Inheritance of useful traits in cassava grown in subhumid conditions

A diallel study among nine parental clones of cassava was conducted in the subhumid environment on the northern coast of Colombia. Analysis of variance suggested significant effects for the six variables analysed: fresh‐root yield, harvest index, root dry matter content, height of first branching, reaction to thrips and plant‐type scores. General and specific combining ability effects and their interaction with the environment were significant for most of the variables as well. Results suggested that dominance plays a particularly important role in the cases of fresh‐root yield and harvest index but had relatively little importance in the reaction to thrips, dry matter content or height of first branching. Specific breeding approaches are suggested for these traits, depending on the relative importance of additive or non‐additive effects in their inheritance. The correlations among different traits were also analysed and in several cases their magnitude reached statistical and biological significance.     

Diallel inheritance of relevant traits in cassava (Manihot esculenta Crantz) adapted to acid-soil savannas

There is a limited knowledge on the inheritance of traits with agronomic relevance in cassava. A diallel study among 10 parental clones was conducted in the acid-soils environment in the eastern savannas of Colombia. Thirty clones were obtained for each F1 cross. Each clone was represented by six plants, which were distributed in three replications at two locations. Therefore, the same 30 genotypes of each F1 cross were planted in the three replications at the two locations. Analysis of variance suggested significant effects for five of the six variables analyzed (harvest index, dry matter content, height of first branching, reaction to super elongation disease, and plant type scores). Fresh root yield showed strong genotype × environment interaction and differences between crosses reached statistical significance in only one of the two environments analyzed. General and specific combining ability effects and their interaction with the environment were significant for most of the variables as well. Results suggested that dominance plays an important role particularly in the cases of fresh root yield and harvest index.     

不同产量水平鲜食型甘薯品种产量形成及品质差异分析

旨在研究不同产量水平的鲜食型甘薯品种生长发育、产量形成及品质的差异,为高产优质鲜食型甘薯品种的选育提供借鉴和指导。以2 个不同产量水平的鲜食型甘薯新品种‘泰薯14’和‘烟薯25’为材料,测定2 个品种不同生长时期茎叶生长、块根膨大速率、以及收获期产量和品质性状,分析其差异。结果表明,‘泰薯14’茎粗、蔓长、茎叶鲜重和单株叶面积均高于‘烟薯25’,‘烟薯25’的地下部发育早于‘泰薯14’,而‘泰薯14’生长后期地下部发育快于‘烟薯25’,栽苗后80 天‘泰薯14’块根单薯重和单株薯重均显著高于‘烟薯25’;‘烟薯25’的块根商品薯率高于‘泰薯14’,‘泰薯14’块根产量高于‘烟薯25’,薯干产量低于‘烟薯25’。‘泰薯14’的干物率、淀粉率和可溶性糖含量显著低于烟薯25,β-胡萝卜素含量显著高于‘烟薯25’,食味评分低于‘烟薯25’。茎叶较长的功能期使‘泰薯14’具有更大的高产潜力,而‘烟薯25’的品质及食味优于‘泰薯14’。     

Combining ability of cassava genotypes for cassava mosaic disease and cassava bacterial blight, yield and its related components in two ecological zones in Ghana

Breeding for resistant genotypes is the best strategy to offset the destructive effects of cassava mosaic disease (CMD) and cassava bacterial blight (CBB). Two sets of diallel parents were selected for the forest and the savannah ecological zones in Ghana based on good levels of resistance to CMD and CBB. Both sets were crossed in a half-diallel design. The first set of seven progenitors and their 21 F1 progenies were planted in a randomised complete block design (RCBD) with three replications in two different locations for two seasons in the forest ecology. The second set of five progenitors and their 10 F1 progenies were planted in a RCBD with three replications in two locations in the coastal savannah ecological zone of Ghana. Both experiments were evaluated for CMD and CBB resistance, fresh root yield, dry root yield, root number, harvest index, dry matter content, plant height at maturity and height at first branching, levels of branching and plant vigour. Results of the combined analysis of variance revealed that the environment effect was significant for all the traits. General combining ability and specific combining ability effects were significant for most of the traits. Narrow sense heritability was significant for plant vigour, root number, CMD and CBB in both the zones. CMD and root number also had a predictability ratio of close to one, indicating the importance of additive gene effects.     

