Fifteen Australian and exotic wheat varieties revealed genotypic differences in their patterns of root development and drought tolerance. Earliness in plant maturity, which accounted for 40 to go per cent of the observed variation in drought tolerance, appeared to be by far the most important characteristic of tolerant genotypes, but there nevertheless appeared to be a measurable relationship between some parameters of root development, water use efficiency and drought tolerance. The impact of study of this kind on the problem of breeding for drought tolerance is briefly discussed.

The results of research done on water relations of rubber are collated and summarised in an attempt to link fundamental studies on crop physiology to crop management practices. Background information is given on the centres of origin (Amazon Basin) and production of rubber (humid tropics; south-east Asia), but the crop is now being grown in drier regions. The effects of water stress on the development processes of the crop are summarised, followed by reviews of its water relations, water requirements and water productivity. The majority of the recent research published in the international literature has been conducted in south-east Asia. The rubber tree has a single straight trunk, the growth of which is restricted by ‘tapping’ for latex. Increase in stem height is discontinuous, a period of elongation being followed by a ‘rest’ period during which emergence of leaves takes place. Leaves are produced in tiers separated by lengths of bare stem. Trees older than three to four years shed senescent leaves (a process known as ‘wintering’). ‘Wintering’ is induced by dry, or less wet, weather; trees may remain (nearly) leafless for up to four weeks. The more pronounced the dry season the shorter the period of defoliation. Re-foliation begins before the rains start. The supply of latex is dependent on the pressure potential in the latex vessels, whereas the rate of flow is negatively correlated with the saturation deficit of the air. Radial growth of the stem declines in tapped trees relative to untapped trees within two weeks of the start of tapping. Roots can extend in depth to more than 4 m and laterally more than 9 m from the trunk. The majority of roots are found within 0.3 m of the soil surface. Root elongation is depressed during leaf growth, while root branching is enhanced. Stomata are only found on the lower surface of the leaf, at densities from 280 to 700 mm−2. The xylem vessels of rubber trees under drought stress are vulnerable to cavitation, particularly in the leaf petiole. By closing, the stomata play an essential role in limiting cavitation. Clones differ in their susceptibility to cavitation, which occurs at xylem water potentials in the range of −1.8 to −2.0 MPa. Clone RRII 105 is capable of maintaining higher leaf water potentials than other clones because of stomatal closure, supporting its reputation for drought tolerance. Clones differ in their photosynthetic rates. Light inhibition of photosynthesis can occur, particularly in young plants, when shade can be beneficial. Girth measurements have been used to identify drought-tolerant clones. Very little research on the water requirements of rubber has been reported, and it is difficult to judge the validity of the assumptions made in some of the methodologies described. The actual evapotranspiration rates reported are generally lower than might be expected for a tree crop growing in the tropics (<3 mm d−1). Virtually no research on the yield responses to water has been reported and, with the crop now being grown in drier regions, this is surprising. In these areas, irrigation can reduce the immaturity period from more than 10 years to six years. The important role that rubber plays in the livelihoods of smallholders, and in the integrated farming systems practised in south-east Asia, is summarized.

This study was aimed at comparing in-field parameters and remote sensing NDVI (normalized difference vegetation index) by both satellite (SAT) and unmanned aerial vehicle (UAV) for the assessment of early nitrogen (N) status and prediction of yield in winter wheat (Triticum aestivum L.). Six increasing N rates, i.e., 0, 40, 80, 120, 160, 200 kg N ha−1 were applied, half at tillering and half at shooting. Thus, when the crop N status was monitored between the two N applications, consecutive N treatments differentiated from each other by just 20 kg N ha−1. The following in-field and remote sensed parameters were compared as indicators of crop vegetative and N status: plant N% (w:w) concentration; crop N uptake (Nupt); ratio between transmitted and incident photosynthetically active radiation (PARt/PARi); leaf SPAD values, an indirect index for chlorophyll content; SAT and UAV derived NDVI. As reliable indicators of wheat N availability, in-field parameters were ranked as follows: PARt/PARi ≅ Nupt > SPAD ≅ N%. The PARt/PARi, Nupt and SPAD resulted quite strongly correlated to each other. At all crop stages, the NDVI was strongly correlated with PARt/PARi and Nupt. It is of relevance that NDVI correlated quite strongly to in-field parameters and grain yield at shooting, i.e., before the second N application, when the N rate can still be adjusted. The SAT and UAV NDVIs were strongly correlated to each other, which means they can be used alternatively depending on the context.

