Slow Vegetative Growth Research Articles

Genetic studies of antifreeze proteins and their correlation with winter survival in wheat

Antifreeze proteins (AFPs) accumulate in the leaves of winter cereals during cold acclimation, where they may inhibit ice recrystallization during freezing and thawing cycles and provide nonspecific disease resistance. In this study, 21 wheat chromosome substitution lines and the parental lines Chinese Spring and Cheyenne wheat were used to determine the heritability of AFPs and the relationship between the accumulation of AFPs and winter survival. In cold-acclimated lines, antifreeze activity in leaf apoplastic extracts ranged from 1 (low) to 5 (high) with an average value of 3.2, and the accumulation of apoplastic proteins ranged from 30 μg (g FW)-1 to 115 μg (g FW)-1 with a mean value of 70 μ (g FW)-1. Examination of the individual lines revealed that Cheyenne chromosomes 5B and 5D carry major regulatory genes that increase both antifreeze activity and the accumulation of antifreeze proteins in plants grown at low temperature. Substitution lines carrying Cheyenne chromosomes 2A, 3A, 6B, and 7A exhibited lower freezing tolerance and also showed a marked decrease in the accumulation of specific AFPs during cold acclimation. Antifreeze activity and apoplastic protein content were not correlated with freezing tolerance (defined as % survival at -11 °C), but they were both significantly and positively correlated with winter field survival rates. Antifreeze activity (positively correlated) and total leaf fresh weight (negatively correlated) together accounted for about 55% of the variation in winter survival, indicating that high antifreeze activity and slow vegetative growth at low temperature are both important quantitative traits for winter survival.

Soil management for irrigated vegetable production. II. Possible causes for slow vegetative growth of lettuce associated with zero tillage

This work explores the consequences of using zero-tillage for irrigated vegetable production. The research is based on the hypothesis that, under the well-watered and adequately fertilized conditions prevailing in commercial vegetable growing, the reduced root density expected under zero-tillage should not limit vegetative growth. To test this hypothesis, irrigated lettuce crops were grown under a range of soil physical conditions on a hardsetting sandy loam soil. Crops were grown during the spring, autumn and summer, in order to compare zero-tillage and cultivation treatments under conditions of low, medium and high evaporative demand. Lettuce yields under zero-tillage were reduced relative to those under cultivation for each of these crops. However, calculations based on a model which describes the movement of water to roots show that the root length measured in the zero-tillage treatment greatly exceeded that required to meet the potential evaporative demand in the field. A possible explanation for the reduced growth is given, based on an inhibitory root signal which has been shown under controlled conditions to reduce shoot growth in hard soil even when the leaves are well supplied with water and nutrients. When lettuce was grown under zero-tillage, but following a crop of subterranean clover (Trifolium subterraneum L.) which senesced in summer to form an in-situ mulch, yield was as high as in cultivated soil. The soil near the surface in this treatment was not only moister and physically weaker than that from the conventional zero-tillage treatment, but also likely to contain biopores which would provide zones of low root impedance in an otherwise hard soil.

STABLE DIPLOIDS FROM INTRAGROUP ZYGOSPORES OF CLOSTERIUM EHRENBERGII MENEGH. (CONJUGATOPHYCEAE)1

ABSTRACTTwo pairs of stable diploid clones were obtained as aberrant forms among F1 progeny of an intragroup (intraspecific) cross between R‐11‐4 (mating type +) and M‐16‐4b (mating type ‐) of Group A of Closterium ehrenbergii Menegh. Each pair was derived from the two germination products of a single zygospore, and both clones were mating type minus. The cell size range of these four diploid minus clones was considerably above that of normal (haploid) Group A clones. Chromosome counts at the second meiotic metaphase indicated that these clones were diploid with approximately 200 chromosomes, which was double the number for normal Group A clones. Diploid minus clones conjugated normally with any haploid Group A plus clones, and yielded many triploid zygospores. Triploid zygospores germinated normally as did intragroup diploid zygospores. In metaphase I preparations, only bivalents were observed except on a few occasions where some uni‐ and multivalents were also detected. Viability of F1 progeny from triploid zygospores (55–74%) was somewhat lower than from diploid zygospores of Japanese Group A populations (65–90%), but higher than intergroup (interspecific) hybrid zygospores from Groups A, B and H (0–12%). In addition to lower viability, some F1 progeny from triploid zygospores exhibited slow vegetative growth. Almost all pairs of F1 clones from single triploid zygospores were of opposite mating type, similar to normal diploid zygospores of the intragroup cross. Morphological variability of F1 progeny of triploid zygospores was great. The apparently normal meiosis of triploid zygospores and the high viability of F1 progeny suggested that the genome of Group A contains several sets of chromosome complements with mechanisms by which bivalents are regularly formed in the first meiotic division.

Survival of Winter Annuals in the Northern Mojave Desert.

Following early autumn germination in Mojave Desert winter annual populations (53 taxa) sampled on 13 plots (total sample size, 16.4 m2 ) in three drainage basins in southern Nevada, 1963-64, there was 38% survival to maturity (plot range 10-63%). Death occurred in early spring, at the time of shift from the slow vegetative growth of winter to the beginning of stem elongation. Despite no marked precipitation deficiencies during the 7- to 8-month growing season, mortality apparently resulted from inadequate soil moisture to meet the demands of seedlings at the point in the life cycle of a manyfold increase in plant volume. Mean percentage survival to maturity of seedlings (58 taxa), sampled on 62 plots (total sample size, 62 m2 ) in sevean drainage basins, following spring germination after rains of 3-5.5 inches in March-April 1965, was 60% (range by basin, 44-83%). Mortality in these populations, whose life cycles were completed in 6-10 weeks, could not be attributed to inadequate moisture levels. In most seasons, regardless of precipitation regimes, the majority of seedlings of desert annuals probably do not survive to maturity.