A few observations on double sigmoid fruit growth

Abstract

Context and purpose ‐ Many fleshy fruit, including the grape berry, exhibit a double‐sigmoid growth (DSG) pattern. Identification of the curious DSG habit has long been attributed to Connors’ (1919) work with peaches. Connors’ description of a three‐stage pattern consisting of two growth stages (Stage I and Stage III) separated by a rest period (Stage II) has become textbook material. The growth of grapes was described similarly by Winkler and Williams (1936), Nitsch et al. (1960), and most subsequent authors. Prior to Connors, grape berry development was described as a two‐stage process, in French periode herbacee and periode maturation, but this description refers to fruit ripening and has little or nothing to do with growth.

Material and Methods ‐ A review of grape literature reveals that the characteristic DSG habit was reported several times prior to Connors’ discovery in peaches. Analyses of berry size, turgor, firmness, and composition during Stage II and into Stage III are interpreted in the context of the growth habit.

Results ‐ It will be argued that one researcher in particular, Carl Neubauer, should be credited with the discovery of DSG and its description as a three‐stage phenomenon in fleshy fruits. It is widely reported that DSG in fleshy fruit is a consequence of within‐fruit partitioning (to endocarp or seed rather than pericarp/flesh). However, DSG is observed in berry dry weight and in seedless berries, which negate the common explanations. Thus, one hundred‐fifty years later, the nature of double‐sigmoid growth is still not understood. It is the resumption of rapid growth that is most curious. Various lines of evidence from our studies suggest that a suite of physiological changes during Stage II lead to the transition from Stage II lag phase to Stage III growth, paradoxically implicating a role of low cell turgor. Turgor declines and berries soften during Stage II. These changes occur in conjunction with increased apoplastic solutes and ABA, followed by increased sugar influx and upregulation of cell wall loosening enzymes. Because growth increases in the face of very low turgor, Stage III growth is hypothesized to result from cell wall loosening or even wall degradation without addition of new wall material.