高产优质淀粉型甘薯新品种‘苏薯24号’的选育研究

为了加快淀粉型甘薯新品种‘苏薯24号’的推广应用,总结淀粉型甘薯品种选育的实践经验与心得,通过2012—2014年国家区试多年多点品比试验,以及抗病性等特性综合鉴定评价。结果表明：‘苏薯24号’在2012—2013年国家甘薯长江流域薯区区域试验中,平均鲜薯产量33534.0 kg/hm²,平均薯干产量11130.0 kg/hm²,淀粉产量7549.5 kg/hm²,比对照‘徐薯22’增产18.27%,增产显著;平均块根干物率和淀粉率为33.08%和22.41%,分别比对照‘徐薯22’高2.84和2.47个百分点。2014年国家长江区试生产试验中,平均鲜薯产量31204.5 kg/hm²,平均薯干产量10194.0 kg/hm²;平均淀粉产量 6883.5 kg/hm²,比对照‘徐薯22’增产23.28%。该品种2015年2月通过国家甘薯新品种鉴定。‘苏薯24号’聚合了双亲高产、高淀粉、丰产稳产、综合性状优良等特点,薯干和淀粉产量均较高,抗病和耐储藏性好,在长江流域薯区及周边地区具有广阔的推广应用前景。最后,笔者提出了高淀粉甘薯品种的选育策略是培育骨干亲本与创建核心种质,组配优势群体且加大选择压力,加速育种进程。     

Genetic variation, heritability and genetic correlations for forage quality and yield traits of Mediterranean tall fescue germplasm

Higher ruminant intake potential and in vivo digestibility as indicated by lower neutral and acid detergent fibre (NDF and ADF), respectively, and higher crude protein (CP), may be valuable targets in tall fescue breeding. Some 105 full‐sib families from a diallel cross of Mediterranean parent germplasm were grown in a heated greenhouse simulating the temperature pattern of a Mediterranean environment, to estimate genetic parameters for NDF, ADF, CP and dry matter (DM) yield in one autumn and one winter harvest. Family x harvest interaction was large for NDF and ADF, moderate for CP and absent for DM yield. Additive genetic variance was mostly larger than specific genetic variance, and tended to be small for quality traits and large for DM yield. Narrow‐sense heritability was low for CP and ADF in autumn and moderate to fairly high in the remaining cases. Predicted gains per selection cycle never exceeded 5% for quality traits while being greater than 10% for DM yield. An inverse genetic correlation between DM yield and any quality trait emerged in the presence of sizeable genetic variation.     

Genetic variation, heritability and progeny testing in meadow bromegrass

Meadow bromegrass is a recently introduced pasture grass in western Canada. Its leafy production and rapid regrowth have made it a major grass species for pasturing beef animals in this region. Because relatively little breeding work has been done on this species, there is little information on its breeding behaviour. The main objective of this study was to estimate genetic parameters and assess breeding methodologies for meadow bromegrass. Forty half‐sib (polycross and open‐pollinated) and full‐sib (selfed) progenies were evaluated for dry matter and seed yield, fertility index, harvest index, plant height, plant spread, crude protein, neutral detergent fibre and acid detergent fibre. Genetic variation for seed yield, harvest index, and plant height was significant in the open‐pollinated and selfed tests. Genetic variation estimates for dry matter yield were negative (polycross), not significant (open‐pollinated), or significant (selfed). Estimates of genetic variation for quality traits were not significant, except for acid detergent fibre (selfed). Correlations among characters indicated that it is possible to simultaneously improve seed and forage yield. Rankings of progenies by the half‐sib tests for forage and seed yield were not altered by the use of the synthetic parental value (SV_i), which includes information from selfed progeny. The polycross progeny test did not discriminate parents as well as the open‐pollinated and selfed tests; this may have been a result of non‐random pollination in the polycross. Correlation among the progeny tests, showed that open‐pollinated and selfed progeny tests agreed for all characters, except for harvest index, while correlation between polycross and selfed tests were significant for six characters but not dry matter yield, and acid and neutral detergent fibres. The most dissimilar tests were polycross and open‐pollinated, with significant correlations found only for fertility index, height, spread and neutral detergent fibre. It was concluded that there was significant variation and moderate heritability for most traits in the meadow bromegrass populations evaluated, and that the open‐pollinated progeny test is the method of choice for selecting parents for synthetics of this species.     

