Matthijs Tollenaar

Source : sink ratio and leaf senescence in maize: II. Nitrogen metabolism during grain filling

Leaf senescence in a recent maize (Zea mays L.) hybrid is delayed relative to that in an older maize hybrid and the trait is associated with an improvement of the ratio of assimilate supply (i.e., source) and demand (i.e., sink) during grain filling. This study examined whether effects of source : sink ratio of leaf longevity in an old and more recent hybrid are associated with changes in leaf nitrogen (N) concentration and N uptake during grain filling. A 3-year field study was conducted with maize hybrids Pride 5 (old) and Pioneer 3902 (recent) grown at two soil-N levels: 150 kg ˇ1 Nh a ˇ1 was broadcast in the high N treatment while none was added to the low N treatment. Four imposed source : sink treatments ranged from partial defoliation to no grain. Leaf N of the control treatments did not differ between the two hybrids, but the decline in leaf N from the control to the no-sink treatment was larger for Pioneer 3902 than for Pride 5. Total N uptake in above-ground portions was 10 and 18% greater in the new than in the old hybrid under low and high soil-N conditions, respectively. The difference in the total N uptake between the two hybrids could be attributed to post-silking N uptake. The proportion of N in the grain derived from post-silking N uptake was 60% for Pioneer 3902 and 40% for Pride 5 and this proportion was positively associated with the source : sink ratio. Higher rates of N uptake in Pioneer 3902 vs. Pride 5 appear to be, in part, the result of higher rates of dry matter accumulation of the newer hybrid during grain filling. # 1999 Elsevier Science B.V. All rights reserved.

Relationship between thylakoid electron transport and photosynthetic CO2 uptake in leaves of three maize (Zea mays L.) hybrids

The introduction of a more efficient means of measuring leaf photosynthetic rates under field conditions may help to clarify the relationship between single leaf photosynthesis and crop growth rates of commercial maize hybrids. A large body of evidence suggests that gross photosynthesis (AG) of maize leaves can be accurately estimated from measurements of thylakoid electron transport rates (ETR) using chlorophyll fluorescence techniques. However, before this technique can be adopted, it will first be necessary to determine how the relationship between chlorophyll fluorescence and CO2 assimilation is affected by the non-steady state PPFD conditions which predominate in the field. Also, it must be determined if the relationship is stable across different maize genotypes, and across phenological stages. In the present work, the relationship between ETR and AG was examined in leaves of three maize hybrids by making simultaneous measurements of leaf gas exchange and chlorophyll fluorescence, both under controlled environment conditions and in the field. Under steady-state conditions, a linear relationship between ETR and AG was observed, although a slight deviation from linearity was apparent at low AG. This deviation may arise from an error in the assumption that respiration in illuminated leaves is equivalent to respiration in darkened leaves. The relationship between chlorophyll fluorescence and photosynthetic CO2 assimilation was not stable during fluctuations in incident PPFD. Since even minor (e.g. 20%) fluctuations in incident PPFD can produce sustained ( > 20 s) departures from the mean relationship between ETR and AG, chlorophyll fluorometry can only provide an accurate estimate of actual CO2 assimilation rates under relatively stable PPFD conditions. In the field, the mean value of ETR / AG during the early part of the season (4.70 ± 0.07) was very similar to that observed in indoor-grown plants in the vegetative stage (4.60 ± 0.09); however, ETR / AG increased significantly over the growing season, reaching 5.00 ± 0.09 by the late grain-filling stage. Differences in ETR / AG among the three genotypes examined were small (less than 1% of the mean) and not statistically significant, suggesting that chlorophyll fluorometry can be used as the basis of a fair comparison of leaf photosynthetic rates among different maize cultivars.

Effects of photoperiod on the phenological development of redroot pigweed (Amaranthus retroflexus L.)

