Ultrasol® K and Ultrasol® SOP 52 positive effect on floral induction

During September 2008 Steve Oosthuyse (SQM Africa) and Claudio Vicencio (SQM Latin America) visited the mango growing regions in Ecuador, Peru, and Northern Brazil. Since these regions are close to the equator, success in inducing flowering in this crop is vital for economic viability. Applying Ultrasol® K foliar at the correct stage of terminal shoot maturation proves to be an essential measure to induce intense flowering. Successful induction depends on the application of a precise sequence of treatments at crucial times during the vegetative flushing period that follows harvest.

Mango trees need to experience stress for the leaves on the terminal shoots to produce flowering hormone. This hormone - referred to as florigen - has not yet been identified but intricate girdling experiments clearly attest to its existence. Terminal shoots experiencing sufficiently reduced temperatures and sufficiently intense water-deficit stress respond by producing florigen. For a bud to develop florally, florigen production must coincide with the early stages of its development. Unbroken buds can either become shoots or inflorescences. Flower primordia are not components of the dormant bud but form when the bud starts breaking, if florigen is received by the newly developing bud in sufficient quantities. Florigen production varies greatly in intensity depending on the severity of stress experienced at a particular point in time. Attenuation is dramatic following stress relief. During periods of relief, which may be short, production typically stops, to restart again once stress is experienced once more. Buds starting their development during intervals of stress relief either develop as shoots or as intermediary inflorescence-shoot structures (Fig. 1).

Mixed inflorescences (right) or shoot (left) arising as a result of variation in the intensity of florigen production
Figure 1. Mixed inflorescences (right) or shoot (left) arising as a result of variation in the intensity of florigen production.

Ultrasol® K and Ultrasol® SOP 52 Have Positive Effect on Mango Floral Induction.

Gibberellic acid (GA) is a potent inhibitor of bud break in mango, and is required to be at low levels for bud break, which usually arises terminally, to occur. The GA level in terminal shoots drops over time from being relatively high in newly developed, but immature shoots. Initial levels are high since newly developing leaves produce GA as they expand. The level becomes low enough for bud break to occur once the leaves become dark green and furthered maturation has taken place.

In tropical Ecuador, Peru and Northern Brazil, growers prune their trees back after harvest, in heading back the outer branches. This action results in synchronized distal bud break and, as a consequence even vegetative flushing. Once the leaves are fully expanded and are about to darken, paclobutrazol - a strong GA synthesis inhibitor - is applied around the tree trunks to be taken up by the shallow feeder roots and carried to maturing terminal shoots in the upwardly moving xylem-water. Once the terminal shoot leaves become dark green, ethephon sprays are alternated with Ultrasol® SOP 52 sprays to prevent terminal bud break and to allow the endogenous state of the mature shoots to become more and more “enabled” regarding florigen production. Moreover, during the period of terminal shoot maturation, drought stress is imposed, and night temperatures are such that they generally drop to below 20 °C. To break bud dormancy and to effect general terminal bud development, Ultrasol® K is spray applied repetitively. Intense flowering is assured if florigen production is sufficiently intense during the bud-break period. The trees are released from drought stress once general bud development has commenced. Figure 2 shows a flowering orchard in Piura (Peru) where some variation in flowering intensity can be noted.

Flowering Kent mango orchard in Piura (Peru). Ultrasol® K spray applications are required to ensure intense and even flowering
Figure 2. Flowering Kent mango orchard in Piura (Peru).
Ultrasol® K spray applications are required to ensure intense and even flowering.

Paclobutrazol is applied about 90 days after post harvest pruning. All of the subsequent sprays are made at medium to full cover. Ultrasol® K sprays are commenced 12 weeks after paclobutrazol treatment, the action of repetitive spraying referred to as induction. 3 or 4 applications are made, weekly, at the rate of 3 or 4% w/v. Drought stress is relieved by copious water application once the terminal buds have started developing. Terminal bud development is first noted 3 to 4 weeks after induction. After paclobutrazol treatment and once the terminal shoot leaves have just become dark green, 2 applications of ethephon (25 ml per 100 l of water) and 2 of Ultrasol® SOP 52 (3% w/v) are made. Ethephon is applied first, and the interval between the sprays is 8 days. Drought stress is imposed once the terminal shoots have just become fully expanded and shortly after paclobutrazol treatment. The rate of paclobutrazol application depends on tree size. 25 or 30 ml of product (Cultar or AuStar) are applied in water around to trunks of 3 to 4 m high trees.

Farmer income is a strong reflection of flowering intensity (Fig. 3). Pruning may give rise to the development of new shoots that are less responsive to inductive treatments due to these shoots being at a lesser stage of maturation when the inductive treatments are carried out (Fig. 4).

Bonanza Estate in Ecuador: Flowering intensity is a direct reflection of farmer income. Here intensity was down to less than 20% due to failure to treat the trees with paclobutrazol
Figure 3. Bonanza Estate in Ecuador: Flowering intensity is a direct reflection of farmer income.
Here intensity was down to less than 20% due to failure to treat the trees with paclobutrazol.

The upper portion of the tree did not produce inflorescences as a result of pruning the branches at the wrong time
Figure 4. The upper portion of the tree did not produce inflorescences as a result of pruning the branches at the wrong time.

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