Water stress during silking or early kernel development decreases the number of kernels set by corn {Zea mays L.) plants. Previous work has suggested that lack of assimilate supply due to water stress at silking was a major factor in the resulting reproductive failure. A greenhouse experiment was conducted to test the hypothesis that sucrose supplementation of water stressed corn plants can prevent decreased kernel set. Sucrose was injected into corn stems at three concentrations [0 (distilled water), 150 and 300 g L"'] for 30 days starting at silking. Water availability was controlled by either maintaining a water table at 50 cm from the soil surface (well watered) or by withholding water starting one week before silking (water stress) until the fifth day after silking. The photosynthesis rate of water stressed plants was 25 % that of well-water plants on the first day of silking. On average, the daily injection rate for distilled water was 1 mL higher than that of the sucrose treatments over a 30 day injection period. No difference in daily uptake rate was observed between the 150 and 300 g sucrose L~' treatments. Over water availability treatments approximately 17 g sucrose were injected into com plants during the 30 day injection period. Corn plants receiving sufficient water supply produced bigger ears, with more seeds and greater 100-seed weight values, leading to higher total plant dry matter accumulation than water stressed plants. Injection of 300 g sucrose L"' increased the weight of the injected internodes by 28 %, compared with distilled water injection. The highest grain yield was for the plants injected with 150 g sucrose L"', but only under sufficient water supply. The plants injected with 300 g sucrose L"' produced the least grain regardless of moisture availability. Thus, the exogenous sucrose supplementation influenced kernel set only under conditions of sufficient soil water supply. These results indicate that plant reproductive development after silking was limited more by water availability than assimilate supply, suggesting that some overall plant response to water stress, perhaps mediated by hormonal signalling, was more important than carbohydrate supply. These results indicated that plant desiccation occurred during floral development or pollination; irreversible loss of florets on unsuccessful pollination could result, thus, grain yield would be limited more by sink size than by availability of photosynthate.

Nitrogen (N) assimilate supply to developing corn (Zea mays L.) ears plays a critical role in grain dry weight accumulation. The use of stem-perfused/ injected 15 N labeled compounds to determine the effects of an artificial N source on the subsequent distribution of injected N and grain weight of fieldgrown corn plants has not been reported previously. Our objective was to assess the distribution of N added via an artificial source. Three soil N fertilizer levels (0, 180, and 270 kg N ha -1 ) and three N solutions (distilled water control and 15 N enriched urea at 15 and 30 mM N) were arranged in a split-plot design. Three N concentrations were injected using a pressurized stem 1 64 ZHOU ET AL. injection technique. The injection started fifteen days after silking and continued until immediately prior to plant physiological maturity. The average uptake volume was 256 mL over the 30-day injection period. The N supplied via injection represented 1.5 to 3% of the total plant N. Neither soil applied N fertilizer nor injected N altered dry matter distribution among plant tissues. As the concentration of N in the injected solutions increased, N concentrations increased in the grain and upper stalks, and % 15 N atom excess in ear+1 leaves and leaves increased. The relative degree of 15 N enrichment for each of the tissues measured was injected internode>grain>upper stalks>leaves>lower stalks>cob>husk>ear+1 leaf>ear leaf. This study indicated that the exogenous N supplied via stem-injection, was incorporated into all the measured plant parts, although not uniformly. The distribution of the injected 15 N was affected both by the proximity of sinks to the point of injection and the strength of the various sinks. The lack of increase in grain N concentration as soil N fertility increased, combined with the increase in grain N concentration as the concentration of the injected N solutions increased demonstrated that the ability of the plant roots to take up from the soil was more limiting to grain N accumulation than was the ability of grain to take up N from the rest of plant.