The results of research into the water relations of cocoa are reviewed in the context of drought mitigation and irrigation need. Background information on the centres of production of the cocoa tree, and the role of water in crop development and growth processes, is followed by reviews of the effects of water stress on stomatal conductance, leaf water status and gas exchange, together with drought tolerance, crop water use and water productivity. Leaf and shoot growth occur in a series of flushes, which are synchronized by the start of the rains following a dry season (or an increase in temperature), alternating with periods of 'dormancy'. Flowering is inhibited by water stress but synchronous flowering occurs soon after the dry season ends. Roots too grow in a rhythmic pattern similar to that of leaf flushes. Roots can reach depths of 1.5-2.0 m, but with a mass of roots in the top 0.2-0.4 m, and spread laterally >5 m from the stem. Stomata open in low light intensities and remain fully open in full sunlight in well-watered plants. Partial stomatal closure begins at a leaf water potential of about −1.5 MPa. Stomatal conductance is sensitive to dry air, declining as the saturation deficit increases from about 1.0 up to 3.5 kPa. Net photosynthesis and transpiration both consequently decline over a similar range of values. Little has been published on the actual water use of cocoa in the field. Measured ETc values equate to <2 mm d −1 only, whereas computed ETc rates of 3-6 mm d −1 in the rains and <2 mm d −1 in the dry season have also been reported. Despite its sensitivity to water stress, there is too a paucity of reliable, field-based published data of practical value on the yield responses of cocoa to drought or to irrigation. With the threat of climate change leading to less, or more erratic, rainfall in the tropics, uncertainty in yield forecasting as a result of water stress will increase. Social, technical and economic issues influencing the research agenda are discussed.

The results of research on the water relations and irrigation need of oil palm are collated and summarized in an attempt to link fundamental studies on crop physiology to drought mitigation and irrigation practices. Background information is given on the centres of origin (West Africa) and of production of oil palm (Malaysia and Indonesia), but the crop is now moving into drier regions. The effects of water stress on the development processes of the crop are summarized followed by reviews of its water relations, water use and water productivity. The majority of the recent research published in the international literature has been conducted in Malaysia and in Francophone West Africa. The unique vegetative structure of the palm (stem and leaves) together with the long interval between flower initiation and the harvesting of the mature fruit (ca. three years) means that causal links between environmental factors (especially water) and yield are difficult to establish. The majority of roots are found in the 0-0.6 m soil horizons, but roots can reach depths greater than 5 m and spread laterally up to 25 m from the trunk. The stomata are a sensitive indicator of plant water status and play an important role in controlling water loss. Stomatal conductance and photosynthesis are negatively correlated with the saturation deficit of the air. It is not easy to measure the actual water use of oil palm, the best estimates for mature palms suggesting crop evapotranspiration (ETc) rates of 4-5 mm d −1 in the monsoon months (equivalent to 280-350 l palm −1 d −1 ). For well-watered mature palms, crop coefficient (Kc) values are in the range 0.8-1.0. Although the susceptibility of oil palm to drought is well recognized, there is a limited amount of reliable data on actual yield responses to irrigation. The best estimates are 20-25 kg fresh fruit bunches ha −1 mm −