食用甘薯品种选育及系谱分析

为了探索食用甘薯品种的选育方法,加快优良食用甘薯品种的选育进程。本研究对2001—2020年育成的22个食用型甘薯品种主要性状和亲本系谱进行了分析。结果表明：22个食用甘薯品种的平均鲜薯产量32689.0 kg/hm²,平均干物率28.1%。其中12个苏薯系列红心食用甘薯品种的平均胡萝卜素含量4.77 mg/100g鲜薯,10个宁紫薯系列紫心食用甘薯品种的平均花青素含量24.49 mg/100g鲜薯。系谱分析表明：苏薯系列红心食用甘薯品种基因源主要来自于美国品种,其中大多数品种都具有‘南瑞苕’血统,宁紫薯系列紫心甘薯品种基因源主要来自于日本品种,同时种质创新和桥梁亲本的应用也对紫心甘薯品种选育起到重要作用。外引资源和创新种质都是食用甘薯育种不可或缺的亲本来源,定向配组杂交是选育优良食用甘薯品种的主要方法。     

Growth Analysis, Herbage and Seed Yield of Certain Forage Legume Species Under Rainfed Conditions

The need to increase forage production for the growing livestock population in the Mediterranean region necessitates the evaluation of various promising forage species with respect to maximum herbage yield, optimum time for harvest and seed yield. Seven forage legumes, woolly pod vetch ( Vicia villosa subsp. dasycarpa accession 683), narbon vetch (V. narbonensis accession 67), common vetch ( V. sativa selections 2541, 2037, 2020), common chickling ( Lathyrus sativus selection 439) and ochrus chickling ( L. ochrus selection 104) were compared over two years with contrasting rainfall. In both years the entries were sown in November and their primary growths sampled periodically during growth. For each sample, phenology, total dry matter, leaf area index (LAI) and, at maturity, total dry matter, seed yield and harvest index, were recorded.
In both seasons, narbon vetch attained the highest dry matter and seed yields, and there was very limited variability in the maximum dry matter yield of the other entries. In general, the maximum dry matter occurred at 20–50 % podding in woolly pod vetch, 100 % podding in common vetch selection 2541 (local), narbon vetch and common chickling and at maturity in common vetch selection 2037, selection 2020 and ochrus chickling.
Throughout the vegetative to early flowering stages, woolly pod vetch had the highest dry matter yield and LAI. Possible uses of each species as animal feed are discussed.     

套作条件下种植密度对紫色甘薯干物质生产的影响

为了研究川中丘陵区旱地套作条件下种植密度对紫色甘薯干物质生产的影响,在与玉米套作条件下,以鲜食型紫色甘薯品种‘南紫薯008’为材料,设置5个种植密度,测定紫色甘薯叶面积指数、群体生长率、群体光合势、干物质积累、产量等指标。结果表明：在套作条件下,随着种植密度的增加,‘南紫薯008’群体叶面积指数有升高的趋势,群体生长率、群体光合势、商品薯率、干物质率、鲜薯产量、淀粉产量均呈先升后降的趋势,而单株结薯数、单株薯鲜重则下降,群体干物质积累量和T/R则随生育期的推进不同密度间表现有所差异。公顷种植密度在3.5×10⁴~4.0×10⁴株时,群体干物质积累量、鲜薯产量、淀粉产量和商品薯率均较高。套作条件下‘南紫薯008’最高鲜薯产量为11816.9 kg/hm²,约为其净作产量的42.6%~53.1%。川中丘陵区旱地套作条件下紫色甘薯适宜的种植密度为3.5×10⁴~4.0×10⁴株/hm²。     

