Effects of Aging and Temperature on Respiratory Metabolism of Green Leaves

Abstract

It has been known since 1921 that the respiration rate of leaves declines during aging (7). Smillie (19, 20, 22) has shown that the amounts of some soluble respiratory enzymes of pea leaves decrease as they grow older and that young leaves are the best source of active mitochondria with high phosphorylat-ing capacity. In addition, adverse effects of relatively high growing temperatures on the yield of active mitochondria have been reported (17) and the effects of exposure of normal tissues to high temperature may be due to a selective inhibition of the Krebs cycle (1, 14). In this work pea plants were grown at 10, 20 (optimal for growth) and at 300. At selected stages of growth, respiration rates of leaf tissue and the biochemical activities of mitochondria isolated from them were measured. The ability of leaf tissue to utilize acetate-ClI and the activities of some mito-chondrial enzymes were also determined. Declining respiratory rates during aging were correlated with striking decreases in mitochondrial activity. These in turn appeared to be related to increased disorgani-zation of the internal structure of the mitochondria. Materials and Methods Plant Material and Sampling Methods. Pea seeds (Pisum sativum L. var. Alaska) were soaked in de-ionized water for 4 to 8 hours, surface sterilized witl either 0.001 M HgCl2 or Spergon, and then sown in sterilized soil or vermiculite. Seeds sown in sterilized soil were watered, as needed, with deionized water while those sown in vermiculite were watered with Withrow A solution (23). Pea plants used to study both temperature and aging effects were grown in 3 controlled climate rooms. The temperature in each room was maintained continuously at a different level, namely 10, 20, and 300. These temperatures are respectively suboptimal, close to optimal and supraoptimal for growth (fig 1). Photoperiod (16 hours), light intensity (400-600 ft-c) and average relative humidity (60 %) were the same in each room. The plants grown at different temperatures were generally uni-1 Received January 13, 1964. 2 Work supported by Atomic Energy Commission Contract AT (11-1)-330. form in internode length and leaf size but plants grown at 300 were considerably lighter green in color. Some plants used to study aging effects only were grown in a greenhouse in early spring. Leaves were sampled in 2 ways: A) Plants at each temperature were grown to predetermined heights of 3, 10, and 23 cm and all of the leaves were removed. Figure 1 describes the growth of pea plants at the 3 temperatures employed and the stages of development at which leaves were sampled are indicated by arrows. For convenience the 3 stages are designated as developmental ages 1, 2, and 3. B) The second sampling method consisted of growing plants to a height of 23 cm at each temperature and detaching leaves at 3 levels. The third plant shown