After one year, the total loss of carbon would be 26.1kg, taking into consideration 52 cycles of regeneration. In the end of the year, there will still be 38.5kg carbon to maintain high adsorption efficiency. So the activated carbon produced from wheat straw is enough to control NOx emission. In fact, the activated carbon can be regenerated many times with the same material, while adsorption tends to improve rather than degrade . Low-N stress is among the major abiotic stresses causing yield reductions in maize grown in the tropics. The access to mineral fertilizers is very unequally distributed among the world’s countries and particularly limited in sub-Saharan Africa. The cultivation of N-efficient cultivars with improved grain yield under low-N conditions could help to alleviate the problem . The breeding process of N-efficient cultivars is more efficient when the selection is performed under the low-N target conditions . However, with decreasing soil fertility the environmental variability increases and thus heritability for grain yield declines. Therefore, secondary plant traits related to N efficiency could be used as selection traits for N efficiency, since these traits are less prone to environmental variability. The main objective of the presented study was the evaluation of N deficiency-induced leaf senescence at the seedling stage as such a trait. In field experiments performed in co-operation with CIMMYT, 16 contrasting maize cultivars were tested for N efficiency and the underlying mechanisms for grain yield formation . N-efficient cultivars were found to have a high N uptake and dry matter accumulation after anthesis,grow raspberries in a pot while N uptake and dry matter production until anthesis were not decisive for N efficiency. Further characteristics of N-efficient cultivars were delayed leaf senescence , a high harvest index and high kernel numbers .
N uptake and dry matter production after anthesis have frequently been found to be decisive for grain yield in cultivar comparisons both under low and high N supply . In almost all these cases, cultivars with an improved performance during reproductive growth were also characterized by delayed leaf senescence. The causal relationships between the different traits are not yet clear. Genotypic differences in delayed leaf senescence might improve dry matter production after anthesis and thus increase harvest index and yield. It may also affect N uptake, due to an enhanced C supply to the roots. This view implies a key role for leaf senescence. However, delayed leaf senescence may also be merely a symptom of increased N uptake. To unravel the relationships between leaf senescence and the other traits decisive for N efficiency, correlation coefficients between the traits were calculated . Leaf senescence score 28 days after anthesis, with a high score representing a high ratio of senescent leaves on the plants, was negatively related to N efficiency in all investigated environments. Close relationships between leaf senescence score and dry matter accumulation and N uptake after anthesis, however, were found only in one of the experiments . This finding only partially support the above described assumption that delayed leaf senescence causes improved reproductive growth and N uptake. Surprisingly, leaf senescence score correlated with kernel numbers and harvest index suggesting that leaf senescence changes the pattern of N remobilization to the kernels. Thus, although delayed leaf senescence appears to be a decisive part of N efficiency and is suited as a selection trait for N efficiency, its physiological action remains to be elucidated. To test if leaf senescence might be a suitable selection trait for N efficiency also in short-term experiments, the same 16 tropical maize cultivars that were used for the field studies were grown in hydroponics .
The progression of leaf senescence was monitored by photosynthesis and leaf chlorophyll measurements that were estimated by SPAD values. Cultivars differed both in SPAD values and photosynthesis rates of old leaves during N deprivation. Photosynthesis rate during leaf senescence proved to be a better indicator for N efficiency in this study than leaf chlorophyll content. Significant negative correlations were found between SPAD values, photosynthesis rates in the nutrient-solution experiment and leaf senescence scores in the field experiments, and positive correlations were found between photosynthesis rates and grain yield under low-N conditions in the field. The data suggests that the assessment of the capacity of a genotype to maintain a higher photosynthetic capacity of old leaves during N deficiency-induced senescence at the seedling stage may be suited as a selection parameter for N efficiency. However, photosynthesis rate during leaf senescence could explain only up to 20 % of the cultivar differences in N efficiency, while leaf senescence in the field experiments could explain 47 % . Enzymes within the chloroplast stroma are degraded early during leaf senescence which could be responsible for the decline in photosynthesis rate . Plant and leafN status at the beginning of the N deficiency period might influence the onset of leaf senescence. Plant-N status is determined by N uptake during early vegetative growth and depends on N supply during that period. An efficient root-N uptake rate during the N depletion period will prolong the N supply to the leaves. Apart from improving leaf-N status, this also increases cytokinin production of the roots , which will also delay leaf senescence . The leaf-senescence rate might also be influenced by the rate of N export from the leaf. The amount of N exported depends upon the breakdown of N compounds within the leaf and thus protease activity, but might also be influenced by sink strength. These findings raise the question A dissection of N import and N export from the senescing leaf during the N-deficiency period was performed by 15N labelling.
