Endogenous root promoting and root inhibiting activity as measured by the mung bean bioassay showed 2 rooting promoters at Rf 0.1-0.2 and at Rf 0.3-0.5, respectively. Extracts were paper chromatographed with isopropanol: water (8:2 v/v). No clear correlation was found between rooting ability and root promoting activity. Root inhibiting activity at Rf 0.5-0.8 was higher in difficult-to-root than in easy-to-root offshoots. The total carbohydrate content was higher in easy-to-root than in difficult-to-root offshoots although the latter contained more reducing sugar.
Root promotion activity in avocado leaf extracts was determined by the mung bean bioassay. Ten different clones representing a wide range of rooting abilities were compared. Following chromatography of methanol extracts in 8 isoprapanol : 2 water (v/v), a positive correlation was found between rooting ability of avocado cuttings and a mung bean rooting promoter at Rf 0.9–1.0 of the chromatograms. The same zone inhibited the straight growth of wheat coleoptile.
Cuttings of 10 different clones of avocado (Persea americana Mill.) were rooted under intermittant mist. A wide range in rooting capability was noted. Rooting percentage of clones was found to be correlated with number of leaves retained on the cuttings. Speed of rooting within the same clone was also determined by number of leaves retained. Carbohydrate content in leaves and cutting bases at the beginning was not associated with rooting capability. Starch accumulation at the cutting bases occurred while cuttings were under mist; this was correlated with the rooting capability. Mineral (N, P, K, Ca, Mg, Na, Cl, B, Fe, Zn, Mn) content of the leaves at the beginning and after a while under the mist did not show any association with rooting except for Mn, which was negatively correlated with rooting capability.
Leaf shedding (LS) of 3 avocado (Persea americana Mill.) clones (‘Northrop 28/5’, ‘Fuchs 20’ and ‘Nahlat 7’) differing in rooting ability was studied. Two distinct processes, leaf blade senescence and formation of the abscission layer, were found to be associated with LS. The rate of these processes differs among the 3 clones, and their combination determines the speed of LS. Leaf senescence was simulated by measuring chlorophyll destruction over time in leaf discs incubated in the light. The speed of abscission layer formation was estimated by measuring the time required for petioles to shed from incubated internodes. The results obtained in these tests were in agreement with the behavior of LS from cuttings of the 3 clones under mist. Auxin and cytokinin alone, or in combination, as measured in the 2 test systems, affected these leaf blade senescence and abscission layer formations at different rates. Six cuttings of the clone ‘Fuchs 20’ were sprayed weekly with combinations of NAA and BA. The optimal combination (1.10-4m NAA + 5.10-4m BA) caused a significant delay in LS and improvement in rooting. Relatively simple tests of leaf senescence and petiole abscission are discussed as potential methods for predicting rooting ability of leafy cuttings.