Soil moisture deficits are one of the most important environmental factors influencing plant growth and reproduction. In many of the fruit-growing regions of the world, including western North America (Barnett et al., 2004), the future availability of water has been questioned as a consequence of climate change and variability. This problem is particularly acute in the southern British Columbia fruit-growing region, where conflicts have arisen between horticulture producers and domestic water users (Neilsen et al., 2006). Thus, for fruit crop production, there is increased interest in imposing controlled moisture deficits to reduce water use as well as limit vegetative growth while potentially improving fruit quality (Behboudian and Mills, 1997).
Manifestations of controlled deficit irrigation have been studied mostly in grape (Dry and Loveys, 1998) and, to a lesser extent, in other fruit crops, including apple (Leib et al., 2006; Mills et al., 1994). There have only been a few studies that investigated both whole-plant physiology and root dynamics (Abrisqueta et al., 2008). Most studies indicate that root systems grow preferentially in moist regions of the soil and exhibit only limited growth in dry soil (Green and Clothier, 1999). However, depending on the volume of soil irrigated, fruit trees can exhibit high architectural plasticity by positioning roots in irrigated soil regions and limiting root growth in dry regions, with little or only relatively small reductions in overall plant water balance (Améiglio et al., 1999). When imposing deficit irrigation, the rate of occurrence of these shifts in root growth may be an important contributor to effective irrigation management, but has not been well studied under field conditions.
In this article, we examine the effects of mulching and restricting irrigation for a 45-d period late in the growing season (August and September) in each of three years on ‘Gala’ apples on M.9 rootstock in sandy soils in southern British Columbia, Canada. Investigations included root growth, shoot water relations, tree branch growth, and fruit yield. We hypothesized that restricted irrigation would increase root growth in the irrigated portion of the soil, and reduce aboveground vegetative growth with only modest reductions in stomatal conductance (gS) and stem water potential. In addition, we hypothesized that mulching would reduce the levels of water stress on the trees caused by restricted irrigation.
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