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Prediction of phenology is based on thermal time (TT) computation, which requires the correct definition of base ([T.sub.B]) and optimum ([T.sub.O]) temperatures. Most information on these traits came from controlled environments using a wide range of mean air temperatures ([T.sub.x]), including [T.sub.x] > [T.sub.o] and [T.sub.x]< [T.sub.B]. These conditions are rarely found in field environments. We assessed the effect on development of day-time temperatures above To during late-vegetative growth of maize (Zea mays L), and established a model based on TT computation on a per hour (T[T.sub.h], in [degrees]C h) rather than per day basis (T[T.sub.d]) for [T.sub.O] estimation. Field experiments included two temperature regimes ([T.sub.C]: control; [T.sub.H]: heated) between [V.sub.11] and tasseling of [T.sub.C]. We registered temperature at ear level, and dates of anthesis and silking. We computed developmental rates (DR), T[T.sub.h] above 8[degrees]C during treatment period (T[T.sub.h1]) and between [V.sub.11] and silking (T[T.sub.h2]), a stress index based on the quotient of differences in T[T.sub.h] ([DELTA]T[T.sub.h]) between [T.sub.H] and [T.sub.C] (SI = [DELTA]T[T.sub.h2][DELTA]T[T.sub.h1]), and [T.sub.O]. Heat stress caused a delay in flowering events, and a decline in DRs. Estimated [T.sub.B] was higher (12.7[degrees]C) than normally used in computations. Estimated [T.sub.O] was within the expected range (36[degrees]C > [T.sub.O] > 30[degrees]C), independently of [T.sub.B]. Stressful temperatures promoted a delayed in silking, identified as an increase of at least 2.14[degrees]C h in T[T.sub.h] for each degree above [T.sub.O]. Estimated [T.sub.O] differed between growing seasons (P = 0.04), suggesting possible variation due to climatic effects. doi: 10.2135/cropsci2009.07.0400

Heat stress around flowering has negative effects on maize (Zea mays L.) grain yield. Most research on this topic focused on the response of pollen viability and pollination constraints, and little is known about the relative response to heat of plant grain yield (PGY) components [kernel number per plant (KNP) and individual kernel weight (KW)] and the physiological determinants of grain yield [light interception efficiency (e,), radiation use efficiency (RUE), and harvest index (HI)]. Field experiments were performed to study the response of physiological traits to contrasting air temperature regimes at ear level [nonheated control ([T.sub.C]) and heated ([T.sub.H]; with air temperature >35[degrees]C around noon)]. Heating was performed during periods of approximately 15 d at two growth stages [presilking (G[S.sub.1]) and postsilking (G[S.sub.2])]. All silked ears received fresh pollen. Heating during G[S.sub.1] caused (i) a larger delay in silking date than in anthesis date, (ii) an increase in male and female sterility, and (iii) a reduction in plant height and leaf area index, but not in % Heating always caused a reduction in (i) plant and ear growth rates (EGR) around silking, (ii) RUE around silking, and (iii) HI and KNP. Final PGY was related to KNP ([r.sup.2] = 0.89, p < 0.001) but not to KW. Variations in KNP were explained ([r.sup.2] = 0.71, p < 0.0001) by variations in EGR post-silking ([EGR.sub.POST]) and not presilking, evidence of long-term effects of heat stress during G[S.sub.1]. Variations in [EGR.sub.POST] depended on variations in RUE postsilking ([RUE.sub.POST]) and not on biomass partitioning to the ear. doi: 10.2135/cropsci2009.10.0574

