Yuko T. Hanba

Leaf Functional Anatomy in Relation to Photosynthesis

Rubisco is a large enzyme with a molecular mass of approximately 550 kD. The maximum rate of CO2 fixation (i.e. ribulose-1,5-bisphosphate [RuBP] carboxylation) at CO2 saturation is only 15 to 30 mol CO2 mol−1 Rubisco protein s−1 at 25°C. Affinity to CO2 is also low, and the K m, K c, at 25°C

Expanding roles of plant aquaporins in plasma membranes and cell organelles

Aquaporins facilitate water transport across biomembranes in a manner dependent on osmotic pressure and water-potential gradient. The discovery of aquaporins has facilitated research on intracellular and whole-plant water transport at the molecular level. Aquaporins belong to a ubiquitous family of membrane intrinsic proteins (MIP). Plants have four subfamilies: plasma-membrane intrinsic protein (PIP), tonoplast intrinsic protein (TIP), nodulin 26-like intrinsic protein (NIP), and small basic intrinsic protein (SIP). Recent research has revealed a diversity of plant aquaporins, especially their physiological functions and intracellular localisation. A few PIP members have been reported to be involved in carbon dioxide permeability of cells. Newly identified transport substrates for NIP members of rice and Arabidopsis thaliana have been demonstrated to transport silicon and boron, respectively. Ammonia, glycerol, and hydrogen peroxide have been identified as substrates for plant aquaporins. The intracellular localisation of plant aquaporins is diverse; for example, SIP members are localised on the ER membrane. There has been much progress in the research on the functional regulation of water channel activity of PIP members including phosphorylation, formation of hetero-oligomer, and protonation of histidine residues under acidic condition. This review provides a broad overview of the range of potential aquaporins, which are now believed to participate in the transport of several small molecules in various membrane systems in model plants, crops, flowers and fruits.

Effects of leaf age on internal CO2 transfer conductance and photosynthesis in tree species having different types of shoot phenology

We examined the changes in leaf anatomy and some physiological characteristics during leaf expansion and maturation. Three deciduous tree species having different types of shoot phenology, maple (Acer mono Maxim.; ‘flush’ type), alder (Alnus japonica(Thunb.) Steud.; ‘successive’ type), and Japanese poplar (Populus maximowiczii A. Henry; ‘successive’ type), were studied. Leaf CO 2 assimilation rate at high irradiance (P max) and CO 2 transfer conductance inside the leaf (g i) varied significantly with leaf development. There were strong positive relationships between P max) and g i for all of the species. The variations in g i were partly related to those in the surface area of chloroplasts facing the intercellular airspaces, while some other factors that related to liquid phase conductance may also contribute to the variation in g i . The developments of mesophyll cells were accompanied by the concomitant increase in chloroplast and Rubisco content in Alnus and Populus (successive types).

The photosynthetic capacity in 35 ferns and fern allies: mesophyll CO2 diffusion as a key trait

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.

Effect of overexpression of radish plasma membrane aquaporins on water-use efficiency, photosynthesis and growth of Eucalyptus trees

Eucalyptus is a diverse genus of flowering trees with more than 700 genotypic species which are mostly native to Australia. We selected 19 wild provenances of Eucalyptus camaldulensis grown in Australia, compared their growth rate and drought tolerance and determined the protein levels of plasma membrane aquaporins (PIPs). There was a positive relationship between the drought tolerance and PIP content. PIPs are divided into two subgroups, PIP1 and PIP2. Most members of the PIP2 subgroup, but not PIP1 subgroup, exhibit water channel activity. We introduced two radish (Raphanus sativus L.) PIPs, RsPIP1;1 and RsPIP2;1, into a hybrid clone of Eucalyptus grandis and Eucalyptus urophylla to examine the effect of their overexpression. Expression of these genes was confirmed by real-time polymerase chain reaction (PCR) and the protein accumulation of RsPIP2;1 by immunoblotting. Drought tolerance was not enhanced in transgenic lines of either gene. However, one transgenic line expressing RsPIP2;1 showed high photosynthesis activity and growth rate under normal growth conditions. For RsPIP1;1-transformed lines, the RsPIP1;1 protein did not accumulate, and the abundance of endogenous PIP1 and PIP2 was decreased. The endogenous PIP1 and PIP2 genes were suppressed in these lines. Therefore, the decreased levels of PIP1 and PIP2 protein may be due to co-suppression of the PIP genes and/or high turnover of PIP proteins. RsPIP1;1-expressing lines gave low values of photosynthesis and growth compared with the control. These results suggest that down-regulation of PIP1 and PIP2 causes serious damage and that up-regulation of PIP2 improves the photosynthetic activity and growth of Eucalyptus trees.

Barley plasma membrane intrinsic proteins (PIP Aquaporins) as water and CO2 transporters.

We identified barley aquaporins and demonstrated that one, HvPIP2;1, transports water and CO2. Regarding water homeostasis in plants, regulations of aquaporin expression were observed in many plants under several environmental stresses. Under salt stress, a number of plasma membrane-type aquaporins were down-regulated, which can prevent continuous dehydration resulting in cell death. The leaves of transgenic rice plants that expressed the largest amount of HvPIP2;1 showed a 40% increase in internal CO2 conductance compared with leaves of wild-type rice plants. The rate of CO2 assimilation also increased in the transgenic plants. The goal of our plant aquaporin research is to determine the key aquaporin species responsible for water and CO2 transport, and to improve plant water relations, stress tolerance, CO2 uptake or assimilation, and plant productivity via molecular breeding of aquaporins.

