Abstract
Maize breeding and product development practices with transgenic and genome edited events use horticultural practices and controlled environment facilities for year-round production. In winter months, when ambient light is limited in the northern regions of the United States, tassel barrenness (lacking anthers) can be problematic due to reduced pollen production. Our objective was to quantify the tassel morphology of two maize inbred lines in response to low daily light integral (DLI) at different growth stages and identify a DLI threshold for tassel quality. Inbreds A and B were analyzed after being transferred from a high DLI equivalent (23.7 mol·m−2·d−1) to a low DLI equivalent (9.3 mol·m−2·d−1) for 7 days starting at vegetative growth stages (V) V4, V5, V6, V7, V8, V9, V10, V11, or V12 then placed back in the high DLI environment. Data were collected to characterize tassel morphology and barrenness in response to the low DLI stress. Inbred A tassels were more barren under low DLI during V7. Tassels of Inbred B plants responded similarly to the low DLI during V6. These results indicate that low DLI stress impacts tassel morphology of the two inbred lines tested during specific growth stages for 7 days. To identify the number of days required to negatively affect tassels, at the low DLI treatment, plants were subjected to 9.3 mol·m−2·d−1 at V6 on Inbred B for 0, 1, 2, 3, 4, 5, 6, or 7 days. Tassel height was shorter (by 9.2, 11.8, or 11.8 cm) for plants that received 5, 6, or 7 days (respectively) of low DLI stress during the V6 developmental stage compared with the high DLI control. These results highlight the importance of providing supplemental lighting if the DLI falls below a critical threshold that is likely well below 23.7 mol·m−2·d−1 for more than 5 days during tassel development. Photoperiod manipulation to increase DLI was investigated as a tool to suppress tassel barrenness. Inbreds A and B were subjected to a 16, 20, or 22-hour photoperiod during vegetative or reproductive growth. A longer photoperiod during vegetative growth resulted in an elongated tassel height for Inbred A (by 4.3 cm). For Inbred B, tassel branch number was less (by 3.5 branches), and barren tassel length was shorter (by 19.1 cm). Only Inbred A tassel morphology responded to the longer photoperiod during reproductive growth stages compared with the control, with a 1.4 cm longer tassel height, from 16 to 20 hours, and a 4.8 cm shorter tassel height from 16 to 22 hours, and a 6.2 cm shorter tassel height from 20 to 22 hours. These results indicate that a longer photoperiod can alter the tassel morphology of the two inbred lines during either vegetative or reproductive growth stages. Increasing the photoperiod too much can decrease tassel quality, for example, tassel branch number and viable tassel length decreased (by 4.5 branches and 23.6 cm) for Inbred A plants as the vegetative photoperiod increased from 20 to 22 hours.
In 2022, corn was the number one field crop planted, covering more than 90 million acres across the United States (US Department of Agriculture 2024). Increasingly, this agronomic crop has horticultural production needs and phases of product development. When plant populations are low or only individual plants are represented from a selection or tissue-culture–generated plantlets, the environmental control and management opportunities afforded within a greenhouse or controlled environment are leveraged. Horticultural practices are used (growth in pots with peat substrates, electric lighting, imposed photoperiods, environmental control, etc.) for what will eventually be a mass-produced agronomic crop. Before it reaches that step, these small-population selections or individual plants are phased up all year in intensively managed facilities. To facilitate the production of traited material and growth during the winter season a controlled environment or greenhouse is used. Although field-grown maize is often open pollinated with relatively unlimited pollen, these early phases of crop selection and seed production require hand-pollinations in the greenhouses to maintain trait purity standards. Pollen availability can be a limiting factor of maize production in a controlled environment setting.
Tassel morphology is both predetermined genetically and influenced by environmental factors. The size and shape of the tassel dictates the pollen potential for the plant (Vollbrecht et al. 2005). Furthermore, pollinations require a minimum quality and quantity of pollen, and if the tassel is small or barren and no pollen is produced, the pollination will not succeed (Tollenaar and Dwyer 1999). As the barren tassel lengthens, the pollen potential decreases and the yield potential of that selection diminishes. Tassels of maize plants grown in greenhouses of the northern United States during winter months are observed with anthers that are underdeveloped or not present, leading to a reduction in pollen production decreasing the yield potential.
