WINDBREAK-CROP INTERACTIONS IN THE SAHEL .2. GROWTH-RESPONSE OF MILLET IN SHELTER

Plant growth behind a windbreak varies considerably with changing macr oclimate, species and windbreak type. This variability can often be ex plained in terms of the microclimate in the lee of a windbreak. This p aper describes an investigation into the modification of microclimate by a windbreak and the influence of the windbreak on growth of a mille t crop in its lee in Niger, West Africa. Growth of millet (Pennisetum typhoides) was reduced by shelter at the start of the season because o f high soil surface temperatures, caused by reduced boundary-layer con ductance, which caused a delay in germination and emergence. Towards t he middle of the season, leaf temperatures were in general lower than at the start of the season, and higher temperatures in shelter increas ed the rate of leaf expansion and senescence above that in the unshelt ered parts of the field. At the start of the season, air and leaf temp eratures in shelter increased whereas ambient vapour pressures remaine d relatively constant. This led to higher vapour pressure deficits at the surface of the leaves (D-1) in shelter than in the unsheltered cro p. In the middle of the growing season, vapour pressures increased in shelter so as to reduce D-1 and this increased the solar radiation con version coefficient, probably because of increases in stomatal conduct ance. There was more transpiration in shelter as a result of both larg er leaf area index in the middle and at the end of the growing season and higher transpiration per unit leaf area relative to the unsheltere d crop. Transpiration per unit leaf area was higher in shelter because of higher leaf temperatures and stomatal conductances than in the uns heltered crop.