Variations in the foliar δ13C and C3/C4 species richness in the Japanese flora of Poaceae among climates and habitat types under human activity

For 383 Poaceae species harvested over the Japanese islands and stored as herbarium specimens along several decades, we determined C3 and C4 types of photosynthesis from leaf stable carbon isotope ratio (δ13C). Then, we sought the relationships between C4 species richness and climatic factors or habitat types. Except for the two Panicum species (P. lanuginosum and P. scoparium) having the possibility of C3–C4 intermediate, 227 and 154 species were classified into C3 and C4. The C4 species richness increased from northern to southern islands in Japan, positively correlated with mean annual air temperature. Greater C4 species richness in the seashore habitats, and smaller C4 species richness in the shaded, wet and highland habitats would be related to the photosynthetic responses to local environmental factors such as irradiance level and temperature regime. No difference of leaf δ-value of C3 Poaceae was obtained between the habitats with different soil water availability, suggesting the less importance of soil water availability on leaf water-use efficiency in C3 Poaceae species in Japan having humid climate. Additionally, possible effects of human activity around the harvested time or site on leaf δ-value were estimated, because the habitat includes the sites with high human activity. Leaf δ-value was decreased with sampling year, and it was higher in the densely inhabited district for both C3 and C4. They are probably due to a historical decrease in the atmospheric δ-value via increasing human activity, and high gas emission at the districts of high human density.

Photosynthetic responses to soil water stress in summer in two Japanese urban landscape tree species (Ginkgo biloba and Prunus yedoensis): effects of pruning mulch and irrigation management

Key messageStomatal regulation involves beneficial effects of pruning mulch and irrigation on leaf photosynthesis inPrunus yedoensisandGinkgo bilobaunder moderate drought.G. bilobashowed conservative water use under drought.AbstractLeaf photosynthesis is highly sensitive to soil water stress via stomatal and/or biochemical responses, which markedly suppress the growth of landscape trees. Effective irrigation management to maintain leaf photosynthesis and information on species-specific photosynthetic responses to soil water stress are essential for the sustainable management of landscape trees in Japan, in which summer drought often occurs. In order to investigate effective irrigation management, we used plants with moderate soil water stress as controls, and examined the effects of daily irrigation and pruning mulch on leaf photosynthesis in container-grown Ginkgo biloba and Prunus yedoensis, which are the first and second main tall roadside trees in Japan. Stomatal conductance was significantly increased by pruning mulch and daily irrigation, with similar increases in leaf photosynthesis being observed in P. yedoensis and G. biloba. In order to obtain information on species-specific photosynthetic responses to soil water stress, we compared the responses of leaf photosynthesis and leaf water status to reductions in soil water content (SWC) between the two species. G. biloba maintained a constant leaf water potential, leaf water content, maximum carboxylation rate, and electron transport rate with reductions in SWC, whereas reductions were observed in P. yedoensis. We concluded that pruning mulch and irrigation effectively offset the negative impact of moderate water stress on leaf photosynthesis in summer in P. yedoensis and G. biloba via stomatal regulation, and also that G. biloba maintained its photosynthetic biochemistry and leaf water status better than P. yedoensis under severe water stress.

Contrasting photosynthetic responses to ambient air pollution between the urban shrub Rhododendron × pulchrum and urban tall tree Ginkgo biloba in Kyoto city: stomatal and leaf mesophyll morpho-anatomies are key traits

Key messageRhododendron × pulchrum avoided air pollution by adjusting its stomatal density, while Ginkgo biloba tolerated air pollution via a small stomatal density and high mesophyll thickness.AbstractInformation on the physiological mechanisms underlying species-specific photosynthetic responses to ambient air pollution is essential for enhancing the multiple services provided by urban trees. We conducted an on-site investigation of Rhododendron × pulchrum and Ginkgo biloba, which are the most common shrub and tall roadside tree used in Japan, in order to clarify their photosynthetic responses at sites with different air pollution levels in Kyoto city, Japan. The shrub tree R. × pulchrum and tall tree G. biloba exhibited contrasting responses to air pollution mainly from automobile exhaust gas. R. × pulchrum had a lower photosynthetic rate and stomatal conductance at high-pollution sites than at low-pollution sites, while no reductions were observed at high-pollution sites for G. biloba. The stomatal density of R. × pulchrum negatively correlated with atmospheric nitrogen oxides (NO and NO2) concentrations. R. × pulchrum avoided the effects of air pollution by reducing stomatal density at high-pollution sites at the expense of reducing CO2 uptake, while G. biloba appeared to have the ability to tolerate high air pollution levels by reducing the pollution load per mesophyll cell surface area with a low stomatal density and large mesophyll thickness. In conclusion, R. × pulchrum and G. biloba both acclimate to urban environments through an avoidance or tolerance strategy for air pollution by regulating stomatal and/or mesophyll morphologies.

Correction to: A hypergravity environment increases chloroplast size, photosynthesis, and plant growth in the moss Physcomitrella patens

The original article can be found online.