Mineral nutrition and growth of tropical maize as affected by soil acidity

Soil constraints linked to low pH reduce grain yield in about 10% of the maize growing area in tropical developing countries. The aim of this research was to elucidate the reasons for this maize yield reduction on an oxisol of Guadeloupe. The field experiment had two treatments: the native non-limed soil (NLI, pH 4.5, 2.1 cmol Al kg^–1, corresponding to 20% Al saturation), and the same soil limed 6 years prior to the experiment (LI, pH 5.3, 0 cmol Al kg^–1). The soils were fertilized with P and N. The above-ground biomass, root biomass at flowering, grain yield and yield components, leaf area index (LAI), light interception, radiation-use-efficiency (RUE), P and N uptake, soil water storage, and soil mineral N were measured during the maize cycle. The allometric relationships between shoot N concentration, LAI and above-ground biomass in LI were similar to those reported for maize cropped in temperate regions, indicating that these relationships are also useful to describe maize growth on tropical soils without Al toxicity. In NLI, soil acidity severely affected leaf appearance, leaf size and consequently the LAI, which was reduced by 60% at flowering, although the RUE was not affected. Therefore, the reduction in the above-ground biomass (30% at flowering) and grain yield (47%) were due to the lower LAI and light interception. At flowering, the root/shoot ratio was 0.25 in NLI and 0.17 in LI, and the root biomass in NLI was reduced by 64% compared to LI. Nitrogen uptake was also reduced in NLI in spite of high soil N availability. Nevertheless, shoot N concentration vs aboveground biomass showed a typical decline in both treatments. In NLI, the shoot P concentration vs above-ground biomass relationship showed an increase in the early stages, indicating that P uptake and root-shoot competition for the absorbed P in the early plant stages controlled the establishment and the development of the leaf area.     

暖温带地区3种森林群落叶面积指数和林冠开阔度的季节动态

以辽东栎(Quercus liaotungensis)为主的落叶阔叶林、华北落叶松(Larix principis-rupprechtii)林和油松(Pinus tabulaeformis)林是暖温带林区具有代表性的森林群落类型。该研究应用国内外流行的半球图方法,通过对这3种森林群落叶面积指数和林冠开阔度的测定和综合比较,分析了叶面积指数和林冠开阔度的季节动态,揭示了暖温带地区不同类型森林群落叶面积指数和林冠开阔度的特征。研究结果表明,落叶阔叶林(优势种为辽东栎、棘皮桦(Betula dahurica)和五角枫(Acer mono))和华北落叶松林两种落叶森林群落的叶面积指数值均随生长季的到来而呈现增长的趋势,最大值出现在8月;林冠开阔度值随着生长季的到来而下降,最大值出现在11月。落叶阔叶林的叶面积指数和林冠开阔度的季节动态较之华北落叶松林明显。油松是常绿树种,其群落叶面积指数和林冠开阔度的变化程度均不明显,但林冠开阔度的变化趋势也是与叶面积指数的变化趋势相反。通过计算得出叶面积指数和林冠开阔度相关显著,并且呈现指数回归的关系。此研究结果为以遥感途径获取暖温带地区叶面积指数提供了地面校正依据,为研究该地区植被林冠的异质性及其造成的影响,以及进一步对该地区林分、景观和区域尺度上碳、水分和通量等方面的模拟提供了基础数据。     

不同放牧制度下呼伦湖流域草原植被冠层截留

冠层截留是降雨过程中的水量分配和流域水平衡的一个重要组成部分,通过水浸泡法和降雨模拟实验研究呼伦湖流域草原3种放牧制度下(休牧、轮牧、自由放牧(超载放牧))植被冠层截留量的变化规律,并利用遥感解译植被归一化指数(NDVI),确定3种放牧制度下草原面积,估算呼伦湖流域草原降雨截留量。研究表明:在休牧、轮牧、自由放牧3种制度下,水浸泡法测定的截留量分别是0.468、0.320、0.271 mm。降雨模拟实验法测得的结果分别是0.957、0.613、0.431 mm。休牧、轮牧、自由放牧草场叶面积指数、盖度、容重、生物量等指标差异显著(P0.05),且单株植被高度、鲜重对截留量影响显著呈线性正相关关系。呼伦湖流域草原一次降雨量为大于等于30 mm全流域降雨,其植被截留量为6.462×106m3。     

