Seasonal variations in the vessel size of temperate, diffuse-porous species and its relationship to leaf water potential

Abstract

Vessel lumen area and leaf water potential (ψleaf) at predawn and midday were measured periodically to reveal seasonal variations in the vessel sizes of diffuse-porous species and to determine the factors that cause this variation. Two deciduous species (Cercidiphyllum japonicum and Liriodendron tulipifera) and one evergreen species (Quercus glauca) were studied. In most of the samples, mean vessel lumen area (MVLA) initially increased then eventually decreased after a period, but this period varied among species. In C. japonicum, the decrease in MVLA coincided with a decrease in predawn ψleaf. In contrast, MVLA variations in most samples of L. tulipifera and Q. glauca did not show such a relationship. However, one of the L. tulipifera samples had an earlier decrease in MVLA that coincided with a lower predawn ψleaf. There was no clear relationship between the variation in vessel size and midday ψleaf, because midday ψleaf showed daily variation depending on vapor pressure deficit, which had less seasonality. Thus, we concluded that there are species-specific annual patterns in vessel size variation that are likely determined by internal factors. Low water availability is one of the external factors inducing the formation of narrower vessels.

散孔材における道管サイズの季節変化, およびその変化を引き起こす要因を明らかにするため, 道管形成の観察と夜明け前と日中の葉の水ポテンシャル (ψleaf)の測定を定期的に行った. 試料には落葉樹のカツラとユリノキ, 常緑樹のアラカシを用いた. 全ての樹種で道管サイズは一年の成長の始めに増加し, ある時期から減少していた. 減少が始まる時期は樹種によって異なっており, カツラでは夜明け前のψleaf の低下と対応が見られた. 一方でユリノキとアラカシでは, 道管サイズと夜明け前のψleaf の対応は見られなかった. ただし夜明け前のψleaf が他よりも低下していた個体では道管サイズの減少が早く始まっていたため, 乾燥ストレスの影響が示唆された. 日中のψleaf は日変動が大きく, 道管サイズとの明確な関連性は見られなかった. したがって, 年輪内の道管サイズの変化は主に内的要因によって決まっており, 乾燥ストレスは外部要因として道管サイズを減少させると考えられた.