Auxin in Relation to Leaf Blade Abscission

Abstract

It is a common observation that a leaf stalk usually abscises shortly after the removal of its blade. LA RUE (2) delayed this abscission of debladed leaf stalks by the application of indoleacetic acid to the stalk. Compounds related to indoleacetic acid also have delayed the premature abscission of leaves and fruits (1, 3). These observations suggest that the auxin normally present in an organ is an important factor in delaying its abscission, and that a decrease in this auxin precedes or accompanies abscission. To test these ideas, determinations were made of the auxin content of bean leaves of various ages. This paper presents the results of the auxin determinations and a discussion of their significance.The first trifoliate leaves of Black Valentine beans, grown in the greenhouse, were used in this investigation. In beans an abscission zone lies between the blade of each leaflet and the leaf stalk. Thus blades differ from stalks in relation to the leaflet abscission zone; tissues of the blade are distal to the zone and tissues of the stalk are proximal to the zone. Samples of blades and stalks were taken from the plants at each age studied, 15, 30, 40, 60, and 70 days after planting. At 15 days the leaves were immature, the blades only partially expanded. At 30 days the leaves were fully expanded. At 70 days the blades were turning yellow and showing other signs of approaching abscission.Each sample was treated in the following manner: Immediately after cutting it was weighed, put into a container, and frozen at -150 C. As soon as convenient it was dried in a lyophilizer. The dried material was ground through a 40-mesh screen in a Wiley Mill, and weighed. Each sample was put into a separate 250-ml. Erlenmeyer flask containing 100-ml. of freshly distilled cold ethyl ether, and the flask kept for two hours in a refrigerator at 00 C. The material was then filtered through No. 1 qualitative paper and the residue washed with three 2-to 3-ml. portions of fresh ether. The ether extract thus obtained was evaporated to a few milliliters and with a pipette transferred to a small test tube where it was evaporated to dryness.A measured amount of lj% agar was added to this dried extract, the mixture shaken vigorously for a few minutes, poured into a 10.8 x 8 x 1 mm. brass mold, and cut into 12 blocks. These blocks were placed on Avena

Applied Auxin Gradients and Abscission in Explants

“…The onset of abscission was found to be correlated with the gradient, or balance, of auxin across the abscission zone, auxin distal to the abscission zone tending to retard abscission, and auxin proximal to the zone tending to accelerate abscission. This general concept is supported by the results of a number of workers with a variety of plant materials from experiments involving extraction, diffusion, applications, and metabolism of auxin (6,14,15,19,21,(26)(27)(28). However, a few experiments have been reported in which proximal applications of auxin have retarded abscission (24), and in some cases distal applications have accelerated abscission (10,12,32 …”

Intact Leaves Exhibit a Climacteric-Like Rise in Ethylene Production before Abscission

“…This correlation raises the question of what cue might initiate ethylene production in an aging leaf in which auxin levels are presumed to be declining. Although auxin levels have been shown to decrease in leaves with age (20,22,24), the analyses have not been frequent enough to eliminate the possibility of a transient rise in auxin levels. Logically, before solution to the question of what cues the presumed rise in ethylene production should be sought, it would be desirable to determine with certainty that ethylene production rates increase in intact leaves on intact plants before abscission.…”

Auxin Physiology in Bean Leaf Stalks.

“…Synthetic compounds (exogenous) will be specifically named; indoleacetic acid will be designated by its abbreviation, IAA. Inactivation is considered a chemical alteration of a compound so that it no longer produces its characteristic chemical or (10) has been found in previous studies with bean leaf stalks on the plant. But a change in auxin concentration in excised bean leaf stalks was found in experiments here reported.…”

“…Little attention, however, has been given to the auxin physiology of tissues with low auxin concentration. This paper reports an investigation of conditions associated with the low auxin concentration foundl in bean leaf stalks, a concentration of 0.4 ug equivalents of IAA per kg fresh weight (10).…”

“…At the end of 24 hours the second and third lots were treated as the first had been, and their auxin extracted. The details of the method by which these extracts were obtained have already been described (10 gests that an oxidative auxin inactivating mechanism is present in bean leaf stalks.…”