Comparative photosynthesis,growth, productivity,and nutrient use efficiency among tall- and short-stemmed rain-fed cassava cultivars

Field trials under rain-fed conditions at the International Center for Tropical Agriculture (CIAT) in Colombia were conducted to study the comparative leaf photosynthesis, growth, yield, and nutrient use efficiency in two groups of cassava cultivars representing tall (large leaf canopy and shoot biomass) and short (small leaf canopy and shoot biomass) plant types. Using the standard plant density (10,000 plants ha⁻¹), tall cultivars produced higher shoot biomass, larger seasonal leaf area indices (LAIs) and greater final storage root yields than the short cultivars. At six months after planting, yields were similar in both plant types with the short ones tending to form and fill storage roots at a much earlier time in their growth stage. Root yield, shoot and total biomass in all cultivars were significantly correlated with seasonal average LAI. Short cultivars maintained lower than optimal LAI for yield. Seasonal P _N, across cultivars, was 12% greater in short types, with maximum values obtained in Brazilian genotypes. This difference in P _N was attributed to nonstomatal factors (i.e., anatomical/biochemical mesophyll characteristics). Compared with tall cultivars, short ones had 14 to 24 % greater nutrient use efficiency (NUE) in terms of storage root production. The lesser NUE in tall plants was attributed mainly to more total nutrient uptake than in short cultivars. It was concluded that short-stemmed cultivars are superior in producing dry matter in their storage roots per unit nutrient absorbed, making them advantageous for soil fertility conservation while their yields approach those in tall types. It was recommended that breeding programs should focus on selection for more efficient short- to medium-stemmed genotypes since resource-limited cassava farmers rarely apply agrochemicals nor recycle residual parts of the crop back to the soil. Such improved short types were expected to surpass tall types in yields when grown at higher than standard plant population densities (>10,000 plants ha⁻¹) in order to maximize irradiance interception. Below a certain population density (<10,000 plants ha⁻¹), tall cultivars should be planted. Findings were discussed in relation to cultivation and cropping systems strategies for water and nutrient conservation and use efficiencies under stressful environments as well as under predicted water deficits in the tropics caused by trends in global climate change. Cassava is expected to play a major role in food and biofuel production due to its high photosynthetic capacity and its ability to conserve water as compared to major cereal grain crops. The interdisciplinary/interinstitutions research reported here, including an associated release of a drought-tolerant, short-stem cultivar that was eagerly accepted by cassava farmers, reflects well on the productivity of the CIAT international research in Cali, Colombia.     

Silicon supply in soilless cultivations of zucchini alleviates stress induced by salinity and powdery mildew infections

In the present study, the hypothesis was tested as to whether silicon supplied via the nutrient solution is capable of enhancing the tolerance of hydroponically grown zucchini squash (Cucurbita pepo L. cv. ‘Rival’) to salinity and powdery mildew infections. Two experiments were conducted involving a low (2.2 dS m^?1, 0.8 mM NaCl) and a high salinity level (6.2 dS m^?1, 35 mM NaCl) in combination with a low (0.1 mM) and a high (1.0 mM) Si level in the nutrient solution supplied to the crop. The exposure of the plants to high external salinity restricted significantly the vegetative growth as well as the fruit yield of zucchini due to a reduction of both the number of fruits per plant and the mean fruit weight. However, the inclusion of 1 mM of Si in the salinized nutrient solution mitigated the salinity-associated suppression of both growth and yield. Part of the growth and fruit yield suppression at high salinity was due to restriction of net photosynthesis. The stomatal conductance was also restricted by salinity, whereas the substomatal CO₂ concentration was not affected by the NaCl or Si treatments. The supply of 1 mM of Si via the nutrient solution mitigated the inhibitory effect of salinity on net photosynthesis and this effect was associated with lower Na and Cl translocation to the epigeous plant tissues. Furthermore, the supply of Si via the nutrient solution suppressed appreciably the expansion of a powdery mildew (Podosphaera xanthii) infection in the leaves at both salinity levels. These results indicate that the supply of at least 1 mM of Si via the nutrient solution is capable of enhancing both tolerance to salinity and resistance to powdery mildew in soilless cultivations of zucchini squash.     