Sunlight is limited during the winter months in the northern regions of the United States (Korczynski et al. 2002). The DLI is the measure of photosynthetic light in a day, and it can range from 10 mol·m−2·d−1 during winter months to 50 mol·m−2·d−1 during the summer season in Iowa (Korczynski et al. 2002). In addition to the low outdoor DLI during winter months, the greenhouse structure can reduce the solar radiation plants receive by 40% to 60% due to glazing and superstructure shadowing (Eddy and Hahn 2010). Even in tropical environments, shading or seasonal weather patterns can reduce DLI inside greenhouse structures to well under 10 mol·m−2·d−1. When other environmental stresses are reduced, it is hypothesized that light is the limiting factor in corn yield (Eddy and Hahn 2010). Because light is one of the most critical growth factors that influence yield, it can be manipulated to increase the yield of maize plants (Blanchard and Runkle 2011; Tollenaar and Aguilera 1992).
Growers can increase their supplemental DLI by operating their lamps longer but increasing the operation time of lamps will also increase the photoperiod. Understanding how plants respond to photoperiod is essential before increasing or decreasing the daylength. In maize, the anthesis-silking interval (ASI) determines whether the plant can successfully self-pollinate, and longer photoperiods are documented having increased the ASI of maize crops, which decreases the pollination success (Warrington and Kanemasu 1983).
Little work has been done to determine susceptible developmental stages to low DLI stress and the effect on tassel quality. We hypothesize that a reduction in DLI to below 10 mol·m−2·d−1 will suppress the anther production of the maize tassel. The objective of this study was 3-fold: to characterize the impact of a low DLI on anther production, to determine the number of consecutive days of low DLI treatment results in suppressed anther production, and the effects of extended photoperiods have on tassel development.
Materials and Methods
Maize inbred lines were obtained from DuPont Pioneer (Inbred A and Inbred B). Both inbred lines are classified within the nonstiff stalk heterotic group; however, Inbred A is a drought-susceptible line that has a relative maturity of 103 d and Inbred B is a drought-tolerant line with a shorter relative maturity of 98 d. Furthermore, the tassel morphology of the two lines is diverse; for example, the number of tassel branches grown by Inbred B is typically half of the number of branches produced by Inbred A. The aim of testing two genetic lines with a diverse tassel morphology and drought response was to detect if genetics were a factor in DLI response.
Maize seed was sown at a 2.5-cm depth into 32-cell flats (90.7-mL individual cell volume) containing a soilless substrate composed of (by vol.) 77% Canadian sphagnum peat, 16% perlite, and 7% vermiculite and adjusted with lime to a pH of 6 and irrigated with municipal water supplemented with 125 mg·L−1 N (Peters Excel © Cal-Mag © 15N–2.2P–12.5K Everris NA, Marysville, OH, USA). Seeds were germinated and grown in a growth chamber (Model BDW160; Conviron ©, Winnipeg, MB, Canada). The growth chamber operation and environmental set points, described below, were programmed and maintained with the growth chamber software (CMP6050 V. 4.06; Conviron ©). The air temperatures, humidity levels, and light intensities were measured with temperature probes, humidity probes, and quantum sensors, respectively, that were built into each growth chamber and connected to data loggers. These sensors were located 50 cm from an outside wall 2 m above the floor. After 2 weeks, seedlings were transplanted into 5.9-L pots containing a soilless substrate composed of (by vol.) 38% Canadian sphagnum peat, 51% composted bark, 8% perlite, and 3% vermiculite and adjusted with lime to a pH of 6. After transplant, plants were irrigated with municipal water supplemented with 95 mg·L−1 N using the same fertilizer. Plants were treated with one of two light treatments: a high DLI (control) chamber and a low DLI chamber. The control chamber provided a light intensity of 550 µmol·m−2·s−1 from metal halide lamps to maintain a DLI of 23.7 mol·m−2·d−1, whereas the low DLI chamber provided a light intensity of 210 µmol·m−2·s−1 to maintain a DLI of 9.3 mol·m−2·d−1. The planting density was not controlled because the plants moved among chambers and environments over the course of the experiments.
