Germination Responses of Cañahua (Chenopodium pallidicaule Aellen) to Temperature and Sowing Depth: A Crop Growing Under Extreme Conditions

Cañahua (Chenopodium pallidicaule) is grown in the Altiplano of Bolivia and Peru, between 3810 and 4200 m a.s.l. Rural indigenous households have cultivated the cañahua as a subsistence crop for millennia. The seeds have a high content and quality of protein. We studied the relation between the following: (i) temperature and seed germination and (ii) the effect of temperature and sowing depth on seedling emergence of five cultivars and one landrace. Three experiments were conducted as follows: (i) seeds of a cultivar were germinated in Petri dishes at six temperatures (3, 5, 10, 14, 20 and 24 °C), (ii) sown at five depths (0, 5, 10, 25 and 50 mm) in a mixed peat soil substrate at three temperatures and (iii) one landrace (Lasta) and 5 cultivars (Lasta and Saihua growth habit) were sown in 6 depth (0, 5, 10, 25, 35 and 50 mm) in a sandy loam at two temperatures (5 and 15 °C). Temperature had significantly effect on the germination percentages of the plants (P < 0.001). Seeds germinated at the lowest temperature (3 °C). The estimated base temperature was close to 0 °C. A polynomial function described well the relation between time to 50% germination (t₅₀) and temperature in the interval from 3 to 24 °C resulting in a linear relationship between germination rate and temperature. Shallow sowing depth (5–25 mm) resulted in 80% germination at 15 °C. There were significant differences of emergence in relationship to burial depth (P < 0.001). Only few seedlings emerged when seeds were sown at 50 mm depth. We did not find significant differences in emergence of seedlings between Lasta and Saihua at 15 °C. Nevertheless, at 5 °C, seedlings of cañahua belonging to the Lasta growth habit form did have higher germination rate as were shown for the Kullaca cultivar and the Umacutama landrace. This may be attributed to larger seed size of these cultivars.     

低温胁迫和赤霉素对花生种子萌发和幼苗生理响应的影响

低温是影响我国广大花生产区春花生发芽的主要因素之一。本文以不同生态区的30个花生品种为实验材料,研究了倒春寒天气诱导的低温胁迫对花生出苗的影响,以出苗率为标准筛选出4个耐低温的花生品种(阜花17、阜花12、冀花16、冀花18)和4个不耐低温的品种(鲁花11、白沙1016、正农黑花生1号、白玉)于温室测定了4℃低温和赤霉素(GA_3)处理后种子发芽相关指标和幼苗生理指标。结果表明, 4℃对耐低温花生品种发芽率、发芽指数影响不显著,但种子活力指数和芽长呈现下降趋势;4℃处理后,不耐低温品种幼苗相对膜透性和MDA含量上升幅度更高,耐低温品种幼苗的可溶性糖和游离脯氨酸含量上升幅度更大。GA_3显著促进4℃低温处理后花生种子萌发和种子活力,抑制了花生幼苗在低温处理后相对膜透性和丙二醛的上升,提高了可溶性糖、可溶性蛋白、游离脯氨酸含量。研究表明,赤霉素促进低温胁迫下种子萌发和幼苗生长的最佳浓度是300μmol L~(–1)。发芽率与相对膜透性和丙二醛含量显著负相关,与可溶性糖、脯氨酸含量显著正相关。温度影响发芽率的品种间差异较大,常温下赤霉素对不同耐低温品种的发芽率影响较小。本研究为耐低温花生种质资源创新和新品种培育提供了理论依据,为研究赤霉素对不同花生品种耐低温性影响的生理机制提供了基础。     

Effect of Sodium Chloride Salinity on Seedling Emergence in Chickpea

Although laboratory (Petri dish) germination as an estimate of seed viability is a standard practice, it may not give an accurate prediction of seedling emergence in the field, especially when saline irrigation water is used. Experiments were conducted to investigate seedling emergence in two chickpea cultivars (ILC 482 and Barka local) in response to varied salinity levels and sowing depths. Seeds were sown in potted soil at a depth of 2, 4 or 6 cm. The salinity treatments were 4.6, 8.4 and 12.2 dS m^–1. Tap water (0.8 dS m^–1) served as the control. Depth of sowing had a significant effect on seedling emergence. Seeds sown 6 cm deep showed the lowest seedling emergence. Similarly, salinity had an adverse effect on seedling emergence. The lowest seedling emergence percentages were obtained at the highest salinity treatment (12.2 dS m^–1). The interaction between salinity treatment and seeding depth was significant. Hypocotyl injury was implicated as a possible cause of poor seedling emergence in chickpea under saline water irrigation and was less severe when pre-germinated seeds were used. ILC 482 appeared to be more tolerant to salinity than Barka local, suggesting that breeding programmes involving regional exchange of germplasm may be helpful.     

