Varietal traits limiting the grain yield of tropical maize

Tropical maize varieties were grown at a tropical lowland location in various -seasons and at a highland location in Mexico to study the general growth pattern of tropical maize. The following results were obtained.

1. The tropical maize variety grown at the lowland location in summer was characterized by (a) a high growth rate and a short growth duration, (b) a small leaf area duration during maturing, (c) a sizable loss of dry weight at late maturity, (d) a low harvest index, and (e) a small number of kernels formed per unit field area. Although these characters may be interrelated, the predominant cause of the former two appear to be environmental and that of the latter three, genetic.

2. The grain yield of tropical maize grown at the lowland location in summer was low mostly because of a small number of kernels and maybe also because of short leaf area duration. The senescence of leaves after silking was rapid and the growth duration was short under high temperatures.

3. Although the number of kernels was small, grain yield was higher at the highland than at the lowland location, and also was higher in the winter than in the summer plantings at the lowland location. At the highland location or in winter, the longer growth duration and the longer leaf area duration compensated for the smaller crop growth rate and resulted in a larger dry matter production after silking and a higher grain yield. Larger kernel size also contributed to the higher yield of highland maize.     

Varietal traits limiting the grain yield of tropical maize IV. Plant traits and productivity of tropical varieties

At Tlaltizapan, Mexico, 26 maize varieties with a wide range of various contrasting traits were grown at three plant densities (25, 000, 50, 000, and 100, 000 plants per ha) to clarify the factors currently limiting grain yield. The following results were obtained.

1. Long-duration varieties compared to short-duration varletles tended to be taller and to produce a larger number of leaves, larger LAl, larger TDW, and higher grain yield. No evidence was obtained to indicate that plant type was important for high-yielding varieties.

2. High grain yield was attributable mainly to the larger number of kernels per unit field area. The sink size is possibly the dominant factor controlllng the grain yield, indicating the importance of studies on the yield components.

For obtaining high-yielding varieties, it would be important to improve a strain so that a single ear became larger and more tolerant to barrenness at a high plant density.     

Varietal traits limiting the grain yield of tropical maize II. The growth and yield of tall and short varieties

Four isogenic varieties with different heights, derived from the Tuxpeño population, were grown in the tropics to evaluate their potential productivity. Short varieties were developed by two means: incorporation of the brachytic-2 gene and current selections of shorter plants.

The tallest, original Tuxpeño produced the highest yield, next was the short-plant selection, while the brachytic varieties yielded least. Yield differences between the original and the short-plant selections were derived from the kernel size and the differences between those and the brachytic varieties, from the number of kernels.

Incorporation of the brachytic gene is unlikely to improve the productivity of varieties because it results in low efficiency of leaves in dry matter production during ripening, caused by the limited sink size and the compact distribution of wide leaves.     

The competitive vs. complementary bacteria‐fungi interactions promote microbial release of Fe(III)‐fixed phosphorus: the roles of exogenous C application

The phosphorus deficiency is very common in Fe(III)‐rich soil, and one of the key point is to clarify the condition in release or desorption of phosphorus from the Fe(III)‐rich minerals. The present study was to explore the effect of labile carbon on microbial reduction of Fe(III) and release of phosphorus in root‐free sub‐tropical soil. A two‐compartment microcosm was collected, in which the roots of Medicago sativa L. cultivar ‘Aohan' were confined within one compartment by a barrier of 30‐μm nylon mesh, while mycorrhizal hyphae could penetrate to the second compartment. Arbuscular mycorrhizal fungi (Funelliformis mosseae) were added to the root compartment and iron‐reducing bacteria (Klebsiella pneumoniae) were added to the hyphal compartment. Hyphal compartments were provided with two levels of additional carbon (0 and 23 mg C kg^?1 soil as sodium acetate) and eight levels of inorganic phosphorus (0 to 35 mg P kg^?1 soil as KH₂PO₄). At low phosphorus levels (< 5 mg P kg^?1 soil), shoot biomass, and total biomass phosphorus were substantially less with added carbon in the presence of iron‐reducing bacteria. Carbon had little effect without iron‐reducing bacteria. At higher phosphorus levels (> 15 mg P kg^?1 soil), the effect of added carbon was reversed; that is shoot biomass and total biomass phosphorus were greater with added carbon. Available soil phosphorus showed a similar response to added carbon—less at low levels of phosphorus and greater at higher levels of phosphorus. Microbial phosphorus in the presence of iron‐reducing bacteria was always higher with added carbon at all corresponding levels of soil phosphorus. Taken together, these results show that some phosphorus mobilized by iron‐reducing bacteria was converted into microbiological phosphorus, but there was an obligatory requirement for labile carbon for this to happen—reducing the amount of phosphorus that was absorbed by the mycorrhizal hyphae. Iron‐reducing bacteria and mycorrhizae showed a competitive interaction at lower levels of available soil phosphorus, and a complementary, or possibly a carbon‐dependent synergistic function at higher levels of available phosphorus. These results demonstrate that phosphorus released from ferralsols by iron‐reducing bacteria is positively mediated by both phosphorus and labile carbon and, hence, that phosphorus release and mobilization by iron‐reducing bacteria is likely to be enhanced by additions of exogenous carbon.