Winter-spring temperature pattern is closely related to the onset of cambial reactivation in stems of the evergreen conifer Chamaecyparis pisifera.

Temperature is an important factor for the cambial growth in temperate trees. We investigated the way daily temperatures patterns (maximum, average and minimum) from late winter to early spring affected the timing of cambial reactivation and xylem differentiation in stems of the conifer Chamaecyparis pisifera. When the daily temperatures started to increase earlier from late winter to early spring, cambial reactivation occurred earlier. Cambium became active when it achieves the desired accumulated temperature above the threshold (cambial reactivation index; CRI) of 13 °C in 11 days in 2013 whereas 18 days in 2014. This difference in duration required for achieving accumulated temperature can be explained with the variations in the daily temperature patterns in 2013 and 2014. Our formula for calculation of CRI predicted the cambial reactivation in 2015. A hypothetical increase of 1-4 °C to the actual daily maximum temperatures of 2013 and 2014 shifted the timing of cambial reactivation and had different effects on cambial reactivation in the two consecutive years because of variations in the actual daily temperatures patterns. Thus, the specific annual pattern of accumulation of temperature from late winter to early spring is a critical factor in determining the timing of cambial reactivation in trees.

Mesophyll conductance to CO2 in leaves of Siebold's beech (Fagus crenata) seedlings under elevated ozone.

Ozone is an air pollutant that negatively affects photosynthesis in woody plants. Previous studies suggested that ozone-induced reduction in photosynthetic rates is mainly attributable to a decrease of maximum carboxylation rate (Vcmax) and/or maximum electron transport rate (Jmax) estimated from response of net photosynthetic rate (A) to intercellular CO2 concentration (Ci) (A/Ci curve) assuming that mesophyll conductance for CO2 diffusion (gm) is infinite. Although it is known that Ci-based Vcmax and Jmax are potentially influenced by gm, its contribution to ozone responses in Ci-based Vcmax and Jmax is still unclear. In the present study, therefore, we analysed photosynthetic processes including gm in leaves of Siebold's beech (Fagus crenata) seedlings grown under three levels of ozone (charcoal-filtered air or ozone at 1.0- or 1.5-times ambient concentration) for two growing seasons in 2016-2017. Leaf gas exchange and chlorophyll fluorescence were simultaneously measured in July and September of the second growing season. We determined the A, stomatal conductance to water vapor and gm, and analysed A/Ci curve and A/Cc curve (Cc: chloroplast CO2 concentration). We also determined the Rubisco and chlorophyll contents in leaves. In September, ozone significantly decreased Ci-based Vcmax. At the same time, ozone decreased gm, whereas there was no significant effect of ozone on Cc-based Vcmax or the contents of Rubisco and chlorophyll in leaves. These results suggest that ozone-induced reduction in Ci-based Vcmax is a result of the decrease in gm rather than in carboxylation capacity. The decrease in gm by elevated ozone was offset by an increase in Ci, and Cc did not differ depending on ozone treatment. Since Cc-based Vcmax was also similar, A was not changed by elevated ozone. We conclude that gm is an important factor for reduction in Ci-based Vcmax of Siebold's beech under elevated ozone.

Agatharesinol biosynthesis-related changes of ray parenchyma in sapwood sticks of Cryptomeria japonica during cell death.

MAIN CONCLUSION: The work demonstrates a relationship between the biosynthesis of the secondary metabolite, agatharesinol, and cytological changes that occur in ray parenchyma during cell death in sapwood sticks of Cryptomeria japonica under humidity-regulated conditions. To characterize the death of ray parenchyma cells that accompanies the biosynthesis of secondary metabolites, we examined cell death in sapwood sticks of Cryptomeria japonica under humidity-regulated conditions. We monitored features of ray parenchyma cells, such as viability, the morphology of nuclei and vacuoles, and the amount of starch grains. In addition, we analyzed levels of agatharesinol, a heartwood norlignan, by gas chromatography-mass spectrometry in the same sapwood sticks. Dramatic changes in the amount of starch grains and in the level of agatharesinol occurred simultaneously. Therefore, the biosynthesis of agatharesinol appeared to originate from the breakdown of starch. Furthermore, we observed the expansion of vacuoles in ray parenchyma cells prior to other cytological changes at the final stage of cell death. In our experimental system, we were able to follow the process of cell death and to demonstrate relationships between cytological changes and the biosynthesis of a secondary metabolite during the death of ray parenchyma cells.