Double Sigmoid Pattern

Another common pattern exemplified by stone fruit also has an initial exponential growth phase during early cell division (Figure F3.3). The fruit growth then slows significantly in midseason and completes a first sigmoid phase. A second rapid increase in diameter and fresh weight growth occurs next, followed by a final slowing of growth as harvest approaches. Together, the seasonal pattern is described as a "double sigmoid."

Unlike the pome fruit pattern, the seasonal pattern of growth of fruit dry weight in stone fruit is somewhat different from the pattern for fresh weight. During the slow period of fresh weight growth in midseason, dry weight growth continues. At this time, growth, called "pit hardening," is primarily in the dense tissues of the seed. The second sigmoidal growth phase is usually related to softening of the fruit and rapid accumulation of sugars. The causes and triggers of this complex pattern of growth are not known.

When considering the energy in the form of tree photosynthesis required for fruit growth over the season, it is theoretically best to measure the energy value of the fruit. However, since that is not easy,

Percent of Growing Season

Percent of Growing Season

FIGURE F3.4. General dry weight growth rate per day for apples and stone fruit, representative of expolinear and double-sigmoid growth patterns. Dry matter growth gains indicate seasonal resource requirements.

the most practical expression of growth is the daily dry weight gain, as it represents most closely the energy in most fruit that contain primarily starch and sugars. For the two main growth patterns described earlier, the seasonal pattern of dry matter growth per day is very different (Figure F3.4). The expolinear growth of pome fruit leads to a rapid increase in dry weight gain per day for the first half or so of the season, followed by a quite constant rate. The pattern for the double-sigmoid growth of stone fruit, however, reflects the two phases of rapid growth with a slower growth rate occurring in midseason and near harvest. These curves represent general patterns for these fruit growing without significant competition and stress. The actual growth curves may vary somewhat with cultivar, and crop load or environmental stress may limit growth at times.

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