Curtis J. Nelson

Response of Fructan to Water Deficit in Growing Leaves of Tall Fescue

Changes in dry matter and water-soluble carbohydrate components, especially fructan, were examined in the basal 25 mm of expanding leaf blades of tall fescue (Festuca arundinacea Schreb.) to assess their roles in plant response to water deficit. Water was withheld from vegetative plants grown in soil in controlled-environment chambers. As stress progressed, leaf elongation rate decreased sooner in the light period than it did in the dark period. The decrease in growth rate in the dark period was associated with a decrease in local relative elongation rates and a shortening of the elongation zone from about 25 mm (control) to 15 mm. Dry matter content of the leaf base increased 23% during stress, due mainly to increased water-soluble carbohydrate near the ligule and to increased water-soluble, carbohydrate-free dry matter at distal positions. Sucrose content increased 258% in the leaf base, but especially (over 4-fold) within 10 mm of the ligule. Hexose content increased 187% in the leaf base. Content of total fructan decreased to 69% of control, mostly in regions farther from the ligule. Fructan hydrolysis could account for the hexose accumulated. Stress caused the osmotic potential to decrease throughout the leaf base, but more toward the ligule. With stress there was 70% less direct contribution of low-degree-of-polymerization fructan to osmotic potential in the leaf base, but that for sucrose and hexose increased 96 and 67%, respectively. Thus, fructan metabolism is involved but fructan itself contributes only indirectly to osmotic adjustment.

EFFECTS OF EXPERIMENTAL PROCEDURES AND CONDITIONS ON BIOASSAY SENSITIVITY OF ALFALFA AUTOTOXICITY

Sampling procedures of the donor material, conditions of the assay, and selection of test organism influence the sensitivity of a bioassay. Autotoxicity affects success of alfalfa (Medicago sative L.) reseeded after old alfalfa. Root length is a more sensitive parameter to autotoxin than germination or hypocotyl length. Our specific objective was to improve the sensitivity of an alfalfa seedling bioassay for a better understanding of the autotoxicity mechanism. Using a petri-dish assay with imbibed seed, we found extract of alfalfa leaves was more autotoxic than either that of stems or seed. Extracts from oven-dried samples were stronger than those from freeze-dried samples. Sterilization through membrane filtering or autoclaving did not alter autotoxicity bioassay. Root length at 25 or 50 seed/plate was more sensitive to the extracts than that at 100 or 200 seed/plate. Roots from older seedlings were more tolerant to the extracts than did those from seed or younger seedling and had more branch roots to escape the autotoxin(s). These results suggest that extracts from leaf samples that oven-dried and sterilized by filtering or autoclaving can improve the consistency and precision of bioassay. The bioassay was suitable with 25 to 50 seed per petri dish to minimize interaction on interference (e.g., competition and autotoxicity from testing seed itself). The bioassay can be improved by using very young roots from imbibed seed which are more sensitive to the autotoxin.

Growth of tall fescue leaf blades in various irradiances

Abstract Information on the effect of irradiance intensity on grass leaf development and spatial distribution of growth is essential to understand the ecology of grazed grasslands. Our objective was to determine how decreasing or increasing irradiance altered elongation and spatial distribution of growth in leaf blades of tall fescue (Festuca arundinacea Schreb.). Three experiments were conducted in growth chambers. In Experiment 1, plants of a low yield per tiller genotype were grown sequentially under decreasing photosynthetic photon flux densities (PPFD ; 400–700 nm) of 550, 190, 50, or 0 μmol m−2 s−1. In Experiment 2, the same plants were grown under increasing PPFD of 50, 130, and 400 μmol m−2 s−1. In Experiment 3, the low yield per tiller genotype and a high yield per tiller genotype were grown under decreasing PPFD of 600, 300, 50, and 0 μmol m−2 s−1. The elongating leaf was measured daily from its tip to the collar of the most recently expanded leaf. Leaf elongation rate was calculated as the linear regression coefficient of leaf length versus time. The spatial distribution of growth in the elongating leaf was determined by measuring short-term displacement of holes pierced through the basal 30 mm. Leaves of the high yield per tiller genotype elongated 40 per cent faster than leaves of the low yield per tiller genotype. The growth zone of elongating leaves was longer at low irradiance, 24 mm versus 18 to 21 mm at high irradiance. The spatial distribution of growth was displaced along a greater length of the elongating zone at low irradiance, whereas at high irradiance elongation was concentrated nearer the ligule. Both genotypes responded in a similar pattern to altered radiation density although there were differences in level of response.