水稻器官形态和干物质积累对穗分化不同时期高温的响应

为明确水稻穗分化期高温对生长发育所造成的影响,以耐热籼稻黄华占和热敏感籼稻丰两优6号为材料,利用人工气候箱在穗分化期进行40℃(10:00-15:00)高温处理,以人工气候箱32℃适温处理为参考,室外环境温度为对照(CK),研究枝梗-颖花分化期(Ⅰ期)和花粉母细胞形成-减数分裂期(Ⅱ期)高温胁迫对水稻器官形态及干物质积累的影响。结果表明:1)Ⅰ期高温抑制幼穗分化,使穗分化期延长2.5~8.8 d;Ⅱ期高温阻碍水稻抽穗,导致穗分化期延长6.8~7.1 d,显著缩短上部茎节,降低抽穗度,其中热敏感品种降幅大于耐热品种;2)穗分化期高温处理显著降低每穗颖花数和颖花大小,颖花数下降原因在不同处理时期间存在差异,Ⅰ期高温减少颖花分化数;而Ⅱ期高温显著加速颖花退化,两个品种退化幅度均达50%以上,并显著降低了花药大小和颖花受精率,热敏感品种丰两优6号受高温影响要大于耐热品种;3)高温对叶片生长有促进作用,Ⅱ期高温处理表现尤为明显,两个时期高温处理对上3叶净光合速率和水稻单茎干物质积累没有显著影响。Ⅰ期高温不同部位间干物质积累无显著差异,而Ⅱ期高温显著降低茎鞘和穗部干物质积累,但高温导致高节位分枝发生,积累了一部分干物质,整体上单茎干物质积累量并没有显著下降。     

水稻分蘖期干旱锻炼对幼穗分化期高温下穗发育和产量形成的影响

【目的】明确分蘖期干旱锻炼对幼穗分化期高温下水稻穗发育和产量形成的影响,并探究其生理机理。【方法】以两优培九(热敏感型)和汕优63(耐热型)为材料,在盆栽条件下设置全生育期淹灌和分蘖期干旱锻炼、幼穗分化期适温和高温共四种水分和温度处理组合,研究干旱锻炼对高温胁迫下水稻颖花分化与退化、花粉活力、颖花育性、颖花大小、产量及穗生理特性的影响。【结果】与淹灌相比,在适温下干旱锻炼对两个品种产量无显著影响,但高温下干旱锻炼后两优培九和汕优63的产量分别显著提高了54.0%和20.1%,这主要是因为结实率和千粒重显著提高。与淹灌相比,幼穗分化期高温下,分蘖期干旱锻炼显著增强两优培九颖花过氧化物酶和超氧化物歧化酶活性,降低了丙二醛含量,其花粉活力和颖花育性分别显著提高了26.0%和39.0%。与适温相比,全生育期淹灌下,高温显著降低了两个品种的颖花大小和总颖花分化数。与淹灌相比,分蘖期干旱锻炼显著提高了幼穗分化期高温下两品种穗非结构性碳水化合物含量和颖花细胞分裂素(反式玉米素+反式玉米素核苷)含量,两个品种颖花长、颖花宽和总颖花分化数平均显著增加了3.2%、4.4%和15.0%。【结论】分蘖期干旱锻...     

外源油菜素内酯缓解水稻穗分化期高温伤害的机理研究

【目的】明确水稻穗分化期高温下喷施2,4-表油菜素内酯(2,4-epibrassinolide, EBR)对穗生长及颖花形成的影响,并探究其生理机制。【方法】以热敏感型水稻IR36为材料,在幼穗分化期设置40℃高温和32℃适温两个处理,并喷施EBR,研究幼穗碳水化合物供应、蔗糖代谢、细胞分裂素代谢及抗氧化能力的变化。【结果】1)高温和适温喷施EBR,水稻每穗粒数分别比不喷施的对照增加13.7%和45.7%,其中以喷施0.15 mg/L效果最好,缓解了高温对水稻幼穗生长的抑制,增加颖花分化数和降低颖花退化率。2)喷施EBR对叶片净光合速率无显著影响,但促进幼穗中干物质和非结构性碳水化合物积累。EBR喷施增加高温下幼穗中蔗糖转运基因OsSUT1、OsSUT2和OsSUT4的表达,并显著提高蔗糖代谢相关酶活性,EBR对高温下碳水化合物利用的促进作用大于适温处理。3)喷施EBR降低高温下细胞分裂素氧化酶基因OsCKX5和OsCKX9的表达量,同时促进细胞分裂素合成和信号调节相关基因的表达,并在适温下也表现出类似的效应。4)喷施EBR降低高温下超氧阴离子含量,增强了超氧化物歧化酶、过氧化氢酶和过氧化物酶活性。【结论】高温下,喷施适宜浓度的EBR促进碳水化合物向幼穗的转运,抑制细胞分裂素分解,同时降低高温引起的过氧化伤害,进而缓解了高温对颖花形成的伤害。适温条件喷施EBR也对颖花形成具有一定的促进作用。     

