Acknowledgements
T. L. Davenport
5.5 Environmental Infl uence on Vegetative and Reproductive Development
Reproductive Physiology 111
(PBZ) reduces the time in rest necessary to allow fl oral induction during warm temperature conditions by c.1 month (Davenport, 2003), thus increas- ing the potential to produce reproductive shoots in younger stems when ini- tiated to grow. PBZ and uniconazole, triazole compounds that inhibit kaurene oxidase in the gibberellin-synthesis pathway (Dalziel and Lawrence, 1984;
Rademacher, 1991), stimulate production of fl owering shoots during weakly inductive conditions (Burondkar and Gunjate, 1991, 1993; Tongumpai et al., 1991a; Voon et al., 1991; Nartvaranant et al., 2000; Yeshitela et al., 2004a).
Application of PBZ to mango trees bearing 1-month-old stems produced infl orescences when bud break was initiated 3 months later by foliar applica- tion of KNO3 (Davenport, 2003).
Vegetative or reproductive induction at the time of shoot initiation is governed by the ratio of the putative fl oral promotive to inhibitory compo- nents (Lang et al., 1977; Lang, 1984; Kulkarni, 1988a; see Bernier et al., 1981 for additional references). The mango fl oral inhibitor should be viewed as an age-dependent VP. The presence of an age-regulated VP in mango leaves, which moves with the temperature-regulated FP and photoassimilates in phloem, may explain the induction of specifi c receptors by this promoter in targeted leaf primordia to cause development of leaves in vegetative or mixed shoots. A gradual decrease in the level or infl uence of the VP may cause vegetative shoots to develop when initiation occurs on 2-month-old stems, and generative or mixed shoots when initiation occurs in stems from 4- to 7-month-old stems, given the constantly warm daily temperatures maintaining a low level of FP in both situations.
5.5 Environmental Infl uence on Vegetative and Reproductive
T.L. Davenport 112
described the vegetative growth and fl owering responses of several mo- noembryonic and polyembryonic cultivars to four temperature regimes rang- ing from vegetatively inductive (30°C day/25°C night) to fl oral inductive (15°C day/10°C night). The effect of temperature on marcotted, container-grown plants that were tip pruned or defoliated in order to stimulate shoot initia- tion was also studied (Davenport, 1987; Núñez-Elisea et al., 1991, 1993, 1996;
Núñez-Elisea and Davenport, 1994b). Mango trees develop vegetative shoots when shoot initiation occurs in warm temperatures (30°C day/25°C night), whereas infl orescences develop when shoots initiate growth in cool tempera- ture conditions (18°C day/10°C night; or 15°C day/10°C night) (Whiley et al.
1989; Núñez-Elisea and Davenport, 1991b, 1995; Núñez-Elisea et al., 1993, 1996; Batten and McConchie, 1995). Bangerth et al. (2004) reported changes in the major phytohormones in stems of containerized mango trees during exposure to cool, fl oral inductive temperatures. The minimum leaf age and time of exposure to a low temperature regime (18°C day/10°C night) required by stems for fl oral induction was examined (Núñez-Elisea and Davenport, 1995). Leaves are competent to respond to cool temperatures at 7 weeks, forming a small percentage of generative shoots. As they age, higher propor- tions of generative shoots are induced and warmer temperatures can stimu- late fl oral induction. The response to temperature is moderated by age of the previous fl ush. Stems that are 4–5 months beyond the limp, red-leaf stage of development will be induced to form generative shoots if initiated to grow at 25–30°C (Davenport, 2003).
Whiley et al. (1988, 1989, 1991) observed that at least 17 weeks are required for initiation of reproductive shoots on non-clipped stems of trees maintained at 15°C day/10°C night. In similar experiments with different cultivars with- out previous clipping of distal leaves to stimulate initiation, infl orescences were observed after 5 weeks at 15°C day/10°C night (Chaikiattiyos et al., 1994). Although inductive conditions were present in each of these studies, shoot initiation was delayed by the presence of distal leaves. The earlier ini- tiation of infl orescence development in tip-pruned or tip-defoliated stems compared to intact ones demonstrates that the fl oral stimulus may be pres- ent, but the buds are not induced until initiation occurs. It demonstrates the importance of stimulating initiation of stems by tip defoliation or pruning at the onset of incubation in controlled environment conditions so that the inductive response can be observed within a reasonable length of time.