The large diversity of farms and farming systems in sub-Saharan Africa calls for agricultural improvement options that are adapted to the context in which smallholder farmers operate. The socio-ecological niche concept incorporates the agro-ecological, socio-cultural, economic and institutional dimensions and the multiple levels of this context in order to identify which options fit best. In this paper, we illustrate how farming systems analysis, following the DEED cycle of Describe, Explain, Explore and Design, and embedding co-learning amongst researchers, farmers and other stakeholders, helps to operationalize the socio-ecological niche concept. Examples illustrate how farm typologies, detailed farm characterization and on-farm experimental work, in combination with modelling and participatory approaches inform the matching of options to the context at regional, village, farm and field level. Recommendation domains at these gradually finer levels form the basis for gradually more detailed baskets of options from which farmers and other stakeholders may choose, test and adjust to their specific needs. Tailored options identified through the DEED cycle proof to be more relevant, feasible and performant as compared to blanket recommendations in terms of both researcher and farmer-identified criteria. As part of DEED, on-farm experiments are particularly useful in revealing constraints and risks faced by farmers. We show that targeting options to the niches in which they perform best, helps to reduce this risk. Whereas the conclusions of our work about the potential for improving smallholders’ livelihoods are often sobering, farming systems analysis allows substantiating the limitations of technological options, thus highlighting the need for enabling policies and institutions that may improve the larger-scale context and increase the uptake potential of options.

Recent literature suggests that to make value chains in changing agrifood systems in sub-Saharan Africa more inclusive, intermediary institutions should foster coordination. The hub concept has been applied as such an intermediary institution that coordinates advisory services, input supply and smallholder access to markets. This study unravels hub coordination in smallholder dairy in Kenya, conceptualising the hub as a mix between a broker of relationships, a one-stop-shop for services and a cluster of producers and service providers, enabling horizontal coordination (between smallholders) and vertical coordination (between smallholders and value chain actors and service providers). Findings indicate that, in resolving challenges that limit smallholders’ integration in value chains, synergies emerged as the hub combined different types of horizontal and vertical coordination. This was done by simultaneously organising clusters of farmers and input and service providers (clustering role) and actively facilitating delivery (broker and one-stop-shop role), where the hub structure stimulated the matching of demand (better articulation) to supply (better organised access). However, tensions emerged in the combination of horizontal and vertical coordination as farmer organisations as hub operators had to balance a role as an honest broker between farmers with the intent of enhancing collective action and as a business-oriented entity which resulted in the exclusion of some farmers who cannot deliver the quantity and quality required to minimise coordination costs. Given these tensions and capacity problems of farmers’ organisations, complementary intermediary arrangements may be necessary to fulfil some coordination roles.

Fertilizer use in the semi-arid tropics is considered risky because of the unreliable rainfall pattern. A flexible system of fertilization was devised which allowed for high yields in wet years, and reduced financial losses in dry years. Relatively immobile nutrients, such as phosphorus, potassium and sulphur, were applied at rates determined from generalized soil properties and the total nutrient uptake required for a crop to achieve its maximum yield potential in a season with average rainfall. Nitrogen was applied as a series of split applications, which were adjusted during the season according to the degree of water stress observed. This approach was tested in on-farm trials for maize production on nutrient-poor sandy soils in three regions of Zimbabwe, and resulted in larger yields and profits than current recommendations, providing an accept able level of financial return. Small further yield increases were possible, at acceptable returns, when these fertilizer practices were combined with the establishment of tied-ridges by ox-drawn implements, 30 days after planting. By careful estimation of yield potential, and appropriate adjustments to soil fertility, this system of soil management could be applied to other cropping situations and other areas with variable rainfall.

