Acknowledgements
T. L. Davenport
5.14 Fruit Set and Retention
Fruit set and retention of mango was recently reviewed by Singh et al. (2005).
Abscission of fl owers and fruitlets is accomplished by rapid formation of a separation layer in the abscission zone in the pedicel-peduncle junction (Bar- nell, 1939). U.R. Singh (1961) described formation of the abscission zone dur- ing fl oral ontogeny and of the separation layer during abscission of fl owers and fruitlets. The majority of panicles lose all fruitlets (Núñez-Elisea and Davenport, 1983). The pattern of fruitlet abscission is asymptotic with the greatest losses occurring during the fi rst weeks after anthesis (Núñez-Elisea and Davenport, 1983; Prakash and Ram, 1984; Searle et al., 1995). Except for the tendency to retain fruit in the distal portion of panicles, abscission of fl owers and fruitlets is random. It can involve fruitlets regardless of size or location.
Of the 8–13% of perfect fl owers setting fruit, < 1% reach maturity (Bijhou- wer, 1937; Sen, 1939; Naik and Mohan Rao, 1943; Mukherjee, 1949b; U.R.
Singh, 1960; Randhawa and Damodaran, 1961a; Singh, 1978; Gunjate et al., 1983; Prakash and Ram, 1984). Generally, most fruit are set on the most distal spike portion of panicles (Chadha and Singh, 1963; Núñez-Elisea and Daven- port, 1983). Fruit loss has been associated with embryo abortion, resulting in blackened or shrivelled embryos (Singh, 1954a, 1964; Chandler, 1958; U.R.
Singh, 1961; Sharma and Singh, 1972; Ram et al., 1976) after the fruit is sepa- rated from the tree (Núñez-Elisea and Davenport, 1983).
Sex ratio
The perfect/staminate fl oral ratio in panicles may infl uence fruit set and pro- ductivity (Naik and Mohan Rao, 1943; Singh, 1954b; Singh and Singh, 1959;
U.R. Singh, 1960). Mallik (1957) noted that more perfect fl owers are formed in ‘on’ than ‘off’ years of alternate-bearing cultivars. Other studies, however,
Reproductive Physiology 141
have demonstrated that the number of perfect fl owers does not correlate with subsequent yield (Randhawa and Damodaran, 1961a) so long as the proportion of perfect fl owers is not < 4% (Singh, 1964, 1971). Most fruit are borne in the distal portion of panicles (Shawky et al., 1977), which may be correlated with the high ratio of perfect to staminate fl owers there. Schole- fi eld and Oag (1984) estimated that one mature fruit is harvested for each 169 perfect fl owers in the distal half of the panicle; whereas 592 perfect fl owers are required to produce one fruit in the proximal half. Therefore, intrinsic factors other than sex ratio regulate fruit set.
Mineral nutrients
Boron is one of seven micronutrients required for normal plant growth. The physiological function of B is unknown (Hu and Brown, 1994), although it is essential for fl oral development, pollen germination, pollen tube growth, embryo development and growth of organs (i.e. fruit) (Vasil, 1963; Agarwala et al., 1981; Dell and Huang, 1997; Shorrocks, 1997). Defi cient soils are com- monly found in mango-producing areas of Australia, Thailand, Central and South America and Africa where symptoms are common (Aitken et al., 1987;
Singh et al., 2005). Boron applications to defi cient mango trees increase normal fruit set (Robbertse et al., 1990; Raja et al., 2005). Fruitlet abscission in mangoes has also been attributed to zinc (Zn) defi ciency (Jiron and Hedstrom, 1985).
Hormonal control
Auxin
Research demonstrating improved fruit set and retention following applica- tion of several auxin analogues to pre-anthesis panicles or to panicles bearing fruitlets of various sizes has been reviewed (Davenport and Núñez-Elisea, 1997; Singh et al., 2005). NAA is the most effective auxin analogue for improv- ing fruit retention (Prakash and Ram, 1986; Khan et al., 1993). Initial fruit set was substantially increased when sprays of 200 mg/l indole acetic acid (IAA) were applied to developing panicles (Singh et al., 1965). A 300–400% increase in fruit set resulted when NAA (40 or 50 mg/l) was sprayed at the pre-anthesis stage (Ram, 1983; Singh and Ram, 1983; Prakash and Ram, 1986). Chen (1981) reported no effect on fruit retention when 5 mg/l of either naphthaleneacet- amide or β-naphthoxoyacetic acid were applied three times at 2-week inter- vals to panicles in which fruit had reached 4 mm in diameter.
Despite increased fruit retention of mango using exogenous applications of auxins, few studies have examined endogenous auxins in fruit as related to retention (Chacko et al., 1970a, b; Ram et al., 1983; Prakash and Ram, 1984).
