The relationship between physiological vitality and age in shoot segments ofPleurozium schreberi(Brid.) Mitt.

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Abstract
Photosynthesis, respiration, net assimilation, chlorophyll b concentration and levels of K, Ca, Mg and Fe were measured in 2 cm shoot segments of P. schreberi (Brid.) Mitt. to examine patterns of physiological decline with aging in a continuously growing species. Photosynthesis and chlorophyll b levels declined rapidly after the apical shoot segment and were undetectable after a distance of 8 cm from the shoot apex. Respiration rate also fell rapidly at first but then declined more gradually and some respiration was detected in all shoot segment samples. K and Mg levels declined approximately linearly in relation to segment number. Ca levels, expressed on a concentration per unit weight basis, became progressively higher in older segments but the Ca content per segment may actually decline with age. The disparity may be due to a decline in the proportion of cytoplasm compared with cell wall material in older segments. No clear division could be made between living and dead shoot segments. Different physiological processes decline at different rates as aging occurs and the importance of the age structure of shoot samples is discussed in relation to design of physiological experiments.