AUXIN CONTROL OF LEAF ABSCISSION. I. EXPERI­ MENTS WITH ERVATAMIA DIVARICATA BURKILL., V AR. FLORE-PLENO M. AcHARYYA CHOUDHURI and S. K. CHATTERJEE Department if Botany, Burdwan University, Burdwan, West Bengal SUMMARY The present study aims to analyse the effects of auxins on abscission of leaves of Ervatamia divaricata Burkill., var. flare-plena (Apocynaceae), with special reference to the structure of different auxins and their relation to abscission activity. Auxins promote and inhibit abscission of leaves, the effect is less pronounced in older leaves than in younger ones. The inhi­ bitory effect of A-stage of leaf cannot be traced in B or C stages of leaves of 3-node twigs. This indicates a lesser degree of auxin control of abscission in older leaves. This is also confirmed in experiments with 2-node twigs which are of different physiological maturation. The occurrence of two distinct physiological steps in the process of abscission of A-stage of leaves has been established. Auxins inhibit the first step and promote the second step of this stage of leaf. Absence of promotion in older leaves (B and C stages) indicates that the ageing of leaves has decreased the sensitivity of the second step towards auxins. The increasing requirement of induction period to cause 50 per cent abscission of the debladed petioles in spring and summer months will suggest a possible correlation of the natural occurrence of two steps of abscission with the metabolic activities of the leaves. In winter months, weaker metabolic activities may lead to an earlier completion of the first step, whereas in summer months the first step of abscission of de bladed petioles is sufficiently prolonged. It appears, that the second step of abscission gradually loses sensitivity towards auxin when the leaves enter winter months of the year. An analysis of the results of the present study carried out in different seasons will point out an involvement of auxins in the abscission process during different seasons. ABSCISSION OF LEAVES 63 The promotive effects of IPA and NOA may be due to the slow transport rate; it is likely that these auxins reach the abscission zone sufficiently later and cause promotion by their effects on the second step. A distinct gradation in the activity of phenoxy auxins (e.g., 2, 4-D; d-2, 4-DP; l-2, 4-DP; and 2, 4, 5-T) having different spatial configurations, has been established by separately following their inhibition and promotion effects on two steps of abscission. An analysis of their effects points out that 2, 4-D is the strongest auxm in the series whereas PAA is the weakest auxin. INTRODUCTION Abscission of leaves takes place by the formation of some specialized cells in the region of abscission zone. The physiologi­ cal basis of the formation of such cells is of special interest. The role of endogenous auxins in this phenomenon has been critically studied by a number of workers (Addicott and Lynch, 1951; Biggs and Leopold, 1958; Jacobs, 1958). Influence of amino acids on the abscission process has been studied by Rubinstein and Leopold ( 1962). Recently Chatterjee et al. (1965) and Chatterjee (1966) have stressed the significance of auxin/amino acid balance on the natural ageing of leaves. ' - Attempts have also been made to isolate some abscission accelerating substances (Carns et al., 1959; Hall et al., 1961 ; Addicott, 1965). ' The auxin control of leaf abscission involves the Coleus occurrence of two distinct physiological steps (AcharyyaChaudhuri and Chatterjee, 1966). The present study aims to analyse the effect of various auxins on the abscission of Ervatamia divaricata Burkill., var. jlore-pleno and their abscission activities have been related to their structures. The abscission behaviour of leaves in different seasons has also been studied. MATERIALS AND METHODS Healthy branches with fully expanded opposite decussate leaves of Ervatamia divaricata Burkill., var.jlore-pleno (Apocynaceae) were used in the abscission tests reported here. The physiological maturation of leaves as reported by the present authors, was also considered in the present study. 