Jon I. Lizaso

How do various maize crop models vary in their responses to climate change factors

Potential consequences of climate change on crop production can be studied using mechanistic crop simulation models. While a broad variety of maize simulation models exist, it is not known whether different models diverge on grain yield responses to changes in climatic factors, or whether they agree in their general trends related to phenology, growth, and yield. With the goal of analyzing the sensitivity of simulated yields to changes in temperature and atmospheric carbon dioxide concentrations [CO2 ], we present the largest maize crop model intercomparison to date, including 23 different models. These models were evaluated for four locations representing a wide range of maize production conditions in the world: Lusignan (France), Ames (USA), Rio Verde (Brazil) and Morogoro (Tanzania). While individual models differed considerably in absolute yield simulation at the four sites, an ensemble of a minimum number of models was able to simulate absolute yields accurately at the four sites even with low data for calibration, thus suggesting that using an ensemble of models has merit. Temperature increase had strong negative influence on modeled yield response of roughly -0.5 Mg ha(-1) per °C. Doubling [CO2 ] from 360 to 720 μmol mol(-1) increased grain yield by 7.5% on average across models and the sites. That would therefore make temperature the main factor altering maize yields at the end of this century. Furthermore, there was a large uncertainty in the yield response to [CO2 ] among models. Model responses to temperature and [CO2 ] did not differ whether models were simulated with low calibration information or, simulated with high level of calibration information.

Putting mechanisms into crop production models

Crop growth models dynamically simulate processes of C, N and water balance on daily or hourly time-steps to predict crop growth and development and at season-end, final yield. Their ability to integrate effects of genetics, environment and crop management have led to applications ranging from understanding gene function to predicting potential impacts of climate change. The history of crop models is reviewed briefly, and their level of mechanistic detail for assimilation and respiration, ranging from hourly leaf-to-canopy assimilation to daily radiation-use efficiency is discussed. Crop models have improved steadily over the past 30-40 years, but much work remains. Improvements are needed for the prediction of transpiration response to elevated CO₂ and high temperature effects on phenology and reproductive fertility, and simulation of root growth and nutrient uptake under stressful edaphic conditions. Mechanistic improvements are needed to better connect crop growth to genetics and to soil fertility, soil waterlogging and pest damage. Because crop models integrate multiple processes and consider impacts of environment and management, they have excellent potential for linking research from genomics and allied disciplines to crop responses at the field scale, thus providing a valuable tool for deciphering genotype by environment by management effects.

Enhancing the ability of CERES-Maize to compute light capture

Abstract Recently it has been proposed to use the relationship between average intercepted photosynthetically active radiation (IPAR) around silking and total number of seeds per plant as the basis to improve kernel number prediction in CERES-Maize. However, there has been no previous evaluation of the accuracy of IPAR predictions in the model. The objectives of this work were to evaluate CERES-Maize predictions of IPAR around silking by testing their components incident PAR, light extinction coefficient (k), and leaf area index (LAI), and to develop alternative methods to simulate PAR and k. Measured IPAR was averaged over a thermal time window of 250 before to 100 growing degree-days after silking and compared with model predicted IPAR averaged over the same thermal time window. Independent data sets were used to develop and test new relationships to predict fraction of PAR to solar radiation as a function of incident total solar radiation, and canopy extinction coefficient as a function of the crop phenological age. The new relationships were incorporated into CERES-Maize and the new IPAR predictions were compared with measured values. We found that the common assumption of predicting PAR as 50% of the solar radiation overestimated PAR for our conditions in Iowa, where a value of 43% worked better. The extinction coefficient changed with crop development, reaching a peak of 0.66 at silking, and being lower early and late in the season. The best IPAR predictions were obtained when the new procedures to convert solar radiation into PAR and estimate k were coupled with the original leaf area model of CERES-Maize.

