The pollen-stigma interaction in the grasses. 6. The stigma (‘silk’) of Zea mays L. as host to the pollens of Sorghum bicolor (L.) Moench and Pennisetum americanum (L.) Leeke

Comparison of the behaviour of Zea. Sorghum and Pennisetum pollens on the stigma of Zea shows that at normal growing temperatures atmospheric humidity is the principal factor controlling germination and pollen-tube penetration; greatest success for all three species is achieved with RH exceeding 70%. While at high humidity levels germination can occur on the stigma axis, it is the stigma trichomes, which have a thin, discontinuous cuticle and bear a surface secretion with protein and polysaccharide components, that provide the most favourable sites for pollen attachment, hydration, germination and penetration. In controlled, single-grain pollinations, the shortest period observed between attachment to a stigma trichome of Zea and germination was 9 min for the pollen of Zea itself, I min 25 sec for that of Sorghum and 1 min 42 sec for that of Pennisetum in RH 70%. The conformation of the basal cell complex of the trichome determines the direction in which the tubes grow into the stigma axis; it is highly effective with Zea tubes, less so with those of Sorghum and Pennisetum. In the stigma axis, Zea pollen tubes reach the transmitting tract by stepwise progression through the intercellular spaces of the cortex. Sorghum tubes are frequently disoriented in the stigma axis, failing to locate the transmitting tract; yet they are capable of high growth rates, and can achieve greater lengths in the Zea stigma than they do in the stigma of Sorghum itself. Pennisetum tubes enter the Zea stigma with greater difficulty, and in the axis tend to grow slowly through the cortical tissues without seeking the transmitting tract. With all three species the entry of the tubes promotes secretion of pectic polysaccharide and protein by the cells of the stigma axis. The paper includes a brief discussion of the significance of the experimental results in relation to the growth physiology of the pollen tubes.