外源油菜素内酯缓解水稻穗分化期高温伤害的机理研究

【目的】明确水稻穗分化期高温下喷施2,4-表油菜素内酯(2,4-epibrassinolide, EBR)对穗生长及颖花形成的影响，并探究其生理机制。【方法】以热敏感型水稻IR36为材料，在幼穗分化期设置40℃高温和32℃适温两个处理，并喷施EBR，研究幼穗碳水化合物供应、蔗糖代谢、细胞分裂素代谢及抗氧化能力的变化。【结果】1)高温和适温喷施EBR，水稻每穗粒数分别比不喷施的对照增加13.7% 和45.7%，其中以喷施0.15 mg/L效果最好，缓解了高温对水稻幼穗生长的抑制，增加颖花分化数和降低颖花退化率。2)喷施EBR对叶片净光合速率无显著影响，但促进幼穗中干物质和非结构性碳水化合物积累。EBR喷施增加高温下幼穗中蔗糖转运基因OsSUT1、OsSUT2和OsSUT4的表达，并显著提高蔗糖代谢相关酶活性，EBR对高温下碳水化合物利用的促进作用大于适温处理。3)喷施EBR降低高温下细胞分裂素氧化酶基因OsCKX5和OsCKX9的表达量，同时促进细胞分裂素合成和信号调节相关基因的表达，并在适温下也表现出类似的效应。4)喷施EBR降低高温下超氧阴离子含量，增强了超氧化物歧化酶、过氧化氢酶和过氧化物酶活性。【结论】高温下，喷施适宜浓度的EBR促进碳水化合物向幼穗的转运，抑制细胞分裂素分解，同时降低高温引起的过氧化伤害，进而缓解了高温对颖花形成的伤害。适温条件喷施EBR也对颖花形成具有一定的促进作用。     

外源油菜素内酯缓解水稻穗分化期高温伤害的机理研究

【目的】明确水稻穗分化期高温下喷施2,4-表油菜素内酯(2,4-epibrassinolide, EBR)对穗生长及颖花形成的影响,并探究其生理机制。【方法】以热敏感型水稻IR36为材料,在幼穗分化期设置40℃高温和32℃适温两个处理,并喷施EBR,研究幼穗碳水化合物供应、蔗糖代谢、细胞分裂素代谢及抗氧化能力的变化。【结果】1)高温和适温喷施EBR,水稻每穗粒数分别比不喷施的对照增加13.7%和45.7%,其中以喷施0.15 mg/L效果最好,缓解了高温对水稻幼穗生长的抑制,增加颖花分化数和降低颖花退化率。2)喷施EBR对叶片净光合速率无显著影响,但促进幼穗中干物质和非结构性碳水化合物积累。EBR喷施增加高温下幼穗中蔗糖转运基因OsSUT1、OsSUT2和OsSUT4的表达,并显著提高蔗糖代谢相关酶活性,EBR对高温下碳水化合物利用的促进作用大于适温处理。3)喷施EBR降低高温下细胞分裂素氧化酶基因OsCKX5和OsCKX9的表达量,同时促进细胞分裂素合成和信号调节相关基因的表达,并在适温下也表现出类似的效应。4)喷施EBR降低高温下超氧阴离子含量,增强了超氧化物歧化酶、过氧化氢酶和过氧化物酶活性。【结论】高温下,喷施适宜浓度的EBR促进碳水化合物向幼穗的转运,抑制细胞分裂素分解,同时降低高温引起的过氧化伤害,进而缓解了高温对颖花形成的伤害。适温条件喷施EBR也对颖花形成具有一定的促进作用。     