Expt. 1 – developmental stages susceptible to low DLI.
Maize phenology is described by the number of leaves that have a fully developed auricle and “collar” at the base of the leaf where it joins the stem (Hanway 1971; Stevens et al. 1986). Using this terminology, a plant with a fully developed leaf with a collar and a still-developing set of young leaves above it is classified as being in V1 or Vegetative Stage with one leaf. A second leaf with a collar is at V2, and so on until the onset of a tassel, above which no more leaves are developed. The stage at which tassels emerge is indicated as VT or “visible tassel.” This terminology is used throughout this article and is a commonly adopted set of terminology for sweet corn, field corn, and popping corn.
For Expt. 1, plants were sown 13 Feb 2015 in Johnston, IA, USA. Growth chamber set points were 12-h daylength, with day/night air temperature 29.1 °C/21.1 °C and a continuous 65% relative humidity [day/night vapor pressure deficit (VPD) of 1.41 kPa/0.87 kPa]. Plants were placed into low DLI conditions when 50% of the plants in a treatment reached V4, V5, V6, or V7 [occurring 11, 14, 18, and 24 d after sowing (DAS), respectively]. After 7 d, plants were returned to the control chamber. A set of plants remained in the control chamber throughout the entire experiment and thus received consistently high DLI. Data were collected 10 d after pollination (DAP) for vegetative traits that included final plant height (from substrate level to the flag leaf junction and the tassel tip) and final ear leaf area index (determined by ear leaf width × ear leaf length) (van Arkel 1978). First silk emergence date and tassel first shed dates (used to calculate ASI) were recorded. At harvest, ear length and kernel number (determined by multiplying the number of kernels running long by the number radial) were collected. Tassel morphology analyses were conducted by taking a digital photo of the tassel, which was held flat and parallel to the camera lens. Tassel length and size were determined using digital images and image-analyzing software (Assess 2.0 Image Analysis Software; CPL Scientific Publishing Services; Newbury, UK). Postharvest tassel analyses included: tassel height (measured from lowest tassel branch node to tassel tip), tassel size (sum of all tassel branch lengths), tassel branch number, barren tassel length (sum of all sterile branch lengths), and viable tassel length (sum of all viable branch lengths). This experiment was designed using a completely randomized design with 12 plants per treatment. Analyses of variance (ANOVAs) and mean separations were performed by Tukey’s honestly significant difference (HSD) test at P ≤ 0.05 (JMP v. 12; SAS Institute, Cary, NC, USA).
Expt. 2 – investigation of additional developmental stages susceptible to low DLI.
The same inbred lines, growing substrate, growth conditions, and fertilizer methods were used as previously described in Expt. 1. Plants were sown 7 Jul 2015 and followed the same manner as Expt. 1. The treatments were similar to Expt. 1, testing V4, V5, V6, V7, and a high DLI control (occurring 16, 21, 27, and 29 DAS, respectively) in addition to V8, V9, V10, V11, and V12 (occurring 31, 34, 36, 38, and 41 DAS, respectively). Plants were monitored for developmental stage and placed in the low DLI treatment individually for Expt. 2, not as a group as in Expt. 1. Each treatment consisted of six plants. In addition to the data collected in Expt. 1, data on total leaf number and ear leaf number (node at which ear developed) were also collected. The statistical analyses of Expt. 2 were identical to Expt. 1.
Expt. 3 – identifying the low DLI threshold of Inbred B.