Uso de fertilizantes para condiciones de precitación variable

A breeding programme for resistance to Striga hermonthica in maize (Zea mays) was initiated in Côte d'Ivoire in 1994. Two extra-early populations, white (TZEE-W Pop STR) and yellow (TZEE-Y Pop STR), were formed from diallel crosses of the best adapted extra-early maturing maize germplasm in the West and Central African subregion. In an effort to improve the populations for Striga resistance, TZEE-W Pop was crossed to inbred 1368 STR (Tzi 3 STR), and TZEE-Y Pop to 9450 STR (Tzi 25 STR). The resulting F1 populations were carried through two backcross (BC) generations. S1 lines were developed from the BC2 and subjected to two cycles of random mating to generate TZEE-W Pop STR C0 and TZEE-Y Pop STR C0. These two populations were subjected to three cycles of S1 recurrent selection under artificial Striga infestation and several experimental varieties were extracted from the different cycles of the selection programme. The original populations (C0), improved populations, and experimental varieties were evaluated in two experiments under Striga-infested and Striga-free conditions. The primary objective was to determine the effectiveness of S1 recurrent selection in developing Striga-resistant extra-early maize varieties. Results of the performance tests showed that the populations were good sources of Striga-resistant varieties with high-yield potential under both Striga-infested and Striga-free conditions. The results have also confirmed the effectiveness of inbreeding, selection and hybridization, as well as the backcross breeding methods, as tools for the development of extra-early populations, synthetic varieties and inbred lines.

Eleven genotypes of soyabean (Glycine max) of tropical, sub-tropical and temperate origin and one accession of G. soja were grown in six locations in Australia during 1986–88, and at one location in Australia and two in Taiwan during 1989–91. Dates of sowing were varied within and among locations so as to expose plants to as many as 32 environments of widely different diurnal temperature and daylength. Times from sowing to flowering (f) were recorded, from which rates of progress towards flowering (1/f) were calculated. These derived data were then related to mean pre-flowering values of temperature (T¯) and photoperiod (P) using a three-plane linear model developed from controlled environment data. Among genotypes, mean values of f varied between 24–49 d and between 139–291 d in the most- and least-inductive environments, respectively. These differences were associated with variations in P from about 11 to 16 h d-1, in daily mean maximum temperatures from about 17° to 36°C, in daily mean minimum temperatures from about 5° to 25°C, and in T¯ from about 11° to 30°C, that is, a very wide range of photothermal regimes. The relations of 1/f to T¯ and P can be described in photoperiod-insensitive genotypes by a thermal plane defined by two constants, a and b, and additionally by a photothermal plane defined by three constants, a′, b′ and c′, in the more numerous photoperiod-sensitive genotypes. If photoperiod-sensitive genotypes are grown in sufficiently long days then a third photoperiod and temperature-insensitive plane is exposed, defined by a constant, d′; this plane indicates the maximum delay in flowering of which the genotype is capable. The constants a′, b′, c′ and d′ define the delay in flowering caused by photoperiod-sensitivity genes. The two intercepts between the three planes define, respectively, the critical photoperiod, Pc, above which increase in daylength delays flowering, and the ceiling photoperiod, Pcc, above which there is no further delay. The values of the six constants for any genotype can be estimated from observations of fin several natural environments. Comparisons between years in Australia and between Australia and Taiwan show that these genotypic constants can predict 1/f, and so the time taken to flower, given data on latitude, sowing date and daily values of maximum and minimum air temperatures. This model is more accurate than an alternative logistic model; we also believe that all six constants in the three-plane rate model described here have biological meaning. They indicate separate genetic control of flowering responses to P and T¯ and could form a rational basis for the genetic characterization and analysis of these responses in the soyabean germplasm.