The variable delays in shoot initiation in these studies occurred because the experimental protocols depended on the plants’ internal initiation cycle to initiate shoots. This cycle slows down when plants are exposed to lower tem- peratures (Whiley et al., 1988, 1989, 1991).
Floral or vegetative induction occurs when shoots are initiated. Resting buds of plants that are exposed to cool temperatures (18°C day/10°C night) for > 3 weeks and then transferred to a warm temperature (30°C day/25°C night) before initiation, produce only vegetative shoots (Núñez-Elisea et al., 1996). Thus, the stems do not ‘remember’ that they had been exposed to fl oral inductive conditions while still in rest. They responded to warm conditions present when shoot initiation occurred.
Reproductive Physiology 113
This response to temperature conditions at the time of shoot initiation extends to the formation of transition shoots if conditions change during early shoot development. First reported by Naik and Mohan Rao (1943), transition shoots are an unusual transition in expression of shoot type during a single growth fl ush (Kulkarni, 1988b; Núñez-Elisea and Davenport, 1989, 1992b; Batten and McConchie, 1995). The transition typically occurs near the middle of the extending shoot. Resting buds possess preformed nodes, each of which contains a primordial leaf or bract and a lateral meristem. The api- cal meristem initiates cell division at the same time or soon after the nodal target tissues begin development (Mustard and Lynch, 1946; L.B. Singh, 1960;
Núñez-Elisea et al., 1996). Vegetative or infl orescence development in the pre-formed primordia is underway before the apical meristem begins to pro- duce differentiating cells. Transfer from a warm, vegetatively inductive con- dition to a cool, fl oral inductive environment at early bud break results in formation of V/F transition shoots (Fig. 5.5). Transfer from cool to warm con- ditions at the same stage of bud break results in formation of F/V transition shoots (Batten and McConchie, 1995; Núñez-Elisea et al., 1996).
The fl owering response to temperature occurs in mangoes growing in subtropical latitudes where cool temperature is the dominant induction fac- tor. Many cultivars fl ower erratically in the low-latitude tropics, providing continuously warm temperatures with high soil and atmospheric moisture.
Under such conditions, the age of stems is the dominant inductive factor (Buell, 1954; Nakasone et al., 1955; Ravishankar et al., 1979; Ou and Yen, 1985;
Issarakraisila et al., 1992), and occasional cool night temperatures in the upper latitude tropics have a positive moderating effect (Davenport, 2003).
Water relations
In the absence of cool temperatures, mango trees in the tropics may fl ower in response to irrigation or rain following periods of water stress lasting 6–12 weeks or more (Pongsomboon, 1991). Plant water stress has been presumed to provide the stimulus for fl owering (reviewed in Whiley, 1993; Chaikiatti- yos et al., 1994; Schaffer et al., 1994; Davenport and Núñez-Elisea, 1997); how- ever, most of these studies have failed to substantiate prolonged tree water defi cit as a successful agent for fl oral induction.
Experiments with container-grown trees fail to produce infl orescences after 8 weeks of water defi cit (Wolstenholme and Hofmeyr, 1985). Under glasshouse conditions (27°C day/22°C night; relative humidity (RH) ≥ 90%), container-grown, monoembryonic cultivars were water stressed through defi cit irrigation for 14 days, resulting in an average leaf xylem water poten- tial of −3.9 MPa (Davenport, 1992; Núñez-Elisea and Davenport, 1992a, 1994b). Following resumption of irrigation, all trees grew vegetatively. Sim- ilarly, only vegetative growth was obtained when container-grown trees were deprived of irrigation for 36 days during summer, although leaf xylem water potentials of −3.78 MPa were attained (Núñez-Elisea and Davenport, 1994b). Water stress imposed on plants during the cool autumn months
T.L. Davenport 114
(night temperatures < 15°C) do not increase the proportion of apical buds forming infl orescences, but expedited shoot initiation after rewatering (Núñez-Elisea and Davenport, 1994b). These results demonstrated that cool temperatures provide inductive conditions, whereas relief of water stress accelerated shoot initiation under cool, inductive temperatures. Flowering was delayed when container-grown monoembryonic mangoes were water- stressed at 18°C day/15°C night (Chaikiattiyos et al., 1994). Water-stressed trees held at 29°C day/25°C night did not fl ower.