Singh and Singh (1974) were unable to detect signifi cant differences in endog- enous auxins or inhibitors when comparing alternate and regular bearing cultivars. Chen (1981) observed lower levels of auxin-like substances in
T.L. Davenport 142
mesocarp and calyx tissues of abscised fruits than those of intact fruits. Simi- lar decreases in auxin and gibberellins with an increase in abscisic acid as fruitlets abscised were reported by Bains et al. (1999). The interaction of auxin in fruit and abscission zones to maintain mango fruit retention is not clear.
Continuous auxin synthesis and basipetal transport to the abscission zone is critical for maintenance of plant organs, including fruit (Crane, 1964;
Nitsch, 1965; Morgan et al., 1977; Davenport et al., 1980; Roberts and Osborne, 1981). Increased mango fruit set and retention in response to exogenously applied auxins confi rms this requirement; however, other hormonal factors also appear to be involved. Developing seeds are rich sources of all the known classes of phytohormones, including auxins (Crane, 1964; Nitsch, 1965; Chacko et al., 1970a, b, c; Chen, 1981). Hence, exogenous enrichment of auxin in the presence of other seed-produced phytohormones facilitates increased fruit retention. In contrast, NAA (10 and 20 mg/l) spray-applied to bagged, self- pollinated fl owers, does not result in development of stenospermocarpic fruits beyond the marble size (Venkataratnam, 1949; Chacko and Singh, 1969a, b). Similarly, applications of 250 or 500 mg/l GA3 or 250 mg/l BA alone to panicles does not promote production of stenospermocarpic fruits (Chacko and Singh, 1969a, b). Supplying exogenous β-naphthoxyacetic acid (10 mg/l), BA (250 mg/l) and GA3 (250 and 500 mg/l) together in multiple sprays until half grown, however, resulted in retention of several seedless fruit to maturity. Chen (1983) and Oosthuyse (1995b) observed that gibberel- lin, cytokinin and auxin reduce fruit drop of open-pollinated fruitlets of some cultivars. Thus, although auxin is important for maintaining the abscission zone, the presence of other phytohormones appears to be important for fruit- let development (Chacko et al., 1970a, b; Ram, 1983; Ram et al., 1983).
Cytokinins
Although cytokinins are not generally thought to be associated directly with abscission, Ram (1983) and Ram et al. (1983) concluded that low cytokinin levels during fruit development might contribute to fruit loss. Chen (1983) observed a correlation of low cytokinin levels in stenospermocarpic fruits with abscission at the marble stage of growth. Application of 250 mg/l BA to bagged panicles does not promote production of seedless fruits (Chacko and Singh, 1969a, b). The synthetic cytokinin, N-(2-chloro-4-pyridyl)-N’-phenylurea (CPPU) also does not improve fruit set when applied alone at a rate of 10 mg/l to post-anthesis panicles (Oosthuyse, 1995b). The role of cytokinins in separation events remains inconclusive.
Gibberellins
Gibberellins do not appear to be directly linked to the onset of abscission (Chacko et al., 1970c, 1972a; Ram and Pal, 1979; Chen, 1981; Ram, 1983). Spray applications of GA3 to pre- and post-anthesis panicles to increase fruit set and retention have been inconsistent. Increased yield (Teaotia et al., 1967;
Singh and Ram, 1983; Rajput and Singh, 1989) and production of seedless fruit (Kulkarni and Rameshwar, 1978) have been reported from these treatments, but Chacko and Singh (1969a, b) observed no such effects. Chen (1983) and
Reproductive Physiology 143
Oosthuyse (1995b) investigated the effects of several foliar applications of GA3 starting at the 4 mm diameter stage, but were unable to improve fruit set.
Several classes of gibberellin-synthesis inhibitors have been tested for reducing fruit drop. The growth retardants, daminozide and cycocel, increased fruit set when applied to post-anthesis panicles (Singh and Ram, 1983). The authors suggested that increased fruit retention might have been mediated through increased cytokinin-like activity of the growth retardants. Although initial fruit set was promoted by PBZ, yield was not improved (Kurian and Iyer, 1993a). It is not clear whether the contrasting results of increased yield (Kurian and Iyer, 1993b) were due to reduced fruit loss or more intense fl ow- ering in response to treatment. Goguey (1990) reported increased fruit set and retention using soil-applied PBZ at 5 g ai/tree. Spray application of uni- conazole, a more biologically active triazole (500–2000 mg/l), reportedly increased fruit set and yield (Galila and El-Masry, 1991). It is diffi cult to resolve the contradictory results demonstrating enhancement of fruit reten- tion by GA3 and inhibitors of its synthesis.
