<b>Improving Phosphorus Use Efficiency and Modeling of Uptake in Corn</b>

<p dir="ltr">Quantifying root length, surface area, average diameter, and volume of fully-matured corn (<i>Zea mays </i>L.) is labor intensive, time consuming, and costly. Accurate and efficient subsampling techniques are needed to overcome these limitations. In this study, eight corn root systems were grown to maturity in a sand-culture hydroponics system to develop and test root system subsampling techniques for accuracy (uncertainty assessment) and efficiency (time). Each entire root system was separated into coarse and fine roots which were then composited into 65 subsamples, either visually or by mass, followed by subsample scanning to quantify root characteristics. A bootstrap non-parametric procedure was used to determine the sample size needed to represent the total root system and quantify the uncertainty based on the number of subsamples analyzed. When subsamples were composited visually, as many as 60 subsamples (92% of the total root system) were necessary to represent the characteristics of the root system within +/- 5% of the true mean at a 95% confidence level. In contrast, when subsamples were composited by equal mass, a maximum of 15 subsamples (23% of the total root system) were needed to be representative, requiring 2 ¼ h per root system. The findings show that separating the entire root system by coarse and fine roots and then weighing into equal mass subsamples before scanning decreased the number of subsamples and time required to accurately estimate corn root characteristics. Thus, this subsampling approach considerably reduced the effort and cost of processing corn root systems.</p><p dir="ltr">The high demands of corn (<i>Zea mays</i> L.) grain production coupled with water quality goals and phosphorus (P) conservation pose a grand challenge to farmers and society, and necessitate improved P utilization efficiency (PU<i>t</i>E: grain yield per mass total P (TP) content). The objective of this study was to evaluate PU<i>t</i>E among six Pioneer corn hybrids released over a span of 75 years. Corn was grown in a sand-culture hydroponics system that eliminated confounding plant–soil interactions, root architecture, and allowed for precise control of nutrient availability. Four P concentration levels (4, 7, 10, and 12 mg P L^-1) were applied to hybrids released in 1936, 1942, 1946, 1952, 2008, and 2011. Nutrients other than P were applied at sufficient levels. Shoots and roots were harvested at maturity (R6) and biomass and P concentration determined. Results showed that total biomass did not differ among hybrids, but partitioning of biomass varied with hybrid. Grain yield varied between hybrids, but there was no trend with year of release. Grain P content was negatively correlated with stem P content (R²=0.91). PU<i>t</i>E differed between the most recently released hybrids (2008 and 2011) whereas older hybrids had intermediate and similar PU<i>t</i>E. Grain yield was not solely determined by TP in the plant, but was strongly influenced by biomass and P partitioning, which was manifested as relative differences in PU<i>t</i>E between hybrids. Findings highlight the critical role of the source-sink relationship in determining PU<i>t</i>E and grain yield.</p><p dir="ltr">Abstract: Phosphorus (P) is critical for maximizing agricultural production and represents an appreciable input cost. Geologic sources of P that are most easily mined are a finite resource, while P transported from agricultural land to surface waters contributes to water quality degradation. Improved knowledge of P timing needs by corn (maize) can help inform management decisions that increase P use efficiency, which is beneficial to productivity, economics, and environmental quality. The objective of this study was to evaluate P application timing on the growth and yield components of corn. Corn was grown in a sand-culture hydroponics system that eliminated confounding plant–soil interactions and allowed for precise control of nutrient availability and timing. All nutrients were applied via drip irrigation and were therefore 100% bioavailable. Eight P timing treatments were tested using “low” (L) and “sufficient” (S) P concentrations. In each of the three growth phases, solution P application levels were changed or maintained, resulting in eight possible combinations, LLL, LLS, LSL, LSS, SLL, SSL, SLS, and SSS, where the first, second, and third letters indicate P solution application levels from planting to V6, V6 to R1, and