Seeds of Inbred B were sown on 23 Jun 2016 in growth chambers. The growing procedures and conditions were as described previously with the exceptions detailed in the following. Seeds were germinated and grown, until transplant on day 14, in a greenhouse propagation room. During the first 2 weeks of growth in a propagation room, set points were 16-h daylength, with day/night air temperature of 26.7 °C/23.9 °C and 80% continuous relative humidity (day/night VPD of 0.69 kPa/0.58 kPa). The supplemental light intensity was 260 µmol·m−2·s−1 provided by high-pressure sodium and metal halide lamps at a ratio of 1 to 5 respectively, resulting in a total combined minimum DLI of 15 mol·m−2·d−1. At the time of transplant, the plants were moved to the control chamber (same as Expts. 1 and 2). All plants were grown in the high DLI chamber (23.7 mol·m−2·d−1) until reaching a specific developmental stage, and plants were moved to the low DLI chamber on an individual plant basis (9.3 mol·m−2·d−1; same as Expt. 2) until the final treatment was carried out. The treatments began when each plant reached V6. Once the plants were moved to the low DLI chamber they remained there for 1, 2, 3, 4, 5, 6, or 7 d, after which time they were returned to the high DLI chamber. A set of plants remained in the high DLI chamber. In addition to the data collected in Expt. 2 25 DAP, data were also collected on stalk diameter. The statistical analyses of Expt. 3 were identical to Expt. 1, with seven plants per treatment.
Expt. 4 – increasing photoperiod to suppress barren tassel length.
The same inbred lines, growing substrate, and fertilizer methods were used as previously described in Expt. 1. Seeds were sown on 23 Jun 2016 and grown in a propagation room and followed the same manner as Expt. 3. On 7 Jul 2016, 2 weeks after sowing, seedlings were transplanted in the same manner as the previous experiments. At transplant, plants were transferred to one of three growth chambers (Model BDW160; Conviron ©), irrigated with municipal water supplemented with 95 mg·L−1 N (Peters Excel © Cal-Mag © 15N–2.2P–12.5K Everris NA). The environment across all three chambers had the same day/night air temperature set points of 29.1/21.1 °C, and 65% relative humidity held constant (day/night VPD of 1.41/0.87 pKa). The chambers were set to a 16-, 20-, or 22-h photoperiod. As the photoperiod lengthened, so too did the hours of day temperature. The light intensity of all three chambers was adjusted to maintain a DLI of 23.04 mol·m−2·d−1, regardless of photoperiods. Light intensity was set in each chamber as follows: 400 µmol·m−2·s−1 for the 16-h chamber, 320 µmol·m−2·s−1 for the 20-h chamber, or 291 µmol·m−2·s−1 for the 22-h chamber. Plants were grown from vegetative stage 2 (V2) until VT, defined as the “vegetative” growth period in each of the chambers, once the plants reached VT, they were randomly divided into groups of six plants each. One group from each photoperiod treatment chamber remained in that chamber to serve as controls, whereas each of the other two groups were placed into the other photoperiod treatment chambers. For example, of the 18 plants treated with a 16-h photoperiod from V2 to VT, six remained at a 16-h photoperiod, six were placed in a 20-h photoperiod, and six were placed in a 22-h photoperiod. This occurred for all three treatments. The “reproductive” growth period began after this plant movement and continued through maturation and harvest (Abendroth et al. 2011). Data were collected the same as Expt. 3. This experiment was designed using a completely randomized design. Each treatment consisted of six plants. Vegetative photoperiod (three levels) and reproductive photoperiod (three levels) were studied. Due to a reproductive photoperiod treatment failure, only two levels (20 and 22 h) were analyzed. ANOVAs and mean separations were performed by Tukey’s HSD test at P ≤ 0.05 (JMP v. 12, SAS Institute).
Results
Expt. 1 – developmental stages susceptible to low DLI
Inbred A.
Transient low DLI stress, when applied at different developmental stages, affected the growth and development of Inbred A across many vegetative, reproductive, and tassel traits (Table 1). Vegetative and reproductive traits unaffected by low DLI include plant height, ASI, ear length, and kernel number. Compared with the high DLI control, ear leaf area was less when the low DLI stress was applied during V6 (by 141.7 cm2) and V7 (by 62.7 cm2) (Table 1). The tassel height was unaffected by the low DLI when applied during any developmental stage (Table 1). Fewer tassel branches developed when the low DLI stress occurred during the V4 and V5 growth stages by 3.2 or 2.4 branches, respectively, compared with the high DLI control. There was no difference when plants were treated at the V6 or V7 stages in comparison with the high DLI control (Table 1). Compared with the high DLI control, viable tassel length was shorter under low DLI stress applied during V4 and V7, 27.2 cm or 24.8 cm, respectively. Low DLI stress at V7 led to a 9.2-cm longer barren tassel length than the high DLI control.
