The effects of canavanine, ~,y-diaminobutyrie acid, homoarginine and lathyrine on the germination of pollen and on in-vitro growth of pollen tubes were studied in the following species: Lathyrus niger, L. 8ilvestris, Vieia uni]nga, Pisum sativum and Citer arietinum.
The effects of these non-protein amino acids depended on their quantity and on the plant species. Every amino acid had a promoting effect on germination and growth at some concentration in some species. Inhibition or promotion of pollen germination and pollen-tube growth were usually parallel. The stronger influence of some amino acid on growth than on germination may be due to slow penetration of the acid into the cell.
Homoarginine and lathyrine had a promoting effect at all concentrations in L. niger, a species in which these amino acids occur naturally. In most other species they had, if anything, a very slight inhibitory effect. ~,y-Diaminobutyric acid and canavanine had the strongest inhibitory effects on the species studied. It seems possible that these amino acids are antimetabolites of common amino acids.
It is obvious that non-protein amino acids can form effective hybridization barriers, although the conditions in nature are more complex than in vitro. The ability to synthesize a new amino acid may therefore be of evolutionary significance in the isolation of new species and genera.