Corn Hybrids Differ in Their Diurnal Pattern of Phytosiderophore Release During Fe-Deficiency Stress.

Iron‑efficient gramineous species respond to Fe‑deficiency stress by releasing phytosiderophores, thus, increasing Fe solubility and uptake of Fe³⁺-phytosiderophore. In small grains, phytosiderophore release peaks in a 4- to 6-h period beginning 2-h after the initiation of light, but in maize (Zea mays L.) phytosiderophore release may be continuous during both light and dark photoperiods. These reported patterns of release are based on one or two hybrids, but our prior work with 11 hybrids suggests possible differences in patterns among hybrids. Our objectives were to examine six hybrids varying in Fe efficiency for differences in 24-h phytosiderophore release patterns and to verify if these patterns differ when phytosiderophore is collected under light and dark photoperiods. Individual maize plants (Fe-deficient) were grown in a collection solution for six consecutive 4-h periods or in some experiments a different plant was grown in collection solution for each of six consecutive 4-h periods. This 24-h measurement cycle was repeated up to fourteen times. Root exudates were analyzed for phytosiderophore release using an indirect Fe-binding assay. Iron-efficient hybrids sustained moderate (P3279, DK655) to high (N7070) levels of phytosiderophore release for the first 12- to 16-h after phytosiderophore collection began, but release declined during the final 8- to 12-h of collection (usually in the dark period). Iron-inefficient hybrids released either high (DK566) or low (N4640Bt, P3489) levels of phytosiderophore during the first 4-h collection, and all three hybrids released low and declining amounts for the next 20-h. However, the consistent decline in later collection periods for all hybrids was observed with plants collected for 24-h whether collection was initiated just before darkness or just after lights came on, or whether micropur was used or was not used during collection. The only factor which eliminated the decline was measuring phytosiderophore on new plants every four hours. This consistent decline in phytosiderophore release is attributed to deactivation of the Fe-stress response mechanism from dissolution of apoplastic Fe during the 24-h collection period (40%  dissolved in 4 h and 58% in 24 h). Although the majority of data suggest that phytosiderophore is released by maize continuously in light as well as dark photoperiods, at least one hybrid produced significantly more phytosiderophore during the light than the dark photoperiod.