Vegetative, reproductive and tassel traits for maize (Zea mays L.) inbred lines A and B grown in Expt. 1 under low (9.3 mol·m−2·d−1) daily light integral (DLI) for 7 d, when 50% of plants reached the different Treatment Growth Stages (TGS; V4, V5, V6, or V7) or an unaltered high DLI control (23.7 mol·m−2·d−1). ASI is defined as the anthesis-silking pollination timing interval. Data were collected 10 d after pollination (DAP). Each treatment consisted of 12 plants and mean separations were performed using Tukey’s honestly significant difference (HSD) test.
Inbred B.
The growth and development of plants from Inbred B were also affected by the low DLI treatment (Table 1). Kernel number and ASI were unaffected by low DLI. Shorter plant height (by 24.4 cm), ear height (by 12.2 cm), and ear length (by 1.7 cm) were recorded for the V7 developmental stage treatment in comparison with high DLI control plants (Table 1). Tassel height was shorter than the high DLI control when the treatment was applied during V6 (by 5.8 cm) and V7 (by 4.7 cm). Plants exposed to the low DLI treatment during V4 developed tassels with 3.2 fewer branches than the high DLI control plants. Plants treated with low DLI during V6 and V7 resulted in suppressed viable tassel length by 46.4 cm and by 41.2 cm, respectively, and an increased barren region of 13.7 cm and of 6.7 cm, respectively. The tassel morphology images illustrate a typical overall reduction in tassel quality in comparison with the high DLI control (Fig. 1).
Tassel height, branch number, viable length and barren length for maize (Zea mays L.) inbred lines A and B grown under low (∼9 mol·m−2·d−1) daily light integral (DLI) for 7 d, when 50% of plants reached V4, V5, V6, or V7, and a high DLI control was also included. Data were collected 10 d after pollination. Tassels imaged above were treated during V6 and V7 compared with the high DLI control.
Citation: HortScience 60, 5; 10.21273/HORTSCI18378-24
Expt. 2 – investigation of additional developmental stages susceptible to low DLI
Inbred A.
Through the testing of additional developmental stages, plant height was found to be susceptible to low DLI for Inbred A (Table 2). Compared with the high DLI control, plant height was shorter when low DLI treatments were applied during V10 and V12 by 19.9 cm and by 19.5 cm, respectively. Vegetative and reproductive traits that were unaffected for Inbred A in response to the low DLI treatment included ear leaf number, total leaf number, ASI, ear length, and kernel number (Table 3). Tassel height, tassel branch number, and viable tassel length of the treated plants were unaffected by DLI (Table 3). Tassel barrenness increased by 11 cm when plants were treated with low DLI conditions during V7 compared with the high DLI control.
Vegetative and reproductive traits for maize (Zea mays L.) inbred lines A and B grown in Expt. 2 under low (9.3 mol·m−2·d−1) daily light integral (DLI) for 7 d, when individual plants reached the Treatment Growth Stage (TGS; V4–V12) or remained in an unaltered high DLI control (23.7 mol·m−2·d−1). ASI is defined as the anthesis-silking pollination timing interval. Data were collected 10 d after pollination (DAP). Each treatment consisted of six plants and mean separations were performed using Tukey’s honestly significant difference (HSD) test.
Tassel height, branch number, viable length and barren length for maize (Zea mays L.) inbred lines A and B grown in Expt. 2 under low (9.3 mol·m−2·d−1) daily light integral (DLI) for 7 d, when individual plants reached different Treatment Growth Stages (TGS; V4–V12) or in an unaltered high DLI control (23.7 mol·m−2·d−1). Data collected 10 d after pollination (DAP). Each treatment consisted of six plants and mean separations were performed using Tukey’s honestly significant difference (HSD) test.
Inbred B.
For Inbred B, none of the vegetative or reproductive traits were affected by low DLI stress when applied beyond V8 when compared with the high DLI control (Table 2). Low DLI at V7 resulted in a shorter ear length by 1.7 cm compared with the high DLI control. Plants were unaffected by the low DLI stress for tassel height, tassel branch number, or viable tassel length (Table 3). Barren tassel length was increased by 13.3 cm for plants treated with low DLI during V6 compared with the high DLI control (Table 3).