Mechanistic weed models focus on determining the outcome of weed and crop interference. An understanding of weed phenology is essential for simulation model development. Phenological development is a major factor determining the outcome of weed–crop competition. Growth cabinet studies were conducted to characterize the influence of photoperiod on the phenological development of redroot pigweed (Amaranthus retroflexus L.). Results indicated that redroot pigweed is a quantitative short-day species. Four development phases of redroot pigweed were described according to its response to photoperiod: (1) a juvenile phase of 1.1 d; (2) a photoperiod-sensitive inductive phase of 7.9 d; (3) a photoperiod-sensitive post-inductive phase of 39.9 d; and (4) a photoperiod-insensitive phase of 2.2 d. This information is useful for the development of mechanistic models and for comprehending the distribution and competitive ability of redroot pigweed with crops. The utilization of these results could help in predicting th...

Mechanisms of Yield Loss in Maize Caused by Weed Competition

Abstract The physiological process underlying grain yield (GY) loss in maize as a result of weed competition is not understood clearly. We designed an experiment to test the hypotheses that early season stress caused by the presence of neighboring weeds will increase plant-to-plant variability (PPV) of individual plant dry matter (PDM) within the population. This increase in PPV will reduce GY through a reduction in harvest index (HI). Field experiments were conducted in 2008, 2009, and 2010. A glyphosate-resistant maize hybrid was cropped at a density of 7 plants m−2. As a model weed, winter wheat was seeded at the same time as maize and controlled with glyphosate at the 3rd or 10th to 12th leaf-tip stage of maize. Weed competition early in the development of maize decreased PDM and GY. This reduction in PDM, which occurred early in the development of maize, was attributed initially to a delay in rate of leaf appearance. Reductions in PDM were accompanied by an increase in PPV of PDM. This increase in PPV, however, did not reduce HI and did not contribute to the GY reductions created by weed competition, as hypothesized. As weed control was delayed, a reduction in fraction of photosynthetically active radiation (fIPAR) accounted for a further reduction in PDM and notably, a reduction in DMA from 17th leaf-tip stage through to maturity. The rapid loss of PDM and the subsequent inability to accumulate dry matter during maturation accounted for a rapid decline in kernel number (KN) and kernel weight (KW). Nomenclature: Glyphosate; maize, Zea mays L. ZEAMX; winter wheat, Triticum aestivum L. TRZAW.

Effect of temperature and photoperiod on the phenological development of common lambsquarters

Abstract The goal of a mechanistic model is to determine the outcome of weed–crop interference. An understanding of weed phenology is essential for construction of such models because phenological development is a major factor determining the outcome of weed–crop competition. Growth cabinet studies were conducted to determine the influence of temperature and photoperiod on the phenological development of common lambsquarters. Common lambsquarters is a short-day species adapted to a temperature range of 6.5 to 44.5 C. Phenological development of common lambsquarters grown under a constant temperature of 20 C and an 8-h photoperiod was described in terms of biological days (Bd: chronological days at the optimum photoperiod and temperature). Three development phases of common lambsquarters were described as (1) a juvenile phase of 6.3 Bd, (2) a photoperiod-sensitive inductive phase of 8.2 Bd, and (3) a photoperiod-sensitive postinductive phase of 34.4 Bd. The photoperiod sensitivity of rate of development did not differ among phases of development across the life cycle. Interpretation of constant sensitivity to photoperiod will simplify simulation of weed phenology in mechanistic models. Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL.

Effect of temperature and photoperiod on the phenological development of wild mustard (Sinapis arvensis L.)

The development of mechanistic weed models focuses on determining the outcome of weed‐crop interference. Phenological development is a major factor determining the outcome of weed and crop competition. The influence of temperature and photoperiod on phenological development of wild mustard (Sinapis arvensis L.) was studied in growth cabinets. The life cycle of wild mustard was defined in terms of biological days (Bd: chronological days at the optimum photoperiod and temperature). Wild mustard was a long-day species adapted to a wide temperature range of 1.5‐488C. Four phases of development of wild mustard were described: (1) a juvenile phase of 12.7 Bd; (2) a photoperiod-sensitive inductive phase of 6.2 Bd; (3) a photoperiod-sensitive post-inductive phase of 12.8 Bd; (4) a photoperiod-insensitive phase of 33.9 Bd. When effects of photoperiod on rate of development were normalized across phases of development, photoperiod sensitivity did not vary among phases of development. Interpretation of constant sensitivity to photoperiod will simplify simulation of weed phenology in mechanistic models. # 2001 Published by Elsevier Science B.V.