significant seasonal variations in temperate zones. It is essential to clarify the seasonal dynamics of photosynthesis to evaluate the CO2 uptake in urban areas. We investigated seasonal variations in the photosynthetic traits of Ginkgo biloba, which is a major urban deciduous tall tree often planted in Japan. Seasonal variations in the leaf photosynthetic traits, including the maximum photosynthesis rate, maximum carboxylation rate, and mesophyll and stomatal conductance, were well fitted to quadratic models, in which they peaked around the summer solstice and then declined with time. Seasonal variations in the environmental variables, such as photoperiod, temperature, and solar radiation, were compared to those of the leaf photosynthetic traits, in which the photoperiod explained well variations in the leaf photosynthetic traits. Seasonal variations in photosynthesis were largely governed by non-stomatal limitations, i.e., mesophyll and biochemical limitations. The high synchrony of the photoperiod and photosynthetic traits during leaf maturation may cause an enhancement in the daily carbon uptake of G. biloba leaves around the summer solstice, which has the longest photoperiod, and thus, will lead to an increase in the annual carbon uptake.