Expt. 3 – identifying the low DLI threshold of Inbred B
Inbred B.
Low DLI stress to Inbred B plants at V6 for 7 d resulted in a smaller stalk diameter by 2.6 cm compared with the high DLI control (Table 4), but all other vegetative traits (plant height, ear height, ear leaf area index, ear leaf number, and total leaf number) were unaffected by the low DLI stress. Reproductive traits (ASI, ear length, and kernel number) were unaffected by the low DLI stress when applied during the V6 developmental stage for 1 to 7 d (Table 4). Tassel heights were shorter for plants that received 5, 6, or 7 d of low DLI stress during the V6 stage resulting in tassel heights 9.2, 11.8, or 11.8 cm shorter, respectively, than the high DLI control (Table 5). Tassel branch number, viable tassel length, and barren tassel length were unaffected when plants were treated for 1 to 7 d of low DLI stress at V6 compared with the high DLI control (Table 5).
Vegetative and reproductive traits for maize (Zea mays L.) inbred line B grown in Expt. 3 under low (9.3 mol·m−2·d−1) daily light integral (DLI) for 1 to 7 d, when individual plants reached V6, or grown continuously in high DLI (23.7 mol·m−2·d−1). ASI is defined as the anthesis-silking pollination timing interval. Data collected 25 d after pollination. Each treatment consisted of six plants and mean separations were performed using Tukey’s honestly significant difference (HSD) test.
Tassel height, branch number, viable length and barren length for maize (Zea mays L.) inbred line B grown in Expt. 3 under low (9.3 mol·m−2·d−1) daily light integral (DLI) for 1 to 7 d, when individual plants reached V6, or grown continuously in high DLI (23.7 mol·m−2·d−1). Each treatment consisted of six plants and mean separations were performed using Tukey’s honestly significant difference (HSD) test.
Expt. 4 – increasing photoperiod to suppress barren tassel length
Inbred A.
Both vegetative and reproductive traits were influenced by photoperiod during both the vegetative and reproductive growth stages, tassel traits specifically included time to shed, tassel height, viable tassel length, and tassel branch number (Table 6). Plants exposed to a 22-h vegetative photoperiod produced an earlier shedding tassel, a longer tassel height, shorter viable branch length, and fewer tassel branches, by 4.8 d, 4.3 cm, 89 cm, and 4.5 branches, respectively, than plants under 20-h vegetative photoperiod (Table 7). As reproductive photoperiod increased from 16 to 20 h, tassel height lengthened by 1.4 cm. As the photoperiod was increased further to 22 h, the tassel height was shorter than 16 h (by 4.8 cm) and 20 h (by 6.2 cm) (Table 8). As photoperiod increased from 16 to 22 h during reproductive growth, barren tassel length was shorter when grown under a 20-h day compared with a 22-h day during vegetative growth stages (Table 9).
Analyses of variance in Expt. 4 of vegetative (V) photoperiod (20 or 22 h) and reproductive (R) photoperiod (16, 20, or 22 h), and their interactions, on both vegetative and reproductive traits of maize (Zea mays L.) Inbred A (drought-susceptible line with a highly branched tassel) and Inbred B (drought-tolerant line with a less-branched tassel). ASI is defined as the anthesis-silking pollination timing interval. Data were collected at 25 d after pollination, with n = 6 plants.
The effect of photoperiod (20 or 22 h) during Expt. 4 on vegetative and reproductive traits for both maize (Zea mays L.) Inbreds A (drought-susceptible line with a highly branched tassel) and B (drought-tolerant line with a less-branched tassel). Data were collected at 25 d after pollination, with n = 6 plants. Data are pooled across reproductive photoperiods.
The effect of photoperiod (16, 20, or 22 h) during Expt. 4 on vegetative and reproductive traits for both maize (Zea mays L.) Inbreds A (drought-susceptible line with a highly branched tassel) and B (drought-tolerant line with a less-branched tassel). Data were collected at 25 d after pollination, with n = 6 plants. Data are pooled across vegetative photoperiods.
