High correlations are often observed between yield and plant seed number. These correlations are frequently interpreted as indicating a causal relationship between plant yield and seed number. It is assumed to increase crop yield, a greater number of seeds need to be generated by the plant. In fact, close examination of seed set and yield indicate that both are closely linked to resource availability in the plant, whether carbohydrate or nutrients. The dependence of both seed set and yield on resource input explains the correlation between the two variables without the existence of a causal linkage. Further, the dependence of yield on resource availability in the plant can be achieved by trade-offs between seed number and average plant seed mass to achieve seed yield. This trade-off has been labeled as component compensation. The ‘packaging’ of seed yield whether achieved by seed number or seed mass generally does not determine crop yield.

Skepticism is a philosophy that traces to the ancient Greeks. It is based on the view that explanations of events are evaluated after collecting all relevant evidence and carefully considering all possibilities. Even then, the explanation should be considered only tentative. This, of course, is fully consistent with the role of hypotheses and their evaluation in science. Hypotheses are tentative explanations and ultimately cannot be proven, but only disproven. Yet, hypotheses are often readily elevated to an accepted descriptive status. A failure to subject hypotheses to critical evaluation—that is, skeptical analysis—seems to be a quirk of the human brain to accept readily available explanations that at least superficially explain the evidence. The skeptic is required to consider all relevant evidence from several viewpoints to judge the relevant merit of competing explanations.

This book serves as a reminder to crop scientists and others that open, clear-minded assessments of the entirety of evidence concerning a hypothesis.

Unfortunately, unconfirmed field observations (UFOs) have become a part of crop science research. One source of UFOs are winning crop yield reports from farmer yield contests. Some of these reported yields are simply much more than is possible for the solar energy input to the crop. Another source of UFOs are attempts to evaluate the yield benefit gained from a genetic transformation. A genotype that is readily suited for transformation is selected, transformed lines are developed, expression of the gene is documented, and plants are grown to test for yield change. These tests can be in controlled environments, which are unlike field conditions in many ways. Some tests are actually done in the field but the field plots ignore long-established protocol to obtain acceptable yield data. For example, bordering on all four sides of a quadrate harvested for yield might be inadequate, or even nonexistent. Further, yield results for a transformation must be tested against the yield of high-yielding commercial cultivars, not the usually weak genotype that was originally transformed. Additional considerations are the number of replications, the size of the plots, and inclusion of a sufficient number of ‘environments’ to reflect the environmental variability to which a commercial product might be eventually subjected.

Increasing crop nitrogen use efficiency appears to be an opportunity to increase crop productivity. However, a close examination of the components of nitrogen use efficiency indicates that plant modification for improving nitrogen use efficiency are quite limited. The uptake of nitrogen is highly regulated so that it aligns with the capacity of the plant to metabolize and store the accumulated nitrogen. Also, modern, commercial crop plants already partition 80% or more of the total nitrogen accumulated by the plant to the seeds. The major opportunity for increasing nitrogen use efficiency is in the soil where losses can be quite high due to soil runoff, leaching through the soil, and denitrification due to soil microbial activity. Often less than 40% of applied nitrogen fertilizer is currently recovered by crops so that major improvements in nitrogen use efficiency seem possible with improvements in fertilizer technology and management.

‘More crop per drop’ has become a popular slogan for the goal of increasing crop water use efficiency (WUE). One common expression for transpiration rate is based on a phenomenological expression that includes a WUE term. Unfortunately, the WUE term in this expression is not derived from mechanistic process and does not give guidance on priority variables to increase crop water use efficiency. To make progress toward of increased crop water use efficiency, it is necessary to have a fundamental, mechanistic understanding of the ratio between plant growth and transpiration rate. A derivation that helps in this understanding is based on basic descriptions of leaf gas exchange and then up-scaled to canopy water loss and crop yield. The derivation showed the importance in understanding temporal dynamics in water loss and the direct impact of vapor pressure deficit (VPD) to which a crop is exposed. Genetic and management approaches to minimize the impact of high VPD on crop transpiration rate, particularly early in the growing season, offer opportunities to conserve soil water and sustain crop physiological activity later in the growing season during the reproductive stages. The conservation of the soil water was proposed to likely result in yield increase.