低温胁迫对北疆棉花出苗的影响及适宜播种期的研究

【目的】为了解决新疆北疆地区棉花播种出苗期低温冷害问题,【方法】利用人工气候室,对种子低温胁迫设计不同温度进行播种出苗试验,并结合近20年农业气象观测资料,研究播种至出苗的适宜播种温度指标、积温指标、确定棉花适宜播种期。【结果】试验结果表明：棉花种子的萌芽率、萌芽天数、萌芽长度、萌芽鲜重及出苗率等各项参数随温度呈非线性有规律变化。棉花种子低温胁迫时间越长出苗越滞后,所需的积温较多。北疆地区棉花主栽品种播种出苗下限温度为12℃,需要活动积温在160℃.d以上,≥12℃有效积温在55℃.d以上。【结论】通过人工气候室试验结合大田实际监测研究,新疆北疆地区稳定通过12℃应作为棉花主栽品种播种的初始期;播种期确定在4月下旬,出苗期可免受春季低温冷害的威胁。     

Hydrotime Analysis of Ethiopian Mustard (Brassica carinata A. Braun) Seed Germination Under Different Temperatures

Ethiopian mustard (Brassica carinata A. Braun) is an oil‐seed species that has recently become an object of great interest as a promising crop for energy purposes (biodiesel and biolubricant production). In semi‐arid regions of South Italy, soil moisture at sowing time is often inadequate, delaying and reducing seed germination. In this study, the effect of reduced water potential on seed germination in three cultivars of B. carinata (ISCI 7, CT 180 and Sincron) was investigated in the laboratory. Germination behaviour at constant temperatures under low water availability was also analysed through the hydrotime model. Six water potentials (ψ_s) in PEG solution (0, ?0.2, ?0.4, ?0.6, ?0.8 and ?1.0 MPa) and three temperatures (10, 20 and 30 °C) were used for the germination tests. A thermo‐inhibiting effect was observed at the highest levels of water stress. The hydrotime analysis revealed that the increase of temperature to 30 °C reduced predicted hydrotime (θ_H) by hastening the rate of germination, but shifted median base water potential (ψ_b(50)) to higher values (less negative), whereas the lowering of temperature increased θ_H. These observations may have a great agronomic impact because although fewer seeds germinated at 30 °C at the highest levels of water stress, they may germinate faster in rapidly drying soil. However, genetic differences were observed among cultivars in terms of estimated θ_H and ψ_b(50). The hydrotime modelling approach offered in this study may help predict B. carinata seed germination under soil water deficit conditions occurring under early or late autumnal sowing.     

不同小麦品种产量对冬前积温变化的响应

为确定河北省中部地区小麦品种适播期,筛选耐迟播品种,2011年秋至2014年夏在河北藁城采用播期×品种二因子裂区试验,研究了不同播期对当地12个主栽小麦品种产量的影响以及不同品种对冬前积温的响应。结果表明,播期对小麦产量有显著影响;不同品种对播期响应差异明显,分为迟钝型、中间型和迟播敏感型。迟钝型品种对播期不敏感,适播期长,冬前≥0℃积温范围为324~560℃,迟播后穗数和产量稳定;中间型品种适播期较长,积温范围为362~566℃,迟播后粒数增加,穗数和产量降低;迟播敏感型品种对播期敏感,适播期较短,积温大于511℃,不宜晚播,晚播后穗数和产量明显下降。在试验地区,推荐小麦适播期为迟钝型品种10月7日至22日、中间型品种10月7日至19日、敏感型品种10月5日至10日。     

Assessment of runner bean (Phaseolus coccineus L.) germplasm for tolerance to low temperature during early seedling growth

