Reproduction at Low and Pollen Water Potentials in Maize

Losses in grain yield are particularly severe when low water potentials (low ψW) occur at anthesis in maize (Zea mays L.). The losses can result from delays in floral development and from a failure in grain development when flowering appears normal. The causes of the latter problem are unknown, but could involve either the male or the female flower. Therefore, we made reciprocal crosses at various stigma (silk) and pollen ψW to determine which flower part failed at low ψW. The plants grew in soil in a controlled environment where silk ψW was between −0.3 and −0.5 MPa and pollen ψW between −1.5 and −12.5 MPa during the day. Water was withheld for a few days at anthesis after which water was resupplied and grain production was evaluated. At high pollen and silk ψW plants produced about 550 grains ear⁻¹. At low pollen ψW (to −12.5 MPa), plants also produced grain at a similar high rate. However, at low silk ψW (−1.2 MPa), grain did not develop. The failure of the grain to grow could not be attributed to insufficient water on the silk surfaces because pollen germinated and the pollen tube grew within the silks. The egg sac invariably was fertilized but the embryo, endosperm, and seedcoat did not develop beyond 2 or 3 days. Therefore, the failure to produce grain at low ψW was attributed to factors in the female flower that allowed fertilization to occur but prevented embryo development. Because both embryonic and maternal tissues were involved, the effect suggests a general starvation for substrate, which could have been caused either by a lack of photosynthate or by a blockage of translocation. Earlier work from our laboratory showed that photosynthesis was inhibited at these ψW, and that carbohydrate reserves were low at this time. However, reserves accumulated at later stages of grain fill and could support grain growth when low ψW occurred. Therefore, a lack of photosynthate rather than a blockage of translocation may have caused the failure in grain development when low ψW were present at anthesis.