Pronóstico del momento de floración II

Growth and development of the tropical grain legumes are generally highly sensitive to photo-thermal regime, so that seasonal and regional effects on phenology and yield potential can be large. Yet failure adequately to recognize and fully exploit the consequences of genotype × latitude/sowing date × density interactions has frequently constrained both agronomic and genetic advance with these species. Thus there is opportunity for short term productivity improvements through agronomic strategies which accept the implications of phenological plasticity, and seek to optimize management practices such as sowing date and sowing density, to exploit more effectively the yield potential and broaden the adaptation of existing cultivars. The greatest physiological potential for genetic improvement in the productivity of the tropical grain legumes lies not with increasing total biomass, but with increasing the proportion of bio-mass partitioned into seed, i.e. with improved harvest index (HI). There are difficulties in selecting for higher HI, including its association with phenology, although the latter provides the most powerful tool for manipulating HI in the short term. As with the cereals, more productive genotypes are likely to be characterized by reduced sensitivity to photothermal conditions, shorter growth duration, a more synchronous reproductive ontogeny, and greater HI, than is typical of traditional cultivars. Accordingly, optimal sowing densities will be higher, and crops will need more inputs, better management and more effective protection, than afforded to them in subsistence systems of production. Moreover, the increases in productivity will be achieved, at least partially, at the expense of the potential for yield homeostasis in adverse environments, and of non-seed components such as forage, fuel or tubers - attributes that are often highly valued in subsistence agriculture where many of the tropical food legumes are presently grown. Improvement programmes will therefore need to adopt strategies in particular situations which reflect local perceptions of the importance of productivity improvement relative to that of minimizing inputs and risk.

Plant breeders cannot respond to every quirk of farmers' circumstances but the relevance of breeding research in poor nations can be improved. Recent innovations in germplasm screening programmes for developing nations highlight important differences in selection criteria between farmers and scientists, and among farmers themselves. Drawing on the authors' African experience with International Agricultural Research Centres working on maize and potatoes, the paper explores how breeding programmes benefit from an understanding of farmers' own detailed knowledge about the crop varieties they already cultivate. Issues considered include farmers' use of cultivar diversity, the trade-off between yield and maturity class as selection criteria, defining appropriate experimental conditions, and innovative techniques to increase farmer participation in germplasm screening.

Yield stability in cassava requires genotypes that produce well under the variable moisture conditions encountered during the growth cycle. Plant characteristics related to yield stability were studied in two cassava clones subjected to 105 days of water stress in a field drainage lysimeter. Stress conditions commenced 117 days after planting, and the plants were allowed to recover at the end of the stress period for the rest of the growth cycle. Water stress restricted the growth of leaves and stems, but root yields were increased or remained unaffected. Leaf water potential varied little with stress, but gas exchange rates were about 75% those of the control throughout the stress period in both cultivars. Under stress, the plants partially closed their stomata and extracted deep soil moisture slowly. A high yield in both wet and stressed environments was associated with high mean LAI, better leaf retention, and greater partitioning of shoot biomass into leaf formation.

Spatial variability in field trials is a reality. A proportion of this is accounted for as inter-block variability by using block (complete or incomplete) designs. A large amount of spatial variability still remains unaccounted for, however, and this may lead to erroneous conclusions. To capture this inexplicable variation (which is mainly due to intra-block variation), yield data from a series of variety yield trials, using cereals and legumes, were analysed using various spatial models. The most suitable of these, selected on the basis of the Akaike Information Criterion, were used to assess the relative performance of genotypes. Although incomplete-block designs have been found to be effective in variety trials, spatial models have added considerable value to trials with legumes and cereals. The ‘best’ spatial models gave efficiency values of over 330% in winter-sown chickpea (Cicer arietinum), 140% in lentil (Lens esculenta), and 150% in barley (Hordeum spp.) trials. Furthermore, the use of these best models resulted in a change in the ranking of genotypes (on the basis of mean yield), which resulted, therefore, in a different set of genotypes being selected for high yield. It is recommended that: (i) incomplete block designs be used in variety trials; (ii) the Akaike Information Criterion be used to select the best spatial model; and (iii) genotypes be selected after the use of this model. The selected model would account most effectively for spatial variability in the field trials, improve selection of the most desirable genotypes and, therefore, improve the efficiency of breeding programmes.

