| Wheat (Triticum aestivum L.) is one of the most essential food crops in the world. Lodging, the permanent displacement of stems from an upright position due to internal and external factors, is an important constraint limiting wheat yields and quality by affecting both the number of grains per m2and the average weight of the grains (AGW) in both developed and developing countries. Lignin, which is frequently a major structural component of secondary cell walls in vascular plants, is not only associated with plant growth and development, but also with conferring mechanical strength to the plant body. The purpose of this study was to evaluate the synthesis and accumulation of lignin in wheat stems and its relation to lodging resistance of wheat, and to test yield sensitivity of different eras wheat cultivars to lodging and the mechanisms involved in yield losses due to lodging in wheat. A greater understanding of that information can provide a theoretical basis to enhance the physical strength of the basal part of culm internode to prevent wheat lodging and improve grain yield and quality.The main results were as follows.(1) Relationship between lignin accumulation and related enzyme activity as well as lodging resistant abilityIn the present study, the data clearly demonstrated that the total lignin accumulation of wheat basal second internode was positively correlated to the breaking strength of stems, suggesting that high lignin accumulation in wheat clums may increase the physical strength of culm internode. Culm lodging resistance index (CLRI) is an important parameter to measure the lodging resistant ability of crops and to evaluate the lodging risk in the agricultural production. In the present study, the total lignin accumulation in basal second internode showed a positive correlations with the CLRI of wheat, and a negative correlation with lodging percentage of wheat at maturity. Therefore, our results suggest that the lignin accumulation is closely related to the lodging resistant of wheat, and higher lignin accumulation in wheat clums could strengthen the lodging resistant ability of wheat.The lignin accumulation of basal second internode increased gradually after forming the second internode, especially increased sharply from0to28days after forming the second internode in all treatments. Our regression analysis indicated the enzyme activities of phenylalanine ammonia-lyase (PAL), tyrosine ammonia-lyase (TAL), cinnamyl alcohol dehydrogenase (CAD), and peroxidase (POD) were significantly correlated to the lignin accumulation in wheat culms. These results suggest that high activities of PAL, TAL, CAD and POD is the enzymology basis for keeping high lignin accumulation in wheat culms, and the lodging resistant ability of wheat could be reflected by the lignin accumulation and the correlated enzyme activities in culms.(2) Lignin accumulation and lodging resistance of wheat due to the application of paclobutrazol (PP333) or gibberellin acid (GA3)In the present study, application of PP333significantly reduced plant height and the length of the second basal internode, while GA3had the opposite effect. Furthermore, our results indicate that PP333significantly increased the diameter, wall thickness, filling degree, lignin accumulation and its related enzymes activities in the second internode. GA3showed the opposite effect to that of PP333. Thus, PP333significantly increased the physical strength of the basal internode, while, GA3decreased it. The exogenous PP333delayed the stage of lodging happened as compared to the control, but the stage of lodging was occurred ahead of time under application of GA3. Based on the present results, it is suggested that, the exogenous PP333or GA3application significantly changed the risk of lodging occurred not only by altering the plant height but also altering the physical strength of the basal part of the culm internode, especially, altering the lignin accumulation and its related enzymes activities in basal internode.In the present study, flat planting with support nets (+net), which completely eliminated lodging, resulted in significantly higher grain yield as compared to the lodging happened treatment of which flat planting without support nets (-net). But under no lodging condition, spraying PP333significantly reduced grain yield and GA3had no significant effect on grain yield. Therefore, whether or not the application of growth retardants has a positive effect on grain yield is dependant on lodge status. Yield loss may be caused by applying retardants in the fields where there is little or no lodging.(3) Lignin accumulation and lodging resistance of wheat in relation to shading Compared with the control, shading obviously decreased the total lignin accumulation and the mean enzyme activities of PAL, TAL, CAD and POD in the basal second internode. Transcript levels for TaPAL, TaCAD and TaPOD in the basal second internode under shading treatment were significantly lower than that under the control treatment after forming the second internode, which indicating that shading could decrease the expression of TaPAL, TaCAD and TaPOD genes in wheat stem, and it is possible that light could regulate PAL, CAD and POD expression in transcript levels.Results showed that shading significantly increased plant height and the length of basal second internode. Shading significantly decreased lignin accumulation and its related enzymes activities in the basal second internode, and also decreased the diameter, wall thickness, filling degree of the internode, thus the breaking strength and lodging resistance of wheat decreased. These results suggested that the shading significantly changed the risk of lodging occurred not only by altering the plant height but also by altering the physical strength of the basal part of the culm internode, especially, altering the lignin accumulation and its related enzymes activities in basal internode.