Inhibitors
Abscisic acid (ABA) is possibly involved in fruitlet abscission. Although cor- relations exist between certain inhibitors and abscission of mango fruitlets, no clear cause and effect relationships have been established. Fruit drop was correlated with levels of an acidic inhibitor, possibly ABA (Chacko et al., 1970b, 1972a; Singh and Singh, 1974; Ram, 1983; Prakash and Ram, 1984).
Chen (1981) reported similar changes in putative ABA with maximum levels occurring during early fruit drop and with advancing age of fruits. Putative ABA levels in abscised and retained fruits were compared and were highest in the calyx and mesocarp of abscised fruitlets.
Ethylene
Ethylene has the greatest immediate impact on fl ower and fruitlet abscission.
Van Lelyveld and Nel (1982) reported higher levels of ethylene in abscised fruitlets compared with those retained on trees. Núñez-Elisea and Davenport (1983, 1984, 1986) examined the dynamics of ethylene production in intact and excised fruitlets from onset to separation. Increased production began in explants about 26 h postharvest and increased logarithmically until fruit sep- aration. Abscission of the fruitlets began 48 h after the onset of enhanced ethylene production. Similar results with avocado fruitlet abscission experi- ments (Davenport and Manners, 1982) indicate that the onset of ethylene production in intact fruitlets is spontaneous in individual fruitlets followed by abscission 48 h later. The pericarp provided the bulk of ethylene for induc- tion of abscission processes; the pedicel produced no ethylene. There was reduced fruit drop in response to inhibitors of ethylene production and action (Singh and Ram, 1983; Naqvi et al., 1990, 1992). Whereas increased peroxi- dase (Van Lelyveld, 1978) and polyphenol oxidase activities have been reported in abscissed mango fruitlets (Van Lelyveld and Nel, 1982), Núñez- Elisea and Davenport (1984) observed no changes in peroxidase activity or protein levels prior to separation of fruitlets.
T.L. Davenport 144
Abscission of stenospermocarpic fruits has been associated with small increments in ethylene production (Núñez-Elisea and Davenport, 1983). Sen- sitivity to low levels of endogenous ethylene may refl ect the absence of seed- produced auxins. Protection of the abscission zone depends on a constant supply of auxin, and ethylene production levels in tissues correlate with endogenous auxin levels (Roberts and Osborne, 1981).
Despite their roles in cell division, cell enlargement and maintenance of the abscission zone in developing fruit, a specifi c recommendation for exog- enous application of plant growth regulators, either alone or in combination, to improve yield of mango has not been adopted. Phytohormones have little residual effect on fruit development, and multiple applications of products to counteract the short-term responses are prohibitively expensive.
Photoassimilates
Wolstenholme and Whiley (1995) discussed the ecophysiology of the mango as a basis for preharvest management. They proposed that the adaptive sur- vival strategies of the mango explain its notoriously poor cropping perfor- mance. Mechanisms that impart tolerance to heat, drought and fl ood stresses, which the tree has developed for survival in harsh environments, have come at considerable carbon cost with the resultant diversion of photoassimilate resources away from fruiting.
There is abundant evidence that heavy cropping in tree crops exhausts stored reserves (Jones et al., 1975; Kaiser and Wolstenholme, 1994; Whiley et al., 1996) and that current photosynthate is often unable to satisfy the demands of fruit set and fruit growth after heavy and prolonged fl owering (Chacko et al., 1982). There are signifi cant genotypic differences in photoassimilation rates between low- and high-yielding cultivars growing in both the tropics and the subtropics of Australia (Chacko et al., 1995; Searle et al., 1995). At each location, photoassimilation rates were considerably greater on the higher- yielding cultivar, and this difference was maintained from fl owering through to fruit maturation. 14C studies during the fruit set and abscission period also demonstrated strong discrimination in the movement of assimilates, which was dominated by randomly located fruit on panicles of the low-yielding cultivar (Chacko et al., 1995). In contrast, assimilate discrimination to fruitlets was less severe in the high-yielding cultivar with a more even distribution of photoassimilates. It was concluded that the availability and distribution of photoassimilates during the fruit set and establishment stages was largely responsible for the yield differences between the cultivars.
Supporting evidence for the role of photoassimilates in fruit set and retention also comes from enrichment studies (Schaffer et al., 1999). Container- grown plants that fl owered in the open were transferred to controlled- environment glasshouse rooms immediately after the completion of anthesis.
Temperatures were 28°C day/20°C night while the atmospheric carbon diox- ide (CO2) concentrations were 350 or 600 Pmol/mol. Photoassimilation of trees in the CO2-enriched rooms was approximately 60% greater than those held at partial pressures of 350 Pmol/mol CO2. Fruit retention and fi nal yield were signifi cantly higher on those trees grown at the partial pressure of
Reproductive Physiology 145
600 μmol/mol CO2. Higher levels of available assimilates during the fruiting cycle appear to benefi t fruit retention and yield.