Field experiments were conducted in 1979 and 1980 to identify visual indicators that coincide with physiological maturity (PM) and 95% maximum kernel dry weight (95% MKW) in hard red spring wheat (Triticum aestivum L.). Dates of PM and 95% MKW were calculated from polynomial regression equations fitted to dry weight accumulation data from eight genotypes differing in several agronomic traits. In 1980, kernels were divided into three groups according to their position on the spike (bottom, middle, and top) to determine the order in which kernels achieve their maximum dry weight.The mean number of days from anthesis to 95% MKW and PM ranged from 22.3 to 33.7 and 27.9 to 40.1, respectively, depending upon planting date and genotype. Kernels matured, in order, from the top of the spike down. The time of 95% MKW preceded PM by about 7 days. Of the 13 visual characteristics observed during the grain‐filling period, first appearance of the pigment strand in the crease most closely coincided with the calculated date of PM. Complete loss of green color from the glumes also occurred close to PM. Complete loss of green color from the flag leaf most consistently coincided with the time of 95% MKW. An abrupt drop in kernel moisture content also occurred at 95% MKW. Complete loss of green color from the flag leaf could, therefore, be used as an indicator of the commencement of rapid kernel dry‐down and the final stage of grain filling which ends when the pigment strand is first visible and the glumes have lost all green color. Kernel moisture percentage at PM and 95% MKW was too variable to be considered a reliable indicator of either PM or 95% MKW.

Tip and middle position kernels from field‐grown ears and in vitro cultured maize (Zea mays L.) were compared for dry matter accumulation and pedicel and endosperm carbohydrate concentration patterns. The experiment was designed to determine whether the growth of tip kernels is limited by their inherent potential to accumulate dry matter or by the supply of assimilate entering the kernel. Tip kernels from field‐grown ears had the lowest linear dry matter accumulation rate, the shortest duration of kernel growth, and therefore, the lowest maximum mass. The mass of tip kernels cultured in vitro was three times greater than tip kernels from field‐grown ears, which indicated that tip kernels on field‐grown ears fail to attain maximum potential mass. The mass of tip kernels cultured in vitro was lower than the mass of middle kernels cultured in vitro or from field‐grown ears. Tip kernels that developed on field‐grown ears had a much lower pedicel fructose, glucose, and sucrose concentration throughout kernel growth than did tip kernels cultured in vitro and middle kernels of both field grown and in vitro culture. Tip kernels from the field also had a reduced maximum endosperm sucrose concentration as compared to tip and middle kernels cultured in vitro and middle kernels from field‐grown ears. The relative endosperm starch accumulation of field‐grown tip kernels was comparable to that of tip kernels cultured in vitro. Starch synthesis in the endosperm of tip kernels from the field was initiated 4 days later than in tip and middle kernels cultured in vitro and middle kernels from field‐grown ears. The low sugar concentrations in the pedicel and the decreased endosperm sucrose concentration in tip kernels from field‐grown ears indicate that an inadequate assimilate supply reaching these kernels may be limiting their growth more than their inherent potential to accumulate dry matter.

Kernels cultured in vitro were induced to abort by high temperature (35°C) and by culturing six kernels/cob piece. Aborting kernels failed to enter a linear phase of dry mass accumulation and had a final mass that was less than 6% of nonaborting field-grown kernels. Kernels induced to abort by high temperature failed to synthesize starch in the endosperm and had elevated sucrose concentrations and low fructose and glucose concentrations in the pedicel during early growth compared to nonaborting kernels. Kernels induced to abort by high temperature also had much lower pedicel soluble acid invertase activities than did nonaborting kernels. These results suggest that high temperature during the lag phase of kernel growth may impair the process of sucrose unloading in the pedicel by indirectly inhibiting soluble acid invertase activity and prevent starch synthesis in the endosperm. Kernels induced to abort by culturing six kernels/cob piece had reduced pedicel fructose, glucose, and sucrose concentrations compared to kernels from field-grown ears. These aborting kernels also had a lower pedicel soluble acid invertase activity compared to nonaborting kernels from the same cob piece and from field-grown ears. The low invertase activity in pedicel tissue of the aborting kernels was probably caused by a lack of substrate (sucrose) for the invertase to cleave due to the intense competition for available assimilates. In contrast to kernels cultured at 35°C, aborting kernels from cob pieces containing all six kernels accumulated starch in a linear fashion. These results indicate that kernels cultured six/cob piece abort because of an inadequate supply of sugar and are similar to apical kernels from field-grown ears that often abort prior to the onset of linear growth.

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This study was designed to compare the uptake and distribution of '4C among fructose, glucose, sucrose, and starch in the cob, pedicel, and endosperm tissues of maize (Zea mays L.) kernels induced to abort by high temperature with those that develop normally. Kernels cultured in vitro at 30 and 35°C were transferred to I'4Clsucrose media 10 days after pollination. Kernels cultured at 35°C aborted prior to the onset of linear dry matter accumulation. Significant uptake into the cob, pedicel, and endosperm of radioactivity associated with the soluble and starch fractions of the tissues was detected after 24 hours in culture on labeled media. After 8 days in culture on I14Clsucrose media, 48 and 40% of the radioactivity associated with the cob carbohydrates was found in the reducing sugars at 30 and 35°C, respectively. This indicates that some of the sucrose taken up by the cob tissue was cleaved to fructose and glucose in the cob. Of the total carbohydrates, a higher percentage of label was associated with sucrose and a lower percentage with fructose and glucose in pedicel tissue of kernels cultured at 35°C compared to kernels cultured at 30°C. These results indicate that sucrose was not cleaved to fructose and glucose as rapidly during the unloading process in the pedicel of kernels induced to abort by high temperature. Kernels cultured at 35°C had a much lower proportion of label associated with endosperm starch (29%) than did kernels cultured at 30°C (89%). Kernels cultured at 35°C had a correspondingly higher proportion of "4C in endosperm fructose, glucose, and sucrose. These results indicate that starch synthesis in the endosperm is strongly inhibited in kernels induced to abort by high temperature even though there is an adequate supply of sugar.Shimamoto and Nelson (13) showed that detectable amounts of radioactive compounds, including sucrose, were taken up into cob, embryo, and endosperm tissue of kernels and cultured in vitro on radioactive media for 7 d. The uptake and incorporation ofL-[35-S]methionine into endosperm protein fractions ofkernels cultured in vitro has also been studied (3). Shannon (11) demonstrated that ['4C]sucrose is cleaved to fructose and glucose by acid invertase in the maternal pedicel and placento-chalazal tissue of kernels before being absorbed by the basal endosperm transfer cells. The fructose and glucose is then used to form sucrose in the endosperm which in turn is used to snythesize starch in the amyloplasts (12).Our goal was to compare the uptake and distribution of 14C into the fructose, glucose, sucrose, and starch fractions in the cob, pedicel, and endosperm tissue of kernels cultured with ['4C]sucrose at 30 and 35°C. Previous research utilizing the in vitro maize kernel culture technique showed that the development of kernels cultured in vitro at temperatures between 25 and 33°C is very similar to kernels from field or greenhouse-grown ears (1-3, 9, 10). Kernels induced to abort by high temperature by culturing at 35°C cease dry matter accumulation during the ...