CHANGES IN RESPIRATION AND CHEMICAL CONTENT DURING AUTUMNAL SENESCENCE OF POPULUS-TREMULOIDES AND QUERCUS-RUBRA LEAVES

Changes in respiration rate, chemical content and chemical concentrati on were measured in leaves of field-grown Populus tremuloides Michx. a nd Quercus rubra L. trees throughout the growing season and autumnal s enescence. Chlorophyll, soluble sugar, N, P, K and Mg contents and con centrations all declined during leaf senescence, whereas Ca content an d concentration increased. Leaf dry mass per area declined 24 and 35% in P. tremuloides and e. rubra, respectively, during senescence. In le aves of both species, respiration rates peaked during leaf expansion i n the spring and then declined, as a result of reduced cytochrome-medi ated respiration, to reach relatively constant rates by midsummer. In senescing P. tremuloides leaves, respiration rates remained relatively constant until mid-October and then declined rapidly. In senescing Q. rubra leaves, respiration rates increased in late September, as a res ult of the appearance of residual respiration that could not be reduce d by respiratory inhibitors, and then declined quickly in early Novemb er. No changes in alternative pathway respiratory activity were observ ed in leaves of either species during senescence until late autumn whe n rates declined. Because respiration rates were correlated with both leaf sugar and nitrogen content during leaf senescence, we conclude th at respiration rates were maintained or increased during leaf senescen ce to supply energy for degradation and mobilization of chemical const ituents.