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.