A broad range of physiological, biochemical and ultrastructural aspects of senescence phenomena in primary leaves of wheat (Triticum aestivum L. Mendos) has been examined. Three distinct types of senescence occurring in the one plant species have been studied. First is natural senescence, which, in leaves of wheat, is of a progressive type. Second is induced senescence of detached leaves incubated in water in the dark, and third is induced senescence of detached leaves incubated under a 16 hr photoperiod. In each case the progress of senescence was different.
Metabolic processes investigated included photosynthesis, transpiration, stomatal diffusive resistances, protein metabolism, chlorophyll concentrations, ribonuclease activities and leucine aminopeptidase activities. In addition, changes in ultrastructure were studied in considerable depth. These ultrastructural studies demonstrated the presence of two structures not previously known to exist in mature or senescent leaf tissue. First are the wavy chloroplast membranes, and second are the cytoplasmic and vacuolar lipid bodies. Furthermore it has been found that chloroplast ribosomes disappear before cytoplasmic ribosomes.
In addition to changes occurring during induced senescence in leaves incubated in water, the effects of the senescence retarding plant growth substance, kinetin, and the senescence accelerating plant growth substance, ABA, on the above mentioned metabolic processes were investigated. As a result of these investigations several new properties of each of these plant growth substances have been discovered. These include effects on transpiration, stomatal diffusive resistance, protein metabolism, ribonuclease activity and ultrastructure, ABA has been shown to inhibit transpiration rates and the rate of photosynthesis and, at the same time, to enhance markedly stomatal diffusive resistance. Furthermore, it has been found that these effects occur particularly rapidly. At the subcellular level electron microscopy has revealed that ABA prevents the temporary increase in starch grains in detached leaves incubated under a 16 hr photoperiod, and also promotes the loss of nuclear chromatin. With dark induced senescing leaves kinetin not only maintains the starch content of the chloroplasts for a considerable time but also maintains both the cytoplasmic and the chloroplast ribosome populations at a level close to that observed in freshly detached leaves. Starch grains are present in the chloroplasts of naturally senescing leaves at a stage when other organelles are showing somewhat advanced signs of senescence.
The chloroplasts of kinetin-treated leaves incubated under a 16 hr photoperiod are packed with large starch grains. Furthermore, these kinetin-treated leaves contain considerably more protein and chlorophyll than freshly detached leaves contain. This quantitative increase in protein levels occurs also in water-treated leaves incubated under a 16 hr photoperiod, while in ABA-treated leaves, protein levels were considerably lower.
The transpiration method of incubating detached leaves and for supplying isotopes was found to be far superior to the floating disc method used by previous investigators. It was established that serious disadvantages are inherent in the floating disc method due largely to edge wound effects. This was particularly apparent when radioactive isotopes were used. For these experiments a system of growing sterile wheat plants was developed and the detached leaves were incubated under sterile conditions. This was found to be essential because contaminating bacteria greatly enhanced the incorporation of labelled precursors into the protein fractions. Experiments carried out under these conditions revealed that kinetin acted to prevent protein hydrolysis. The ability of naturally senescing leaves to incorporate labelled precursors into the protein fraction was greater than that of dark induced senescing leaves at a stage when chlorophyll levels were similar in both groups of leaves.
Ribonuclease activities in dark induced senescing leaves were found to be considerably higher than in naturally senescing leaves from the same harvest. Furthermore, it was found that kinetin markedly suppressed the increase in ribonuclease activity that accompanied leaf detachment and dark incubation and that ABA, on the other hand, enhanced ribonuclease activity. Also, ribonuclease activities in naturally senescing leaves declined with advancing senescence whereas a steady rise was recorded in dark induced senescing leaves.
Results obtained from all aspects of senescence investigated have been correlated, wherever possible, in order to obtain a broader and deeper understanding of the processes involved.