Synchronous pollination improves maize (Zea mays L.) kernel set, but the physiological reasons behind this response are yet unclear. We registered ovary fresh weight evolution at three floret positions along the ear (base, middle and tip) of plants subjected to natural (NP) or synchronous (SP) pollination during two growing seasons. Synchronous pollination of ears bagged before silking was performed five days after silking (DAS). Ovary weight increased exponentially at all floret positions (P [less than or equal to] 0.01), but a lag in this trait was detected among those at the base and middle of SP ears. At each sampling date, florets along the ear differed (P < 0.01) in ovary weight, but differences were larger for NP than for SP plants. At pollination of each floret position, however, the range in ovary weight was smaller in NP ears (ca. 4 to 5 mg [ovary.sup.-1]) than in SP ears (4 to 9 mg [ovary.sup.-1]). Contrarily, just pollinated ovaries at the tip of the ear of NP plants (i.e. on ca. 4-5 DAS) experienced the high growth rate of those already pollinated at the base (ca. 0.7 mg [d.sup.-1] for the former and ca. 3.7 mg [d.sup.-1] for the latter). The range in ovary growth rate along the ear was drastically reduced under synchronous pollination on 5 DAS (ca. 0.55 mg d 1 for the base and ca. 0.47 mg [d.sup.-1] for the tip). The larger the tip-to-base ratio in ovary growth rate, the larger the number of kernels set per plant ([r.sup.2] = 0.94; P = 0.03).

A better understanding of the physiological processes related to nitrogen (N) metabolism in maize (Zea mays L.) inbred lines is important for increasing the efficiency of breeding programs targeting low-N environments. This study analyzed the response to contrasting N availability of morphophysiological traits in a set of 12 maize inbred lines, from different origins (USA and Argentina) and breeding eras (from 1952 onward). Traits included in the analysis were related to canopy structure, light interception, shoot biomass production, and grain yield. Our results indicate that (i) the start of N effects on canopy size was more related to a threshold crop leaf area index (about 2) than to a given leaf stage (i.e., [V.sub.n]), (ii) the light attenuation coefficient value was not affected by N availability, (iii) variations in kernel number per plant were explained by prolificacy ([r.sup.2] = 0.59), and (iv) differences in harvest index were related to kernel number per plant ([r.sup.2] = 0.77). The most important finding of our research was the detection in some inbreds of a particular response of kernel number to plant growth rate around silking, different from the general model established for hybrids. In these inbreds an additional effect of N availability was detected as reduced kernel set at a given plant growth rate under N deficient conditions (i.e., reduced reproductive efficiency). This result highlights the need of more research on reproductive sink development in this type of germplasm., WORLDWIDE, nitrogen, together with phosphorous, is one of the macronutrients that is most limiting to maize grain yield. Large amounts are applied to maize fields in most regions with a [...]

In maize (Zea mays L.), the gradient in floret development and silk length along the ear at silking determines a time lag between early- and late-appearing silks, which results in pollination asynchrony between them. This asynchrony is partially responsible of reduced kernel set at the ear tip, and hybrids differ in this trait. The objective of this work was to analyze the pattern of floret and silk differentiation and elongation at different spikelet positions ([S.sub.n]) along the apical ear of two hybrids of contrasting ear size (DEA ≡ 500 spikelets [ear.sup.-1]; DK696 ≡ 800 spikelets [ear.sup.-1]). At silking, both hybrids had reached approximately the same proportion of final ear length (about 44%), but DK696 had differentiated a greater number of spikelets [row.sup.-1] (46 spikelets) than DEA (33 spikelets). Silk initiation rate was always faster than spikelet initiation rate, and silk extension dynamics was similar for all spikelet positions. Silks from the base of the ear were always longer than those from the tip ([S.sub.25] in DEA or [S.sub.35] in DK696). Before pollination, silks experienced an early phase of exponential elongation followed by a phase of linear growth. A drastic reduction in elongation rate followed silk emergence, which did not occur when ears were bagged and pollination was prevented. Convergence in silking among spikelets along the ear could be attained by ii) synchronous silk initiation among spikelet positions, followed by a similar pattern of silk elongation in all florets (hybrid DEA), or (ii) increased silk elongation rate in apical florets (hybrid DK696)., IN MAIZE, the harvestable organ is produced by an axillary inflorescence, commonly known as the ear and botanically described as a modified spike. The ear consists of a set of [...]

