Response of apple rootstocks to irrigation in south-east England

Abstract

Growth, cropping and some aspects of the water relations of apple trees grown on contrasting rootstocks ranging from the dwarfing M.9, through M.26 and MM. 106 to the invigorating MM.lll were compared over five years in a field experiment at East Mailing, Kent. Irrigation increased growth and cropping similarly for trees on all rootstocks, with no evidence for rootstock x irrigation interactions. The rate of soil moisture depletion in the absence of irrigation increased with rootstock vigour, though that of M.9 tended to increase to the level of the intermediate rootstocks towards the end of the experiment. There was no evidence that leaf conductance to water vapour (g,) was controlled by leaf water potential in the normal summer conditions in south-east England. However, there were differences in g, between rootstocks, the highest leaf conductances being observed in more invigorating rootstocks (e.g. MM.lll being up to 19% more than the others) and with an indication that these rootstocks also had the largest stem hydraulic conductance. The results confirm suggestions that growth and development can respond sensitively to soil moisture status and that measured leaf water potential (i)),) is not a sensitive indicator of stress.