The physiological basis for differences in yields from well-watered and draughted plants of four contrasting clones of tea was studied in terms of light interception, dry matter production and partitioning at a high altitude site in Southern Tanzania where there are marked seasonal variations in rainfall and temperature. The plant dry weights, including roots, were measured eight months after field planting and subsequently at intervals of three to four months, corresponding to the different seasons, during the following two years. Fully irrigated plants of one clone (S15/10) were also harvested after four years in the field. Clones differed in the rates of canopy spread and hence in their capacity to intercept solar radiation. The ‘radiation use efficiency’ (the net total dry matter production per unit of intercepted short-wave radiation) was similar for the four well-watered clones and ranged from 0.40 to 0.66 g MJ−1, which corresponds closely to values reported for other woody tropical plants. A 16-week drought treatment imposed two years after planting reduced the mean light interception of the four clones by about 25% and the mean radiation use efficiency by 78% to 0.09 g MJ−1. Clone S15/10, a cultivar from Kenya which produces large yields, partitioned a greater proportion of dry matter to leaves and harvested shoots than the other clones, and correspondingly less to large structural roots. This resulted in a maximum harvest index of 24%, substantially greater than other values reported in the literature. There were seasonal differences in partitioning, with more dry matter being diverted to roots and less to shoots during the cool season. Although the drought treatments had no significant effect on root growth, the amount of dry matter partitioned to leaves, stems and harvested shoots declined by 80–95%. The roots of all four clones extended in depth at similar rates (about 2 mm d−1), those of Clone S15/10 reaching 2.8m after four years. The results are discussed in terms of appropriate field cultural practices and possible selection criteria for high yielding clones.

Simulations of the dynamics of the seed bank of Orobanche crenata Forsk. under different crop rotations are presented. Rotations studied involved four host species, lentil (Lens culinaris Medik.), chickpea (Cicer arietinum L.), faba bean (Vicia faba L.) and woolly-pod vetch (Vicia villosa subsp. dasycarpa (Ten.) Cavill.), and non-host species. Simulation showed that the Orobanche seed bank dynamics in three-course crop rotations would result in a high Orobanche seed population and hence in low yields of the respective crops. Replacing the susceptible by resistant legumes such as woolly-pod vetch in some of the cycles of the rotations would keep the Orobanche infestation at a low level without reducing the proportion of legumes. A three-course crop rotation with faba bean would have to be changed to a 12-coursc rotation in which faba bean was grown every twelfth year but was replaced by woolly-pod vetch or other resistant legumes in seasons 3, 6, 9; 15, 18, 21; and so on. In the three-course rotations with chickpea or lentil, these susceptible legumes would be grown every ninth year but would have to be replaced in seasons 3 and 6; 12 and 15; 21 and 24; and so on, thus changing these three-course rotations into nine-course rotations.

Over several decades agronomists have developed a mineral nutrition management method for perennial oil crops based on leaf analysis combined with reference field trials. This method has been used in a coconut plantation on the peats of the east coast of Sumatra to optimize mineral nutrition in a very short time, at minimum cost and with maximum reliability, through successive adjustments of the fertilizer schedules. Two reference trials were set up to study a wide range of mineral nutrients assumed to be deficient on this area. Further experiments were set up as new nutritional problems occurred at the plantation, and as soon as the first results were obtained from these two trials. In this way, the iron and copper deficiency problems at the plantation were dealt with satisfactorily taking an experimental approach. New experiments were set up recently to test the hypothesis of a silicon deficiency. The fertilization schedules for the commercial plantation were developed by successive adjustments based on the trial results. Leaf analysis data showed that the coconut palms in the commercial plots always complied with the experimental models. In addition to being rapid and reliable, this method was also cost-effective, especially when larger areas were involved.