Root zone temperature alters storage root formation and growth of sweetpotato

Large mounds amended with plant residues are a common cultivation method for sweetpotato (Ipomoea batatas) in the western parts of the Papua New Guinea highlands, with a suggested benefit of elevated root zone temperature (RZT) from decomposition. This study aimed to evaluate whether RZT affects the formation and growth of sweetpotato storage roots. Experiments evaluated the effects of a constant elevated RZT on the Beauregard cultivar and of a short‐term increase in RZT on the Beauregard and Northern Star cultivars. Plants were grown in 18‐L pots divided into a control compartment (constant RZT at 20°C) and a treated compartment (RZT at 30, 40 or 50°C). Constant elevated RZT was maintained until harvest, while short‐term increase in RZT occurred in weeks 2 and 3 only. Constant elevated RZT did not affect the total number of storage roots, but RZT of 30°C and above reduced the dry‐biomass of storage roots. The short‐term increase in RZT did not alter the total number or dry‐biomass of storage roots in Beauregard, while both of these attributes were reduced at RZT of 40°C or above in Northern Star. Elevated RZT inhibited dry‐biomass accumulation of storage root, with a more pronounced impact at constant elevated RZT. These results demonstrate that any benefits of the mound culture on sweetpotato yield production do not arise through the influence of temperature elevation.     

Carbon Isotope Discrimination and Productivity in Field-Grown Barley Genotypes

Five barley genotypes of different origins were used to examine the relationships between carbon isotope discrimination (Δ) and grain yield, yield components, dry aerial biomass and harvest index under rainfed and irrigated Mediterranean conditions. High positive correlations were found between Δ and grain yield and harvest index under both water conditions, suggesting that Δ may represent efficiency of dry matter partitioning to the grain. This hypothesis was also supported by the positive correlation found between Δ and thousand-grain weight when all the data were considered. The two drought-tolerant genotypes LM2887 and Tadmor presented the smallest difference between the two treatments for traits related to productivity as well as the highest Δ values. These results highlight the potential of Δ as a selection criterion in barley breeding in Mediterranean regions. They also encourage the study of the genetics of Δ and the identification of molecular markers linked to its variation in available progenies derived from crosses between the genotypes used in the present paper.     

Genotypic Variability in Adaptation Responses of Selected Clones of Cassava to Drought Stress in the Sudan Savanna Zone of Nigeria

Identification of drought‐tolerant and ‐resistant varieties of cassava is of paramount importance for the maximization of productivity potential in drought‐prone areas. Nine improved cassava varieties developed in humid and subhumid agro‐ecologies were evaluated for their growth responses and adaptability in the dry savannas of Nigeria. Field trials were set up in two cropping seasons at three different locations. Parameters evaluated included fresh root yield (FRY), fresh shoot yield (FSY), cumulative leaves formed (CLF), cumulative leaf scars (CLS), root dry‐matter content (DMC), harvest index (HI), and root cyanogenic potential (CNp). Results demonstrated that varietal response differed with water table site, implying different adaptation responses of varieties to the water regime. Variations in all characters studied were significantly (P<0.05) influenced by genotype, suggesting a strong genetic basis for the phenotypic differences amongst varieties. Compared with variation attributable to location, the genotypic components were strong for FRY, FSY, CLF, CLS, HI and DMC. HI and DMC were not significantly affected by location. CNp tended to increase with increasing drought stress. The significance of genotype × environment effects for FRY suggested that genotypes may be selected for specific adaptation for drought‐prone environments. The poor adaptability responses of the majority of the varieties implied that an agro‐ecologically targeted breeding scheme for the dry savanna would be a more appropriate approach for the development of drought‐tolerant varieties.     

木薯单株产量与主要农艺性状的相关性研究

探讨木薯单株产量与主要农艺性状的相关性,以提高木薯育种效率,为木薯品种选育奠定基础。采用13 份木薯种质资源为试验材料,通过相关与通径分析等方法,探讨了11 个主要农艺性状与单株产量之间的相关性以及不同种质资源单株产量之间的差异性。结果表明：NK-2、NK-6、NK-7、NK-10、NK-11、‘新选048’与‘华南205’之间的单株产量呈显著差异;单株产量与块根数呈极显著正相关,与收获指数、生物产量、最长薯长、主茎高度呈显著正相关,与干物质含量呈显著负相关;通径分析结果表明,各农艺性状对单株产量的直接通径系数和决定系数的大小顺序一致,依次为：块根数、最长薯长、生物产量、主茎高度、叶保留高度、干物质含量、收获指数、地上部鲜重、淀粉含量、主茎粗、株高。由此表明,选育高产木薯需要关注的主要农艺性状有：块根数、最长薯长、生物产量、主茎高度、叶保留高度、干物质含量、收获指数等。     

