Oecologia (Berlin) (1986) 70:568-572 Oecologia 9 Springer-Verlag 1986 Stigmatic inhibition of pollen grain germination - its implication for frequency distribution of seed number in pods of Leucaena Ieucocephala (Lam) de Wit K.N. Ganeshaiah, 1 R. Uma Shaanker 2, and G. Shivashankar 1 1 Department of Agricultural Botany, University of Agricultural Sciences, Hebbal, Bangalore 560 024, India 2 Department of Crop Physiology, University of Agricultural Sciences, GKVK Campus, Bangalore 560 065, India Summary. Seed number per pod is negatively skewed in 12 Leucaena leucocephala. Post-fertilisation abortion of pods 11 does not completely explain the skewed distribution. It is shown that a pre-fertilisation regulation effectively restricts 10 the formation of pods with less than a certain number of 9 seeds. The regulation is rendered through the inhibition a of pollen grain germination by the stigmatic fluid when the pollen grain is less than a certain critical load in the ~ 7 stigma. Such a control is shown to have implications in ~ 6 the evolution of pollen package units, generation of male x 5 gametophytic competition and the economy of packing seeds in pods. 3 Key words: Seeds per pod - Stigmatic inhibition - Pollen grain germination - Seed packing - Leucaena 2 1 a 0 The preferential maturation of many-seeded as compared to few-seeded fruits has been reported in a number of mul- tiovulated species (Janzen 1982; Lee 1984; Bawa and Webb 1984). The selective elimination of fruits with relatively few seeds has been attributed to post-fertilisation abortion of zygotes or embryos (Stephenson 1981; Lee and Bazzaz 1982a, b; Bawa and Webb 1984). Such abortions necessari- ly involve a certain cost to the female parent in terms of the resource invested in aborted seeds or fruits. The cost could however be reduced by preventing the formation of fruits with fewer seeds through a pre-fertilisation control. Here we show that the negatively skewed distribution of seeds per fruit in Leucaena leueoeephala results from a pre- fertilisation control. We also show that this control operates through the stigmatic inhibition of pollen grain germination when the number of pollen grains on the stigma is below a certain threshold. System L. leucoeephala, a legume, produces approximately 200-250 flowers in globose heads, of which only 1%-2%, on aver- age, develop into mature pods. These pods show a negative- ly skewed distribution of seeds per pod. For instance, the seed number in a clone, KB-8, ranged from 8 to 27 with the mode at 20 (Fig. 1 a), while in another clone, K-100, the seed number ranged from 11 to 29 with the mode at 28 (Fig. 1 b). Thus, there was a total absence of pods with Offprint requests to: K.N. Ganeshaiah 1 2 3 ~,5 6 7 8 9 101112131/,1516171819202122232/,252627 Seed number per pod 22 20= I 18' 16 1l, / ,,'- 10! 8 2 f - ' ] | ~ b 0 .......... ~ , ["~, [ r~, . . . . . . . . . . . 1 2 3 4 5 6 7 8 9 1011 12131t,15161718 1920212227,2t,.2526272829 Seed number per pod Fig. l. a Frequency distribution of seed number per pod in KB-8 clone, b Frequency distribution of seed number per pod in K-100 clone less than 8 seeds in KB-8 and 11 in K-100. Our study per- tains to clone KB-8 only. The study was conducted at the "Leucaena farm" at the University of Agricultural Sciences, Hebbal, Bangalore (12~ 77~ India. Data were collected on ten ran-