Determination of pineapple (Ananas comosus, MD-2 hybrid cultivar) plant maturity, the efficiency of flowering induction agents and the use of activated carbon

Abstract

Natural flowering is one of the major agronomic problems in pineapple (Ananas comosus) cultivation. It causes heterogeneous flowering and fruit development with multiple harvest passes of the same field as an inevitable consequence. To avoid natural flowering, pineapple plants are induced to synchronize flowering by external ethylene treatments. In this research it is shown that pineapple plants (MD-2 hybrid cultivar) are already sensitive to external ethylene treatments at an early developmental stage, i.e. 3 months after planting, although no natural flowering occurs during this early stage of plant development. These results indicate that young pineapple plants already posses all the necessary factors to induce flowering in response to ethylene treatments. Additionally the efficiencies of flowering induction of different external ethylene treatments, including a novel agent developed at our lab, called ‘zeothene’, were evaluated. Zeothene (=zeolite containing ethylene gas) and ethylene dissolved in water (both applied in the central cup of the plant) were proved to be very efficient flowering induction agents. The commercial cultivation practice, in which ethylene gas is sprayed with water over whole plants, was found less efficient confirming that central cup applications are more efficient than whole plant spraying. Cup applications allow the active ingredient (ethylene or ethephon) to be taken up immediately by the apical meristem resulting in an efficient flowering induction signal. The addition of activated carbon to enhance the flowering induction treatment was found to be useful only with a very high dose of activated carbon (5%) and a long interaction time (at least 5–30 min) between the activated carbon and the flowering induction solution.

Introduction

Ananas comosus is the most important representative of the Bromeliaceae family and is cultivated worldwide all around the tropical and subtropical regions for local consumption and international export. Pineapple holds the third rank in world tropical fruit production only preceded by banana and citrus (Uriza Avila, 2005). The MD-2 hybrid cultivar, developed by Del Monte Fresh Produce International Inc., has gained market share over the last few years, especially on the international fresh fruit market (Rebolledo-Martínez et al., 2005). This cultivar possesses a high production capacity and good fruit quality characteristics compared with the ‘Smooth Cayenne’ cultivar (Chan et al., 2003).

A remarkable hallmark of Bromeliaceae species is the possibility to induce flowering by external ethylene treatments. This feature is exploited worldwide by commercial pineapple growers to synchronize flower and fruit development resulting in a single harvest pass. This synchronization is of crucial importance in pineapple cultivation due to the non-climacteric nature of the fruit. However, natural flowering can occur before a flower induction treatment is applied, resulting in unsynchronized flower and fruit development and necessitating multiple harvest passes of the same field. Natural flowering is one of the long lasting agronomic problems in pineapple cultivation causing considerable losses and a disrupted market supply (Min and Bartholomew, 1996, Wang et al., 2007).

Natural flowering can occur under different environmental conditions, namely low night temperatures (van Overbeek and Cruzado, 1948a, Friend, 1981), short photoperiods (Friend and Lydon, 1979), water deficit (Py et al., 1984), flooding (Py et al., 1984), geotropic stimulation (van Overbeek and Cruzado, 1948b) and mechanical stress, e.g. hitting the stem base (Ruiz, J., personal communication). These inductive conditions trigger ACC synthase gene expression, resulting in increased ACC production and consequently augmented ethylene biosynthesis (Botella et al., 2000, Trusov and Botella, 2006). Ethylene is responsible for signaling the switch from vegetative into flowering stage (Burg and Burg, 1966, Min and Bartholomew, 1997). The developmental stage at which the plant is susceptible to natural flowering conditions or external ethylene applications remains to be revealed. Das et al. (1965, cited by Norman, 1982) stated that pineapple plants should have at least 21 leaves before they are susceptible to flowering induction. Py et al. (1984) suggested that plants (‘Smooth Cayenne’) should weigh at least 1 kg before applying flower induction treatments. However, no representative maturity level has yet been determined so far. The plant maturity level can be defined as the moment on which the plant becomes susceptible to flowering induction signals and switches from juvenile (vegetative development) into mature stage (generative development) (Larcher, 1995).

Environmental conditions promoting natural flowering also increase the plants sensitivity towards external flowering induction applications. Flowering induction can be achieved by a range of commercial induction agents, thoroughly summarized by Bartholomew et al. (2003). The most frequently used chemicals in present pineapple cultivation are ethephon (2-chloroethylphosphonic acid) and ethylene gas.

Ethephon is a pH dependent ethylene-releasing chemical which is stable at low pH (≤3) and breaks down under more alkaline conditions (pH ≥ 5) to release ethylene gas (Yang, 1969, Dass et al., 1976). The product is sprayed over entire plants by large boom sprayers. Multiple applications may often be necessary to secure a successful flower induction since unfavorable climate conditions such as high temperatures and low relative humidity can cause failure in ethephon uptake (Turnbull et al., 1999).

Ethylene gas is more effective than ethephon to achieve flowering induction but is difficult to apply (Py et al., 1984). Ethylene gas is properly applied with a pressurized sprayer which uses water as carrier for the gas. Ethylene can partially dissolve in the water droplets which will make contact with the plant tissue. The solubility of ethylene gas in water is temperature dependent, i.e. 61.6 g/dm³ at 30 °C, 76.7 g/dm³ at 20 °C and 101.8 g/dm³ at 10 °C (L’Air Liquide, 1976). For this reason late evening or night applications of ethylene gas flower induction treatments are favored. To enhance the efficiency of the flowering induction treatment activated carbon is added to the water because it increases ethylene absorption and consequently total amount of ethylene exposed to the plant. As the activated carbon will slowly release the gas, the exposure time of ethylene gas to the plant is extended (Py et al., 1984, Soler et al., 2006).

In this article we report on a study to unravel the exact MD-2 hybrid pineapple plant maturity level. In addition, the efficiency of five different flower induction agents, including four known and one novel (zeothene), were tested on pineapple plants cultivated on a commercial plantation. Finally the use of activated carbon, to enhance the flower induction reaction, was evaluated.