Shoot and root competition in potato/maize intercropping: Effects on growth and yield

Interspecific competitive relationships and their effect on yield have been analysed in the association of potato and maize, two species with contrasting patterns of root and shoot systems establishment. Greenhouse experiments were carried out under three configurations (NC: no interspecific competition; FC: shoot and root interspecific competition; SC: shoot-only interspecific competition). Despite large variations between replicate experiments associated with seasonal effects, the study revealed consistent patterns of competition for above- and below-ground resources. Light interception in FC and SC was dominated by potato (60%) during the first 45 days after planting and by maize thereafter (80%). The extra shade caused by the companion crop increased soil moisture by up to 10% in SC treatments. The yield of the two species responded in opposite ways to SC, which was consistent with asymmetric patterns of competition between the two species. In potato, FC reduced tuber yield (number and size) by 4–26%, while SC increased tuber size (compared to NC) by 3–39%. In maize, FC reduced LAI and plant height by up to 45%, shoot and root dry mass, nutrient content, yield, the weight of 100 grains and harvest index by ca. 30–100%, while SC affected all but LAI and plant height. It appears that the contrast between the progressive installation of the maize root system and the rapid early extension of the potato root system is amplified by the restriction of maize root development under competition, which leads to close interdependencies between root and shoot competitive relationships. Although the specific effects of root competition cannot be uncovered by this set of experiments, competition effects on maize in the potato/maize intercropping seem to primarily related to light availability in the mixed canopy.     

小麦LAI-2000观测值对辐亮度变化的响应

利用LAI-2000植物冠层分析仪和ASD光谱仪,通过固定点拔节期冬小麦叶面积指数(LAI)观测实验和同步光谱辐亮度实验,测量了晴天条件下,固定点冬小麦从中午至傍晚24个不同时刻的LAI值及对应的辐亮度。继之分析了此段时间内测得的定点冬小麦LAI值分别与对应时刻可见光和近红外谱段的天空光辐亮度、总辐亮度和太阳直射辐亮度值之间的相关性。结果表明,无论波长小于490nm的谱段,还是波长大于490 nm的谱段,LAI与天空光辐亮度、总辐亮度和太阳直射辐亮度都呈负相关,相关系数(R2)高达0.8左右;尤其LAI与天空光辐亮度的负相关性最高,这种相关性随着波长的增大而减小。LAI与各谱段天空光辐亮度的相关性特征可为LAI-2000晴天观测的LAI值归一化修正处理提供一种新思路和技术途径,以消除太阳直射光的影响,从而解决LAI-2000只能在晴天观测的局限性,拓展LAI-2000在晴天观测条件下的适用性。     

Treatment by acidification followed by solid–liquid separation affects slurry and slurry fractions composition and their potential of N mineralization

The aim of the present work was to assess the effect of treatments by acidification, solid–liquid separation or acidification followed by solid–liquid separation on the physical and chemical composition of pig slurry (S) and pig slurry fractions (non acidified and acidified solid (SF and ASF) and liquid (LF and ALF) fractions), as well as on the potential of N mineralization of these pig slurry derived materials. Acidification strongly decrease the inorganic carbon content of S, SF and LF and it also affects the distribution of P, Ca and Mg between the solid and liquid fraction leading to an ALF more equilibrated than LF in terms of nutrients. Acidification increases the potential of organic N mineralization in SF and decreases the potential of N immobilization in S and LF. It can be concluded that the proposed treatment generates valuable slurry fractions with distinct characteristics and potential of N mineralization that may be incorporated to soil at different periods after sowing to comply with plant nutrient requirements.     

Physiological N response of field-grown maize hybrids (Zea mays L.) with divergent yield potential and grain protein concentration

