Polyamines in kiwifruit and as a putative seed-produced factor

There is a correlation between seed number and fruit size in kiwifruit Actinidia chinensis var. deliciosa. Investigations also suggest there may be a “seed-produced factor” released by kiwifruit seeds that could promote cell division in surrounding tissue. This study was carried out to determine the concentrations of free polyamines in kiwifruit seeds compared with other fruit tissue and whether these polyamines could potentially be the putative seed-produced factor. Fruit were periodically harvested from -7 to 168 days after full bloom (DAFB), separated into seed, inner pericarp (IP) and outer pericarp (OP) from 17 DAFB, and the concentrations of the polyamines putrescine (PUT), spermidine (SPD) and spermine (SPM) measured. Concurrently, fruit were harvested periodically between 30 and 168 DAFB with seeds removed, floated on nutrient solution for 6 days and seed diffusates were analysed for polyamines. Results showed that PUT and SPD were the polyamines present in highest concentration in all fruit parts. PUT and SPD concentrations were significantly greater in seed than IP or OP tissue at their peak, between 17 and 30 DAFB, before falling gradually to similar concentrations at 48 DAFB. Seeds extracted from fruit showed that all three polyamines diffused out in vitro, with PUT diffusing at the fastest rate over the critical cell division period and peaking in seeds from fruit harvested at 50 DAFB. Possible roles of polyamines in kiwifruit growth are discussed.