高产优质紫心甘薯新品种‘宁紫薯6号’的选育及育种策略探讨

旨在选育高产优质紫心甘薯新品种,加快优良品种的更新换代。以‘苏薯8号’为母本,‘渝紫薯7号’为父本,通过有性杂交,采用多环境交替定向选择、高低世代组合筛选方法,育成紫心甘薯品种‘宁紫薯6号’。研究表明,‘宁紫薯6号’聚合了双亲的优良性状,平均鲜薯产量40100.40 kg/hm²,较对照品种增产21.09%。平均干物质含量29.36%。食味评分77.8,比对照高7.8。花青素含量17.68 mg/100gFW,大中薯率84.7%。抗黑斑病和中抗茎线虫病。该品种高产稳产、食味品质优异,抗性较好,2019年通过国家非主要农作物品种登记,可作为鲜食及食品加工型紫薯品种,在长江流域薯区推广种植。笔者通过对育种策略的讨论,认为核心亲本的高效利用将显著提高育种效率,“参与育种模式”将加速品种的推广应用。     

Root characteristic system improves drought tolerance in orchardgrass

Plant genotypes with higher drought tolerance through improved root characteristics are poorly studied in orchardgrass. In the current research, 30 orchardgrass genotypes were polycrossed and the resulting half‐sib families evaluated under both normal and water stress environments. Under water stress conditions, values for most root traits decreased at 0–30 cm soil depth, while at 30–60 cm depths, the root length (RL), root area (RA), root volume, percentage of root dry weight (RDW) and the ratio of root to shoot were increased. We identified drought‐tolerant genotypes with a high combining ability for root characteristics and a high yield potential. High estimates of heritability as well as genetic variation for root traits indicated that phenotypic selection would be successful in order to achieve genetic progress. Indirect selection to improve dry matter yield was most efficient when selecting for RL and RDW under water stress conditions. Significant associations between a drought tolerance index and RL, RA and root volume confirmed the importance of these traits in conferring drought tolerance of orchardgrass.     

Introgression of genes for dry matter content from wild cassava species

Cassava cultivars often have poor resistance to biotic stresses and lack good quality traits. Wild species of cultivated crops have frequently been used as an important source of genetic diversity. Cassava breeders are becoming increasingly interested in incorporating genes of wild relatives. In 2000 the International Center for Tropical Agriculture (CIAT) initiated a programme to introgress genes for several root yield and quality traits from wild cassava relatives into its germplasm collection. The objectives of this study were to evaluate one resulting inter-specific cross with high variability for dry matter content (DMC) and to assess the effect of such a cross on other yield related traits. Crossing of the elite cultivar MTAI 8 to the wild relative Manihot tristis increased the percentage DMC above the normal average of about 35%, with percentage DMC ranging from 34.39 to 42.73. The crosses, however, were accompanied by some detrimental effects, most noticeable the reduction in harvest index (HI). It is apparent that when selecting for DMC, caution should be taken and HI and fresh root yield should be monitored. Regression analysis singled root weight, percentage DMC and fresh root yield out as the most important contributors to dry root yield. Principal component analysis indicated that root weight, roots per plants and DMC contributed most to storage root yield.     

氮肥施用量对不同紫甘薯品种产量和氮素效率的影响

选取紫甘薯品种浙紫1号、宁紫2号和紫菁2号,设置3个施氮处理,即0 (N0)、75 (N1)和150 (N2) kg hm^–2纯氮,于2013-2014年2个生长季在青岛农业大学现代农业科技示范园进行大田试验,研究不同氮肥用量对块根产量、干物质累积速率、氮素累积量及氮素效率的影响。结果表明,施用氮肥不同程度地降低了浙紫1号和紫菁2号的薯块产量,其中,浙紫1号的N1、N2处理分别较N0处理降低12.64%和13.32%,紫菁2号分别降低3.94%和29.06%;宁紫2号N1处理产量略高于N0处理,两年分别较N0处理提高8.5%和3.4%,而N2处理块根产量显著低于N0处理。茎蔓生物量和氮素累积量随着施氮量的增加而增加,而收获指数、氮素收获指数和氮素利用效率逐渐降低。第1年N1、N2处理的茎蔓干物质累积量分别较N0处理提高2.7%~20%和12.3%~36.4%,第2年分别提高12.6%~51.9%和28.7%~85.5%。相关分析表明,块根产量与氮素效率各指标均呈显著或极显著正相关;而茎蔓生物量与收获指数、氮素收获指数及氮肥利用效率均呈极显著负相关(r = –0.615^**, –0.704^**, –0.663^**)。肥沃土壤上施用氮肥会造成浙紫1号和紫菁2号的茎蔓旺长,光合产物向薯块转运比例降低,导致源库比例不协调,块根产量下降。宁紫2号对氮肥的需求相对较高,施用氮肥75 kg hm^–2时鲜薯产量提高,而施氮量过高时薯块产量降低。因此,紫甘薯在含氮量较高的肥沃土壤上种植时,对氮肥的需求量较低,茎蔓和薯块的协调生长是提高块根产量和氮素利用效率的保障。     

