Gynoecium structure and extra-gynoecial pollen-tube growth in an apocarpous species, Sagittaria trifolia (Alismataceae)
2017
Huang, Lan-Jie | Wang, Xiao-Fan
Apocarpy is regarded as an original feature obtained during the evolution of angiosperms. Compared with syncarpous plants, apocarpous plants have some adaptive disadvantages in apocarpous plants, for example, the number of offspring is lower under conditions of uneven pollen-tube distribution. However, in some apocarpous species, extra-gynoecial pollen-tube growth (EGPG) may remedy this disadvantage. We conducted micro-observations and field studies of Sagittaria trifolia, to investigate the gynoecium structure and the pathway of pollen-tube growth in the entire gynoecium. In a single-carpel pollination experiment, we found that the extra-gynoecial pollen tubes from a carpel of S. trifolia were able to fertilize approximately 13 carpels. Simulated EGPG in the entire gynoecium of S. trifolia revealed that its effect on the seed set could be divided into two stages: stage of low/high-level stigmas pollination, in which the cutoff point was about 0.1. The seed set would be markedly improved during the low-level stigmas pollination stage by EGPG when the maximum distance of extra-gynoecial pollen tubes could span three carpels, as in the present experiment. Our simulation also showed that the high pollen load could enhance the effect of EGPG on the seed set, and if the number of germinating pollen is triple the carpel number in the gynoecium, a 100% seed set rate would be obtained when approximately 50% of the stigmas are pollinated.
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