不同施肥方式对不同密度下直播稻的产量及群体光合物质生产的影响

以常规粳稻品种云粳41为试验材料,采用两因素裂区设计,研究不同密度下水稻一次性施用控释肥和分次施用尿素对直播稻产量及其构成因素、群体光合物质生产的影响,并探讨了不同施肥方式对不同密度下直播稻的产量性状和产量之间的相关性。结果表明:(1)不同施肥方式下,密度相同时,分次施肥的处理比控释肥一次性基施的处理和不施肥对照增产,产量随着施氮量的增加而增加,分次施肥处理的产量显著高于控释肥一次性基施的处理;在相同氮肥条件下,直播稻产量随着直播密度的增加而降低,以基本苗2万/667 m~2的处理产量最高,6万/667 m~2的处理产量居中,10万/667 m~2的处理产量最低。(2)在施用控释肥的处理中,密度相同时,增加施氮量能够显著提高直播稻的有效穗数和每穗颖花数;施氮量相同时,单位面积有效穗数随密度的增加而提高,每穗颖花数刚好相反。单位面积颖花量和实际产量之间呈极显著正相关关系。在形成适宜穗数的基础上提高每穗颖花量,从而增加单位面积的颖花量是提高直播稻产量的主要途径。(3)当密度相同时,抽穗至成熟期总干物质积累量、群体生长率、群体叶面积和总颖花数都随着施氮量的增加而提高。本试验中,以精确定量栽培技术为核心的分次施肥A1RPQ4处理的群体结构优于其他处理,群体茎蘖消长动态最为平稳,抽穗期至成熟期的总干物质积累量、颖花/叶比和粒质量/叶比最大,产量最高,达到875.14 kg/667 m~2。     

不同时期弱光胁迫对晚稻不同耐弱光品种源库特征及叶片保护酶活性的影响

目的 本研究旨在明确弱光胁迫下不同弱光耐性品种在源库特征及叶片保护酶活性变化方面的差异,揭示其耐弱光胁迫的机制,为双季稻高产稳产新品种的选育及抗逆栽培提供理论指导。方法 以2个对弱光胁迫耐性存在显著差异的晚稻品种为材料,通过设置不同时期的遮光处理,研究弱光胁迫时期对晚稻不同耐性品种的源库特征及叶片保护酶活性的影响。结果 结果表明,不同时期弱光胁迫对晚稻生长有重要影响,分蘖期弱光胁迫下晚稻分蘖数、每穗粒数及总颖花量减少,叶面积下降,产量降低;幼穗分化期弱光胁迫下晚稻分蘖数和每穗粒数减少,总颖花量、叶面积、结实率均下降,产量降低;灌浆期弱光胁迫下水稻叶片净同化率降低,结实率和千粒重下降,产量降低。不同时期弱光胁迫对晚稻产量的影响表现为灌浆期>幼穗分化期>分蘖期。结论 耐弱光胁迫型品种能通过增加叶片的长宽度减少叶面积的下降,叶片含氮量保持相对稳定,叶片中SOD、CAT等保护酶活性保持在较高水平,POD活性保持相对稳定,叶片净同化率下降幅度小,在弱光胁迫下实现“强源”,减产率相对较低。弱光胁迫下具有较强的叶面积补偿能力和适宜氮含量以及较高的保护酶活性是水稻耐弱光胁迫的重要机制。     

超级稻新品种南粳9108产量稳定性及高产特征分析

对超级稻新品种南粳9108在2011-2012年江苏省区域试验中产量的稳定性和2013-2014年江苏省机插高产栽培示范方中高产形成的特征特性进行了分析,以期为南粳9108的超高产栽培及示范推广提供理论依据。研究结果表明:(1)南粳9108的丰产性、稳产性和适应性均优于对照淮稻9号,产量潜力较高,通过改善栽培技术或环境能显著增加产量。(2)南粳9108从中高产到高产到更高产再到超高产,群体颖花量不断提高,且不同产量等级间差异显著,而结实率和千粒重在4个产量等级间略有增减,但差异不显著。群体颖花量的提高在由中高产提高到高产的水平上,主要依靠单位面积有效穗数的增加,而由高产提高到更高产再提高到超高产水平,则主要依靠在稳定足够穗数基础上增加每穗粒数。南粳9108产量与群体总颖花量和每穗粒数呈显著正相关,与有效穗数、千粒重和结实率呈不显著正相关。通径分析显示,群体总颖花量对产量的直接作用最大,其次是每穗粒数和有效穗数。千粒重和结实率通过影响有效穗数、每穗粒数和总颖花量而间接影响产量。因此,南粳9108超高产栽培应以足量的穗数获取较大的穗型,保证群体具有较大的总颖花量,并保持正常的结实率与千粒重。     