Atmospheric CO(2) enrichment generally stimulates plant photosynthesis and nutrient uptake, modifying the local and global cycling of bioactive elements. Although nutrient cations affect the long-term productivity and carbon balance of terrestrial ecosystems, little is known about the effect of CO(2) enrichment on cation availability in soil. In this study, we present evidence for a novel mechanism of CO(2)-enhancement of cation release from soil in rice agricultural systems. Elevated CO(2) increased organic C allocation belowground and net H(+) excretion from roots, and stimulated root and microbial respiration, reducing soil redox potential and increasing Fe(2+) and Mn(2+) in soil solutions. Increased H(+), Fe(2+), and Mn(2+) promoted Ca(2+) and Mg(2+) release from soil cation exchange sites. These results indicate that over the short term, elevated CO(2) may stimulate cation release from soil and enhance plant growth. Over the long-term, however, CO(2)-induced cation release may facilitate cation losses and soil acidification, negatively feeding back to the productivity of terrestrial ecosystems.

Predicting future plant and ecosystem responses to elevated CO(2) also requires an understanding of the role of other factors, especially soil nitrogen. This is particularly challenging for global aridlands where total N and the relative amounts of nitrate and ammonia vary both spatially and seasonally. We measured gas exchange and primary and secondary C metabolites in seedlings of two dominant aridland shrub species (Prosopis flexuosa [S America] and P. glandulosa [N America]) grown at ambient (350 ppm) or elevated (650 ppm) CO(2) and nitrogen at two levels (low [0.8 mM] and high [8.0 mM]) and at either 1 : 1 or 3 : 1 nitrate to ammonia. Whereas elevated CO(2) increased assimilation rate, water use efficiency, and primary carbon metabolites in both species, these increases were strongly contingent upon nitrogen availability. Elevated CO(2) did not increase secondary metabolites (i.e., phenolics). For these important aridland species, the effects of elevated CO(2) are strongly influenced by nitrogen availability and to a lesser extent by the relative amounts of nitrate and ammonia supplied, which underscores the importance of both the amount and chemical composition of soil nitrogen in mediating the potential responses of seedling growth and establishment of aridland plants under future CO(2)-enriched atmospheres.

Mycorrhizas are ubiquitous plant-fungus mutualists in terrestrial ecosystems and play important roles in plant resource capture and nutrient cycling. Sporadic evidence suggests that anthropogenic nitrogen (N) input may impact the development and the functioning of arbuscular mycorrhizal (AM) fungi, potentially altering host plant growth and soil carbon (C) dynamics. In this study, we examined how mineral N inputs affected mycorrhizal mediation of plant N acquisition and residue decomposition in a microcosm system. Each microcosm unit was separated into HOST and TEST compartments by a replaceable mesh screen that either prevented or allowed AM fungal hyphae but not plant roots to grow into the TEST compartments. Wild oat (Avena fatua L.) was planted in the HOST compartments that had been inoculated with either a single species of AM fungus, Glomus etunicatum, or a mixture of AM fungi including G. etunicatum. Mycorrhizal contributions to plant N acquisition and residue decomposition were directly assessed by introducing a mineral N-15 tracer and C-13-rich residues of a C-4 plant to the TEST compartments. Results from N-15 tracer measurements showed that AM fungal hyphae directly transported N from the TEST soil to the host plant. Compared with the control with no penetration of AM fungal hyphae, AM hyphal penetration led to a 125% increase in biomass N-15 of host plants and a 20% reduction in extractable inorganic N in the TEST soil. Mineral N inputs to the HOST compartments (equivalent to 5.0 g N m(-2) yr(-1)) increased oat biomass and total root length colonized by mycorrhizal fungi by 189% and 285%, respectively, as compared with the no-N control. Mineral N inputs to the HOST plants also reduced extractable inorganic N and particulate residue C proportion by 58% and 12%, respectively, in the corresponding TEST soils as compared to the no-N control, by stimulating AM fungal growth and activities. The species mixture of mycorrhizal fungi was more effective in facilitating N transport and residue decomposition than the single AM species. These findings indicate that low-level mineral N inputs may significantly enhance nutrient cycling and plant resource capture in terrestrial ecosystems via stimulation of root growth, mycorrhizal functioning, and residue decomposition. The long-term effects of these observed alterations on soil C dynamics remain to be investigated.