Seedling tubers of three true potato seed (TPS) crosses raised in a seedbed soil mixture of clay, sand and dry leaves produced about 3.1 kg of seedling tubers m−2 in the growing season of 1988–89 and between 4.3 and 5.1 kg m−2 in 1989/90. The seedling tubers produced were stored and their use as seed tubers was subsequently compared with the locally popular variety Alpha. The mean tuber yields of the hybrids ranged between 9.4 and 11 t ha−1 in 1990/91 and between 21.9 and 22.9 t ha−1 in 1991/92. Mean tuber yields of cultivar Alpha were 14.3 t ha−1 in 1990/91 and 24.1 t ha−1 in 1991/92, showing that tuber yields comparable to those of imported seed tubers could be obtained from seedling tubers of TPS origin.

Nitrogen efficiency from Azolla microphylla or Sesbania rostrata green manure, rice straw, and inorganic fertilizer-N was compared in two long-term experiments with irrigated lowland rice (Oryza sativa L.). Treatments included a control and each nitrogen source alone or in combinations that provided 50% of the total applied nitrogen from an organic and inorganic nitrogen source. All nitrogen sources were applied at equivalent nitrogen rates to 19–22 consecutive rice crops. Residual effects were assessed in two subsequent cropping seasons at one site. Lower grain yield, agronomic efficiency (Δgrain per kg total applied nitrogen), and apparent nitrogen uptake were obtained from green manure and rice straw nitrogen as sole or dual nitrogen sources rather than from a standard split application of prilled urea. Compared to prilled urea, residual effects from green manure or rice straw included a significant increase in soil organic carbon and total nitrogen, and greater extractable soil nitrogen in the vegetative growth period. After panicle initiation there was no residual effect on the rate of crop nitrogen accumulation, and final grain yields were similar regardless of previous nitrogen source. Recycling of rice straw appeared to have greater potential for reducing fertilizer-N requirements than use of green manure because rice straw is often a wasted resource in irrigated rice systems of the humid tropics, the efficiency of rice straw nitrogen in combination with prilled urea is comparable to green manure nitrogen, and the increase in soil nitrogen from rice straw was 50–150% greater than from green manure.

Farmer participatory approaches for the identification or breeding of improved crop cultivars can be usefully categorized into participatory varietal selection (PVS) and participatory plant breeding (PPB). Various PVS and PPB methods are reviewed. PVS is a more rapid and cost-effective way of identifying farmer-preferred cultivars if a suitable choice of cultivars exists. If this is impossible, then the more resource-consuming PPB is required. PPB can use, as parents, cultivars that were identified in successful PVS programmes. Compared with conventional plant breeding, PPB is more likely to produce farmer-acceptable products, particularly for marginal environments. The impact of farmer participatory research on biodiversity is considered. The long-term effect of PVS is to increase biodiversity, but where indigenous variability is high it can also reduce it. PPB has a greater effect on increasing biodiversity although its impact may be limited to smaller areas. PPB can be a dynamic form of in situ genetic conservation.

Farmer participatory varietal selection (PVS) was used to identify farmer-acceptable cultivars of rice and chickpea. Farmers' requirements in new crop cultivars (varieties) were determined, a search was carried out for released and non-released cultivars that matched these needs, and they were tested in farmer-managed, participatory trials. Farmer-acceptable cultivars were found amongst released material, but not among the recommended material for the area. Lack of adoption is, therefore, because resource-poor farmers have not been recommended or exposed to the most appropriate cultivars under the existing system of varietal identification and popularization. Adoption rates of cultivars would be improved by increased farmer participation, the systematic testing in zonal trials of locally popular cultivars to define their domains properly, a more liberal release system, and a more open system of providing seeds of new cultivars to farmers.

A participatory plant breeding (PPB) programme was conducted for the high altitude areas of Nepal. The aim was to breed acceptable varieties with minimum use of resources and to utilize farmers' knowledge in the PPB programme. Farmer participation began at the F5 stage and progress was followed over two seasons in two villages. Farmers proved to be willing participants and made selections in the segregating material, often with great success. Large differences in farmers' preferences between the F5 bulks were found and the most preferred were adopted rapidly. The most adopted variety, Machhapuchhre-3, performed well in the formal trials system, and much better than the products from conventional centralized breeding. The PPB programme increased biodiversity in the two participating villages.