不同比例红蓝光及光照强度对金线莲生理及叶绿素荧光特性的影响

为探讨LED光源在金线莲工厂化栽培中的应用,利用研发的具有不同比例红（R）、蓝光（B）的LED灯作为光源,以白色荧光灯为对照（CK）,设置以下5个试验处理,T₁：R/B(3/7), PPFD 20 μmol/(m²·s);T₂：R/B(7/3), PPFD 20 μmol/(m²·s);T₃：R/B(3/7), PPFD 30 μmol/(m²·s);T₄：R/B(7/3), PPFD 30 μmol/(m²·s);T₅：R/B(7/3), PPFD 50 μmol/(m²·s),研究不同比例红蓝光源及其光照强度对金线莲的生长、光合作用、叶绿素荧光反应和生理特性的影响。结果表明：与对照处理相比,T₄、T₅处理的金线莲株高、茎粗、植株干鲜重显著提高;T₅处理的金线莲叶片净光合速率显著高于其他处理,而T₁、T₃处理间差异不显著,但显著高于T₂、T₄处理。不同比例红蓝光处理下金线莲叶片的叶绿素含量显著高于对照处理,在相同的光照强度下,R/B(7/3)处理的金线莲叶片叶绿素含量大于R/B(3/7)处理。不同比例红蓝光源处理的金线莲叶片最大光化学效率、光系统II活性均显著高于对照处理;红光比例减少,蓝光比例增加,可降低金线莲叶片最大光化学效率、光系统II活性,同时光系统II实际光化学效率和光合电子传递效率也随之降低。T₂处理可有效提高金线莲叶片SOD和CAT活性,但POD活性降低;T₄处理下金线莲叶片POD和CAT活性降低,SOD活性升高;与对照相比,T₁处理的金线莲叶片MDA含量显著增加,较对照提高19.4%,而T₅处理与对照差异不显著。综合各处理金线莲生长来看,T₅处理R/B(7/3), PPFD 50 μmol/(m²·s)的金线莲生长最好,保持较高的光合速率及生理活性。     

土壤铜含量对水稻生长发育和产量形成的影响

 在盆栽土培条件下，以武香粳14号和汕优63为材料，研究了土壤铜含量对水稻生长发育和产量形成的影响。结果表明：(1) 土壤铜含量100、200 mg/kg处理对水稻株高、主茎总叶数、伸长节间数和抽穗期几乎没有影响，土壤铜含量达到400 mg/kg及其以上时，随着土壤铜含量的增加，水稻株高显著变矮、主茎总叶数和伸长节间数显著变少、抽穗期显著推迟；(2) 土壤铜处理使水稻产量显著低于CK，且随着土壤铜含量的提高水稻产量下降幅度显著增大；(3) 土壤铜含量100、200 mg/kg处理使水稻减产是由于每穗颖花数减少所致，土壤铜含量400 mg/kg及其以上处理使水稻减产是由于穗数和每穗颖花数共同减少所致；(4) 土壤铜处理使穗数减少主要是由于返青活棵慢、分蘖发生迟、高峰苗少所致，每穗颖花数减少主要是由于抽穗期单茎干物质量小和单位干物质量形成的颖花数少所致；(5) 随着土壤铜含量的增加水稻生物产量显著下降，经济系数呈下降趋势，但只有土壤铜含量达到800 mg/kg及其以上时经济系数才显著下降。     

Pre-anthesis non-structural carbohydrate reserve in the stem enhances the sink strength of inferior spikelets during grain filling of rice