The runner bean requires moderately high temperatures for optimum germination and growth. Low temperature at sowing delays both germination and plant emergence, and can reduce establishment of beans planted early in the growing season. The objective of this work was to identify potential runner bean germplasm with tolerance to low temperature and to assess the role of this germplasm for production and breeding. Seeds of 33 runner bean accessions were germinated in a climate-controlled chamber at optimal (17°C-day/15°C-night) and at sub-optimal (14°C-day/8°C-night) temperature. The low temperature tolerance was evaluated on the basis of germination, earliness, ability to grow and vigor. Differences in agronomical characters were significant at low temperatures for germination, earliness, ability to grow and early vigor except for emergence score. The commercial cultivars Painted Lady Bi-color, Scarlet Emperor, the Rwanda cultivar NI-15c, and the Spanish cultivars PHA-0013, PHA-0133, PHA-0311, PHA-0664, and PHA-1025 had the best performance under cold conditions.     

Screening of Chickpeas for Adaptation to Autumn Sowing

Dry matter accumulation was determined in 27 chickpea (Cicer arietinum) lines in time‐of‐sowing field trials and in controlled‐environment chambers at day/night temperatures of 13/5, 18/8 and 23/13 °C to assess tolerance to growth‐inhibiting temperatures. Field trials were based at Narrabri, NSW, Australia, in a region of summer‐dominant rainfall where winter crops are grown on stored soil moisture. Percentage emergence was lower than expected in some field trials and in the coolest controlled environment. Subsequent dry matter accumulation showed the effects of poor crop establishment until the onset of flowering. Kabuli types were more susceptible to poor emergence than desi types. Different lines yielded the greatest dry matter production at different stages of growth. In the seedling phase, to 30 days after emergence, kabuli accessions SP1.563 and Garnet showed significantly greater dry matter accumulation than all other accessions in all controlled environments, suggesting broad adaptation. One desi accession, Gully, was almost as productive as these two kabuli accessions in the intermediate environment but was much poorer in the other environments, indicating very narrow adaptation. In the vegetative phase, the greatest relative growth rates were found in the desi accessions. Line 940‐26 was identified as highly productive in both field and controlled‐environment experiments. Dry matter accumulation was not significantly affected by temperature, although it was slightly greater in the coolest controlled environment than in the other two. The accession by temperature interaction was not significant, showing that the breadth of adaptation was similar in all accessions during this growth phase. The optimum time of sowing for dry matter accumulation was late May, 4–6 weeks before the winter solstice. The results showed that chickpeas are well adapted to germination and seedling establishment in moderate conditions, followed by vegetative growth in cooler conditions. These conditions are typical following autumn sowing in a Mediterranean or temperate environment. Kabuli types appear to have stronger growth during the seedling phase and desi types during the vegetative phase. Recombination of these traits could lead to more productive cultivars.     

播期对小麦冬前幼苗生长和积温的调控效应

为了研究播期对小麦冬前幼苗生长和积温的调控效应,以‘光明麦1号’为试验材料,设计不同播期试验,旨在探讨播期对春性小麦冬前幼苗生长发育和积温的影响。结果表明,随播期推迟,出苗率下降,且冬前积温降低,显著影响小麦冬前生育进程,植株叶龄减小,分蘖发生数下降,LAI和干物质积累降低。播种至出苗阶段日均温与出苗天数呈指数相关关系,出苗至越冬始期≥0℃积温与此期天数、叶龄、茎蘖数、LAI和干物质积累呈显著线性正相关。本试验条件下,播期每推迟6天左右,播种至出苗日均温下降1.0~2.5℃,≥0℃积温上升5~15℃,播种至出苗所需天数延长2~5天,出苗至分蘖所需天数延长1~10天,出苗至越冬始期的天数缩短6~11天,出苗率下降1.5~3.5个百分点,越冬始期叶龄减少0.4~1.0,茎蘖数、叶面积指数和干物质积累分别下降100~150万/hm²、0.04~0.51和30~200 kg/hm²。实现春性小麦壮苗的冬前≥0℃积温为500~670℃,适宜播期10月23日—11月4日,最适播期是10月27日—10月29日。     

Estimation of key parameters for weed population dynamics models: Base temperature and base water potential for germination