The role of water in the development and yield of the coee crop (Coea arabica L.) is reviewed. A period of water stress, induced either by dry soil or dry air, is needed to prepare¯ower buds for blossoming that is then stimulated by rain or irrigation. Although attempts have been made to quantify the intensity and duration of stress required, these have not yet been speci®ed in ways that are commercially useful. Water must be freely available during the period of rapid fruit expansion to ensure large, high-quality seed yields. Depending on the time and uniformity of¯owering this can occur at times when rainfall is unreliable, particularly in equatorial areas.Although there are dierences in their responses to drought, commercial cultivars have retained many of the characteristics adapted to the shady environment of the forests in the Ethiopian highlands in which C. arabica is believed to have originated. These include partial closure of the stomata when evaporation rates are high as a result of large leaf-to-air saturation de®cits (41.6 kPa), even if the soil is at ®eld capacity. This is thought to be an adaptive mechanism that minimizes transpiration at high irradiances when the leaves are light-saturated.Our understanding of the actual water use of coee crops grown in diverse ways is imperfect. For mature crops, well supplied with water, the crop coecient (Kc) appears to have a value in the range equivalent to 0.7±0.8 times the evaporation from a US Weather Bureau Class A pan. There is some evidence that Kc values are less than this on days when evaporation rates are high (47 mm d 71 ). For immature crops allowance has to be made for the proportion of the ground area shaded by the leaf canopy, but this alone may underestimate rates of water use. Present methods of calculating crop water requirements for the purposes of irrigation scheme design and management are imprecise and, probably, subject to large errors depending on local circumstances.The need for irrigation, and its role in controlling the timing of¯owering, varies depending on the rainfall distribution, the severity of the dry season, and soil type and depth. Two geographic areas need to be distinguished in particular; those close to the equator with a bi-modal rainfall pattern and those at higher latitudes with a single rainy season and an extended dry season. Despite the international importance of irrigation in coee crop production, the bene®ts to be derived from irrigation, in yield and in ®nancial terms, have not been adequately quanti®ed in either location. Allowable soil-water de®cits have been speci®ed for deep-rooting crops (2±3 m) on water retentive soils, usually linked to conventional over-tree sprinkler irrigation systems. Other, potentially more ecient, methods of irrigation are now available for coee grower use, in particular, micro-jet-and drip-irrigation systems. However, there appears to be little advice, based on sound experimental work, on how to design and operate these to best advantage.There is a need to interpret and apply the s...

The yield responses of clonal tea (Clone 6/8) to irrigation and fertilizer were studied in a field experiment (based on the line-source technique) at a high altitude site (1840 m) in the Southern Highlands of Tanzania over a three-year period. In this area (latitude 8°33'S) the annual dry season can last up to six months with potential soil water deficits reaching 600 to 700 mm. In the third year of the experiments yields for the fully irrigated, well fertilized (375 kg N ha" 1 ) treatments had reached 4.9 t ha" 1 of made tea. These were reduced by about 2.9 kg ha" 1 for each mm increase in the potential soil water deficit. For tea with little or no fertilizer applied the loss of yield was about 1.4 kg ha" 1 mm" 1 . These figures provide a basis for assessing the potential benefits from irrigation where other factors, such as large saturation deficits of the air, do not restrict shoot extension and yield. Yield responses to nitrogen (applied as N:P:K 20:10:10) were essentially linear up to a maximum of about 375 kgNha"' in the fully irrigated plots, and 300 kg N ha" 1 for the unirrigated plots. Irrigation increased the proportion of crop harvested during the dry season, up to 45% in the fully irrigated treatments. The commercial implications of these results for 'high' and 'low' input producers are discussed.Respuestas del te (Camellia sinensisj al riegoy la fertilization. I. Rendimiento RESUMEN Las respuestas de rendimiento del te clonal (Clon 6/8) respecto del riego y la fertilization fue estudiada en un experimento de campo (en base a la tecnica de linea-fuente) en un emplazamiento de elevada altitud (184 m) en las Tierras Altas del Sur en Tanzania, durante un perfodo de tres anos. En esta zona (latitud 8°33S) la estacion seca anual puede durar hasta un maximo de seis meses con un potencial deficit de agua en la tierra de hasta 700 mm. Durante el tercer ano, los rendimientos del experimento para los tratamientos con riego total y buena fertilization (375 kg N ha" 1 ) alcanzaron las 4,9 t ha" 1 de te hecho. Estos resultados se redujeron aproximadamente en 2,9 kg ha" 1 por cada mm de incremento en el deficit potencial de agua en la tierra. Para el te que recibio poco o ningun fertilizante la perdida de rendimiento fue de aproximadamente 1,4 kg ha" 1 mm"'. Estas cifras proporcionan una base para evaluar los potenciales beneficios del riego en los casos en que otros factores, tal como vastos deficits de saturation del aire, no limitan la extension y production de brotes. Las respuestas de rendimiento al nitrogeno (aplicado como N:P:K 20:10:10) resultaron esencialmente lineales hasta un maximo de aproximadamente 375 kg N ha" 1 en los lotes con riego total, y 300 kg N ha"' en los lotes sin riego. El riego incremento la proportion de la cosechas recogidas durante la estacion seca en hasta un 45% en los tratamientos con riego total. El articulo discute las implicancias comerciales de estos resultados para los productores de 'alta' y 'baja' aplicacion de factores externos. 178

