Fine structural quantification of drought-stressed Picea abies (L.) organelles based on 3D reconstructions.

Ultrastructural investigations of cells and organelles by transmission electron microscopy (TEM) usually lead to two-dimensional information of cell structures without supplying exact quantitative data due to the limited number of investigated ultrathin sections. This can lead to misinterpretation of observed structures especially in context of their three-dimensional (3D) assembly. 3D investigations and quantitative morphometric analysis are therefore essential to get detailed information about the arrangement and the amount of subcellular structures inside a cell or organelle, respectively, especially when the plant sample was exposed to environmental stress. In the present research, serial sectioned chloroplasts, mitochondria, and peroxisomes from first year spruce needles (Picea abies (L.) Karst.) were 3D reconstructed and digitally measured using a computer-supported image analysis system in order to obtain a detailed quantitative characterization of complete cell organelles including precise morphological data of drought-induced fine structural changes. In control plants, chloroplast volume was composed of 56% stroma, 15% starch, 27% thylakoids, and 2% plastoglobules. In drought-stressed chloroplasts, the relative volume of both the thylakoids and the plastoglobules significantly increased to 37% and 12%, respectively. Chloroplasts of stressed plants differed from control plants not only in the mean thylakoid and plastoglobules content but also in the complete lack of starch grains. Mitochondria occurred in variable forms in both control and stressed samples. In stressed plants, mitochondria showed a significant smaller mean volume which was only 81% when compared with the control organelles. Peroxisomes were inconspicuous in both samples and their volume did not differ between control and drought-stressed samples. The present study shows that specific subcellular structures are subject to significant quantitative changes during drought stress of spruce needles giving a detailed insight in adaptation processes of the investigated cell organelles.