Sink strength plays an important role in grain filling of cereals but how it is related to the pre-anthesis non-structural carbohydrate (NSC) reserves is not clear. This study investigated if and how an increase in NSC reserves could enhance sink strength, and consequently improve grain filling of later-flowering inferior spikelets (in contrast to the earlier flowering superior spikelets) for rice varieties with large panicles. Two “super” rice varieties (the recently bred high-yielding rice) and two New Plant Type (NPT, named in IRRI for the extra-large panicle) rice lines were compared with two elite inbred varieties under field-grown conditions. Three nitrogen (N) treatments, applied at the stages of panicle initiation, spikelet differentiation or both, were adopted with no N application during the mid-season as control. Both super rice and NPT rice showed a greater yield capacity as a result of a larger panicle than the elite inbred rice. However, a lower percentage of filled grains limited the realization of higher yield potential in super rice and especially in NPT rice, due to their lower grain filling rate and the smaller grain weight of their inferior spikelets. The low grain filling rate and small grain weight of inferior spikelets are mainly attributed to a poor sink strength as a result of small sink size (small number of endosperm cells) and low sink activity, e.g. low activities of sucrose synthase (SuSase) and adenosine diphosphoglucose pyrophosphorylase (AGPase). The amounts of NSC in the stem and NSC per spikelet at the heading time are significantly and positively correlated with sink strength (number of endosperm cells and activities of SuSase and AGPase), grain filling rate, and grain weight of inferior spikelets. Nitrogen application at the spikelet differentiation stage significantly increased, whereas N application at the panicle initiation or at both panicle initiation and spikelet differentiation stages, significantly reduced, NSC per spikelet at the heading time, sink strength, grain filling rate, and grain weight of inferior spikelets in super rice. The results suggest that pre-anthesis NSC reserves in the stem are closely associated with the sink strength during grain filling of rice, and N application at the spikelet differentiation stage would be a good practice to increase pre-anthesis NSC reserves, and consequently to enhance sink strength for rice varieties with large panicles, such as super rice varieties.     

Effects of Soil Copper Concentration on Growth, Development and Yield Formation of Rice （Oryza sativa）

Pot experiments were conducted in 2002 and 2003 to investigate the effects of soil copper（Cu） concentration on growth, development and yield formation of rice by using the japonica cultivar Wuxiangjing 14 and hybrid rice combination Shanyou 63. The plant height, leaf number, elongated internode number and heading date of rice plants were not affected at soil Cu levels below 200 mg/kg, but affected significantly at above 400 mg/kg. The inhibitory effects on rice growth and development were increased with the increment of soil Cu levels. The grain yields decreased significantly with raising soil Cu levels. The main reasons for the grain yield reductions under lower soil Cu levels （100, 200 mg/kg） were mainly due to the decrease of number of spikelets per panicle, however, under higher soil Cu levels （more than 400 mg/kg）, both panicle number and number of spikelets per panicle contributed to the yield loss. The decreases of panicle number by Cu stress were mainly attributed to slow recovery from transplanting, delayed tillering and reduced maximum tiller numbers. The reduction of number of spikelets per panicle under soil Cu stress resulted from the decreases of both shoot dry weight （SDW） at the heading date and the ratio of spikelets to SDW. Total biomass at maturity decreased significantly with the increase of soil Cu levels, while economic coefficient showed non-significant decrease except under soil Cu levels above 800 mg/kg.     

Hormonal changes are related to the poor grain filling in the inferior spikelets of rice cultivated under non-flooded and mulched condition

Non-flooded plastic mulching cultivation (PM) of rice can save much irrigation but usually exhibits a poor grain filling and low grain weight when compared to traditional flooding cultivation (TF). This study measured the variations of plant hormones during grain filling and investigated whether they were related to the grain-filling problem under mulching in a field experiment. Hormonal levels in the grains and the grain development of both superior (early flowered) and inferior spikelets (later flowered) were monitored continuously during the grain filling period. The contents of indole-3-acetic acid (IAA), zeatin riboside (ZR) and abscisic acid (ABA) were higher in superior grains than in inferior ones at early grain-filling stage. For individual spikelets, the peaks of IAA and ZR contents appeared just before the peak grain-filling rate but the peak ABA content matched it. The earlier flowered superior spikelets did not show much difference in grain filling characteristics and hormonal changes under the two cultivation treatments. Distinctively with the inferior spikelets in a panicle, PM led to lower grain weight, higher grain-filling rate at early stage and shorter active grain-filling period than the TF. PM also resulted in less IAA and ZR but more ABA contents in the grains than TF at early and middle grain filling stages in these inferior spikelets. Peaks of these hormone contents in the PM inferior grains usually appeared earlier and also disappeared more quickly than those in TF inferior grains. When exogenous IAA was applied to plants at the initial grain-filling stage, IAA and ZR were increased and ABA was reduced in the inferior grains. The treatment prolonged the active grain-filing period and reduced the peak grain-filling rate of inferior grains, similarly as the case with TF. As a result, IAA spraying enhanced the weight of PM inferior grains but reduced that of TF ones. Exogenous ABA spraying led to the opposite effects to those of IAA. The results suggest that the shortened grain-filling period in the inferior grains under PM is related to the increased ABA and reduced IAA and ZR in the grains. Regulation of the ratio of ABA to IAA in grains could potentially increase the weight of inferior grains under the water-saving cultivation with film mulching.     