The effect of the interaction between vegetative photoperiod (V) and reproductive photoperiod (R) on barren tassel length of maize (Zea mays L.) Inbred A (drought-susceptible line with a highly branched tassel) during Expt. 4. Data were collected at 25 d after pollination, with n = 6 plants.
Inbred B.
Both vegetative and reproductive traits were influenced by photoperiod during both the vegetative and reproductive growth stages, tassel traits specifically included tassel branch number and barren tassel length (Table 6). Compared with the control, the tassel branch number was fewer by 3.5 branches as the reproductive photoperiod increased from 20 to 22 h. Similarly, the barren tassel length was shorter from 20 to 22 h, by 19.1 cm (Table 7).
Discussion
Plants grown under low DLI conditions produced greater barrenness likely due to diminished anther production compared with those under a higher DLI. Expt. 1 aimed to quantify the effect of low DLI on tassel morphology for developmental stages V4 to V7 and subsequently expanded in Expt. 2, V8 to V12. Tassel barrenness was greatest for Inbred A when a low DLI was experienced during V7. Some of the traits that were affected in Expt. 1. were unaffected in Expt. 2. The Inbred A plants were affected by low DLI across both experiments showing that ear leaf area index was smaller (V6) and barren tassel length grew (V7). Traits that were affected across both experiments for Inbred B was a shorter ear length (V7) and a longer barren tassel length (V6). Shorter ear length under low light conditions was also documented for maize plants grown under 70% shade (30% transmittance) during vegetative growth in the field (Earley et al. 1966), and for maize plants grown under 50% shade during reproductive growth stages (Zhou et al. 2013). Barren tassel length was longer after with 7 d of low DLI stress for both Inbred A (V7) and Inbred B (V6). This tassel stress response during these vegetative growth stages is supported with literature that reports tassel morphology and development occurring between V5 and V7, making the tassel highly susceptible to stress during these stages (Phillips et al. 2011; Ritchie et al. 1992). Fewer tassel branches and the decreased number of anthers on maize plants treated with 65 µmol·m−2·s−1 and a 12-h photoperiod (a DLI of 2.8 mol·m−2·d−1) at V6 and V7 has been previously documented (Bechoux et al. 2000). Our research supports these findings, with a 16-h photoperiod and light intensity of 260 µmol·m−2·s−1 (a DLI of 14.9 mol·m−2·d−1), indicating that the reduction of DLI during vegetative growth stages diminishes tassel quality by increasing the barren tassel length and lowering the pollen production potential.
In addition to quantifying the impact of DLI on tassel morphology, we aimed to understand the effects of the duration of low DLI stress on tassel quality. More than 5 d of low DLI at V6 suppressed the tassel height of Inbred B (Table 5). Tassel height was also shorter in field maize under shaded conditions (50% transmittance) during VT and reproductive growth stages compared with nonshaded plants (Zhou et al. 2013). Tassel development at V6 (Phillips et al. 2011; Ritchie et al. 1992) increases the susceptibility of the tassel to environmental stress, such as low DLI, supporting the hypothesis that high light is an important factor for tassel quality. Previous work on the effect of low light intensity during vegetative growth of field-grown maize demonstrated that the stalk diameter above the primary ear became smaller as light is decreased (Earley et al. 1966). This positive relationship between stem diameter and DLI agrees with our results. For both inbred lines, there were three traits that never responded to the low DLI stress across developmental stages (V4–V12): ASI, total leaf number, and kernel number. Low light has been documented to decrease ear row number and kernel number (Zhou et al. 2013), which disagrees with our findings that the kernel number was unaffected by low DLI conditions. The discrepancy between these studies may be due to experimental design; their low light treatments were applied during VT and reproductive growth stages, which is later than any stage tested in this study.
Inbreds A and B were more susceptible to a longer photoperiod during vegetative growth than reproductive growth (Table 6). Furthermore, none of the tassel or ear traits were affected by a longer photoperiod during reproductive growth. Tassel morphology and ear architecture are already developing by the time the plant reaches VT (Bechoux et al. 2000). The longer ear leaf length, ear length, and plant height can all be attributed to lower light intensities under the longer photoperiods during vegetative growth (Bechoux et al. 2000; Moe and Heins 1990). The similar response of these traits to the longer photoperiods of both inbred lines suggests that the responses are conserved through the genetics of the two inbred lines.
