Hollow Heart in Triploid Watermelons

Successful fruit set and development in triploid watermelons consists of eight phases: 1) flowering of triploids and diploid pollenizers, 2) pollen production by diploid pollenizers, 3) pollen transfer from diploids to triploids, 4) pollen germination and pollen tube growth, 5) fertilization-like event, 6) hormone release, 7) cell division, and 8) cell expansion and fruit enlargement.  Photosynthate production and movement to dseveloping fruits is critical in the cell division and cell expansion stages.  It is theorized that a reduction in the amount of pollen that germinates and successfully produces pollen tubes will cause reduction in hormones released during the fertilization-like event and limit initial cell division leading to smaller fruits and hollow heart disorder.  Experiments have shown that limiting pollen increases hollow heart in triploid watermelons. Experiments were conducted from 2011-2013 where pollenizers were spaced at a 1:10 ratio with selected triploids.  In 2011 there was increased hollow heart incidence (hhi) in triploid fruits starting 1.5 m from a pollenizer crown: at 1.5 m hhi was 12%; at 3.0 m, 28%.  In a 2012 study, there was delayed fruit set and increased hhi with increasing distance from pollenizer plants.  This relationship was linear in the cultivar ‘Liberty’ but not in the cultivar ‘SS7187’.  In 2013, it was shown that under pollen limited conditions, less dense fleshed varieties (Liberty, SS7187) had 312-432% more hollow heart than more dense fleshed watermelon varieties (Crunchy Red and AC9651).  Hand pollination studies in 2011 and 2013 showed reduced hhi with increased amounts of pollen. Pollen transfer studies showed that ~500 pollen grains are necessary for triploid watermelon fruit set but ~1000 pollen grains were necessary for full fruit size and reduced hhi. Growth regulator studies from 2011-2013 showed that auxin (2,4-D, IBA) and cytokinin (6BA, CPPU) applications improved early fruit set.  The theory that pollination, hormone activity, and cell division in early watermelon fruit development determines hollow heart incidence and severity will be discussed.  Recommended management to reduce hollow heart includes insuring adequate pollen availability with matched pollenizer selection, use of mixed pollenizers with different flowering peaks, planting extra pollenizers, maintaining vigor of vines and planting to avoid cold weather at pollination. To improve pollen transfer, place extra bees, place hives in several locations around or in the field, consider using bumblebees for plantings where flowering occurs in colder weather, time bee placement properly, and manage pesticides to reduce effects on bees.