Effects of Moisture Deficits on 14 C Translocation in Corn ( Zea mays L.)

Abstract

Corn plants (Zea mays L.) were grown in the field on two soils. On a droughty soil, water was withheld from some plants during the grain-filling period while other plants were irrigated. Carbon-14 was fed to the leaves, and translocation to different plant parts was determined. Translocation appeared to be more sensitive to moisture stress than was photosynthesis. More radioactive carbon was retained in both the fed portion and the nonfed portion of the leaf of stressed plants than in nonstressed plants. The stalk segment between the treated leaf and ear-node also contained less radioactivity in stressed plants than in nonstressed plants. On a soil with higher water-holding capacity, moisture stress was imposed on plants by root pruning. Plants under severe stress continued to translocate photo-synthetically assimilated 14C nearly as well as nonstressed plants for 90 minutes. Between 90 and 120 minutes after labeling, there was a major reduction in amount translocated in stressed plants compared to the nonstressed plants. At longer transloca-tion times the rates of translocation appeared again to be more nearly equal. Almost every plant process is affected directly or indirectly by water deficits. Some processes are quite sensitive to water stress, but others are relatively insensitive. When plants are subjected to water stress, there is a decrease in photosynthesis and cell enlargement (10). There is considerable retention of carbohydrates in photosynthetic tissues (8, 10). Although trans-location proceeds, its rate is reduced (3, 6, 10). Reduced trans-location is rarely mentioned as a factor in reduced plant growth under limited moisture. This is surprising, since the pressure flow hypothesis would require a pressure potential gradient between the leaves and the receiving organs before any trans-location of sugars could occur. Such a pressure potential gradient would be reduced as the internal moisture deficit increased. If this hypothesis is correct, translocation could be one of the chief physiological factors limiting growth under unfavorable moisture conditions. Quantitative measurements of water stress imposed on plants during a translocation study are rare. This study was conducted to determine the effects of plant water stress on the translocation and distribution of 14C photosynthate in corn plants during the grain-filling period. MATERIALS AND METHODS Experiment I. Corn seeds (Zea mays L., PAG SX-29 single cross) were planted by hand, two seeds per hill, on June 1, 1971. Row spacings of 75 cm, with 60 cm between hills within a row gave a population of 44,440 plants/ha. Each of four plots contained 19 rows with 26 plants/row. Two plots were irrigated and two were not irrigated ("dry treatment"). The soil was Warsaw sandy loam with a low water-holding capacity. Treatments were imposed after 54 days growth. For the dry treatment, the soil was completely covered with plastic sheets 90 cm wide which did not allow the penetration of rain. The plants in the irrigated treatment were watered regularly using perforated plastic hose lines between rows. Measurements of soil moisture content were made with a neutron probe (Troxler Model 104) at 30-cm increments of soil depth via an aluminum tube installed in the center of each plot to a depth of 120 cm. Observations were made at 7-day intervals. Leaf water potential values were determined by the pressure chamber technique developed by Scholander et al. (7). Paired plants in each hill allowed the determination of leaf water potential on one plant while the other was being fed 14C02. The second leaf above the ear was used for both leaf water potential measurements and feeding '4CO2-Pollination started on July 28, and silking dates were recorded for each plant. On August 19 and 22, 19 days after pollination, 40 plants with one ear were selected at random. A segment (10 X 7 cm) of the second leaf above the ear (approximately 60 cm from the leaf collar) was enclosed in a Plexiglas chamber (total internal volume approximately 190 cm') in which approximately 50 ,c of "4CO were generated. The leaf was exposed to "CO, for 5 min, after which the chamber was removed. All labeling was done between 10:30 AM and 2:30 PM. Translocation times, which did not include feeding time, were: 0, 60, 120, and 360 min, selected on the basis of Eastin's results (2). Six plants were used for each translocation time. The temperature during the experiment varied between 26 and 32 C. Solar radiation during the feeding period averaged 65.4 and 54.7 langleys/hr on August 19 and August 22, respectively. The plants were harvested and individually separated into