Crop yield response to water - Cra

quinoa, or vegetables as in the Andean and East Africa highlands. Two crops per year are notuncommon at lower elevations.Growth and developmentPotato is grown on ridges or on flat soil. For rainfed production in dry conditions, such as insouthwest China and at higher elevations of semi-arid cool tropics, flat planting tends to givehigher yields as a result of soil-water conservation. Under irrigation the crop is mainly grownon ridges. Although potato can be planted as botanical seeds, in nearly all cases it is plantedas tubers. The tuber seed should have two to three eyes, from which the new buds and shootssprout. The sowing depth is generally 5 to 10 cm, while deeper planting is preferred in dryareas. Plant density ranges from 30 000 to 60 000 plant/ha and varies depending on seed size,cultivar, and use. Higher plant density is used under irrigation and lower density in rainfedconditions. Cultivation during the growing period must avoid damage to roots and tubers,and often ridges are earthed up to avoid greening of tubers.The plant emerges about 7 days after planting under good conditions. Emergence may bedelayed by lower soil temperature, limited soil moisture, unsprouted or young tubers, anddeep planting. Initial canopy cover (CC o ) depends on seed tuber density and the numbers ofdeveloping shoots per seed tuber, and whether beds are widely or narrowly spaced. Cultivarsmay also differ in this regard. The time from planting to maximum canopy cover (CC x ) can beshortened by denser planting, and by practices conducive to earlier plant emergence, warmtemperatures, and good water supply. At maximum canopy, the percentage of canopy coverranges from 45 percent to nearly 100 percent in the cool tropics. At the early vegetative stage,between emergence and tuber initiation, stolon formation begins, followed by tuberizationand tuber enlargement. The duration of the growth cycle and total tuber production dependson cultivar, temperature and day length. Tuberization comes before flowering and floweringis not necessary to produce tubers.As is true for other crops, the rooting depth of potato depends strongly on soil conditions, theabsence of impeding layers and soil temperature. Root water extraction has been shown to godeeper than 1.8 m under favourable conditions (Wolfe et al., 1983), although often potato issaid to be shallow rooted. Normally, when the upper soil profile is not dry, 70 percent of thetotal water uptake occurs from the upper 0.30 m and nearly 100 percent from the upper 0.40to 0.60 m soil depth. The uptake pattern is, however, also dependent on the location of waterand nutrients, with water extraction mainly from deeper layers when the upper part of thesoil profile is dry.The subspecies tuberosum tuberizes under the 14 to 17 hours photoperiod prevalent in thehigher latitude regions but is grown worldwide as it adapts to shorter photoperiods. Somecultivars of S. tuberosum tuberosum, when grown under short days may initiate tubers earlier,have a reduced canopy with little or no flowering, and mature earlier but with less tuber yieldpotential (Condori et al., 2010). S. tuberosum andigena and the other seven cultivated Solanumspecies (other than Solanum tuberosum) tuberize under 10 to 12 hours photoperiod prevalentin the tropical region and are grown in the Andes. Hybrids of tuberosum and andigena arebeing increasingly grown in developing countries where and when day length is short. S.tuberosum andigena and its hybrids, when grown under long days, may have a late tuber186crop yield response to water