Although there were only small net changes in leaf-N content during the N-deficiency period at N1, considerable N amounts were exported from and imported into the leaf during this time span . Leaf-N contents before the onset of leaf senescence were more than two times higher at high compared to low-N supply. The amount of N exported during N deprivation was nearly four times higher at high N compared to low N. These results suggest that N export was mainly governed by N availability in the leaf. Cultivar differences in leaf-N content prior to leaf senescence had no impact on leaf-N content during leaf senescence . Unexpectedly, N import represented a quantitatively not negligible part of total leaf-N even during leaf senescence, and cultivar differences in N import were also important for differences in total leaf-N during leaf senescence. Since N import was not related to total plant N uptake , it was probably governed by leaf-inherent factors. Some observations made by quantifying leaf and plant-N flows during N deficiency-induced leaf senescence were unexpected. First, photosynthesis rate decreased earlier and stronger than leaf-N content . This could be due to the degradation of N-containing enzymes within the chloroplast stroma . Alternatively, the declining photosynthesis rate induced leaf senescence and consequently N remobilisation from the leaf . Our results suggest that the decrease in photosynthesis rate might have been caused by a negative feedback regulation due to an accumulation of C assimilates in the leaves,best grow pots since specific leaf weight increased during N deprivation . Leaf-area growth and thus shoot growth is strongly decreased by N deficiency mediated via cytokinins produced in the roots . A poor leaf growth will lead to a low carbohydrate demand and a low phloem-sap flow from matured leaves to growing leaves. This will also affect N retranslocation, since it could be shown that a low phloem-sap flow also decreases amino acid translocation . Thus, N retranslocation from senescing leaves under N deficiency might be delayed due to a low sink-N demand. N import might have played a decisive role for the induction of leaf senescence, since nitrate influx regulates the induction of leaf senescence . In this study, N import was probably governed by leaf-inherent factors instead of reflecting total plant-N uptake. Nitrate-N enters the leaf by the transpiration stream. Therefore, a decrease in stomatal conductance affects N import. This might be due to the decreased photosynthesis rate or mediated by abcisic acid , which is known to induce stomatal closure.
Indeed, differences in ABA contents have been found in senescing leaves of an early-senescing and a stay-green phenotype of maize . The possible carbon and nitrogen flows in the plants which might influence leaf senescence of vegetative plants are summarized in Figure 2. Photosynthesis rates and leaf-N contents of plants pre-cultured at low or high N supply were significantly related to leaf senescence scores at anthesis of the same cultivars grown at low-N stress in the field . However, only photosynthesis rates during leaf senescence of plant pre-cultured at low N supply reflected leaf senescence score during reproductive growth and N efficiency in the field experiments. Therefore, cultivar differences in leaf senescence during reproductive growth can only partly be reproduced in a short-term nutrient-solution experiment. Several differences between vegetative and reproductive growth might influence the induction and development of leaf senescence: first, although leaf senescence might be induced by N shortage both in hydroponics and under field conditions, the timing of N shortage is dependent upon different factors. In the field, the exploration of N sources in deeper soil layers might play the most important role for N uptake during reproductive growth . Thus in the field, root growth and morphology are the most important plant traits, which play only a minor role for N uptake in hydroponics. Secondly, source-sink relationships differ distinctly between vegetative and reproductive growth, both for carbohydrates and as a consequence also for N. The changes in assimilate flows might influence the development of leaf senescence, or at least the parameters used to characterize leaf senescence. However, the fact that photosynthesis rate during late stages of leaf senescence was significantly correlated to leaf senescence in the field experiments and to grain yield at limiting N supply suggests that cultivar differences in specific steps of leaf senescence related to the breakdown of the photosynthetic apparatus contribute to N efficiency in the field. While occupational exposure and tobacco products are associated with a high risk of Cd poisoning, consumption of contaminated plant-based foods represents the major source of Cd exposure in the general public . Many cases of widespread cadmium poisonings have been attributed to consumption of contaminated seeds in Thailand, China, Japan, and Australia . However, the molecular mechanisms and genes mediating the loading of both essential and nonessential heavy metals into seeds remain largely unknown. Metal accumulation and distribution in plants consist of several mechanisms, including: metal uptake into roots, xylem-loading and transport to the shoot, and phloem-mediated redistribution of metals from mature leaves to sink tissues, including younger leaves, roots, and seeds . Cadmium enters the root through the Fe transporter IRT1, which shows broad substrate specificity towards divalent metals including Fe2+, Zn2+, Mn2+, and Cd2+ . Once inside the cell, metals bind to different ligands, according to specific affinities, and these metal–ligand complexes can be stored in different cellular compartments or distributed to other tissues through the vasculature . Because of the broad substrate specificity of IRT1 for divalent metals, transcriptional regulation of the Fe-deficiency response, including up-regulation of IRT1, will also have an impact on the uptake of non-essential heavy metals such as Cd. In plants, the root iron-deficiency response is regulated by local signals within the root and also by systemic signals originating from leaves . Two major transcriptional networks have been identified to mediate the Fe-deficiency response at the root level in Arabidopsis: the FIT network and the POPEYE network . The components of the systemic shoot to-root Fe signaling on the other hand remain largely unknown. The identification of mutants showing a constitutive Fe-deficiency response even when Fe is supplied in sufficient amounts plus experiments where the constitutive root response is restored by foliar application of Fe suggest that mobile Fe is required for proper shoot-to-root signaling . However, the transporters, ligands, and the chemical speciation of the putative phloem-mobile molecule mediating the systemic Fe signaling have not yet been clearly identified. Here, we report that opt3-2, an Arabidopsis mutant carrying an insertion in the 5’ UTR of the oligopeptide transporter gene OPT3 , over-accumulates significant levels of Cd in seeds.