Mango trees growing in the low-latitude tropics may fl ower after an extended period of mild water stress (Harris, 1901; Collins, 1903; Kinman, 1918; Gangolly et al., 1957; Gangolly, 1960; L.B. Singh, 1960). Pongsomboon et al. (1991) observed fl owering in fi eld-grown trees in the tropics following 6 weeks of withholding water. The primary impact of water stress appears to be prevention of shoot initiation during stress. The accumulating age of stems is greater in water-stressed trees than in trees maintained under well-watered conditions that promote frequent vegetative fl ushes (Davenport, 1992, 1993;
Schaffer et al., 1994). This delay in fl ushing may provide more time for accu- mulation of a putative FP (Schaffer et al., 1994) or reduction in the level of a putative VP (Davenport and Núñez-Elisea, 1997; Davenport, 2000). Some cultivars appear to be better adapted to such delays in growth and perform better in dry environments in the tropics.
Effect of N on fl owering
Subsequent to the discovery of ethephon to stimulate mango fl owering (Gon- zalez, 1923; Alcala and San Pedro, 1935), Barba (1974), Bueno and Valmayor (1974), Astudillo and Bondad (1978), Bondad et al. (1978), Bondad and Apos- tol (1979), Pantastico and Manuel (1978) and Bondad and Linsangan (1979) reported that KNO3 could be used for the same purpose. This has been exploited in the low- and mid-latitude tropics (Mosqueda-Vázquez and de los Santos de la Rosa, 1981; Mosqueda-Vázquez and Avila-Resendiz, 1985;
Núñez-Elisea, 1985, 1986; Ou and Yen, 1985; Winston and Wright, 1986;
Tongumpai et al., 1989; Goguey, 1993; Ravishankar et al., 1993; Sergent et al., 1996; Yeshitela et al., 2004b, 2005). The nitrate (NO3–) anion is the active component of KNO3 (Bueno and Valmayor, 1974), and ammonium nitrate (NH4NO3) is twice as effective as KNO3 (Núñez-Elisea, 1988; Núñez-Elisea and Caldeira, 1988). In the low- and mid-latitude tropics, receptive trees respond by developing visible reproductive buds within 2 weeks after application. The effective spray concentration is 1–10% KNO3, depending on the age of the trees and climate. Two to four per cent KNO3 or calcium nitrate (Ca(NO3)2) and 1–2% NH4NO3 are effective for stimulating fl owering in most conditions. The physiological and temporal timing of application is important. Old trees, non-vigorous trees, and trees in which vegetative fl ushes have been discouraged by low water potentials produce the best response to NO3– induction (N. Golez, personal communication, the Philip- pines, 1989).
Reproductive Physiology 115
Chemical bud forcing is most effective in the tropics where distinct wet and dry seasons prevail. The response to chemical bud forcing by NO3− and ethephon diminishes at latitudes > 22° N or S (Mosqueda-Vázquez and de los Santos de la Rosa, 1981). Their effect may involve the decline of night temperatures from ≥ 20°C around the equator to ≤ 10°C between 22° and 27°
N or S latitude during winter months or by late summer vegetative fl ushes.
Trees in the wet or dry subtropics at 25° N or S have not responded to treat- ments (Davenport, 1993).
