Using a malate dehydrogenase isozyme system, it was possible to identify the pollen donor parent in progenies of avocado (Persea americana Mill.) obtained by enclosing 2 cultivars of complementary flower type in a screenhouse with a beehive. The observed percentage of cross-pollination ranged from 7% to 92%. Isozymes are demonstrated to be a practical way of identifying hybrids.
Outcrossing rate in a solid block of ‘Hass’ avocado (Persea americana Mill) was determined using the isozyme systems of malate dehydrogenase (MDH) (EC 188.8.131.52), leucine aminopeptidase (LAP) (EC 184.108.40.206), and triosephosphate isomerase (TPI) (EC 220.127.116.11), for which evidence concerning the genetic control of the latter is presented. Almost all ‘Hass’ mature fruits were found to have resulted from cross-pollination. Among the potential pollen donors—‘Hass’, ‘Ettinger’, and ‘Reed’—‘Ettinger’ excelled, producing almost all of the hybrid fruits.
Pollination of lychee (Litchi chinensis Sonn.) by the honeybee was studied in Israel's two commercial cultivars, `Mauritius' and `Floridian'. Pollination rate, which was determined in a mixed `Mauritius' and `Floridian' plot, followed a consistent pattern: it was low at the first male (M,) `Mauritius' bloom and reached a high value only when the pseudohermaphroditic (M2) `Mauritius' bloom started. Pollen density on bees collected from `Mauritius' inflorescences was very low during the M, bloom and increased to very high values during the M2 bloom. These results indicate that the `Mauritius' M, bloom does not play an important role as a source of pollen for pollination. Pronounced, significant, and consistent differences in nectar volume per flower and sugar concentration in the nectar were found between M1, M2, and female (F) `Mauritius' flowers. Values were very high in F flowers, medium in M2 flowers, and low in M, flowers. Accordingly, the density of bees found on inflorescences was high during the F bloom, intermediate during the M2 bloom, and low during the M1 bloom. The positive correlation between bee density and sugar concentration in the nectar was highly significant for M2 and F `Mauritius' flowers. The nectar contained three sugars: glucose (43%), fructose (39%), and sucrose (18 %). This ratio was the same in nectar from M1, M2, and F `Mauritius' flowers.
The lychee (Litchi chinensis Sonn.) has two types of pollen-releasing flowers—M1 and M2. We compared the morphology and viability of these two pollen types, mainly for the two commercial cultivars in Israel: `Mauritius' and `Floridian'. Observation by scanning electron microscope did not reveal any consistent morphological differences between the two pollen types. However, M2 pollen was found to have a consistent and significant advantage over M1 pollen in in vitro germination tests. M2 pollen from `Mauritius', `Floridian', `No Mai Chee', `Wai Chee', and `Early Large Red' had a much higher germination rate at 15, 20, 25, 30, and 35 °C than M1 pollen from those same cultivars. The optimal incubation temperature for in vitro pollen germination was 30 °C for M2 pollen of all five cultivars studied; adequate germination rates were also found at 35 and 25 °C. The optimal temperature for M1 pollen germination was also 30 °C for `Mauritius' and `No Mai Chee', but was not well defined for the other three cultivars. No pronounced advantage of M2 pollen-tube growth could be discerned 48 h after hand pollination. However, final fruit set was consistently and significantly higher after hand pollination with M2 pollen, relative to M1 pollen. Hot (32/27 °C) and warm (27/22 °C) regimes during flower development had a pronounced detrimental effect on pollen viability compared to a cool (22/17 °C) regime. `Floridian' was much more susceptible than `Mauritius' in this respect.
Outcrossing rates in ‘Hass’ and Fuerte’ avocado (Persea americana Mill.) blocks were determined during fruit development using the isozyme systems of malate dehydrogenase (MDH, EC 18.104.22.168), leucine aminopeptidase (LAP, EC 22.214.171.124), and triosephosphate isomerase (TPI, EC 126.96.36.199) as genetic markers. Abscission of avocado fruitlets was selective and was greatly influenced by the pollen parent. Thus, in ‘Hass’ trees subjected to crosspollination by ‘Ettinger’ and ‘Fuerte’, the population of ‘Hass’ fruitlets 1 month after fruit set consisted mainly of ‘Hass’ selfs; however, during fruit development, the rate of ‘Hass’ selfs decreased and the rate of ‘Hass’ hybrid fruitlets produced by ‘Ettinger’ and ‘Fuerte’ increased. By the end of fruit abscission, the surviving mature ‘Hass’ fruits were mostly ‘Ettinger’ hybrids. The ‘Hass’ fruit yield was found to correlate significantly with the rate of outcrossing with ‘Ettinger’. When ‘Ettinger’ served as a pollen parent for ‘Fuerte’, the outcrossing rate in trees adjacent to ‘Ettinger’ was about 40%, which shows that cross-pollination among avocado cultivars of the same flowering group can be substantial at close proximity. The outcrossing rate in ‘Fuerte’ diminished with increasing distance from ‘Ettinger’, but no effect on yield was observed.
The seed influences rate of growth, size, shape and maturation of avocado fruits. Seeded fruits are 8-10 times larger than seedless ones and contain more and larger cells. The growth pattern of seeded and seedless fruits is similar from June until maturation, when growth rate of seeded fruits decreases. Fruit maturation is characterized by rapid accumulation of oil in the mesocarp, preceded by shriveling of the seed coats, and the discontinuation of the seeds’ influence on fruit growth. The role of the seed in the development of avocado fruit is discussed.
Evidence is presented suggesting that genetic selection could be an important factor in avocado fruitlet abscission. ‘Ettinger’ embryos (Persea americana Mill.) at different stages of fruit development were classified according to their leucine aminopeptidase (LAP) electrophoretic pattern in the Lap-2 locus. Analysis of several fruitlet populations showed significant deviations from the expected Mendelian ratio. The genotypic ratios at the different stages indicate genetic selection during fruitlet abscission.
Most fruit-tree breeding projects are based on selection of seedlings in regard to their performance. The selected seedlings are vegetatively propagated, usually by grafting. It is highly important for the breeder to know whether the performance of the grafted tree will resemble the performance of the original seedling. In this study the performance of avocado and mango seedlings was compared with that of their grafted duplicates. Significant differences were found in only 8 out of 36 avocado traits and 2 out of 10 mango traits. Significant seedling x graft interaction was detected in 10 other avocado traits. These differences were considered of no practical significance, since their magnitude was of minor importance for the breeder. The conclusion for avocado and mango breeders is that for most traits selection could be carried out on ungrafted seedlings.
Avocado (Persea americana Mill.) progeny that originated from 11 crosses (both self-pollinations and crosses between cultivars) were evaluated for the length of their juvenile period. Time to first flowering, “flowering age,” and time to first fruit production, “fruiting age,” were recorded for each progeny. The mean values for both ages, the sd, and the progeny distribution were calculated. Significant statistical differences in flowering age and fruiting age between various progeny populations were detected. No differences were detected between self-pollinated plants and crosses. The time until first flowering was found to be the limiting factor in evaluation of seedlings.
Forty-one (Mangifera indica L.) cultivars were characterized electrophoretically using the isozyme systems aconitase, isocitrate dehydrogenase, leucine aminopeptidase, phosphoglucose isomerase, phosphoglucomutase, and triosephosphate isomerase. The outcross origin of some of the mango cultivars was supported by the isozymic banding patterns. Reported parentage of some other cultivars was not consistent with their isozymic banding patterns.