Weed‐crop competition studies in swedes.

SUMMARY The developmental morphology and anatomy of swede cv. Ruta Otofte from seedling emergence to vegetative maturity is described, with particular emphasis on development of leaves at the shoot apex and of storage tissue in the hypocotyl and root. Most of the growth of the storage organ takes place by proliferation of parenchyma from secondary cambia scattered throughout the inner secondary xylem. The effects of weed competition on these processes were studied by means of hand-weeding experiments in field-grown swede crops in 1981 and 1982. Weed competition greatly reduced leaf area per plant from about 9 wk after sowing onwards, through restriction of both number and size of leaves. Only leaves which had not fully emerged by the onset of weed competition 6 wk after sowing were reduced in area. Weed removal at 9 wk allowed some recovery of leaf number but not of leaf size; weed removal at 12 wk was similar in effect to leaving the crop unweeded all season. Effects of weed competition on storage organ growth were later to appear than effects on leaf area. Weed competition reduced both the number of cambia initiated and the proliferation of parenchyma from these cambia; again if weed removal was delayed until 12 wk the effect was similar to leaving the crop unweeded but some recovery took place if weeds were removed at 9 wk. Artificial shading had similar effects on swede growth to weed competition, except that leaf number was more strongly affected than leaf size and no recovery in cambial initiation took place if shading was removed at 9 wk. Growth analysis showed that shading affected net assimilation rate to a greater extent than leaf area, at least in the early stages, whereas weed competition had previously been found primarily to affect leaf area. The failure of swedes to make any recovery from weed competition if this was removed at 12 wk was primarily due not to depletion by weeds of soil nutrient resources nor to restricted sink capacity in the storage organ, but to limitation of source capacity. Swedes, unlike sugar beet, apparently cannot rapidly generate new leaves in mid-season to take advantage of weed removal, though there is a large reserve of leaf primordia at the shoot apex. This difference between swedes and sugar beet is discussed in terms of relative sink strength of shoot apex and storage organ.