'A' represents the apical or the youngest pair of leaves, 5-8 days old; 'B' represents the middle pair of leaves, 15-18 days old; and 'C' represents the third pair of leaves, 30-35 days old. In 64 M. ACHARYYA CHOUDHURI AND S. K. CHATTERJEE addition, two-node twigs were also used in some of the experi­ ments. The physiological age of the youngest pair (A,) in such a twig varied from two to three days and that of the second pair (B,) varied from 10-12 days. The leaves were debladed and 1 em length of the petioles was left on the main stem. The abscission times (time required for 50 per cent of the debladed petioles to abscise) for the de bladed petioles of different nodes were separately recorded in each case. The following auxins were used Indole-acetic acid (IAA) Indole-propionic acid (IPA) a-Naphthalene acetic acid (NAA) a-Naphthoxy acetic acid (NOA) Phenoxy acetic acid (P AA) 2, 4-Dichlorophenoxy acetic acid (2, 4-D) 2, 4, 5-Trichlorophenoxy acetic acid (2, 4, 5-T) d-2, 4-Dichlorophenoxy-2-propionic acid (d-2, 4-DP) l-2, 4-Dichlorophenoxy-2-propionic acid (l-2, 4-DP) Different concentrations of IAA, IPA, NAA and NOA were applied in lanolin paste to the freshly cut ends of the de bladed petioles of both three-node and two-node twigs and the abscission times were recorded. In the experiments with phenoxy auxins, only A-stage of leaf of three-node twigs was considered. In the studies relating to the occurrence of two physio­ logical steps in the process of abscission, the method described by Rubinstein and Leopold (1963) and modified by the present authors ( 1966) had been followed. In seasonal studies, an initially inhibitory concentration of NAA (0·25 per cent) was applied distally to the petioles after 12, 14, 16 ...... 36, 38 and 40 hours of deblading. These experiments were conducted at different times of the year. In studies with phenoxy auxins, two sets of experiments were performed. In the first set, 0 · 25 per cent of each of the auxins was applied to the freshly cut ends ofthe petioles of A-stage of leaf and later the source of auxins was removed by trimming off 2 mm portion of the petioles after stipulated periods. In the second set of experiments, the debladed petioles were subject­ ed to a sufficiently prolonged induction period (36 hours) and later 0 · 25 per cent of each ofthe auxins was applied distally for 2, 4, 8 and 12 hours. The abscission times were noted as usual. The plants were maintained on a 12-hour photoperiod at a temperature of 25°-27°C and the relative humidity of l ABSCISSION OF LEAVES 65 70-80 per cent. Abscission readings, i.e. the detachment of the petioles from the main stem under slight pressure, were recorded at 12-hour intervals and the time taken for 50 per cent abscission was calculated. The experiments were replicated at least thrice with comparable results. RESULTS The effects of IAA, IPA, NAA and NOA on abscission of debladed petioles of three-node and two-node twigs are presen­ ted in Figs. la and 1b. Both IAA and NAA inhibited abscission of debladed petioles of A-stage of leaf markedly. Complete inhibition of abscission was obtained with IAA in concentrations of 0 · 25 per cent and 1 · 0 per cent and with 1·0 per cent NAA only. In case of two-node twigs, abscission of debladed petioles of both A, and B, stages of leaf was inhibi­ ted completely by IAA (0 · 25 per cent and 1 · 0 per cent) and by NAA (1·0 per cent). Partial inhibition was obtained with 0·25 per cent NAA, 0·062 per cent of both IAA and NAA and • CEI • = 113111 -80 ' .. ,6Q .. 0 .. .. .. +40 .. .. 1 ~~~ ABC A,B, .\BCA,B, ABC A,B, ABC A,B, -20 ""·'· •oc : ~ '' 'oc '" I -·40 IAA -60 - -~._._____ - I ·O 0"25 o·062 O·OI5 I ·O 0•25 0·062 o·OI5 Concentration (per cent) Fra. Ia. Effects of different concentrations ofiAA and NAA on abscission times for 50 per cent of the debladed petioles of the two types of twigs. (Solid bars indicate the abscission times for the petioles of three-node twigs and dotted bars indicate the abscission times for two-node twigs.) >14 Control times for 50 per cent abscission of th(debladed petioles of A -stage of leaf - 96 hours B - ........... - 88 hours C - ........... - 78 hours A 1 - ••••••••••• - 98 hours B1 - .• , •••••••• - 90 hours 66 7\L ACHARYYA CHOUDHURI AND S. K. CHATTERJEE IPA NOA A,H ABC .~.ll, ·\BC A,ll, ABC .\,B. ABC .\,B, ABC .\,8, 1 ·U 0·25 0·062 I 0 l) l'• 0·062 0 015 Concentration (per cent) FIG. I b. Effects of different concentrations of IPA and NOA on abscission for 50 per cent ofthc debladed petioles of the two types of twigs. (Solid bars for three-node twigs and dotted bars for two-node twigs.) with 0·015 per cent IAA and NAA in A., A, and B, stages of leaves. The petioles of B and C stages of leaves of three-node twigs showed little or no change in their abscission behaviour in different treatments of these t\VO auxins. Both IPA and NOA were effective in promoting abscission. The promotion of abscission was obtained with all the concen­ trations used and the extent of promotion was maximum with 1·0 per cent IPA and NOA and minimum with 0·015 per cent of these auxins in A, B and C stages of leaves of three-node twigs and A, and B, stages of leaves of two-node twigs.