An increasing amount of evidence indicates that N can be transferred between plants. Nonetheless, a number of fundamental questions remain. A series of experiments was initiated in the field to examine N transfer between N 2 -fixing soybean ( Glycine max [L.] Merr.) varieties and a non-nodulating soybean, and between N 2 -fixing peanut ( Arachis hypogaea L.) or soybean and neighboring weed species. The experiments were conducted in soils with low N fertilities and used differences in N accumulation and/or 15 N natural abundance to estimate N transfer. Mixtures of N 2 -fixing and non-nod soybean indicated that substantial inter-plant N transfer occurred. Amounts were variable, ranging from negligible levels to 48% of the N found in the non-nod at maturity. Transfer did not appear to strongly penalize the N 2 -fixing donor plants. But, in cases where high amounts of N were transferred, N content of donors was noticeably lowered. Differences were evident in the amount of N transferred from different N 2 -fixing donor genotypes. Results of experiments with N 2 -fixing crops and the weed species prickly sida ( Sida spinosa L.) and sicklepod ( Senna obtusifolia [L.] Irwin & Barneby) also indicated substantial N transfer occurred over a 60-day period, with amounts accounting for 30–80% of the N present in the weeds. Transfer of N, however, was generally very low in weed species that are known to be non-hosts for arbuscular mycorrhizae (yellow nutsedge, Cyperus esculentus L. and Palmer amaranth, Amaranthus palmeri [S.] Watson). The results are consistent with the view that N transfer occurs primarily through mycorrhizal hyphal networks, and they reveal that N transfer may be a contributing factor to weed problems in N 2 -fixing crops in low N fertility conditions.

In low fertility conditions, germinating seedlings are heavily dependent upon N from seed reserves for growth and development. Experiments were conducted to examine the influence of seed N content and mobilization on leaf development and the ability to take up nitrate and up-regulate leaf growth processes during recovery from the period of N deprivation. Genetically homozygous soybean lines with a range of seed N contents were grown in hydroponics culture without an external N source. Microscopic analyses of the apical meristem and plant exposure to labeled 15N-nitrate allowed precise descriptions of changes in leaf initiation and nitrate uptake during recovery. In seedlings grown without N, inhibitions of leaf expansion and leaf initiation were detectible in the 2nd week after germination, and both were inversely related to the amount of N released from the seed. The S/R ratio decreased, indicating early N stress, but the S/R adjustments were not proportional to the degree of N stress as occurs with older plants, suggesting limitations in C availability. When external N was supplied to plants after 15 or 23 days of N deprivation, the induction period for nitrate uptake was longer than that normally observed with N-replete seedlings, and rates of growth during recovery from the N stress were correlated with initial seed N contents. The inhibition of individual leaf expansion was released only if leaves were at an early developmental stage. Leaf initiation at the apical meristem immediately responded to the presence of external nitrate, with initiation rates approaching those for +N controls. The results indicate that seedling vigor in infertile conditions and the ability to respond to available N are strongly influenced by seed resources.

ABSTRACTEvidence from past studies suggests that loblolly pine may be tolerant of Al. The experiments described in this manuscript were initiated to examine Al tolerance and Al accumulation in the pine root and the degree of Al accumulation in fungal hyphae when pine roots were colonized with the ectomycorrhiza Pisolithus tinctorius. The experiments used lumogallion staining and confocal microscopy to localize Al in root and fungal structures. The results clearly showed that loblolly pine seedlings were highly resistant to Al. A decrease in primary root extension could not be detected until Al+3 activities approached 40 µmol L−1, and extension was suppressed only 30% at an Al+3 activity of 580 µmol L−1. This contrasted with the response of the Al‐sensitive ‘check’ species soybean, where primary root extension was severely restricted at Al+3 activities lower than 5 µmol L−1. Tissue Al measurements and lumogallion fluorescence of longitudinal sections of the pine root tip indicated that tolerance was associated with both Al exclusion from the tip region and compartmentalization of absorbed Al in peripheral cell areas outside of the meristem. In lateral roots colonized with ectomycorrhizae, lumogallion fluorescence showed that large amounts of Al accumulated at the fungal mantle and in areas with the Hartig net. At higher magnification, lumogallion indicated substantial Al accumulation inside hyphae. Little Al could be detected in lateral root cells. The results show that pine possesses multiple mechanisms that can contribute to Al tolerance in acid field soils.