In fields, the timing of weed emergence flushes is mostly related to the timing and rate of seed germination, which depend on soil temperature and water potential as well as soil tillage and crop sowing date. Base temperature and base water potential for seed germination are parameters required in weed dynamics models to take into account the effects of soil conditions on weed demography. Since these parameters are difficult to measure, our objective was to develop a method for estimating them from easily accessible information.Data on base temperature and base water potential for germination were collected from the literature for 28 weed species. Correlations were then searched for between these parameters and (i) the species ecological indicator values of Landolt and Ellenberg for temperature and moisture and (ii) dates of the onset and end of weed emergence based on experts’ knowledge.Both base temperature and base water potential for germination were partially correlated with the species indicator values found in the literature. These correlations explained however only a small proportion of the variance of base temperature and water potential. In contrast, the base temperature of spring-emerging species was highly correlated with the mean soil temperature at the mean onset date of each species’ spring emergence flush. Water potential for germination was higher for species requiring high base temperature. The date of spring emergence onset of a species, which can be easily assessed from experts’ knowledge, can thus be used to estimate their base temperature and then their base water potential for germination.     

AlomySys: Modelling black-grass (Alopecurus myosuroides Huds.) germination and emergence,in interaction with seed characteristics,tillage and soil climate: II. Evaluation

Weed emergence models are increasingly necessary to evaluate and design cropping systems. The model AlomySys was developed for a frequent and harmful weed, i.e. Alopecurus myosuroides Huds. [Colbach, N., Dürr, C., Roger-Estrade, J., Caneill, J., 2005a. How to model the effects of farming practices on weed emergence. Weed Res. 45, 2–17; Colbach, N., Dürr, C., Roger-Estrade, J., Chauvel, B., Caneill, J., 2005b. AlomySys: modelling blackgrass (Alopecurus myosuroides Huds.) germination and emergence, in interaction with seed characteristics, tillage and soil climate. I. Construction. Eur. J. Agronomy] and is based on sub-models predicting (a) soil environment (climate, structure) resulting from the cropping system and external climate, (b) vertical soil seed distribution after tillage, and (c) seed survival, dormancy, germination and pre-emergent growth depending on soil environment, seed depth, characteristics and past history. In the present paper, the model was evaluated by comparing its simulated output to independent field observations, as a function of sowing/tillage date, seed depth, seed history and characteristics, soil climate and soil structure. The evaluation showed that the survival of the seed bank in top layers was systematically overestimated. In addition, some minor aspects were identified for future improvements, such as the pre-emergent seedling mortality in very compacted soil structures or germination rates of seeds that had not been previously stimulated by light. Despite these few shortcomings, the model simulates the timing and relative importance of the emergence flushes very well; in addition, the actual emerged seedling densities are usually predicted satisfactorily. Consequently, the model can be used to simulate the effects of cropping systems on black-grass emergence.     

不同抗旱性花生品种根系形态及生理特性

以12个花生品种为试验材料, 在人工控水条件下, 通过苗期及结荚期干旱试验, 对比分析花生品种苗期根系性状与抗旱性的关系。结果表明, 花生苗期与结荚期抗旱性基本一致。利用产量抗旱系数可把12个花生品种的抗旱性划分为强、中、弱3级, 抗旱性强的品种为A596、山花11和如皋西洋生, 中度抗旱品种为花育20、农大818、海花1号、山花9号和79266, 抗旱性弱的品种有ICG6848、白沙1016、花17和蓬莱一窝猴。山花11可作为花生强抗旱性鉴定的标准品种, 79266可作为花生弱抗旱性鉴定的标准品种。山花9号、山花11、花育20的根系抗旱机制为较大的根量及根系吸收能力, 而A596、如皋西洋生、农大818、山花11为较强的根系抗氧化能力及膜稳定性。相关分析表明, 苗期重度干旱胁迫下的单株根系干重、体积、总吸收面积、超氧化物歧化酶(SOD)活性和丙二醛(MDA)含量与品种抗旱系数的相关性达极显著水平, 对照与重度干旱胁迫下的以上性状呈极显著正相关。因此, 在花生出苗后10 d进行40%土壤相对含水量的干旱胁迫, 持续胁迫至出苗24 d的单株根系干重、体积、总吸收面积、根尖SOD活性和MDA含量可鉴定花生品种的根系抗旱能力, 正常水分下的性状值也能反映根系性状的抗旱级别。山花11可作为花生根系形态及生理优异抗旱性状鉴定的标准品种。     