Maize is a high-light plant, with photosynthesis saturating at ∼2000 µmol·m−2·s−1 (Fletcher et al. 2008). Tassel and ear structures are growing and developing during the vegetative growth stages, making these organs vulnerable to environmental stress (Bechoux et al. 2000). Flowering time hastened as the photoperiod grew longer during vegetative growth stages for Inbred A (Table 6). Although flowering occurred earlier, ASI was unaffected because the interval between the silk and pollen shed dates were conserved. In maize, flowering time is reportedly hastened by both photoperiod and temperature (Bechoux et al. 2000; Craufurd and Wheeler 2009). It is important to note that in this study the average daily temperature (ADT) increased as photoperiod increased (see Materials and Methods). The difference between the lowest (25.8 °C) and highest (28.4 °C) average daily temperatures were less than 3 °C, yet it is still worth considering as a factor that could speed up the response of the flowering traits (Craufurd and Wheeler 2009). With this study, however, we did not observe a negative impact on ASI due to longer photoperiods, which gives us more confidence to use daylength extension to increase DLI without negatively altering the ASI. For Inbred A plants, tassel height increased as the reproductive photoperiod increased from 16 to 20 h but was suppressed at 22 h. Leaf damage has been documented when the plants have been exposed to continuous (24-h) lighting (Eddy and Hahn 2010). It is possible that a 22-h daylength is too long for the maize plant and may explain the severe deformity of the leaves to be caused by calcium deficiency symptoms (Eddy and Hahn 2010). During the domestication of maize, adaptation to longer daylengths was required to move the species from tropical to temperate regions of the Americas (Goodman and Galliant 2008; Gouesnard et al. 2002; Matsuoka et al. 2002; van Heerwaarden et al. 2011; Xu et al. 2012).
Tropical maize is taller with more leaves and later flowering when daylengths are >13 h compared with temperate maize (Warrington and Kanemasu 1983). Our research found that Inbreds A and B were not as susceptible to longer photoperiods during reproductive growth. With photoperiod playing a role in plant quality to varying degrees depending on genetics, it is important to quantify the effects of photoperiod on different genetic backgrounds.
Conclusions
Maize plants negatively respond to low DLI stress in several ways. As the number of days of low DLI stress increased, tassel height was suppressed for Inbred B, producing the shortest tassel under 7 d of low DLI stress (Table 5). The growth and development of Inbreds A and B under limited light conditions demonstrates that genetics also influences the responses. Both inbred lines responded with longer barren tassel lengths (Table 3). Tassel barrenness of Inbred A is most susceptible to low DLI stress during V7, whereas Inbred B is most susceptible during V6. This supports our hypothesis that the timing of the low DLI stress is critical to tassel quality. As barren tassel length increases the pollen potential of the plant decreases. The lack of anthers present would lead to a decrease in pollen available for pollination. The kernel yield for both farmers and breeders would suffer if relying only on the pollen from the barren tassel, such is the case with breeding programs crossing two genetic lines. This research demonstrates the importance of low DLI stress on plant quality, and more specifically tassel morphology. With only 7 d of low DLI stress, the plant loses yield potential. In production growing settings, low DLI stress is not conveniently limited to 7 d, but rather a cyclic stress which could amplify the negative impact of the low DLI stress. To better alleviate this problem, environmental conditions must be optimized for each growing season, such as low light winter months.
Our work demonstrated that Inbreds A and B are susceptible to longer photoperiods during vegetative growth stages, but less so during reproductive growth stages. Caution must be used when increasing photoperiod above 20 h during reproductive growth stages due to the negative plant responses such as a reduction in ear leaf width and area index, along with a shorter tassel for Inbred A. Increasing lamp operation time to increase the DLI in greenhouses can be used during light-limiting seasons in northern climates, but the developmental growth stages should be taken into consideration to limit negative responses to a longer daylength.
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