Fertilizer N availability impacts photosynthesis and crop performance, although cause–effect relationships are not well established, especially for field-grown plants. Our objective was to determine the relationship between N supply and photosynthetic capacity estimated by leaf area index (LAI) and single leaf photosynthesis using genetically diverse field-grown maize (Zea mays L.) hybrids. We compared a high yield potential commercial hybrid (FR1064 x LH185) and an experimental hybrid (FR1064 x IHP) with low yield potential but exceptionally high grain protein concentration. Plant biomass and physiological traits were measured at tassel emergence (VT) and at the grain milk stage (R3) to assess the effects of N supply on photosynthetic source capacity and N uptake, and grain yield and grain N were measured at maturity. Grain yield of FR1064 x LH185 was much greater than FR1064 x IHP even though plant biomass and LAI were larger for FR1064 x IHP, and single leaf photosynthesis was similar for both hybrids. Although photosynthetic capacity was not related to hybrid differences in productivity, increasing N supply led to proportional increases in grain yield, plant biomass, LAI, photosynthesis, and Rubisco and PEP carboxylase activities for both hybrids. Thus, a positive relationship between photosynthetic capacity and yield was revealed by hybrid response to N supply, and the relationship was similar for hybrids with a marked difference in yield potential. For both hybrids the N response of single leaf CER and initial Rubisco activity was negative when expressed per unit of leaf N. In contrast, PEP carboxylase activity per unit leaf N increased in response to N availability, indicating that PEP carboxylase served as a reservoir for excess N accumulation in field-grown maize leaves. The correlation between CER and initial Rubisco activity was highly significant when expressed on a leaf area or a total leaf basis. The results suggest that regardless of genotypic yield potential, maize CER, and potentially grain yield, could be improved by increasing the partitioning of N into Rubisco.     

Analyses of the effects of potato cyst nematodes (Globodera pallida) on growth, physiology and yield of potato cultivars in field plots at three levels of soil compaction

Field experiments were carried out in 1991 and 1992 on sandy soil highly infested with the potato cyst nematode Globodera pallida. Half the trial area was fumigated with nematicide to establish two levels of nematode density. Three levels of soil compaction were made by different combinations of artificial compaction and rotary cultivation. Two potato cultivars were used in 1991 and four in 1992. Both high nematode density and soil compaction caused severe yield losses, of all cultivars except cv. Elles which was tolerant of nematode attack. The effects of the two stress factors were generally additive. Analysis of the yield loss showed that nematodes mainly reduced cumulative interception of light while compaction mainly reduced the efficiency with which intercepted light was used to produce biomass. This indicates that nematodes and compaction affect growth via different damage mechanisms. Nematodes reduced light interception by accelerating leaf senescence, by decreasing the specific leaf area and indirectly by reducing overall crop growth rate. Partitioning of biomass between leaves, stems and tubers was not affected by nematode infestation but compaction decreased partitioning to leaves early in the growing season while increasing it during later growth stages. The effects of nematodes and compaction on root length dynamics and nutrient uptake were also additive. This suggests that the commonly observed variation in yield loss caused by nematodes on different soil types is not related to differences in root system expansion between soils of various strength. Cv. Elles, which showed tolerance of nematodes by relatively low yield losses in both experiments, was characterised by high root length density and thick roots. These characteristics did not confer tolerance of soil compaction, since compaction affected root lengths and tuber yields equally in all cultivars. In the first experiment only, high nematode density led to decreased root lengths and lower plant nutrient concentrations. The yield loss which occurred in the second experiment was attributed to the effects of nematodes on other aspects of plant physiology.     

Comparison of the nutrient ecology of coastal Banksia grandis elfinwood (windswept shrub‐like form) and low trees,Cape Leeuwin‐Naturaliste National Park,Western Australia

Abstract Trees growing along windy coasts often have canopies that are greatly reduced in size by the sculpting effects of wind and salt spray. Trees with environmentally reduced stature are called elfinwood (windswept shrub‐form or krummholz) and are ecologically important because they represent outposts growing at the limit of tree success. The purpose of this study was to assess if Banksia grandis elfinwood growing at Cape Leeuwin had a different nutrient status than normal low‐form (LF) trees growing nearby, and if nutrient deficiencies, toxicities and/or imbalances were among the limiting factors imposed on elfinwood. The concentrations of N, P, K, Ca, Mg, Na, Cl^–, Fe, Mn, Zn, Cu, Mo and B were analysed for mature green foliage, immature foliage, foliage litter, flowers and soil. When the elfinwood and LF trees were compared, the foliar nutrient status was generally similar, except that elfinwood foliage had significantly higher mean concentrations of N, Zn and Cu, while LF trees had higher Fe and Mn contents. Many nutrients were conserved before leaves were shed in both elfinwood and LF trees, including N, P, K, Na, Cl^–, Mn and Cu (LF trees also conserved Ca and Mg). However, elfinwood and LF tree‐litter contained significantly higher Fe concentrations than green foliage (elfinwood litter also had higher levels of Mg and B). It is tempting to suggest that the translocation of Fe into leaves before they were shed is a regulation mechanism to prevent Fe toxicity, or imbalance in the Fe : Mn ratio. Proteoid roots strongly acidify the soil to mobilize P, which also chemically reduces Fe⁺³ to plant‐available Fe⁺². The increased supply of Fe⁺² in the rhizosphere, caused by the action of proteoid roots, might tend to defeat self‐regulation of Fe uptake. It is possible that excess Fe accumulation in the plant might be regulated, in part, by exporting Fe into the leaves before they are shed. The nutrient status of B. grandis elfinwood is compared with mountain elfinwood of North America. The extreme habitat of coastal elfinwood provides many theoretical pathways for nutrient limitation, but B. grandis elfinwood at Cape Leeuwin does not appear to be nutrient deficient.     

