Diurnal changes in microscopic structures of mesophyll cells of Norway spruce, Picea abies (L.) Karst., and the effects of ozone and drought

Microscopy has been used to diagnose the effects of air pollutants on conifers. As sampling may be slow, it is important to know whether the structure of needles changes during the day. Using light- and electron microscopy, diurnal changes in the structure of mesophyll cells of current year needles From 6-year-old Norway spruce saplings were studied on one day in early autumn, 1995. Saplings had been grown in open-top chambers and exposed to ozone and drought stress during the 1992-1995 growing seasons. In all treatments, the proportion of cells with small cytoplasmic vacuoles and with large cytoplasmic lipid deposits decreased during the day. Mitochondria increased in size from early morning to midday, after which time they shrank progressively. Ozone affected the shape of mitochondria: the proportion of tube-shaped mitochondria decreased from early morning in control saplings; whereas it increased in ozone-treated saplings. Starch accumulated during the day, but to a lesser extent in drought-stressed saplings than well-watered controls. Microbodies were smaller in drought-stressed saplings than in controls early in the morning, but their size then increased compared to that of the controls. Cellular and sub-cellular disorganization, probably due to suboptimal fixation procedures, were detected in all treatments, especially in the morning. but these decreased later. particularly in well-watered saplings. Results show that microscopy is a sensitive and valid tool For studying stress responses in plants, However. the time of sample collection must be considered and documented to avoid misleading conclusions due to actual diurnal changes and fixation problems. (C) 2001 Annals of Botany Company.