Stems must be suffi ciently mature, dark green with a minimum age of 4 months since the previous limp, red-leaf stage in easily induced cultivars and 5 months for more recalcitrant cultivars to obtain a reproductive shoot response in the low-latitude tropics (Davenport, 2003). Bueno and Valmayor (1974) indicated that leaves must be brittle when hand-crushed. Núñez- Elisea (1986, 1988) reported that stems must be at least 6 months old. Trees that experience autumn dry periods become responsive to treatments as early as October (northern hemisphere). Groups of stems within tree cano- pies are produced through asynchronous fl ushes of growth, and vary in age;
only a few are responsive to the fi rst inductive spray. Subsequent biweekly applications cause fl owering in canopy sectors as they reach the age-depen- dent requirement for initiation. Early and out-of-season fl owering and fruit- ing can thereby be achieved.
KNO3 may be fl oral inductive in mango (Barba, 1974); however, trees in the upper latitude tropics typically fl ush vegetatively rather than produce bloom when either KNO3 or NH4NO3 is sprayed between June and Septem- ber (N. Golez, personal communication, the Philippines, 1989). The warm, rainy season producing frequent fl ushes of growth during this period is con- ducive to a vegetative response to the sprays. These results indicate that KNO3 and NH4NO3 stimulate shoot initiation but do not determine bud morphogenesis. In buds released after KNO3 or NH4NO3 treatments, the ratio of leaf-generated FP to VP and not NO3– causes initiating buds to become reproductive. Kulkarni (1988b, 2004) suggested that the fl oral stimulus is present in stems when buds are forced in response to KNO3 and suggested that KNO3 may also sensitize buds to the fl oral stimulus. Davenport (2003), T.L. Davenport and J. Oleo (2006, unpublished data) and F. Ramirez and T.L.
Davenport (submitted for publication) observed 100% vegetative shoots when 4% KNO3 was foliar applied to 2-month-old stems; whereas, applica- tion of the same spray treatment to 4.5-month-old stems on trees in the same orchards resulted in 100% reproductive shoots.
Trees with high leaf N levels rarely fl ower in the tropics. Lack of fl ower- ing is always due to frequent vegetative fl ushes of growth, especially during the rainy season. Mango trees must have leaf N levels of 1.4% or less in order to suppress frequent fl ushes of vegetative growth (Davenport, 2003). Leaf N levels of < 1.1% suppress frequent fl ushes but also provide insuffi cient nutri- tion to support good cropping. Thus, 1.1–1.4% N levels in leaves appear to be optimum for good commercial production and control of fl owering time in a managed orchard. The application of KNO3 to the foliage of the resting stems 4–5 months after the limp, red-leaf stage will cause a fl owering response.
T.L. Davenport 116
Photoperiod
Flowering in most trees does not appear to be under photoperiodic control (Kozlowski et al., 1991). Mango cultivation is concentrated between 27° N and 27° S where the shortest annual photoperiod is c.10.5 h and the longest photoperiod is c.13.5 h. Cultivars in the upper-latitude tropics and subtropics fl ower during the winter when photoperiods are short; however, trees in the low-latitude tropics, where a 12-h photoperiod is nearly constant, can fl ower at any time of the year. Furthermore, fl owering on spring-initiated shoots in the subtropics occurs during summer (Schaffer et al., 1994). Studies have failed to demonstrate a correlation between 8-h photoperiods and fl owering (Maiti, 1971; Maiti and Sen, 1978; Maiti et al., 1978). Núñez-Elisea and Daven- port (1995) studied the effects of 11-, 12-, 13- and 24-h photoperiods at 18°C day/10°C night, or 11- and 13-h photoperiods at 30°C day/25°C night on fl owering of container-grown trees. Photoperiod had no effect on the fate of buds, and the promotive effect of cool temperatures on fl owering was inde- pendent of photoperiod. Photoperiods of 11-, 12- or 13-h with 18°C day/10°C night, caused fl owering in trees within 40 days. The 24-h photoperiod with 12-h thermoperiods of 18°C and 10°C caused fl owering of trees within 35 days. Photoperiods of 11- or 13-h at 30°C day/25°C night resulted in vegeta- tive growth only. With warm temperatures, vegetative shoots were produced in 17 days. These results confi rm that fl oral induction is caused by cool tem- peratures and not by short photoperiods and that warm temperature, not a long photoperiod, caused vegetative induction.