华北落叶松人工林蒸散及产流对叶面积指数变化的响应

定量评价林地蒸散和产流等水文过程对冠层叶面积指数(LAI)的响应,对于深入认识森林植被的生态水文过程及其发生机制,实现半干旱区林水综合管理和区域可持续发展是非常必要的。应用集总式生态水文模型BROOK90,模拟分析了不同降水年型(丰水年、平水年、枯水年)下,位于半干旱区的宁夏六盘山叠叠沟小流域内华北落叶松(Larix principis-rupprechtii)人工林的水文过程对冠层LAI变化的响应关系。结果发现:林分总蒸散量、冠层截留量、蒸腾量与LAI都呈显著的正相关关系(R~20.99,P0.01),而土壤蒸发量、产流量则与LAI均呈显著的负相关关系(R~20.99,P0.01);在不同的降水年型下,各水文过程变量与LAI的关系都可以很好地用指数函数来表达,且都存在着一个LAI阈值。当LAI低于阈值时,各水文过程变量随LAI的变化幅度较大;但高于阈值时,各变量的变化十分缓慢并趋于稳定。在不同降水年型下,各变量LAI阈值之间存在着一定的差异。一般地,丰水年各变量的LAI阈值要大于枯水年,尤其是冠层截留和土壤蒸发。在丰水年,各水文过程变量随LAI增加而变化的速率要比在平水年、枯水年更快,说明在水分充足年份中各变量的波动更多取决于LAI变化,而在水分亏缺的年份中则可能更多地受到水分条件的限制。模拟结果表明,通过减少冠层LAI(如间伐)导致的林分的降低蒸散耗水和增加产流的作用是有限的,这是由于林分蒸散降低的幅度要比LAI降低的幅度小。例如,在平水年,当LAI从4.2变为2.0(减少幅度52.4%)时,林分年蒸散仅从357.2 mm减少至333.9 mm(减少幅度6.5%)。     

Aerated subsurface irrigation water gives growth and yield benefits to zucchini, vegetable soybean and cotton in heavy clay soils

Inadequate oxygen concentration in the root zone is a constraint to plant performance particularly in heavy, compacted and/or saline soils. Sub-surface drip irrigation (SDI) offers a means of increasing oxygen to plant roots in such soils, provided irrigation water can be hyper-aerated or oxygenated. Hydrogen peroxide (HP) at the rate of 5 litre ha⁻¹ at the end of each irrigation cycle was injected through SDI tape to a field-grown zucchini (courgette) crop (Cucurbita pepo) on a saturated heavy clay soil in Queensland, Australia. Fruit yield, number and shoot weight increased by 25%, 29% and 24% respectively due to HP treatment compared to the control. Two pot experiments with vegetable soybean (Glycine max) and cotton (Gossypium hirsutum) compared the effectiveness of HP and air injection using a Mazzei air injector (a venturi), throughout the irrigation cycle in raising crop yield in a heavy clay soil kept at saturation or just under field capacity. Fresh pod yield of vegetable soybean increased by 82–96% in aeration treatments compared with the control. The yield increase was associated with more pods per plant and greater mean pod weight. Significantly higher above ground biomass and light interception were evident with aeration, irrespective of soil water treatment. Similarly cotton lint yield increased by 14–28% in aeration treatments compared with the control. The higher lint yield was associated with more squares and bolls per plant which accompanied greater above ground biomass and an increase in root mass, root length and soil respiration. Air injection and HP effected greater water use, but also brought about an enhancement of water use efficiency (WUE) for pod and lint yield, and increased leaf photosynthetic rate in both species but had no effect on transpiration rate and stomatal conductance per unit leaf area. Aeration-induced enhanced root function was arguably responsible for greater fruit set and yield in all three crops, while in vegetable soybean greater canopy cover, radiation interception and total vegetative biomass were responsible for additional yield benefit. Increased aeration of the root zone in heavy clay soils employing either air injection or HP proved beneficial to SDI irrigated crops, irrespective of the soil water conditions, and can add value to grower investments in SDI.     