甘薯苗期根系农艺性状与内源激素的动态变化研究

为了解甘薯苗期根系生长发育与内源激素之间的关系,通过水培的方法分析了根系农艺性状与内源激素的动态变化,结果表明：在栽后25天内,‘烟薯25’的根长指标一直保持最高,色素提取型甘薯品种Aya和‘济黑1’的平均根鲜重最高,但其平均单株根干重却最低。淀粉型甘薯品种（‘徐薯18’、‘济徐23’）根系在栽后10天就已块根分化完成,并开始了干物质的积累,早于鲜食型甘薯品种（‘济薯22’、‘烟薯25’）和色素提取型甘薯（Aya、‘济黑1’）的根系分化过程。对不同节位根系的内源激素分析发现,尽管6个品种的不同时期、不同节位上根系的内源激素含量变化存在品种间差异,但3个节上根系生长促进类激素(IAA、ZR、GAs)含量在栽后15天左右达到高峰,倒三节根系中的ABA含量低谷也出现在栽后15天左右。对甘薯苗期不同节位根系农艺性状与内源激素间的相关性分析发现,从栽后5天至栽后25天,倒一节根系体积的增大主要与生长类内源激素的平衡紧密相关,生长较为缓慢,而薯苗的倒二节和倒三节根系在栽后20天左右进入了生长的旺盛期,干物质积累迅速。倒一节至倒三节根系分化的时间分别为栽后20天、栽后15天和栽后10天左右,从薯苗基部向上,各节位根系分化时间有逐渐提前的趋势。总之,甘薯苗期不同节位根系的生长、分化特点以及内源激素调控存在时间和空间的差异。     

不同产量水平甘薯品种光合产物分配差异及其原因

选取我国北方薯区主栽的食用型甘薯品种龙薯9号、红香蕉、泰中6号、苏薯8号、遗字138和北京553,于2011—2012年2个生长季在山东农业大学农学试验站进行大田试验,研究不同产量水平甘薯品种光合产物的分配差异及其原因。在甘薯主要生长时期测定茎蔓各部位蔗糖含量和基部茎粗、在块根膨大过程中对功能叶进行13C标记,探索不同品种13C同化物分配特点,同时研究上述项目与收获指数和块根产量的关系。结果表明,高产品种龙薯9号、红香蕉、泰中6号和苏薯8号产块根均在60 t hm–2左右,而中、低产品种遗字138和北京553产块根在50 t hm–2以下。高产品种的收获指数显著高于中、低产品种。在块根膨大前期高产品种的块根成为光合产物的分配中心,而中、低产品种的块根则在块根膨大后期成为分配中心;在块根膨大前、中期,高产品种13C同化物在块根中的分配比例显著高于中、低产品种,即高产品种光合产物由功能叶向块根的转运能力强于中、低产品种。供试品种茎蔓基部的蔗糖含量均高于顶部,且中、低产品种的蔗糖浓度差大于高产品种;随着甘薯生长发育,中、低产品种茎蔓基部增粗速率高于高产品种。相关分析表明,块根产量与收获指数呈极显著正相关(r=0.98**,2011;r=0.93**,2012);块根13C分配率与块根产量和收获指数均呈显著正相关(0.87*和0.91**,2011;0.92**和0.99**,2012);生长中后期,基部茎粗与块根产量和收获指数均呈显著负相关(–0.78*和–0.83*;–0.88**和–0.95**)。块根成为光合产物分配中心的时间早、光合产物由功能叶向块根的转运能力强是高产品种收获指数显著提高的主要原因,而中、低产品种光合产物由叶片向块根转运能力差的主要原因是块根中光合产物卸载不畅。