高温对玉米籽粒淀粉合成及产量的影响

以农大108和山农3号为试材，采用田间生长箱增温的方法，研究了不同生育时期高温对玉米淀粉合成及产量的影响。结果表明，第1期（出苗后0~28 d）高温处理对玉米的产量没有显著影响，第2期（出苗后29~57 d）和第3期（出苗后58~86 d）高温处理显著降低玉米产量，以第2期对产量的影响最大；同时高温降低1,6-二磷酸酯酶（FBPase）、磷酸蔗糖合成酶（SPS）、蔗糖合成酶（SS）和腺苷二磷酸葡萄糖焦磷酸化酶（ADPGPPase）的活性，且以第2期降低的幅度最大；籽粒淀粉含量，以第1期高温处理高于对照，其他2个处理显著低于对照。     

播种期温度变化对玉米出苗速率的影响

为了应对气候变化,掌握玉米播种出苗期的温度特性,科学调整玉米播种期提供依据,采用分期播种方法,研究温度变化对玉米出苗速率的影响。结果表明,随着春季气温的升高,玉米出苗速率加快。玉米出苗速率与4月中旬—6月上旬平均气温存在对数函数关系,与≥12.0℃有效积温存在线性函数关系,出苗天数与活动积温存在指数函数关系。稳定通过12.0℃为玉米播种初始温度,14.0~18.0℃为较适宜温度,18.0℃以上为播种适宜温度。耕层温度与气温对出苗速率的影响相同。播种期初始温度应从稳定10.0℃调整至稳定12.0℃为宜。     

种子盐引发对转基因抗虫棉耐盐性的影响

以2个转基因抗虫陆地棉品种为材料,以其遗传背景品种为对照,研究种子引发对转基因抗虫棉在NaCl胁迫条件下的种子萌发和田间出苗的影响。结果表明,0.5%的NaCl处理显著地降低陆地棉种子发芽势和发芽率,其中转基因抗虫棉品种(中棉所41和浙905)的种子萌发和田间出苗率差于遗传背景亲本(中棉所23)。种子盐引发处理可以有效地缓解NaCl的胁迫作用,显著地提高种子的发芽势、发芽率和田间出苗率,显著地提高棉花种子的耐盐性。种子盐引发处理对转基因抗虫棉品种的效果优于非转基因抗虫棉的背景品种。引发剂种类对于其耐盐性的影响有一定的差异,以K2SO4的效果更好,且引发剂的浓度与引发时间之间存在着一定的互作效应,高浓度引发剂的处理时间宜短,而低浓度引发剂的处理时间应适当延长。     

低温胁迫对棉花种子萌发的影响

为了研究低温胁迫对棉花种子萌发特性的影响,以新陆中88为试验材料,采用纸培和砂培的方式进行种子萌发试验,将棉花种子的发芽温度设置为5℃、10℃和15℃不同的低温,低温持续时间分别为1 d、2 d、3 d、4 d、5 d。结果表明,在同一温度下低温持续时间对种子发芽率、发芽势、幼苗高度、主根长度、叶面积、电导率均有所影响,持续时间越久抑制越严重;同一持续时间下温度越低导致发芽率越低、幼苗高度越矮、主根长度越短、叶面积越小。因此,适当推迟棉花播期,可以预防棉花苗期的冻害,有效防止苗期病害的发生,并且有利于棉花出苗整齐、长势健壮,提高幼苗的抗逆能力。     

Comparative Effects of NaCl and Polyethylene Glycol on Germination, Emergence and Seedling Growth of Cowpea

Seeds of Paceño and Cuarenteño cultivars of cowpea (Vigna unguiculata L. Walp.) were tested for salt and drought tolerance at germination, seedling emergence and early seedling growth in NaCl and PEG‐8000 solutions of different osmotic potentials (0, ?0.2, ?0.4, ?0.6, and ?0.8 MPa). Daily and final germination and emergence percentage, as well as germination and seedling emergence rate, and seedling growth were recorded under controlled conditions. Results showed that germination and emergence rate were delayed by both solutions in both cultivars, with differences between cultivars among growth stages, given that cultivar Cuarenteño, showed a higher germination rate than Paceño in NaCl, but Paceño was less affected by NaCl and PEG solutions at the emergence stage. Sodium chloride had a lesser effect on both cultivars in terms of germination rate, emergence rate and the final germination and emergence percentage than did PEG‐8000. This conclusively proves that the adverse effect of PEG‐8000 on germination, emergence and early seedling growth was due to the osmotic effect rather than the specific ion. Seedling growth was reduced by both stresses, but NaCl usually caused less damage than PEG to cowpea seedlings, suggesting that NaCl and PEG acted through different mechanisms.     

