No abstract

Ethephon [(2‐chloroethyl) phosphonic acid] has recently been introduced in North America as a regulator to control lodging in cereals. A 3‐yr study (1983‐1985) was conducted to determine how widely grown spring wheat (Triticum aestivum L.) and spring barley (Hordeum vulgare L.) cultivars in the North Central United States responded to ethephon. Nine randomized complete‐block, split‐plot experiments with wheat, and seven with barley, were conducted at the Crookston (soil classification, Aeric Calciaquoll), Morris (Aeric Calciaquoll), St. Paul (Typic Hapludoll), and Waseca (Aquic Hapludoll) Experiment Stations in Minnesota. Ethephon was applied at a rate of 0.42 kg a.i. ha−1 in all years as well as 0.28 kg a.i. ha−1 in 1985. When lodging occurred, ethephon treatment at either rate lessened its severity. Ethephon shortened crop height, more so when applied at the higher rate. Effects of ethephon on grain yields varied from significant reductions (average 13% for wheat, 9% for barley) to significant increases (average 12% for wheat, 13% for barley). Increases were most common when control plots lodged, although higher yields in response to reduced lodging were not obligatory. When lodging did not occur, ethephon treatment tended to result in reduced yields. Among barley cultivars, ‘Robust’ was most likely to exhibit reduced yields. Genotypic variability for ethephon sensitivity among wheat cultivars was less evident. In most experiments, ethephon treatment lowered kernel numbers per spike or mass per kernel. We conclude that ethephon use is most reasonable when the production practices followed, or environmental conditions, assure the likelihood of significant lodging. Further research investigating cereal responses to lower ethephon rates, as well as interactions between ethephon and plant stress, is needed.

Grain yield in maize (Zea mays L.) is a function of the relationship between assimilate supply to the grain and inherent potential of the grain to accommodate this assimilate. In this study we sought to ascertain the effects of altering assimilate supply per kernel during early and late kernel development on kernel growth rate and final weight. We also sought to elucidate the effect of such alterations on the plant's compensatory responses, specifically kernel abortion and carbohydrate/ N redistribution. The single‐cross hybrid M14 ✕ W64A, selected because of intermediate weight per kernel was grown in field plots in 1979, 1980, and 1981. Assimilate enhancement per kernel was achieved by ear tip removal, which resulted in a 20 to 40% reduction in kernels per ear. Total defoliation was the assimilate reduction treatment. Both treatments were applied 12 days (early) and 24 days (late) after mid‐silking. Early defoliation greatly reduced both kernel growth rate and filling period duration, resulting in lower weight per kernel. These defoliation effects occurred despite large compensatory reductions in number of kernels per ear on treated plants. Soluble carbohydrate content of the internode above the ear of defoliated plants declined following defoliation, probably in conjunction with remobilization of stem reserves. This decline was more rapid for the later defoliated plants. In contrast, N concentration of this stem internode of defoliated plants was higher than in control plants and did not decline after treatment. Soluble carbohydrate and N concentration in the internode above the ear of plants in which ear size was reduced was higher than in control plants. Nitrogen concentration of the grain was also increased. However kernel growth rate, final kernel weight, and soluble carbohydrate concentration at maturity were not affected by this treatment. These results show that the remobilization patterns for stems in response to changes in assimilate supply, differs for N as compared to soluble carbohydrates. In addition, the data suggest that kernel growth rates and final seed size may be already approaching the upper limits of their potential in this hybrid. Undetermined factors other than assimilate supply appear to be restraining higher kernel growth rates and final weights.

SUMMARYCommunities of vesicular-arbuscular (VA) mycorrhizal fungi were studied in a long-term crop rotation experiment at two locations (Waseca and Lamberton, Minnesota, USA). Spores of mycorrhizal fungi were counted and identified in experimental plots with a cropping history of either corn (Zea mays L.) or soybean \Glycine max (L.) Merrill], Mycorrhizal fungal communities were affected by both location and cropping history. At Waseca, Glomus aggregatum Schenck & Smith, G. leptotichum Schenck & Smith and G. occulttmi Walker spores were more abundant in soil with a corn history than a soybean history, while spores of G. microcarpum Tul. & Tul. exhibited the reciprocal pattern. Approximately 90 % of the spores recovered at Lamberton were G. aggregatum and did not vary with crop history. However, the spores of three other species: G. albidum Walker & Rhodes, G. mosseae Gerdemann & Trappe, and G. occultum, were more abundant in plots with a corn history than a soybean history. Densities of G, aggretatiim spores were negatively correlated with soil pH at Waseca, but were unrelated to pH at Lamberton were the mean soil pH was lower. Our results indicate that mycorrhizal fungal species are individualistic in their responses to cropping history and edaphic factors.

Supplemental far-red (FR) illumination of light-grown grass seedlings inhibits tiller production while enhancing leaf elongation. Although much is known about FR enhancement of internode elongation in dicots, relatively little research has been conducted to determine the effects of FR on monocot development. In growth chamber experiments, fibre optics were used to direct supplemental FR to elongating leaf blades, main stem bases and mature leaf blades of light-grown barley (Hordeum vulgare L.) seedlings. Our objective was to identify specific sites of perception for FR enhancement of leaf elongation and inhibition of tiller production, and to assess potential FR effects on tiller senescence. Far-red illumination of elongating leaves or of the main stem base reduced the total number of tillers per plant, primarily by reducing secondary and tertiary tiller production, and enhanced leaf elongation. However, leaf elongation was less sensitive to stem base treatments than to illumination of the elongating blade. Increased leaf length resulted from increased leaf elongation rate, while the duration of leaf elongation was unaffected. Exposure of mature leaf blades to FR had no effect on tillering or leaf elongation. None of the FR treatments led to tiller senescence. Localization of FR perception in vertically oriented tissues such as elongating blades and stem bases permits early detection of reflected light from neighbouring plants, allowing rapid response to impending competition.