Two types of design are described, in which the number of plants per unit area varies systematically within any one plot in the field, the plant arrangement being held constant. Any desired range of spacing can be covered in either design and the calculations necessary to establish specific sets of dimensions are given. In addition descriptions are given of the laying-out, planting (or sowing), management and recording of experiments using these designs.

Field experiments during 1979 and 1980 on a Vertisol soil at ICRISAT Centre, Hyderabad, India, examined the residual effect of pigeonpea (Cajanus cajan) on growth and yield of a following maize crop. Pigeonpea was grown alone or as an intercrop with sorghum fertilized or not with 80 kg N ha−1. Sorghum alone and fallow treatments were included for comparison. In the first year, the sorghum/pigeonpea intercrop produced the largest grain and dry matter yields, but the yield of intercropped pigeonpea was about 50% less than that produced by the sole crop. Pigeonpea alone had a large residual effect on maize, increasing grain yield by 57% and total plant dry matter by 32% compared with corresponding values after fallow. In comparison, intercrop pigeonpea had little residual effect. Maize following either fallow, sorghum grown alone, with or without N, or the sorghum/pigeonpea intercrop, again with or without N, required fertilizer equivalent to 38–49 kg N ha−1 in order to attain yields similar to that of unfertilized maize following sole crop pigeonpea.

Lentil frequently suffers from drought stress towards the end of the growing season in rainfed Mediterranean farming systems. This study aimed to quantify the contribution of yield potential under irrigated conditions and of drought escape to the adaptation of lentils to drought, and to identify traits of value in selection for adaptation to a rain-fed Mediterranean environment. Twenty-five diverse lentil lines were sown under both irrigated and rainfed conditions in the 1987/88 and 1988/89 seasons in northern Syria. In 1987/88 the total seasonal rainfall was 403 mm and average rainfed biomass and seed yields were 5.0 and 1.8 t ha-l, respectively. In contrast, in 1988/89 the total season rainfall was 180 mm and average biomass and seed yields were only 0.67 and 0.12 t ha-1, respectively. Early vigour (estimated as a visual score) and percentage ground cover were strongly associated with final biomass, and were of value in predicting economic yield (seed + straw). Early maturity was correlated with seed yield. In the dry season 1988/89, 49% of the variation in seed yield among lines was accounted for by variation in flowering time. Drought escape was clearly the key response to drought. For severely drought-prone areas, selection for early flowering is therefore required. Variation in flowering time and seed yield under irrigated conditions accounted for 62% of seed yield variation under drought stress.

This paper addresses the issue of genotype by environment (G × E) interaction in relation to the distribution of germplasm from international to national breeding programmes. Theoretical aspects of G × E interaction and of the selection versus the test environment are reviewed. The importance of specific adaptation to maximize yield and yield stability under stress conditions is highlighted in barley and lentil. We conclude that repeated cycles of selection at a few sites have a high probability of reducing the frequency of genotypes specifically adapted to environmental and/or agronomic conditions not represented at those sites. Therefore a wider and earlier devolution of the selection work done by International Agricultural Research Centres (IARCs) to plant breeders in national programmes will increase the chances of exploiting positive G × E interactions. It will also address the needs of small, resource-poor, subsistence farmers and reduce the danger of narrowing genetic diversity within crops and regions. This is an efficient way of making breeding programmes in less developed countries more self-reliant, which is a major objective of International Agricultural Research Centres.

Programas internacionales de reproducción

Photosynthetic rates of coffee plants grown under the full sun and shade were compared. The saturating irradiance of 300 μE−1 m s−1 of shade plants shifted to near 600 μE in sungrown plants, but shade plants possessed substantially higher photosynthetic rates. Photosynthetic rates decreased above 25°C, which was apparently due to a decline in the mesophyll conductance, as stomatal conductance remained more or less unchanged between 25° and 35°C. Most of these measurements were made on seedlings in plastic bags. It is concluded that coffee is more suited to high density plantings where mutual shading, resulting in low light intensities and lower leaf temperatures, are likely to produce a favourable environment for efficient photosynthesis and growth.

