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 conif ers. As sampling may be slow, it is important to know whether the structure of needles changes during the day. Using light- and electron microscopy, d iurnal 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 aut umn, 1995. Saplings had been grown in open-top chambers and exposed to ozon e and drought stress during the 1992-1995 growing seasons. In all treatment s, the proportion of cells with small cytoplasmic vacuoles and with large c ytoplasmic lipid deposits decreased during the day. Mitochondria increased in size from early morning to midday, after which time they shrank progress ively. Ozone affected the shape of mitochondria: the proportion of tube-sha ped 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 contr ols. Microbodies were smaller in drought-stressed saplings than in controls early in the morning, but their size then increased compared to that of th e controls. Cellular and sub-cellular disorganization, probably due to subo ptimal fixation procedures, were detected in all treatments, especially in the morning. but these decreased later. particularly in well-watered saplin gs. 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 di urnal changes and fixation problems. (C) 2001 Annals of Botany Company.