Nutrient movement from turfgrass systems into surface and ground water is a public concern. Data indicate that actively growing turf rapidly immobilizes applied N, thus restricting nutrient movement. It is possible, however, that growth suppression with plant growth regulators (PGRs) could reduce N demand and thus N uptake, resulting in greater leaching losses. An experiment was conducted with column lysimeters to investigate the effects of trinexapac‐ethyl (TE) on nitrate leaching and N‐use efficiency in Tifway bermudagrass (Cynodon dactylon × C. transvaalensis). The experiment was conducted in a growth chamber with day/night temperature set at 29/24°C and a 12‐h photoperiod. Trinexapac‐ethyl was applied twice at 4‐wk intervals at 0.11 kg a.i. ha−1 Ammonium nitrate (AN) was applied at 50 kg N ha−1 2 wk after each TE application, and again 6 wk after the second TE application. Separate sets of columns received 15N‐labeled AN for the first two applications. Irrigation was scheduled to provide a leaching fraction of ≈50%; leachate was collected after each irrigation and analyzed for nitrate and ammonium. Cumulative nitrate leaching was unaffected by TE after the first two N applications, but was reduced ≈60% by TE following the third N application. Trinexapac‐ethyl reduced 15N allocation to clippings by ≈25% and increased 15N allocation to roots and rhizomes; total recovery of applied 15N in tissues was ≈65%. Results demonstrate chemical growth suppression with TE does not reduce N uptake or increase nitrate leaching from bermudagrass.

The effect of differences in nitrogen (N) availability and source on growth and nitrogen metabolism at different atmospheric CO(2) concentrations in Prosopis glandulosa and Prosopis flexuosa (native to semiarid regions of North and South America, respectively) was examined. Total biomass, allocation, N uptake, and metabolites (e.g., free NO(3)(-), soluble proteins, organic acids) were measured in seedlings grown in controlled environment chambers for 48 d at ambient (350 ppm) and elevated (650 ppm) CO(2) and fertilized with high (8.0 mmol/L) or low (0.8 mmol/L) N (N(level)), supplied at either 1 : 1 or 3 : 1 NO(3)(-) : NH(4)(+) ratios (N(source)). Responses to elevated CO(2) depended on both N(level) and N(source), with the largest effects evident at high N(level). A high NO(3)(-) : NH(4)(+) ratio stimulated growth responses to elevated CO(2) in both species when N was limiting and increased the responses of P. flexuosa at high N(level). Significant differences in N uptake and metabolites were found between species. Seedlings of both species are highly responsive to N availability and will benefit from increases in CO(2), provided that a high proportion of NO(3)- to NH(4)-N is present in the soil solution. This enhancement, in combination with responses that increase N acquisition and increases in water use efficiency typically found at elevated CO(2), may indicate that these semiarid species will be better able to cope with both nutrient and water deficits as CO(2) levels rise.

In plants experiencing phosphorus (P) stress, ATP concentrations can be reduced significantly and shoot growth is strongly restricted, raising the possibility that energy availability is responsible for the growth inhibition. Experiments were conducted to investigate the relationship between P deprivation and energy availability in tissues involved in the growth response. Young soybean (Glycine max [L.] Merr. cv. Ransom) plants were deprived of P for 32 days. Leaf initiation and individual leaf expansion were followed along with localized P and ATP concentrations. Tissue analyses revealed preferential distribution of P to the root, which accompanied a decline in the shoot to root dry weight ratio. Even though P concentrations in all shoot tissues dropped sharply, ATP concentrations and energy charge in the shoot meristem region were maintained similar to controls for an extended period when leaf initiation slowed. In the first trifoliolate leaf, ATP and energy charge remained at control levels during the expansion phase, but expansion was inhibited by 50%. Furthermore, ATP levels in root tips were decreased almost 30%, yet growth of the root system was equal to or greater than the control. The absence of a positive correlation between ATP levels and growth responses in the different tissues suggests that energy availability is not a primary factor limiting growth under P stress conditions. The results, along with others from previous experiments, support the notion that a signaling mechanism, as yet unidentified, controls down regulation of cell division in shoot growth regions.

Experiments were conducted in environmental chambers to the evaluate effects of photoperiod and temperature on Florida betony growth and development. Plants were exposed to two photoperiods, short day (9 h) and long day (9 + 3 h night interruption), and three day/night temperature regimes, 18/14, 22/18, and 26/22 C. After 10 wk of growth, shoot length and weight were 3.4 and 3.5 times greater, respectively, in the long-day photoperiod and with the 26 and 22 than with the 22 and 18 C day and night temperature regime, respectively. Shoot number, however, was greatest in the short-day photoperiod and at a lower temperature of 22/18 C. Shoot number in long day 22/18 C and 26/22 C environments increased asymptotically. No difference in root weight was observed between long- and short-day environments, but root weight increased with increasing temperature. Flowering and tuber production only occurred in long-day environments, with greater production of both at higher temperatures. Results provide a general framework for understanding Florida betony growth and development characteristics in the field and provide insights that should be considered in developing control strategies.