Silk and Pollen Water Potentials in Maize¹

Sexual reproduction depends on the transport of the male gamete to the egg along an aqueous path. In plants, this process causes reproduction to be particularly vulnerable to low water potentials (low ψ W ). Crops experience large yield losses when low ψ W occur during reproduction, but the ψ W are unknown for most floral parts in relation to the vegetative plant. Therefore, in maize ( Zea mays L.) we measured the ψ W of the pollen and stigmas (silks), two structures important to the fertilization process, and compared the ψ W to those in the leaves. Plants were grown in the field and in a controlled environment in soil. When water was supplied, leaf ψ W varied diurnally between −0.15 and −0.7 MPa. However, silk ψ W showed little diurnal variation and ranged between −0.3 and −0.6 MPa. Freshly released pollen began the day at much lower ψ W (−1.2 MPa) and decreased to very low ψ W (−12.5 MPa) as the day progressed. Similar ψ W were observed in the controlled environment and in the field. When water was withheld, leaf ψ W decreased to about −1.7 MPa, silk ψ W decreased to about −1.2 MPa, but pollen ψ W was unaffected. Silk ψ W followed changes in leaf ψ W (r = 0.77) but pollen ψ W did not (r = 0.28). These observations indicate that stigmatic tissues (silks) are in moderate hydraulic contact with the vegetative plant but the mature male gametophytes (pollen) are not. As a consequence, depletion of soil water affects the silks more than the pollen. This may explain earlier reports that low leaf ψ W . had no effect on in vitro pollen germination. Also, the data show that pollen ψ W is always substantially lower then silk ψ W , assuring that water moves from the silks to the pollen. Because pollen having ψ W as low as −12.5 MPa has been reported to be viable, pollen desiccation should not be a factor limiting grain production in maize in the field.