(4) Changes of wheat grain yield caused by lodging treatmentsLodging markedly reduced the grain yield and the relative yield loss were difference under different lodging conditions, as the earlier of lodging happened, the more serious of the yield loss. The loss of both number of grains per m2and average grain weight is the cause of yield reduction when wheat under lodging conditions. The earlier of lodging happened (lodged from jointing to maturity), the more serious of the yield components reduction than the condition of lodging happened later. At the early lodging condition, the rank of the relative decrease about the average grain weight was markedly lower than that about the number of grains per m2. This would suggest that the main reason for yield loss of earlier lodging happened was the decrease of the number of grains per m2, and the main reason for yield loss of later lodging happened was the decrease of the average grain weight. In spite of the maximum yield in older and newer varieties is different, the rank of yield loss in older varieties was higher than it in newer varieties.The contents of water soluble carbohydrates in wheat stem at anthesis was marked reduced when lodging happened earlier. Compared with the control, lodged from jointing to maturity and from anthesis to maturity also markedly decreased the contents of water soluble carbohydrates at maturity, but lodged from milk stage to maturity showed no significant effect on water soluble carbohydrates at maturity. The lodging markedly influenced the total biomass and harvest index, the earlier of lodging happened, the more serious of the biomass reduction than the condition of lodging happened later, lodging happened earlier influenced the growth of the whole growth period, and lodging happened later only influenced the growth of post-anthesis period. The earlier of lodging happened, the more serious of the harvest index reduction than the condition of lodging happened later. Under all lodging conditions, the relative loss of harvest index in older varieties was higher than in the newer varieties.Under all lodging conditions, trimming at anthesis obviously increased the average grain weight. The increase of the average grain weight in lodging happended later was lower than in lodging happended earlier. The decrease of the average grain weight was associated with both a decrease in wheat growth and in carbohydrates accumulation that for grain filling.(5) Changes of starch content and granule size distribution in wheat grains caused by lodging treatments after anthesisGranule size distribution of wheat starch is an important characteristic that may affect the functionality of wheat products. In the present study, both for cultivars and lodging treatments, a typical bimodal curve was found in the volume distribution, in which the valley particle diameter is the cut-off of A-type and B-type granules,9.9μm. And a unimodal curve in number distribution with the peak value diameter<0.8μm was found, indicating that the wheat starch granules were comprised of A-type and B-type granules, and that the diameter of most starch granules was<0.8μm.The present results have shown that the volume and surface area proportions of A-type and B-type starch granules can be significantly influenced by lodging during grain filling. Lodging also led to an increase in the proportion (both by volume and by surface area) of A-type (>9.9μm in diameter) starch granules, with a significant reduction in those of B-type (<9.9μm) starch granules. However, lodging treatments had little effect on the proportional number of B-type starch granules. It was suggested that, under lodging conditions, the limited substrate for starch accumulation was mainly partitioned for growing starch granules, not for producing more starch granules. The high medium diameter meant low percentage of small granules, and vice versa. Therefore, the increase of medium diameter indicated that lodging decreased the volume percentage of <9.9μm granules. This effect could also be found in surface area and number distributions.Lodging, especially for early lodging treatment, decreased the starch yield markedly at maturity, and also decreased the contents of amylose, amylopectin and starch at maturity. However, the ratio of amylose to amylopectin were increased significantly by lodging treatments. In this study, we observed that the amylose content was positively correlated with the volume proportion of granules2.8-9.9μm and negatively correlated to volume proportion of granules9.9-22.8μm. The ratio of amylose to amylopectin was negatively correlated with the volume proportion of granules<9.9μm and positively correlated to volume proportion of granules22.8-42.8μm. This indicates that an increase in volume proportion of granules22.8-42.8μm enhanced the ratio of amylose to amylopectin in wheat grain induced by lodging treatments. The results mentioned above are valuable for obtaining high quality for different end-users in wheat production of areas where lodging often occurs. |
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