Canopy Apparent Photosynthetic Characteristics and Yield of Two Spike-Type Wheat Cultivars in Response to Row Spacing under High Plant Density

In northern China, large-spike wheat (Triticum aestivum L) is considered to have significant potential for increasing yields due to its greater single-plant productivity despite its lower percentage of effective tillers, and increasing the plant density is an effective means of achieving a higher grain yield. However, with increases in plant density, the amount of solar radiation intercepted by lower strata leaves is decreased and the rate of leaf senescence is accelerated. Row spacing can be manipulated to optimize the plant spatial distribution under high plant density, therefore improving light conditions within the canopy. Consequently, field experiments were conducted from 2010 to 2012 to investigate whether changes in row spacing under high plant density led to differences in canopy apparent photosynthesis (CAP), individual leaf photosynthesis and grain yield. Two different spike-type winter wheat cultivars, Jimai22 (a small-spike cultivar as a control cultivar) and Wennong6 (a large-spike cultivar), were grown at a constant plant density of 3,600,000 plants ha^–1 (a relatively higher plant density) over a wide range of row spacing as follows: 5-cm row spacing (R₀), 15-cm row spacing (R₁), 25-cm conventional row spacing (R₂), and 35-cm row spacing (R₃). The two-year investigations revealed that increased row spacing exhibited a significantly higher light transmission ratio (LT), which improved light conditions within the canopy; however, excessive light leakage losses in R₂ and R₃ treatments were not favorable to improved irradiation energy utilization efficiency. Aboveground biomass accumulation was influenced by row spacing. Two spike-type wheat accumulated greater biomass under 15-cm row spacing compared to other row spacing treatments, although a markedly improved photosynthetic rate (P_N), effective quantum yield of photosystem II (Φ_PSII) and maximal efficiency of photosystem II photochemistry (F_v/F_m) in the penultimate and third leaves were observed in R₂ and R₃ treatments. At the same time, a longer duration of CAP and green leaf area was maintained in R₁ during grain filling. Compared with conventional row spacing, Wennong6 in R₁ treatment obtained 21.0% and 19.1% higher grain yield in 2011 and 2012, respectively, while for Jimai22 it increased by 11.3% and 11.4%, respectively. A close association of yield with CAP and LAI at mid-grain filling was observed. In conclusion, for the tested growing conditions, decreasing the row spacing to an optimal distance (15 cm) maintained a longer duration of LAI and CAP during grain filling, made a better coordination of group and individual leaf photosynthesis, and accumulated higher aboveground biomass, leading to a greater grain yield. In addition, Wennong6 had a more rational canopy architecture than Jimai22 (improved LT and higher LAI) and CAP under 15-cm row spacing, leading to a higher grain yield, which indicated that the large-spike type cultivar has the potential to obtain higher yields by increasing plant density through optimum row spacing allocation (15 cm).     

Reproductive phenology in three Genisteae (Fabaceae) shrub species of the W Mediterranean Region

We studied the flower duration, the phenology of flowering and fruiting, and flower, fruit, and seed production in three Genisteae shrub species of the W Mediterranean Region: Cytisus multiflorus, C. striatus , and Retama sphaerocarpa . Flower duration was negatively correlated with temperature, and in the case of C. striatus it was also influenced by pollination. In Cytisus multiflorus , which flowers during winter in the population studied, two floral morphs were recognized differentiated by flower size, phenological pattern, and production of the reproductive organs: morph LF (large flowers) and morph SF (small flowers). The former is earlier in phenology but its reproductive success is less than the second since it not only produces significantly fewer flowers/plant, but also a smaller crop of fruit and seeds. The flowering of the LF morph lasted some three months and of the SF morph 11 weeks. The winter-spring flowering C. striatus , with a four month duration of flowering time, is sympatric with C. multiflorus and their flowering periods overlap, but the former is more successful reproductively. Retama sphaerocarpa is clearly spring flowering, with an extremely short duration of flowering (c. 6.5 weeks), but, unlike the other two species which disperse their seeds in the same season in which they are produced, it staggers the dispersal of its diaspores over more than one year. Despite the great flower per plant production, especially in Retama sphaerocarpa , and of the number of fruit initiated, the final crop of fruit and seed/plant is low in the three species (C. multiflorus : fruit 8.40% and seeds 1.96%; C. striatus : fruit 13.09% and seeds 4.12%; Retama sphaerocarpa : fruit 5.65% and seeds 1.33%).     