EARLY FLOODING OF TWO CULTIVARS OF TROPICAL MAIZE. I. SHOOT AND ROOT GROWTH

Maize (Zea mays L.) crop growth and development are disrupted when the soil is subjected to transient flooding, especially if the stress occurs early during the vegetative phase. Our understanding however, is limited regarding the physiological basis of field tolerance to flooding. Closer examination of adapted tropical cultivars is also required, because limited-resource farmers find drainage systems unaffordable. In this study we examine the short-term damage and acclimation of two tropical varieties of maize, during a six-day period of soil flooding, and their recovery, early during the vegetative stage. Two Venezuelan varieties, one considered tolerant and the other susceptible to poor soil drainage, were planted in 10 kg pots, and were flooded at the seventh leaf-tip (V4) stage for six days, and their responses compared to corresponding non-flooded plants. Shoot biomass was reduced in both varieties by 20 to 50% because of flooding. The tolerant variety however, placed a larger proportion of biomass into the root system, both during the flooding and afterwards. Leaf area was reduced by 40% in both cultivars. Plants continued elongating their youngest adventitious roots when flooded. These roots developed increased porosity (aerenchyma) and only first order laterals. The tolerant cultivar produced 30% more aerenchyma than the susceptible one, but maintained lower rates of respiration. Older adventitious roots and seminal roots experienced extensive damage by flooding and did not recover 10 days after drainage. Flooding accelerated root development, by early activation of new nodes with adventitious roots, while shoot development was less affected. Upon drainage, root systems proliferated very rapidly, retaining the aerenchymatous tissue. Our results indicate that tolerance of maize plants to soil inundation was associated with moderate root respiration rates and abundant root aerenchyma.

CHANGES OF pH AND AVAILABLE PHOSPHORUS AND CALCIUM IN RHIZOSPHERE OF ALUMINUM-TOLERANT MAIZE GERMPLASM FERTILIZED WITH PHOSPHATE ROCK

Availability of phosphorus (P) and calcium (Ca) is a common constraint in tropical acidic soils limiting the potential of those soils for crop production. Under such conditions, phosphate rock (PR) could become a cheap source of P and Ca for crops. The dissolution of PR, however, is a slow process that has limited its use for annual crops requiring ready-available nutrients. This study was conducted to examine the effect of aluminum-tolerant maize genotypes on the rhizosphere pH and the consequent release of P and Ca from PR. Six aluminum-tolerant maize inbreds from CIMMYT and one from the local program of FONAIAP-CENIAP were used. Sources of P were Riecito PR (PR) and commercial triple super phosphate (TSP), applied at a dose of 400 mg kg1. Individual pregerminated seeds were placed in small glass boxes 16×14×1 Cm filled with 200 g of a plinthic paleustult soil, with pH of 4.4 and very low P and Ca. Where PR was applied, rhizosphere pH increased between 0.14 and 0.38 pH units as a consequence of the increase in Ca concentration from the dissolution of PR. Differences among genotypes in P and Ca uptake were associated with soil pH and characteristics of the root system, especially total root surface. Inbreds 4, 5, and 17 were more efficient using PR than using TSP, suggesting the possibility of breeding maize for an efficient use of field applied PR.

EARLY FLOODING OF TWO CULTIVARS OF TROPICAL MAIZE. II. NUTRITIONAL RESPONSES

The level of oxygen in soils affects the bio-availability of nutrients as well as the ability of root systems to uptake and transport water and mineral nutrients. However, efforts addressing management practices to reduce yield losses after transient flooding have had limited success. Since after-drainage nitrogen (N) fertilization has been proposed to mitigate crop damage, a closer examination of plant nutrient acquisition during this period is required. In this work, we compare the short-term changes in the tissue levels of macronutrients [N, phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg)] in two varieties of tropical maize differing in tolerance to poor soil drainage, after a six day period under water saturated conditions, early during the vegetative growth. Two Venezuelan varieties, one labeled as tolerant and the other as susceptible to limited soil drainage, were planted in 10-kg pots and flooded at the seventh-leaf-tip (V4) stage. Treatments included a post-drainage N fertilization. Plant responses were compared to corresponding non-flooded plants. Flooding the soil reduced concentrations of macronutrients in shoots, compared to well aerated plants. Calcium and Mg levels were also reduced in roots, whereas K concentrations increased. After a post-drainage recovery period, nutrient concentration in shoots of flooded plants were above those of non-flooded ones, due to higher uptake rates. The only exception was P, where reduced acquisition appears to limit plant recovery. A post-flood N-urea fertilization increased the concentrations of N, Ca and Mg in shoots, but failed to increase shoot growth after 15 days. Differences in the pattern of Ca accumulation suggested a possible role of Ca nutrition in the tolerance of maize to flooding.

