It is sometimes assumed that two plants won’t cross because of genetic incompatibility. That’s simple enough, so we just move on.
But there are cases where the failure is due to something that can be remedied. Maybe it’s pH, temperature or the absence of a key nutrient/element.
I happened across a paper that surprised me more than a little. Gudin and Arene (1991), working at Sélection Meilland, Antibes, France, observed the growth of pollen tubes from a rose designated ‘P 30’. They cultured the pollen tubes artificially, and used it on six cultivars. Three of the cultivars had stigmatic exudate (stigma goo) with a pH of 5, while the others had a pH of 9. I had no idea that the pH range in roses varied so widely. They found that ‘P 30’ pollen grew fastest and fertilized most successfully at pH 5. It grew slowly at pH 3 and 9.
I will venture to guess that a species with pH 9 stigma goo will also produce pollen that will grow in it.
http://bulbnrose.x10.mx/Roses/breeding/GudinpHRosaPollen1991.html
The above two authors were joined by Bulard, in the same year, in a study of changes in pollen quality over the seasons. Success of fertilizations varied with the seasons. Temperature was likely involved in some of this variation.
http://bulbnrose.x10.mx/Roses/breeding/GudinPollen1991b/GudinPollen1991b.html
Pollen quality can vary from specimen to specimen and from branch to branch. It may vary among flowers in the same cluster, and even on different anthers in the same flower. Yeamans (2014), studied pollen quality in Mimulus gutattus, and found that the raw protein content could vary from 15% to 45%. The higher protein content was positively associated with pollen viability.
http://bulbnrose.x10.mx/Heredity/YeamansPollen2014.html
http://bulbnrose.x10.mx/Heredity/AlonsoPollen2013.html
And speaking of temperature, Končalová (1975) in Czechoslovakia, studied a specimen of Rosa hugonis that had been collected as a seedling in 1918, but had never produced hips until 1973. In that year the weather was unusually warm at the time the pollen was developing, and when fertilization occurred.
She also compared the growth of Hugonis pollen in various concentrations of sugar, and at different temperatures.
http://bulbnrose.x10.mx/Roses/breeding/KoncalovaHugonis1975.html
Končalová, Jičínská and Sýkorov (1976) expanded the earlier study to include six other Rosa species, and to observed the effect of calcium as well as sugar on the growth of their pollen tubes
“The stimulating effect of calcium was generally most pronounced in the pollen from roses of hybrid nature, such as R. jundzillii, R. canina, and especially in the case of the calciphilous species R. eglanteria.”
This fact was established in vitro, but there might be some way to add a bit of calcium with the pollen to aid some crosses.
http://bulbnrose.x10.mx/Roses/breeding/KoncalovaRosePollenCa1976.html
In other experiments, boron was the limiting factor in pollen tube growth.
Fang: Boron and apple pollen (2016)
http://bulbnrose.x10.mx/Heredity/FangBoronApplePollen2016.html
Cheng: Boron and wheat pollen (1993)
http://bulbnrose.x10.mx/Heredity/ChengBoronWheatPollen1993.html
Wang: Boron and spruce pollen (2003)
http://bulbnrose.x10.mx/Heredity/WangBoronSprucePollen2003.html
de Wet: Temp, Boron and Mango pollen (1989)
http://bulbnrose.x10.mx/Heredity/deWetBoronMangoPollen1989.html
Peñaloza: Boron and pepper pollen (2018)
http://bulbnrose.x10.mx/Heredity/PenalozaBoron2018.html
Muengkaew: Calcium-Boron and Mango pollen (2017)
http://bulbnrose.x10.mx/Heredity/CalciumBoron2017.html
Finally, the odd occurrence of “hybrid vigor” in pollen. This seems contradictory at first glance, because pollen tubes should not be able to express “heterosis” or “heterozygosity”. Apparently the hybrid plants pack more nutrients into their pollen. This seems to confirm the old idea that crossing with species is easier with plants that are “a little bit hybridized”. That is, crosses between closely allied species, or between different geographical selections of the same species. The “hybrid” pollen gains vitality, and in many cases is more tolerant of environmental conditions than that of the parents.
Jóhannsson and Stephenson: Pollen Vigor (1998)
http://bulbnrose.x10.mx/Heredity/JohannssonPollen1998.html
Johnson & Mulcahy: Pollen vigor in inbred plants (1978)