Crop yield response to water - Cra

Generally, the growing season begins in December in most of the Southern African regionwhereas in parts of Eastern African farmers may plant in two seasons around December andJune. When water supply is unlimited, the rate of germination is dependent on temperature,genetic variability, and seed size and age. Germination generally takes 7 to 15 days and themaximum rate of seed germination occurs around 30 ºC, with no germination below 9 ºCor above 45 ºC (Massawe et al., 2003). The length of the main growth stages is reported inTable 1. Once established, the seedling enters a period of leaf initiation with very little rootgrowth. By about 35 days, the plant has 6-10 leaves with a shallow root system. From thisstage onward leaf and root growth occur simultaneously with the above ground parts makingup an increasing proportion of total plant dry weight. The evolution of canopy cover (CC) istemperature dependent and, at optimal temperature, maximum CC (CC x ) is reached afterabout 60 days in field conditions. Onset of flowering ranges between about 45 and 70 daysafter sowing. Bambara groundnut is an indeterminate crop, with overlapping of vegetativeand reproductive phases, as leaf production continues alongside flowering and podding.The total growing period ranges from 120 to 140 days. Across landraces, stands of bambaragroundnut effectively extract water down to at least 0.90 m under drought whilst when wateris unlimited most extraction occurs in the top 0.50 m of the soil profile.A major labour requirement is the practice of earthing up, also known as ridging, which involvescovering the developing pods with soil. Different reasons are given for this practice, includingbetter pod development, protection of the pods against pests and disease and avoidanceof waterlogging where rainfall intensities are high. However, there is no clear quantitativeevidence for the benefits of earthing up across different environments.Water Use & ProductivitySeasonal transpiration under adequate water supply varies between 500 to more than 600 mm,depending on land races and environment. There is little information on the transpiration ofwater-limited crops. Water productivity (WP Y/ET ) varies with landrace and possibly ranges from2-3 kg/m 3 .Response to StressesBambara groundnut tolerates a wide range of agroecological conditions and poor soilsand this resilience to variable and low-input systems makes it popular among farmers withlimited resources. In particular, it exhibits a number of drought tolerance traits that conferagronomic advantages over other legumes in low and variable rainfall areas. Nevertheless,severe drought during the vegetative phase affects leaf and dry matter production. Wateravailability influences the allocation of dry matter to reproductive yield with harvest index(HI) reduced by up to 16 percent under water limitation.There are reports of significant effect of heat stress on pod formation in some landraces(e.g. Uniswa Red from Swaziland). Experimental evidence from controlled environmentglasshouses at Nottingham, United Kingdom showed that, whilst Uniswa Red produced amassive vegetative canopy through unrestricted leaf production and expansion, it exhibiteda 45 percent reduction in HI when exposed to temperatures above 33 ºC. Whilst high224crop yield response to water