R1 to R6, respectively. All other nutrients were applied at sufficient levels. Sacrificial samples were harvested at V6, R1, and R6 and evaluated for various yield parameters. Plants that received sufficient P between V6 and R1 produced a significantly higher grain yield than plants that received low P between V6 and R1 regardless of the level of P supply before V6 or after R1. The grain yield of plants that received sufficient P only between V6 and R1 did not differ significantly from plants that received only sufficient P (SSS), due to (1) a greater ear P concentration at R1; (2) an efficient remobilization of assimilates from the stem and leaf to grains between R1 and R6 (source–sink relationship); (3) a higher kernel/grain weight; and (4) less investment into root biomass.</p><p dir="ltr">Phosphorus (P) is a vital nutrient for corn (<i>Zea mays</i> L.) growth and yield, influencing the uptake and availability of other macro- and micronutrients through synergistic and antagonistic interactions. Understanding the critical timing for P availability can improve nutrient use efficiency in corn production. The objective of this study was to evaluate P application timing on the dynamics of macro- and micronutrients concentration and contents in corn. Corn was grown in a sand-culture hydroponics system that eliminated confounding plant–soil interactions and allowed for precise control of nutrient availability and timing. All nutrients were applied via drip irrigation and were therefore 100% bioavailable. Eight P timing treatments were tested using “low” (L) and “sufficient” (S) P concentrations. In each of the three growth phases, solution P application levels were changed or maintained, resulting in eight possible combinations, LLL, LLS, LSL, LSS, SLL, SSL, SLS, and SSS, where the first, second, and third letters indicate P solution application levels from planting to V6, V6 to R1, and R1 to R6, respectively. All other nutrients were applied at sufficient levels. Plants were sacrificially harvested at V6, R1, and R6 and evaluated for biomass and nutrient concentration. At V6, sufficient P increased shoot macronutrient concentrations, while micronutrients remained unchanged. At R1, macronutrient concentrations in leaves were similar across treatments, but stem and ear N concentrations were greater in LL and LS plants than in SL and SS plants. Total macro- and micronutrient contents in all plant parts were higher in SL and SS plants at V6 and R1. Despite lower grain N concentration in LSL plants, their overall nitrogen utilization efficiency (NUtE) were superior to SSS plants. These findings indicate that supplying sufficient P between V6 and R1 enhances nutrient remobilization and utilization efficiency without requiring continuous high solution P concentration throughout the growing season.</p><p dir="ltr">Phosphorus (P) influx kinetics such as <i>I</i>_max (maximum uptake rate) and K_m (P concentration where the uptake rate equals half of <i>I</i>_max) play an important role in P use efficiency. In addition, knowledge of I_max and K_mat various developmental stages of corn hybrids can be used in modelling P uptake and predicting optimal soil P. The objective of this study was to evaluate how I_max and K_m vary across growth stages and among different corn hybrids. Four corn hybrids, released in different years, were grown in sand culture hydroponics to various developmental stages (V4, V7, V12, R1, R3, R4, R5, and R6). After non-destructive harvesting, the roots of intact plants were washed free of sand and placed in solution to measure P uptake using the solution nutrient depletion method to determine <i>I</i>_max and K_m. The K_m and <i>I</i>_max (per-plant basis) was similar between corn hybrids. However, in comparing growth stages, values increased until reaching between V12 and R4, and remained relatively stable throughout the reproductive stages (R1–R6). When expressed per unit root surface area (RSA), <i>I</i>_max peaked at the initial measurements (V4 and V7), declined drastically until V12, and remained relatively stable between R1 and R6. <i>I</i>_max expressed per unit RSA was also similar across hybrids. These specific values will be used in developing P uptake models, which have previously assumed constant <i>I</i>_max and K_m. Also, these results demonstrated how P influx was partly dictated by root and shoot P concentration, rather than total biomass. Future corn genetic development should consider targeting P influx kinetics parameters for improving P use efficiency.</p>