Research and observations on how climate effects the growth and yield of the tea plant are reviewed. Despite differences in the types of tea grown and the cultural techniques practiced in the different tea areas, an attempt is made to define quantitatively the climatic conditions needed to maintain growth rates at a high level. The need for critical work in the future is emphasized.

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SUMMARYShoot extension rates and numbers recorded over an 18 month period in Tanzania from three contrasting clones were analysed to determine variation between and within seasons. Clonal differences in base temperatures for shoot extension ranged from 10.3 to 14.5°C, whilst variability in the response of shoot growth rates to temperature could be ascribed to the shoot selection technique employed. Considerable clonal variation in shoot population densities occurred, with maxima ranging from 200 to 1200 shoots m−2. Total active shoot extension, the product of shoot growth rates and population densities, varied between 4 and 35 m m−2 week−1, shoot numbers being the dominant component. The implications of these results are discussed in terms of shoot measurement techniques, clonal selection criteria, yield modelling and harvesting policies.

The results of research on the water relations and irrigation need of banana are collated and summarised in an attempt to link fundamental studies on crop physiology to irrigation practices. Background information on the ecology of the banana and crop development processes, with emphasis on root growth and water uptake, is presented, followed by reviews of the influence of water stress on gas exchange (stomatal conductance, photosynthesis and transpiration), crop water use, and yield. Emphasis is placed on research that has international relevance and, where appropriate, three geographical areas (the tropics, subtropics and Mediterranean climates) are considered. Although roots can extend to depths of 1.0-1.5 m, the 'effective' depth of rooting is usually taken to be 0-0.40 m, sometimes extending to 0.60 m. Stomatal conductance is a sensitive measure of soil water availability and plant water status, whilst transpiration rates can be limited by dry air (saturation deficits >2.0 kPa). In the subtropics, there are seasonal differences in the crop coefficient (Kc) with values ranging from 0.6 in the winter months to about 1.0 Epan in the summer. It is difficult to draw generic conclusions with wide applicability from the irrigation experiments as they were reported. All the components of marketable yield can be enhanced by irrigation whilst applying insufficient water delays crop development. Annual yield responses to irrigation are variable, but water use efficiencies of 40 kg ha −1 mm −1 (fresh fruit/water applied) have been achieved in the tropics and subtropics (and elsewhere up to 80 kg ha −1 mm −1 with 'partial' replacement of the soil water deficit). To ensure large yields of (marketable) fruit, soil water deficits must be kept low ( m > −20 kPa at 0.2 m depth). In the subtropics, this means irrigation intervals should not exceed 2-3 d during the summer. The cooling effect of irrigation with micro-sprinklers on the soil and pseudostem temperatures, compared with drippers, can delay crop development and reduce annual yields by 30%. There is some (limited) evidence that the presence of the B genome contributes to drought tolerance. Yield response factors to irrigation for different growth stages have yet to be confirmed. Opportunities to improve the water productivity of the many, diverse banana cultivars need to be explored further. I N T RO D U C T I O NAccording to Turner (1995), water is probably the most limiting non-biological factor affecting banana production. The need for irrigation varies between locations, whilst its justification in commercial terms depends on the purposes for which a crop is grown and its market value. For crops that have international commercial importance, integrated approaches to irrigation research should complement site specific, single discipline, usually empirical methodologies, which are still too common. To avoid duplication of effort, the first need is to collate existing (published) information on the water relations and irrigation needs of individual crops, and to attem...