农业措施对小麦田主要禾本科杂草出苗的影响

为探明播种、耕作等农业措施对小麦田主要禾本科杂草出苗的影响,对西安地区小麦田主要禾本科杂草多花黑麦草、节节麦、野燕麦、日本看麦娘、假看麦娘的田间落种状态、形态特征及不同土壤深度下出苗规律进行研究。结果表明,小麦机播深度(4~5 cm)对上述5种杂草出苗均无影响;小麦撒播深度(0~1 cm)出苗时间最早,但因表层土壤湿度较差导致出苗量低;旋耕深度(10~15 cm)出苗最晚,该深度对多花黑麦草、节节麦、野燕麦的出苗量无影响,但对日本看麦娘、假看麦娘出苗影响较大,尤其对个体小、重量轻的假看麦娘影响最大;深翻深度(30~35 cm)对5种杂草种子萌发均有抑制作用。     

江苏不同熟期粳稻品种的齐穗期和安全播期预测

为明确当前江苏各地不同熟期粳稻品种的安全播期,以江苏的4个熟期类型的5个常规粳稻品种为材料,在江苏连云港(34°83′N)、淮安(33°34′N)、扬州(32°25′N)和苏州(31°18′N)等4地,进行自5月10日起每10天1期,共8个播期的播种试验,得出各播期的相应齐穗期。以齐穗期温光预测模型y=a＋b₁x₁＋b₂x₂拟合其实得的播期与齐穗期资料。用该模型拟合4个试验点、5个供试品种计20组数据,得出20个预测式,其拟合度R²在0.9927~0.9998之间,均达极显著水平。用各试点所在地近25年各年粳稻品种安全齐穗期及逐日平均温度,依齐穗期光温模型预测式经多次迭代推得各年的安全播期,进而求得90%保证率的安全播期并作各地生产应用分析。在江苏淮北的北部,麦稻两熟田水稻的可播栽始期为6月15日,中熟中粳的安全播期为6月21日,此类品种可用于该地直播;其他各熟期类型品种安全播期均早于当地可播栽始期,故不能用于直播。在淮北南部稻麦两熟田的可播栽始期为6月12日左右,中熟中粳和迟熟中粳的安全播期分别在7月4日和6月22日,此两类品种可用于该地麦茬直播;早熟晚粳安全播期在6月17日、18日,在大面积生产中因接茬时间过紧应慎用于麦茬直播;中熟晚粳的安全播期为6月8日,不可用于直播。在苏中南部,水稻可播栽始期为6月8至11日,中熟中粳、迟熟中粳、早熟晚粳类型两品种的安全播期分别在7月12日、3日、6月25日和28日,可用于该地直播;中熟晚粳的安全播期为6月11日,用于直播接茬时间太紧,不可应用。在苏南,水稻的可播栽始期为6月5日,供试4熟期类型品种的安全播期分别为7月18日、13日、9日至10日和7月3日,仅考虑安全齐穗各熟期类型品种均可用于麦茬直播,但因中粳类型品种直播稻生育期过短、产量不高,不宜应用。     

苗期地温对棉花前期生育进程的影响研究

为研究苗期地温变化对棉花出苗和生长状况的影响,采用田间试验,设置3个不同播期,调查地温与棉花生长变化的关系。结果表明,苗期5 cm地温受气温影响波动要比10 cm和15 cm土层大,且地温相对较高,随播期推后,不同土层温差变化减小;不同播期约4~5天可见苗,受积温影响不大,但随播期推后,进入苗期时间增长;从第一真叶到见蕾一、二播期仅差1天,出叶时间间隔变短,而二、三播期的间隔则变长;不同播期中第三播期棉花株高最高,高出其他播期达20 cm,差异显著,株宽和始节高度差值也达5.5 cm和5.3 cm,且随播期推迟这种差异增大。本试验认为,不论播期怎样变化,只有地温变化趋缓,才利于棉花出苗和稳健生长,构建理想群体结构。