Four maize (Zea mays L.) inbreds representing genetic differences in seedling cold tolerance were used to determine the effect of growth temperatures on dry weight accumulation and mitochondrial properties, especially the altemative oxidase capacity. Seedlings were grown in darkness at 30°C (constant), 140C (constant), and 150C for 16 hours and 80C for 8 hours. lnbreds B73 and B49 were characterized as cold tolerant while G50 and G84 were cold sensitive. Shoot growth rate of coldsensitive inbreds in the lower temperatures was slower relative to the tolerant inbreds. Mesocotyl tissue was particularly sensitive to low temperatures during growth after germination. There were no significant differences in relative rates of mitochondrial respiration in the cold-tolerant compared to cold-sensitive inbreds measured at 25°C. Mitochondria from all seedlings grown at all temperatures had the ability to phosphorylate as indicated by the observation of respiratory control. This result indicated that differences in low temperature growth were probably not related to mitochondrial function at low temperatures. Altemative oxidase capacity was higher in mitochondria from seedlings of all inbreds grown at 14°C compared to 300C. Capacities in seedlings of 14°C-grown B73 and G50 were higher than in B49 and G84. Capacities in seedlings grown for 16 hours at 150C and 8 hours at 80C were similar to those from 14°C-grown except in G50 which was lower and similar to those grown at 30°C. Mesocotyl tissue was the most responsive tissue to low growth temperature. Coleoptile plus leaf tissue responded similarly but contained lower capacities. Antibody probing of westem blots of mitochondrial proteins confirmed that differences in altemative oxidase capacities were due to differences in levels of the altemative oxidase protein. Male sterile lines of B73 were also grown under the three different temperature regimes. These lines grew equally as well as the normal B73 at all temperatures and the response of alternative oxidase capacity and protein to low growth temperature was similar to normal B73.Germination and early seedling growth are major determinants of stand establishment. Thus, these developmental events are particularly important for commercial production of maize (Zea mays L.). Recent trends in agronomic practices have been to plant maize earlier in the spring to take advantage of more optimal summer rainfall and temperatures and to avoid hot, dry periods during pollination and fertilization. A second recent trend in agronomic practice has been toward various forms of conservation tillage. These practices result in slower warming of soils in the early spring compared to more extensive tillage, particularly that done in the previous fall. Thus, physiological characteristics that improve germination and early seedling growth at low temperatures are increasingly of interest to maize producers (10).The capacity and activity of the mitochondrial alternative oxidase in plants has been associated with growth at low seedling becaus...

Respiration rates of Zea mays L. seedling tissues grown at 30 and 140C were measured at 250C at different stages of seedling growth. Accumulation of heat units was used to define the developmental stages to compare respiration between the two temperatures. At both temperatures, respiration rates of most tissues were highest at the youngest stages, then declined with age. Respiration rates of mesocotyl tissue were the most responsive to temperature, being nearly twofold higher when grown at 14 compared to 300C. Alternative pathway respiration increased concomitantly with respiration and was higher in mesocotyls grown in the cold. When seedlings were started at 30 then transferred to 140C, the increase in altemative pathway respiration due to cold was not observed unless the seedlings were transferred before 2 days of growth. Seedlings transferred to 140C after growth at 300C for 2 days had the same altemative oxidase capacity as seedlings grown at 300C. Seedlings grown at 140C for 10 to 12 days, then transferred to 300C, lost alternative pathway respiratory capacity over a period of 2 to 3 days. Western blots of mitochondrial proteins indicated that this loss of capacity was due to a loss of the alternative oxidase protein. Some in vitro characteristics of mitochondna were determined. The temperature optimum for measurement of alternative oxidase capacity was 15 to 200C. At 41°C, very little altemative oxidase was measured, i.e., the mitochondrial oxygen uptake was almost completely sensitive to cyanide. This inactivation at 410C was reversible. After incubation at 410C, the alternative oxidase capacity measured at 250C was the similar to when it was measured at that temperature directly. Isolated mitochondria lost alternative oxidase capacity at the same rate when incubated at 410C as they did when incubated at 250C. Increasing the supply of electrons to isolated mitochondria increased the degree of engagement of the alternative pathway, whereas lower temperature decreased the degree of engagement. Lower temperatures did not increase the degree of engagement of the pathway in intact tissues. We interpret these observations to indicate that the greater capacity of alternative oxidase in cold-grown seedlings is a consequence of development at these low temperatures which results in elevated respiration rates. Low temperature itself does not cause greater capacity or engagement of the alternative oxidase in mitochondria that have developed under warm temperatures. Our hypothesis would be that the low growth temperatures require the seedlings to have a higher respiration rate for some reason, e.g., to prevent the accumulation of a toxic metabolite, and that the alternative pathway functions in that respiration.

Field emergence of maize (Zea mays L.) can be severely reduced by flooding shortly after planting. This study was conducted to characterize the response of several maize inbreds to soaking injury and to examine the importance of ethanol and acetaldehyde accomulation as determinants of tlooding tolerance. Seeds of four maize inbreds (Z38, B49, G49, and GS0) were soaked from 0 to 7 d 27 °C. Subsequent germination of Z38 and IM9 was relatively unaffected by this treatment, while germination of G49 and GS0 was reduced to <10%. Soak temperature affected germination, with minimal inhibition at 15 to 20 °C and maximal effects at 0.1 and 40 °C. All inbreds excreted ethanol and acetaldehyde during soaking, but amounts excreted were not significantly correlated with subsequent germination. Purging the soak water with air or N2 gas during soaking resulted in subsequent improvements in germination. Soak‐induced inhibition of germination was partially reversed by venting the seed after soaking but prior to planting. Venting was accomplished with flowing air, N2 gas, or a vacemn treatment. Treatment with ≈0.15 MPa (1.5 atm) of total pressure resulted in no improvement in germination after soaking. These data are consistent with the concept that soak‐induced inhibition of germination results from accumulation of volatile metabolite(s) other than ethanol or acetaldehyde.

Flooding of recently planted fields of maize (Zea mays L.) in the spring results in poor emergence of some germplasm. Previous studies have shown genetic variation for flooding tolerance of maize inbreds and hybrids. This study was conducted to identify a biochemical basis of reduced germination after flooding and determine differences between tolerant and sensitive inbreds. Seeds soaked in nonaerated water accumulated CO~. Reduction of germination after soaking was partially reversed by venting the seed with ambient air after soaking and prior to planting. Respiration was measured immediately after soaking and after venting. Respiration rates of flood tolerant inbreds increased faster and to a higher level after flooding and venting than those of sensitive inbreds. The apparent Michaelis constants for succinate oxidation by isolated intact mitochondria of flood sensitive inbreds were higher than those of tolerant inbreds. Succinate oxidation was competitively inhibited by bicarbonate. As the apparent quantitation of inhibition (Ki) bicarbonate increased, flooding sensitivity decreased. Genetic differences in flooding tolerance of maize inbred seed germination were at least in part influenced by sensitivity of respiration to COz.