Maize (Zea mays L.) pollen production has been found not to limit kernel set, but there is scarce information on pollen production of modern hybrids and the effect that breeding for reduced tassel size has had on this trait. Our objectives were to study genotypic differences in pollen production and flowering dynamics, and to estimate pollen availability per exposed silk. Four F1 hybrids were grown at different plant densities (between 2.5 and 12.5 plants [m.sup.-2]) in two distinct environments (cool Midwest USA and temperate Argentine Pampa). Pollen availability was also modified by delayed plantings and detasseling treatments. We measured the dates of anthesis and silking of individual plants, the anthesis-silking interval (ASI), the number of exposed silks per apical ear, the number of pollen grains per square meter (PGM), and kernel number per ear. The number of pollen grains produced per tassel (PGT) and per exposed silk were estimated. Increased plant density promoted an increase in ASI, matched by an enhanced interplant variability in this parameter, and a reduction in PGT. The latter was reduced from 10.3 x [10.sup.6] or 11 x [10.sup.6] at 2.5 plants [m.sup.-2] to about 3 x [10.sup.6] at 12.5 plants [m.sup.-2] (USA), and from 9.7 x [10.sup.6] or 113 x [10.sup.6] at 3 plants [m.sup.-2] to 4 x [10.sup.6] or 3.6 x [10.sup.6] at 9 plants [m.sup.-2] (Argentina). We estimated two thresholds beyond which kernel set could be affected: (i) 227 pollen grains [cm.sup.-2] [d.sup.-1] at the end of pollen shedding and (ii) two pollen grains per exposed silk. Some cropping conditions were closer to the second threshold than others., A SHORT anthesis-silking interval (ASI = anthesis date minus silking date) is a key trait for obtaining high grain yield in maize (Bolanos and Edmeades, 1993). An increase in this [...]

Maize production in Argentina has increased in recent years, following the global gain (GG) in grain yield (GY). The GG in GY depends on genetic progress (GP), which requires frequent quantification to detect potential plateaus and variations in its contribution to the GG. In this sense, hybrid adoption in Argentina shifted from double- and three-way to single cross (F1) hybrids in the 1990s, thereby increasing the level of heterosis of released hybrids. Since heterosis increase may have had a greater impact on estimates of maize GG than those based on a single cross type, GP based exclusively on F1 hybrids and its contribution to GG could be lower than that including different types of crosses. The main objectives of this work were to (i) analyze grain yield GP, (ii) dissect grain yield GP into the corresponding trends in its physiological determinants and numerical components, and (iii) estimate the relative contribution of GP to GG in the main maize producing region of Argentina for the period marked by significant changes in heterosis level (i.e. from 1960s to 1990s) and the period of massive adoption of F1 hybrids (from 1990s onwards). We used a dataset obtained at the core of the mentioned region (INTA Pergamino; 33°56'S, 60°34'W) from era-decade experiments (ERA) including 24 hybrids released between 1965 and 2016 and grown with no abiotic or biotic restrictions. We quantified GY, its numeric components (KN: kernel number m−2, KW: individual kernel weight) and its physiological determinants (B T : total shoot biomass at maturity, HI: harvest index). A GP of 0.84 % y−1 was computed for GY from 1965 to 1993, and of 0.51 % y−1 thereafter. We also detected genetic progress for KN (0.53 % y−1) and B T (0.26 % y−1) but not for KW. A GP of 0.66 % y−1 was computed for HI up to 1993, which plateaued thereafter. A 40 % contribution of GP to GG was estimated for the Pergamino site from 1965 to 1993, predominantly driven by changes in heterosis level. The contribution dropped to 32 % from 1993 onwards, when F1 hybrids were massively adopted by the farmers. This contribution was much smaller (e.g. 19 %) in areas less representative of the target population of environments of dominant breeding programs. Our findings underline that the relative contribution of GP to GY improvement at the regional level varies markedly depending upon the period included in the analysis and the environment used for evaluation. The decline observed at less representative locations of the main target population of environments may guide breeders in their decision for developing new programs. • Maize yield improvement in Argentina was driven by the increase in kernel number. • Harvest index increased up to 1993 and then plateaued among Argentine hybrids. • Breeding contribution to global yield gain declined when based only on F1 hybrids. • ERA studies are a robust approach to the estimation of breeding contribution. [ABSTRACT FROM AUTHOR]