Mycorrhiza Effect on Maize P Uptake from Phosphate Rock and Superphosphate

Low available phosphorus (P) is a serious constraint for crop production in acidic tropical soils. Economical yields in these environments require application of large amounts of costly nitrogen (N) and P fertilizers. Although phosphate rock (PR) has been proposed as a less expensive P source, the slow P release to the soil limits its use for annual crops. The objective of this work was to examine the effect of inoculating a nonsterile acidic soil with vesicular arbuscular mycorrhizal (VAM) Gigaspora margarita on PR dissolution and P uptake by aluminum (Al)–tolerant maize inbreds. Three maize inbreds from CIMMYT, at Cali, Colombia, ranked as Al‐tolerant and one local breed ranked as Al‐susceptible were seeded in 4‐kg pots filled with a soil of pH 4.1 and 2.5 mg kg−1 available P. Inoculants (Gigaspora margarita and indigenous VAM), P fertilizer (Riecito phosphate rock and triple superphosphate), and the four inbreds were arrainged in a factorial design (2 × 2 × 4) with four replications. Plants were harvested 35 days after seeding, and P was determined in shoots. Four 2.5‐cm‐diameter soil cores were obtained from each pot to determine root length (two cores), root colonization (one core), and available P (one core). The inoculation with Gigaspora margarita caused a reduction in root length but better root colonization, 55% increase in P uptake, and 27% increase in shoot growth. When PR was used as fertilizer, plant growth was reduced in both roots and shoots. However, when PR was used in the presence of Gigaspora margarita, inbreds had 13% longer roots and shoot growth was the same as shoots fertilized with triple superphosphate. Our data suggest that inbreds exhibit different abilities to acquire P from PR under the influence of Gigaspora margarita fungi.

USING THE NORMALIZED DIFFERENCE VEGETATION INDEX AND A CROP SIMULATION MODEL TO PREDICT SOIL SPATIAL VARIABILITY

Improved understanding and better techniques to measure spectral reflected radiation must translate into practical applications to better manage cropping systems. This study developed a simple procedure to predict the unknown underlying patterns of soil variation using an airborne multispectral image. A crop simulation model (CERES–Maize) was used to establish the “base line” effects of genotype and population on crop growth, and on the reflected radiation from maize (Zea mays L.) canopies. The normalized difference vegetation index (NDVI) was calculated from an aerial spectral image obtained at silking. The NDVI–based image analysis indicated the areas of the field supporting crop growth above or below the “base line” for each treatment, thereby revealing the spatial patterns of soil variability. The relative growth map obtained by NDVI analysis compared well with a relative growth map derived from field measurements of leaf area index at silking. The procedure offers potential for target–oriented soil and crop sampling for spatial models, site–specific management, and also identifying patterns of crop–limiting factors not related with the soil, such as pests or diseases.

Diverging importance of drought stress for maize and winter wheat in Europe

Understanding the drivers of yield levels under climate change is required to support adaptation planning and respond to changing production risks. This study uses an ensemble of crop models applied on a spatial grid to quantify the contributions of various climatic drivers to past yield variability in grain maize and winter wheat of European cropping systems (1984–2009) and drivers of climate change impacts to 2050. Results reveal that for the current genotypes and mix of irrigated and rainfed production, climate change would lead to yield losses for grain maize and gains for winter wheat. Across Europe, on average heat stress does not increase for either crop in rainfed systems, while drought stress intensifies for maize only. In low-yielding years, drought stress persists as the main driver of losses for both crops, with elevated CO2 offering no yield benefit in these years.Drivers of crop yield variability require quantification, and historical records can help in improving understanding. Here, Webber et al. report that drought stress will remain a key driver of yield losses in wheat and maize across Europe, and benefits from CO2 will be limited in low-yielding years.