استكشف المجموعة الواسعة من العناوين المتاحة.

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.

The results of research on the water relations and irrigation need of Citrus spp. are collated and reviewed 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 (south-east Asia) and of production of citrus (areas with subtropical Mediterranean-type climates). The effects of water stress on the development processes of the crop are summarised followed by reviews of the plant water relations, crop water requirements, water productivity and irrigation systems. The topic is complicated by the diversity of species and cultivars (including rootstocks) that are embraced within Citrus spp. The effects of water availability on vegetative growth are understood in general terms, but the relationships have not yet been quantified. Similarly, the need for a ‘rest period’ to induce flowering is understood, but its magnitude (in terms of a drought stress index or day-degrees) does not appear to have been specified with precision. Again, the effects of drought on flower and fruit formation and retention are understood in general terms, but the relationships have not been quantified in useful ways for specific cultivars. Rooting depth and distribution have only been described in a limited number of situations. Environmental factors influencing stomatal conductances are generally well described and relationships with some growth processes established. Compared with other crops, low stomatal/canopy conductance restricts water use of Citrus spp. Some (limited) progress has been made in quantifying crop water requirements in specific conditions. Despite many recent attempts to specify how little water can be applied at specific growth stages to optimise water productivity through regulated deficit irrigation, no consensus view has emerged. The yield response to ‘full’ irrigation is of the order 6–7 kg fresh fruit m⁻³ as a result of an increase in the number of fruit of marketable size. There are also improvements in fruit quality. The most effective way of irrigating a citrus orchard is with a microirrigation system (drip or microsprinklers), but both methods require answers to the question: what proportion of the root zone needs to be irrigated? Both methods, especially drip, allow water to be applied (with fertigation) at very frequent intervals (including several times a day), although formal evidence of the benefits to be obtained from this level of intensification is lacking.

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−1 (or a yield loss of about 10% for every 100 mm increase in the soil water deficit). These increases are only realized in the third and subsequent years after the introduction of irrigation and follow an increase in the number of fruit bunches as a result of an improvement in the sex ratio (female/total inflorescence production) and a reduction in the abortion of immature inflorescences. There is no agreement on the allowable depletion of the available soil water, or on the associated optimum irrigation interval. Drip irrigation has been used successfully on oil palm.

The results of research into the water relations and irrigation requirements of lychee are collated and reviewed. The stages of plant development are summarised, with an emphasis on factors influencing the flowering process. This is followed by reviews of plant water relations, water requirements, water productivity and, finally, irrigation systems. The lychee tree is native to the rainforests of southern China and northern Vietnam, and the main centres of production remain close to this area. In contrast, much of the research on the water relations of this crop has been conducted in South Africa, Australia and Israel where the tree is relatively new. Vegetative growth occurs in a series of flushes. Terminal inflorescences are borne on current shoot growth under cool (<15 °C), dry conditions. Trees generally do not produce fruit in the tropics at altitudes below 300 m. Poor and erratic flowering results in low and irregular fruit yields. Drought can enhance flowering in locations with dry winters. Roots can extract water from depths greater than 2 m. Diurnal trends in stomatal conductance closely match those of leaf water status. Both variables mirror changes in the saturation deficit of the air. Very little research on crop water requirements has been reported. Crop responses to irrigation are complex. In areas with low rainfall after harvest, a moderate water deficit before floral initiation can increase flowering and yield. In contrast, fruit set and yield can be reduced by a severe water deficit after flowering, and the risk of fruit splitting increased. Water productivity has not been quantified. Supplementary irrigation in South-east Asia is limited by topography and competition for water from the summer rice crop, but irrigation is practised in Israel, South Africa, Australia and some other places. Research is needed to determine the benefits of irrigation in different growing areas.

