Auxin Control of Protein-Levels in Detached Autumn Leaves

Abstract

IT is well established that when mature leaves are detached from the plant the protein-level in the blade falls¹. It has also been demonstrated by both Vickery, Pucher, Wakeman and Leavenworth² and Mothes and Engelbrecht³ that an external supply of carbohydrate or of nitrogenous substances such as nitrates, ammonium salts or urea, will only slow down the rate of the protein degradation. If detached leaves are maintained in nitrogen, however, the fall in protein can be considerably retarded without the addition of metabolites⁴, while the reduction in protein-level can be entirely suppressed under normal aerobic conditions once root initials have been formed on the petiole³. The retention of protein in detached but rooted leaves caused Chibnall⁵ to propose that a root factor, possibly hormonal in nature, is essential for the synthesis of protein in leaf tissue. In this connexion the findings of Richmond and Lang⁶ and Person, Samborski and Forsyth⁷ are of considerable interest. The former workers showed that when detached unrooted leaves of Xanthium were kept with their petioles in 5 or 10 mgm./l. kinetin the loss of protein was retarded, and the chlorophyll and green colour of the blade were maintained. Person et al. ⁷ reported similar effects on detached wheat leaves floated on solutions of benzimidazole. Later, Mothes, Engelbrecht and Kulajewa⁸ elegantly demonstrated that when solutions of kinetin were applied locally to the surface of mature tobacco leaves the original levels of protein and chlorophyll were maintained in the treated areas ; furthermore, nitrogenous compounds applied to other parts of the leaf moved towards the parts treated with kinetin. In a recent communication⁹, one of us directed attention to the importance of auxins in the control of physiological changes occurring during the autumn senescence of both attached and detached leaves. Local applications of the n-butyl ester of chlorinated phenoxyacetic acids to the surface of green autumnal leaves of cherry resulted in a retention of photosynthetically active chlorophyll and the conservation of alcohol-insoluble nitrogen in the treated areas. In contrast, untreated parts and control leaves became completely yellow and showed a greater reduction in the protein-level. These investigations have now been extended, and more detailed work has revealed that the total protein-levels in the auxin-treated areas of the leaf may be wholly maintained for periods of more than two weeks, during which time the total protein in control leaves and untreated parts of the treated leaf may fall to half the original values.