Changes in Gas Exchange and in the Activities of Proteolytic Enzymes during Senescence of Wheat Leaves (Triticum aestivum L.)

Feller, Urs; Erismann, K. H. (1978). Changes in Gas Exchange and in the Activities of Proteolytic Enzymes during Senescence of Wheat Leaves (Triticum aestivum L.). Zeitschrift für Pflanzenphysiologie, 90(3), pp. 235-244. Gustav Fischer Verlag 10.1016/S0044-328X(78)80236-0

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Gas exchange rates, nitrogen an chlorophyll contents and the activities of aminopeptidase, carboxypeptidase and neutral endopeptidase were measured in the different leaves of field grown wheat (Triticum aestivum L.) during grain development. Like the macroscopic senescence criteria (yellowing, drying) the physiological changes occurred in strong gradients in the whole plant. First the bottom Teaves and finally the flag leaf senesced. Early in senescence net photosynthesis decreased and chlorophyll was degraded, but the leaf remained turgid and dark respiration was only partially reduced. The C02 compensation concentration increased during this period and finally no defined value could be obtained, because no equilibrium existed between carboxylating and decarboxylating processes. Aminopeptidase activity increased during leaf development, readied a maximum and decreased simultaneously with the mobilization of leaf nitrogen. Carboxypeptidase also increased during leaf development and decreased during senescence, but this loss occurred later than that of aminopeptidase. In contrast to the two exopeptidases, the endopeptidase activity increased with the beginning of senescence and peaked during protein mobilization. Maximal activity appeared first in the lowest leaf and then sequentially in the upper leaves. Inside the leaf the activity increase progressed from the tip to the base and finally the leaf sheath mobilized its nitrogen. The endopeptidase appears to be associated with senescence mobilization of proteins.

During wheat leaf senescence first net photosynthesis and chlorophyll content decreased and the endopeptidase activity and the CO2 compensation concentration increased. Then dark respiration, aminopeptidase and a little later, also carboxypeptidase activity decreased, while endopeptidase was still increasing. Finally leaf fresh weight decreased, endopeptidase reached its maximum and then it also lost its activity. When the breakdown of the photosynthesis apparatus is progressing, although the cells are still turgid and active, macromolecules can be degraded and the breakdown products can be transferred into translocation compounds and loaded into the phloem.

It is known, that in cereals under certain conditions (i.e. water stress, artificial senescence of detached leaves) the pattern of proteolytic enzymes during protein mobilization is not or only slightly changed in the manner mentioned above. A possible mechanism of regulation in the whole plant is the flux of assimilates into the grains affecting their requirement for nitrogen compounds. For a slow export from the leaves to the grains enough amino acids may be produced by regular protein turnover, but for a rapid mobilization from a healthy leaf a more effective mechanism (i. e. increasing endopeptidase activity) may be required.

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