Spatial and temporal variability of canopy structure in a tropical moist forest

Tree fall gaps are widely considered to play a prominent role in the maintenance of species diversity, while the spatiotemporal variability of canopy structure within closed forest stands is largely ignored. In this study we examined the vertical and horizontal components of canopy structure and its seasonal variability in a tropical wet semideciduous rainforest in Panama. Leaf area indices (LAI) were derived from measurements of diffuse radiation and empirically-based leaf angle distribution by mathematical inversion of a light interception model. Vertical distribution of LAI was non-homogeneous with 50% of the leaf area being concentrated in the uppermost 5 m of the canopy. In the wet season, when foliage is most abundant, the horizontal distribution of LAI in a 2100 m² plot ranged widely from 3 to 8, with a mean of 5.41. Changes in mean LAI between wet and dry seasons were small but highly significant. While ca 40% of the area was not affected by local changes in LAI, sizeable small scale changes in LAI did occur between wet and dry season in some locations. Local changes in LAI ranged from –2.3 to 2.4. These changes resulted in a 50% or more increase in light reaching the forest floor at 29% of the measuring locations, and a doubling or more at 13% of the location. Our results imply that structural heterogeneity by simple tree fall gaps do not adequately describe the dynamics of forest canopies.     

有机营养液灌溉频次和灌水量对设施甜瓜产量、品质及肥水利用效率的影响

为研究高产优质甜瓜的有机管理模式,以甜瓜为试材,采用基质盆栽方式,设3种有机营养液灌溉频次(施用8次,每次每株750 mL, F₁; 施用12次,每次每株500 mL, F₂; 施用16次,每次每株375 mL, F₃)与2种单株灌水量(果实膨大前按120%日蒸腾蒸发量(ET)灌溉,之后按140%ET灌溉, W₁; 果实膨大前按140%ET灌溉,之后按160%ET灌溉, W₂),共6个试验处理,随机区组试验设计,研究了不同处理对设施甜瓜光合特性、产量、品质及肥水利用效率的影响.结果表明: 少量多次施用有机营养液可以显著提高甜瓜叶片光合速率,低灌水量显著提高果实产量和水分利用效率;高灌水量和中等有机营养液灌溉频次可使肥料利用率达到最高;少量多次有机营养液施用且相对适宜的水分供给提高了果实品质.回归分析发现,甜瓜果实维生素C含量与有机营养液灌溉频次呈指数函数y=0.214e^0.18x (R²=0.851)相关.综合考虑产量、品质、水分利用效率等因素,F₃W₁处理可在保证产量的前提下,提高果实品质,且水分利用效率最大,可以实现设施有机甜瓜肥水高效管理.     

Response of cowpea to variations in planting density and nutrient level

A greenhouse study was carried out using cowpea (Vigna unguiculata (L.) Walp.) grown in Perlite^® and inoculated with Nitragin^® to investigate the concentration of plant nutrients and planting density required for optimum biomass production. Five concentrations (full, 0.5, 0.2, 0.1 and 0.05 strength) of Bisseling's nutrient solution and five planting densities (one to five plants per pot) were tested in a factorial randomized Graeco-Latin square design. Growth was determined as fresh and dry weights of leaves, stems, petioles, roots, flowers and pods, and whole plant.Optimum biomass production was found at 0.5 strength nutrient solution and a density of one plant per pot. Plants were more sensitive to higher planting density than to alterations of nutrient level. Over a twenty-fold range of nutrient supply, whole plant biomass yield varied at most by 44%, whereas increasing planting density from one to five plants per pot decreased biomass production by as much as 77%. There is a decrease in the shoot/root ratio as nutrient level decreases. The data suggests a potential for higher seed production at the higher densities and lowest nutrient levels, but this data was inconclusive.