亚种间杂交稻大穗形成机理研究I．颖花的形成特点

比较了亚种间杂交稻赣化７号与品种间杂交稻汕优６３的枝梗和颖花的分化及发育特性，分析了颖花的分化、发育及一、二次枝梗对大穗形成的作用，提出了增加颖花数的主攻方向。     

Changes in Enzyme Activities Involved in Starch Synthesis and Hormone Concentrations in Superior and Inferior Spikelets and Their Association with Grain Filling of Super Rice

The changes in activities of key enzymes involved in sucrose-to-starch conversion and concentrations of hormones in superior and inferior spikelets of super rice were investigated and their association with grain filling was analyzed.Four super rice cultivars,Liangyoupeijiu,IIyou 084,Huaidao 9 and Wujing 15,and two high-yielding and elite check cultivars,Shanyou 63 and Yangfujing 8,were used.The activities of sucrose synthase (SuSase),adenosine diphosphoglucose pyrophosphorylase (AGPase),starch synthase (StSase) and starch branching enzyme (SBE),and the concentrations of zeatin + zeatin riboside (Z + ZR),indole-3-acetic acid (IAA) and abscisic acid (ABA) in superior and inferior spikelets were determined during the grain filling period and their relationships with grain filling rate were analyzed.Maximum grain filling rate,the time reaching the maximum grain-filling rate,mean grain filling rate and brown rice weight for superior spikelets showed a slight difference between the super and check rice cultivars,but were significantly lower in the super rice than in the check rice for inferior spikelets.Changes of enzyme activities and hormone concentrations in grains exhibited single peak curves during the grain filling period.The peak values and the mean activities of SuSase,AGPase,StSase and SBE were lower in inferior spikelets than in superior ones,as well as the peak values and the mean concentrations of Z + ZR and IAA.However,the peak value and the mean concentration of ABA were significantly higher in inferior spikelets than in superior ones and greater in the super rice than in the check rice.The grain filling rate was positively and significantly correlated with the activities of SuSase,AGPase and StSase and the concentrations of Z + ZR and IAA.The results suggested that the low activities of SuSase,AGPase and StSase and the low concentrations of Z + ZR and IAA might be important physiological reasons for the slow grain filling rate and light grain weight of inferior spikelets in super rice.     

Response of Spikelet Number per Panicle in Rice Cultivars to Three Transplanting Densities

Abstract

Spikelet number per panicle (SPP), differentiated spikelet number per panicle (D-SPP), and preflowering aborted spikelet number per panicle (A-SPP) were examined in five rice cultivars at three planting densities (HD; high, MD; medium, LD; low planting density) in the field condition. Rice plants at LD produced a higher panicle number per plant but lower panicle number per unit area, accompanied by higher D-SPP and SPP, on average. A-SPP and the ratio of A-SPP to D-SPP (%A) showedno consistent trends. There was a broader range of D-SPP values at LD than at HD because of larger D-SPP in higher order panicles (panicles with a higher spikelet number). D-SPP was smaller in lower order panicles in all cultivars and years, whereas %A increased. D-SPP and SPP of each panicle were positively correlated with tiller size (tiller height, leaf area, and neck internode diameter). Spikelet production efficiency for D-SPP or for SPP (spikelet number per leaf area) of each tiller was higher in IR65564-44-51 (NPT65) and Akihikari than in the other cultivars, indicating a greater capacity of tillers to produce spikelets or support spikelet growth. In each cultivar except NPT65, spikelet production efficiency for D-SPP increased as panicle order decreased, whereas spikelet production efficiency for SPP remained constant or decreased. This finding indicates that irrespective of planting density, lower order panicles produce more spikelets than they can afford physiologically, but they were regulated downward to a nearly constant value in four cultivars. In NPT65 different from other cultivars, spikelet production efficiency for D-SPP was lower in lower order panicles.