The results of research on the water relations and irrigation need of avocado are collated and reviewed in an attempt to link fundamental studies on crop physiology to irrigation practices. Background information is given on the centre of origin (Mexico and Central America) and the three distinct ecological areas where avocados are grown commercially: (1) Cool, semi-arid climates with winter-dominant rainfall (e.g. Southern California, Chile, Israel); (2) Humid, subtropical climates with summer-dominant rainfall (e.g. eastern Australia, Mexico, South Africa); and (3) Tropical or semi-tropical climates also with summer-dominant rainfall (e.g. Brazil, Florida and Indonesia). Most of the research reported has been done in Australia, California, Israel and South Africa. There are three ecological races that are given varietal status within the species: Persea americana var. drymifolia (Mexican race), P. americana var. guatemalensis (Guatemalan race) and P. americana var. americana (Antillean, West Indian or Lowland race). Interracial crossing has taken place. This paper summarises the effects of water deficits on the development processes of the crop and then reviews plant–water relations, crop water requirements, water productivity and irrigation systems. Shoot growth in mature trees is synchronised into flushes. Flower initiation occurs in the autumn, with flowering in late winter and spring. Flowers form on the ends of the branches. A large heavily flowering tree may have over a million flowers, but only produce 200–300 fruits. Fruit load adjustment occurs by shedding during the first three to four weeks after fruit set and again in early summer. Water deficits during critical stages of fruit ontogeny have been linked to fruit disorders such as ring-neck. Reproductive growth is very resistant to water stress (compared with vegetative growth). Avocado is conventionally considered to be shallow rooted, although roots extend to depths greater than 1.5 m. The majority of feeder roots are found in the top 0.60 m of soil and root extension can continue throughout the year. Leaves develop a waxy cuticle on both surfaces, which is interrupted by stomata on the abaxial surface (350–510 mm−2), many of which are blocked by wax. Stomata are also present on the sepals and petals at low densities (and on young fruit). During flowering, the canopy surface area available for water loss is considerably increased. Stomatal closure is an early indicator of water stress, which together with associated changes in leaf anatomy, restricts CO2 diffusion. There have only been a few attempts to measure the actual water use of avocado trees. In Mediterranean-type climates, peak rates of water use (in summer) appear to be between 3 and 5 mm d−1. For mature trees, the crop coefficient (Kc) is usually within the range 0.4–0.6. The best estimate of water productivity is between 1 and 2 kg fruit m−3. Soil flooding and the resultant reduction in oxygen level can damage roots even in the absence of root rot. Avocado is particularly sensitive to salinity, notably that caused by chloride ions. Rootstocks vary in their sensitivity. Both drip and under-tree microsprinklers have been/are successfully used to irrigate avocado trees. Mulching of young trees is a recommended water conservation measure and has other benefits. A large proportion of the research reviewed has been published in the ‘grey’ literature as conference papers and annual reports. Sometimes, this is at the expense of reporting the science on which the recommendations are based in peer-reviewed papers. The pressures on irrigators to improve water productivity are considered.

The results of research on the water relations and irrigation need of pineapple are collated and summarised in an attempt to link fundamental studies on crop physiology to irrigation practices. Background information on the centres of origin (northern South America) and of production (Brazil, Thailand and the Philippines) of pineapple is followed by reviews of crop development, including roots, plant water relations, crop water requirements and water productivity and irrigation systems. The majority of the recent research published in the international literature on these topics has been conducted in the United States (Hawaii) and Brazil. Pineapple differs from most other commercial crops in that it has a photosynthetic adaptation (crassulacean acid metabolism (CAM)) that facilitates the uptake of carbon dioxide at night, and improves its water-use efficiency under dry conditions. The crop is propagated vegetatively. The succulent leaves collect (and store) water in the leaf axils, where it is absorbed by surrounding tissue or by aerial roots. There is little published information on the effects of water deficits on vegetative growth, flowering or fruiting. Water stress can reduce the number of fruitlets and the fruit weight. After harvest, one or two ratoon crops can follow. Roots originate from just behind the stem-growing point, some remaining above ground (aerial roots), others entering the soil, reaching depths of 0.85–1.5 m. Root growth ceases at flowering. The ratoon crop depends on the original (plant crop) root system, including the axillary roots. Stomata are present on the abaxial leaf surfaces at relatively low densities (70–85 mm−2). They are open throughout the night, and close during the day before reopening in mid-afternoon. The degree to which CAM attributes are expressed depends in part on the location (e.g. tropics or subtropics), and possibly the cultivar, with the total amount of carbon fixed during the night varying from <3% to >80%. There are surprisingly few published reports of field measurements of crop water use and water productivity of pineapple. Two reports show evapotranspiration only occurring during the daytime. There is more uncertainty about the actual water use of pineapple, the value of crop coefficient (Kc) and relative rates of water loss (transpiration) and carbon gain (net photosynthesis), during the daytime and at night, under different water regimes. This is surprising given the amount of fundamental research reported on photosynthesis of CAM plants in general. Although pineapple is mainly a rainfed crop, it is widely irrigated. Drip irrigation is successfully used where the water supply is restricted, the cost of labour is high and cultivation techniques are advanced. Micro-jets can also be used, as can any of the overhead sprinkler systems, provided wind distortion is not a problem. There is a lack of reliable published data quantifying where irrigation of pineapple is likely to be worthwhile, how it is best practised and the benefits that can be obtained. This is remarkable considering the importance of pineapple as an internationally traded commodity.

Irrigation has been used for thousands of years to maximize the performance, efficiency and profitability of crops and it is a science that is constantly evolving. This potential for improved crop yields has never been more important as population levels and demand for food continue to grow. Recognising the need for a coherent and accessible review of international irrigation research, this book examines the factors influencing water productivity in individual crops. It focuses on nine key plantation/industrial crops on which millions of people in the tropics and subtropics depend for their livelihoods (banana, cocoa, coconut, coffee, oil palm, rubber, sisal, sugar cane and tea). Linking crop physiology, agronomy and irrigation practices, this is a valuable resource for planners, irrigation engineers, agronomists and producers concerned with the international need to improve water productivity in agriculture in the face of increased pressure on water resources.