Cleistogamy and seed heteromorphism in Triplasis purpurea (Poaceae)

Two populations of the annual grass Triplasis purpurea on Staten Island, New York, were examined to determine the importance of cleistogamy (CL) to caryopsis (seed) production, and whether CL was associated with a position-dependent seed heteromorphism, as noted in other CL grasses. The percentage of tillers with terminal panicles bearing chasmogamous spikelets was low in both populations (less than or equal to 35%), and at least 72% of the seeds produced by tillers with terminal panicles were matured on axillary panicles in CL spikelets enclosed by leaf sheaths. A marked seed hrteromorphism exists in T. purpurea: from the lower to the upper nodes along a tiller, there was an increase in seed number with a concurrent decrease in mean seed mass. This heteromorphism was further investigated in a greenhouse experiment where plants were subjected to low, medium, and high levels of soil nutrients by periodic addition of N-P-K fertilizer. Again, there was a progressive increase in seed number with a concordant decrease in mean seed mass from the lower to the upper tiller nodes. Differences in seed number among nutrient treatment groups were greatest in the upper nodes, but differed very little in the lower nodes. This is the first report of seed heteromorphism in this species. It is suggested that the position-dependent variation in seed mass reflects differences in the timing and length of the maturation period. From an evolutionary perspective, this pattern of seed heteromorphism may represent an adaptation to an unpredictable, highly-disturbed environment.