Induced senescence of intact wheat seedlings and its reversibility.

Intact wheat seedlings (Triticum aestivum L.) were induced to senesce by placing them in the dark and at various stages of senescence were placed back in the light and their recoverability observed. Seedlings demonstrated complete recovery of chlorophyll, protein, and rate of photosynthesis after 2 days in the dark, but were unable to recover fully after 4 days. This suggests the onset of an irreversible stage in senescence by day 4. Foliar applied cytokinins delayed senescence, and zeatin at 0.1 mm delayed the onset of the irreversible stage for 6 days. In addition to delaying the loss of total soluble protein, zeatin maintained the net protein recovery capacity of the tissue. Control seedlings, however, lost their potential for net protein recovery at a rate similar to their loss of total soluble protein. Treatment with zeatin had no apparent effect on dark respiration during senescence, and although treatment did delay the loss of membrane permeability to substrate, the change in permeability occurred too late to have a causal role in senescence.