To measure the yield and financial returns from five grain legume–maize intercrop combinations over 12 years of cropping, a field experiment was conducted on a loamy sand soil in the subhumid unimodal rainfall environment of Domboshava in north-central Zimbabwe. Inputs and management followed smallholder practice, including partial grazing of crop residues and a zero mineral fertilizer treatment. The intercropped legumes grew moderately well most years. Cowpea averaged the highest grain yield (0.244 t ha−1) and haulm yield (1.54 t ha−1) over the 12 years, followed by pigeonpea and sugar bean. Intercropped pigeonpea yield was the least variable of the legumes over the years. Maize grain yield was highly variable across years with or without fertilizer and was reduced in years of low (533 mm) and high (1313 mm) rainfall. The pigeonpea–maize intercrop grown without fertilizer produced 0.11 t ha−1 (6.25 %) more maize grain yield per year than sole crop maize, in addition to pigeonpea grain and haulms. Intercropped cowpea (which yielded more than double the above-ground non-grain biomass of pigeonpea) had less effect on maize grain yield. There was no trend to greater benefits from the legumes on maize yield after more years of intercropping. Net present values of annual margins accumulated over the 12 years for sole maize with fertilizer (US$1719 ha−1) and without fertilizer (US$935 ha−1) were higher than the fertilized and unfertilized intercropping options (US$1017 and US$745 ha−1). Pigeonpea or cowpea–unfertilized maize generated more financial returns than the other intercrops, but the low yields and high labour costs for the legumes made the intercrops financially unattractive. We conclude that regularly intercropped pigeonpea or cowpea can to a small extent help to maintain maize yield when maize is grown without mineral fertilizer on sandy soils in sub humid zones of Zimbabwe, and simultaneously provide some nutritious food, but that financial considerations will encourage smallholder farmers to persist with growing low input sole crop maize.

Pulses of potassium (K+) applied to columns of repacked calcium (Ca2+) saturated soil were leached with distilled water or calcium chloride (CaCl2) solutions of various concentrations at a rate of 12 mm h−1. With increased Ca2+ concentration, the rate of movement of K+ increased, as did the concentration of K+ in the displaced pulse, which was less dispersed. The movement of K+ in calcite-amended soil leached with water was at a similar rate to that of the untreated soil leached with 1 mM CaCl2, and in soil containing gypsum, movement was similar to that leached with 15 mM CaCl2. The Ca2+ concentrations in the leachates were about 0.4 and 15 mM respectively, the expected values for the dissolution of the two amendments. Soil containing native K+ was leached with distilled water or CaCl2 solutions. The amount of K+ leached increased as Ca2+ concentration increased, with up to 34 % of the exchangeable K+ being removed in five pore volumes of 15 mM CaCl2. Soil amended with calcite and leached with water lost K+ at a rate between that for leaching the unamended soil with 1 mM CaCl2 and that with water. Soil containing gypsum and leached with water lost K+ at a similar rate to unamended soil leached with 15 mM CaCl2. The presence of Ca2+ in irrigation water and of soil minerals able to release Ca2+ are of importance in determining the amounts of K+ leached from soils.The LEACHM model predicted approximately the displacement of K+, and was more accurate with higher concentrations of displacing solution. The shortcomings of this model are its inability to account for rate-controlled processes and the assumption that K+: Ca2+ exchange during leaching can be described using a constant adsorption coefficient. As a result, the pulse is predicted to appear a little earlier and the following edge has less of a tail than that measured. In practical agriculture, the model will be more useful in soils containing gypsum or leached with saline water than in either calcareous or non-calcareous soils leached with rainwater.

At Samaru (11° 10′N, 7° 39′E) the growing season for rain-fed annual crops is 180 to 200days. Sole cropping, relay cropping and intercropping of long season sorghum, early millet or maize and late planted cowpeas were evaluated for most effective use of this period, all crops being cultivated mechanically. Relay cropping and intercropping gave 59 and 80 per cent more return per acre respectively than a sole crop of sorghum, the increases coming mainly from higher cereal yields. Some criteria applicable to intercropping are recognized.