Organic Pineapple Working Group

[Organic Pineapple Working Group <opwg AT lists.ibiblio.org>]

Dear colleagues (for those not interested in the subject, I apologize for the intrusion):

I want to thank all who responded; the response has been almost overwhelming. Rather than try to reply to all who wrote on subject, I have summarized my thoughts on this issue below. I want to bring closure to the subject until someone is able to conduct a successful experiment to clearly demonstrate the cause. I will be repeating some thoughts shared by others but no one has applied a detailed plant physiological analysis in attempting to understand the disorder. I welcome your rebuttals, criticisms and new insights on the following.

Please don't write to me just to tell me that you agree or disagree with my theoretical explanation.

Genetic predisposition seems to be a factor because while stem cracking has been observed in Smooth Cayenne most people who reported seeing the disorder noted that the cracking was seen in the Queen cultivar, or in the case of Tainon 17, a Queen hybrid of which there are several but only the one mentioned is widely grown.

Based on the responses I have received and what I know about plant physiology, I believe the primary cause has to be the development of mild to moderate water stress in the fruit stem (peduncle) at a time when cell expansion is occurring and believe a similar explanation applies where fruit splitting is observed.

From basic plant physiology it is known that all growth is the result of cell division and cell expansion. Cell division probably is a minor factor in peduncle splitting (actually lateral cracking) because it is one of the processes most sensitive to water stress (deficit) but also because by the time the fruit emerges in the leaf whorl cell division in the fruit stem likely has mostly ceased and further growth is the result of cell expansion. I hypothesize that cell division has mostly ceased because a study of fruit developmental anatomy (M. Okimoto, Bot. Gaz., 1948 ) showed that cell division of the fruit has ceased by the time flowering is completed.

Cell expansion is turgor (water pressure) dependent and in a well-watered plant where large water concentration gradients don't exist, cell expansion is a well studied and orderly process. As a result of a water concentration gradient between water conducting tissue (xylem) and the expanding cell, water enters the expanding cell causing internal pressure on the cell wall to increase. Growth regulators cause the cell wall to weaken so fibers can slip past each other and allow the cell to expand without rupturing. When plants are exposed to a gradually decreasing water supply, relatively high (perhaps very high; I don't believe the subject has been studied) concentrations of soluble substances likely accumulate in the peduncle tissues because photosynthesis is less affected by mild to moderate water deficits than is cell expansion. If heavy rain or irrigation rather suddenly eliminates the water deficit, high concentration gradients suddenly (probably minutes to a few hours) develop between water conducting tissue and the surrounding (parenchyma) tissues. Under such conditions water would move so rapidly into the parenchyma tissues that the cell walls would not have time to adjust to the rapidly increasing internal (turgor) pressure so the cells would burst, and in the specific case of pineapple at a susceptible growth stage, result in stem cracking. This phenomenon also is well studied in fruit crops, cherries and tomatoes come to mind.

I can't think of any way that high or low fertilization, or temperature for that matter, would cause such a rapid rate of cell enlargement that it would cause cells to rupture and produce the splitting. Nutrition might be a secondary factor where calcium and perhaps boron could affect cell wall strength, thus making tissues more susceptible to cracking in the above scenario.  A lack of nitrogen or potassium, or both, might slow photosynthesis, which could reduce the concentration of soluble materials flowing upward in the phloem of the peduncle and thus make it less likely that large concentration gradients would develop as described in the scenario above. Without a large gradient at the time a water deficit is relieved, there would be no rapid influx of water into cells and no uncontrolled cell expansion.

The responses from those who have actually seen the disorder seems to confirm that most cracking is associated with increased water availability.

If someone collects solid data on the subject I hope they will close the discussion permanently by writing a note about their results for Pineapple News.

Best regards,

Duane