Matthijs Tollenaar

Gains in maize yield from the US Corn Belt have been attributed to agricultural technologies. A study now shows that solar brightening was responsible for approximately 27% of yield growth from 1984 to 2013. Predictions of crop yield under future climate change are predicated on historical yield trends1,2,3, hence it is important to identify the contributors to historical yield gains and their potential for continued increase. The large gains in maize yield in the US Corn Belt have been attributed to agricultural technologies4, ignoring the potential contribution of solar brightening (decadal-scale increases in incident solar radiation) reported for much of the globe since the mid-1980s. In this study, using a novel biophysical/empirical approach, we show that solar brightening contributed approximately 27% of the US Corn Belt yield trend from 1984 to 2013. Accumulated solar brightening during the post-flowering phase of development of maize increased during the past three decades, causing the yield increase that previously had been attributed to agricultural technology. Several factors are believed to cause solar brightening, but their relative importance and future outlook are unknown5,6,7,8,9, making prediction of continued solar brightening and its future contribution to yield gain uncertain. Consequently, results of this study call into question the implicit use of historical yield trends in predicting yields under future climate change scenarios.

Heterosis in maize (Zea mays L.) confers stress tolerance. To better understand the physiological mechanisms underlying the differential response of a maize hybrid (CG60 × MBS1236) and its parental inbred lines to shading stress, studies were conducted in a field hydroponic system in Ontario, Canada, from 2004 to 2006. Shading stress consisted of a 55% reduction in incident solar radiation and was implemented either for a 30‐ to 33‐d period before silking starting at the 7‐leaf tip stage, a 21‐d period during silking, or a 21‐d period after silking. Mean reduction in total dry matter at maturity (TDM) due to the shading treatments was 18%, and this reduction was similar for the three shading periods. Heterosis for grain yield was greater when plants were exposed to shading during the presilking and silking periods compared to the unshaded control. This increase was attributable to increased heterosis for both harvest index and TDM. In contrast, shading during the grain‐filling period did not increase heterosis for grain yield. Heterosis for grain yield was highly associated with heterosis for kernel number. Heterosis for kernel set was attributable, in part, to the relationship between kernel number and plant growth rate (PGR) during the period bracketing silking and the inherent lower PGR of the inbred lines as compared to the hybrid. Kernel set was also affected by shading during the presilking period, in particular, in one of the two inbred lines.

Genetic yield improvement in maize (Zea mays L.) has been associated with heterosis and increased tolerance to high plant densities, but the physiological processes underlying heterosis for tolerance to plant density stress have not been identified. The objective of this study was to quantify the response of heterosis for grain yield to increasing plant density, and to examine the processes underlying this response. Field experiments were conducted in Ontario, Canada, during the 2005 and 2006 growing seasons in which the hybrid CG60 × MBS1236 and its parental inbred lines were grown at a low plant density (4 plants m⁻²), a high plant density (12 plants m⁻²), and a plant density resulting in an approximately equal leaf area index (LAI) for all three genotypes. Increasing plant density from 4 to 12 plants m⁻² resulted in an increase in heterosis for grain yield and harvest index (HI), but did not affect heterosis for dry matter at maturity. Heterosis for dry matter accumulation did not differ between the low and the high plant density treatment, increased from the presilking to the postsilking period, and was only to a small extent attributable to a higher maximum LAI and light interception by the hybrid as compared to its parental inbred lines. Increased heterosis for HI was associated with a greater plant‐to‐plant variability and a higher threshold plant dry matter for HI at maturity in the inbred lines as compared to the hybrid.

Conflicting results have been reported on the effects of spacing and emergence variability on grain yield in maize (Zea mays L.). Effects of spacing and emergence variability on maize grain yield are the net result of the responses of all plants within the stand. The objective of this study was to quantify effects of spacing and emergence variability on crop yield in terms of increased or decreased crowding stress on resource capture (i.e., dry matter accumulation) and resource utilization (i.e., dry matter partitioning) of the individual plants within the crop canopy. Results of previously reported studies were analyzed in terms of plant dry matter accumulation, leaf area, plant growth rate during the critical period for kernel set bracketed by silking (PGR_s), grain yield, and harvest index, that is, the proportion of dry matter partitioned to the grain at maturity. Results show that a moderate increase in plant‐spacing variability does not influence maize grain yield at the canopy level because reductions in grain yield of plants that experience enhanced crowding stress is compensated, in part, by increased yield of plants that experience reduced crowding stress; crowding stress affected dry matter accumulation but did not affect harvest index. In contrast, plant‐emergence variability reduced grain yield at the canopy level because the reduction in grain yield was attributable, in part, to a reduction in harvest index of plants with PGR_s less than the threshold for kernel set. Hence, plants can compensate for factors that influence resource capture, but cannot compensate for a reduction in factors that influence resource utilization.

