Morphological characteristics of pollen from triploid watermelon and its fate on stigmas in a hybrid crop production system

Hybrid crop production is more reliant on pollinators compared to open-pollinated crops because they require cross-pollination between a male-fertile and a male-sterile line. Little is known about how stigma receipt of pollen from male-sterile genotypes affects reproduction in hybrids. Non-viable and non-compatible pollen cannot fertilise plant ovules, but may still interfere with pollination success. Here we used seedless watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai) as a model hybrid plant, to evaluate the morphology, physiology, and movement of pollen from inter-planted genotypes (diploids and triploids). We found that pollen from triploids (‘Exclamation’ and ‘Royal Armada’) and diploids (‘SP-6’, ‘Summer Flavor 800’, and ‘Tiger’) was visually distinguishable. Pollen in triploids had more deformities (42.4–46%), tetrads (43–44%), and abnormal growth of callose plugs in pollen tubes. The amount of pollen in triploids to germinate on stigmas was low (8 ± 3%), and few pollen grains produced pollen tubes (6.5 ± 2%). Still, contrary to previous reports our results suggest that some viable pollen grains are produced by triploid watermelons. However, whilst honey bees can collect and deposit pollen from triploids onto stigmas, its effect on hybrid watermelon reproduction is likely to be minimal due to its low germination rate.与开放授粉（open-pollinated）作物相比，杂交作物生产对传粉媒介（pollinators）的依赖程度更高，因为杂交作物需要在雄性可育（male-fertile）与雄性不育（male-sterile）品系之间进行异花授粉。目前学界对雄性不育基因型的花粉被柱头接收后，会如何影响杂交品种的繁殖过程仍知之甚少。无活力（non-viable）且不亲和（non-compatible）的花粉虽无法使植物胚珠受精，但仍可能干扰授粉成功率。本研究以无籽西瓜（*Citrullus lanatus* (Thunb.) Matsum. & Nakai）作为杂交植物模型，探究间行种植的二倍体（diploids）与三倍体（triploids）两种基因型的花粉形态、生理特性与运动规律。研究发现，三倍体品种（‘Exclamation’与‘Royal Armada’）与二倍体品种（‘SP-6’、‘Summer Flavor 800’及‘Tiger’）的花粉可通过肉眼区分。三倍体西瓜的花粉存在更多畸形（42.4%~46%）、四分体（tetrads，43%~44%）以及花粉管中胼胝质塞（callose plugs）的异常生长。三倍体西瓜的花粉在柱头上的萌发率极低，仅为(8±3)%，且仅有极少数花粉粒能长出花粉管，比例为(6.5±2)%。尽管如此，与既往研究结论相悖的是，本研究结果表明三倍体无籽西瓜仍可产生部分有活力的花粉粒。不过，尽管蜜蜂（honey bees）能够采集并将三倍体西瓜的花粉沉积到柱头上，但由于该花粉萌发率极低，其对杂交西瓜繁殖的影响极微。