Accurate estimates of total leaf area and the vertical leaf area profile are important in process‐based crop growth models. The bell‐shaped function that quantifies the area‐per‐leaf profile of a maize (Zea mays L.) plant can be used to estimate the area‐per‐leaf profile. The objectives of this study were to quantify the effects of maize hybrid, soil N, plant density, and row spacing on the coefficients of the bell‐shaped function. The coefficients of the bell‐shaped function that quantify (i) the breadth of the area‐per‐leaf profile, (ii) the skewness of the area‐per‐leaf profile, and (iii) the position of the largest leaf were estimated using nonlinear regression in four datasets. Datasets consisted of the fully expanded leaf areas of all leaves on maize plants grown in studies performed in Ontario, Canada, between 1997 and 2001 that included combinations of maize hybrids, plant densities, N levels, and row spacing. Observations fitted well to the bell‐shaped function (r² > 0.95). The breadth of the area‐per‐leaf profile decreased under high soil N level and high plant density, and was lower for a newer than an older hybrid, whereas the opposite occurred with the position of the largest leaf. In contrast, the degree of skewness was not significantly altered by any of the factors examined in this study. Because of the relatively small impact of the examined agronomic factors on the coefficients of the bell‐shaped function, a general model using mean coefficient values was validated with independent datasets. Results showed that this general bell‐shaped function is a robust predictor of the area‐per‐leaf profile in maize.

Heterosis for grain yield in maize (Zea mays L.) manifests itself through its effects on the components of grain yield, dry matter accumulation at maturity, harvest index, and its effects on physiological processes underlying these components, such as leaf CO₂ exchange rate (CER). The objectives of this study were (i) to quantify the pattern of leaf CER throughout the grain‐filling period in maize hybrids and their parental inbred lines, and (ii) to determine the mode of inheritance of leaf CER during the grain‐filling period. Studies were performed with 12 F₁ hybrids and their seven inbred parents grown hydroponically in the field at the Cambridge Research Station, ON, Canada, in 2002. Data were recorded on leaf CER from silking to maturity, and grain yield, aboveground dry matter, and root dry matter at maturity. Mean leaf CER of hybrids was not different from that of their parental inbred lines at silking. However, significant differences became apparent 2 wk after silking and became increasingly larger as plants advanced toward maturity. In general, leaf CER differed among inbred lines but did not differ among hybrids. Combining ability analysis showed that predominantly additive genetic effects influence the expression of leaf CER late in the season. Finally, the maintenance of leaf CER throughout a plant's life cycle, rather than potential leaf CER, is positively associated with dry matter accumulation during the grain‐filling period and grain yield.

Grain yield improvement of maize (Zea mays L.) hybrids has been associated with delayed leaf senescence. The objective of this study was to quantify the vertical profile of leaf senescence during the grain‐filling period in an older hybrid (‘Pride 5’) and two more recent maize hybrids (‘Pioneer 3902’ and ‘Pioneer 3893’). Leaf senescence was rated visually from silking to maturity on each individual leaf across the vertical leaf‐area profile along the stem of maize plants growing in the field at 1, 3.5, and 12 plants m⁻² near Elora, ON, Canada, during the 1999 to 2001 growing seasons. Maximum leaf area index (LAI) at silking was greater in newer hybrids than in the older hybrid. Rate of leaf senescence across hybrids and plant population densities progressed at a linear rate of 0.44% d⁻¹ during the first half of the grain‐filling period, whereas the rate was 1.87% d⁻¹ during the second half of the grain‐filling period. Rates of leaf senescence were 3.4 and 2.1 times greater in the older hybrid than in the newer hybrids during the first and second half of the grain‐filling period, respectively. During the first half of the grain‐filling period, leaf senescence increased from the medium to the highest plant population density, whereas rates of senescence during the second half of the grain‐filling period declined with an increase in plant population density for the older hybrid and rates were lowest at the medium plant population density for the newer hybrids. A top–bottom profile of leaf senescence was observed during the second half of the grain‐filling period, with leaves in the central section of the canopy being the last leaves to senesce, and this phenomenon was more marked in the newer hybrids.

Light quantity (photosynthetic photon flux density, PPFD) and quality (red:far‐red ratio, R:FR) may affect phenological development of weed species growing under a crop canopy. An indoor study was conducted to quantify the effects of incident PPFD and R:FR on development and dry matter accumulation of redroot pigweed (Amaranthus retroflexusL.). Pigweed was grown in growth cabinets from the one‐leaf stage to the initiation of seed set under three different PPFD/R:FR treatments: (i) high PPFD (550 μmol m⁻²s⁻¹) and high R:FR (1.4) (HH), (ii) low PPFD (180 μmol m⁻²s⁻¹) and high R:FR (1.4) (LH), and (iii) low PPFD (180 μmol m⁻²s⁻¹) and low R:FR (0.8) (LL). The experiment was undertaken at 12‐ and 16‐h daylengths with three replications. Rate of leaf appearance (RLA) was accelerated with an increase in PPFD (HH vs. LH) at both daylengths. The FR enrichment (LL) negated the effect of low PPFD on RLA under the 12‐h but not under the 16‐h daylength. Low PPFD delayed the occurrence of floral primordia, flowering and initiation of seed set. Plant height was a result of the complementary effects of PPFD and R:FR. Total dry matter accumulation and partitioning, with the exception of dry matter accumulation to the stem, were influenced by PPFD only. Results of this study show that both light quality and quantity influence the phenology of pigweed.