GROWTH AND NITROGEN FIXATION IN LUPINUS ARBOREUS AS AFFECTED BY SHADING AND WATER SUPPLY

Abstract

SUMMARYThree experiments were carried out to investigate the effects of shading and water supply on growth and nitrogen fixation by young plants of Lupinus arboreus (up to 4 months old). Firstly, two light treatments, 20,000 and 6500 lux, were given under controlled environment conditions, and plants sampled weekly for 7 weeks. Secondly, four shading treatments (100%, 37%, 19% and 10% full daylight) were given to plants grown outside and samples taken after 4, 8 and 10 weeks. Thirdly, response to and recovery from moisture stress were investigated under controlled‐environment conditions.Results from the two shading experiments showed that L. arboreus responds to shade as a typical mesophyte. Growth (measured as dry weight) was proportional to the logarithm of relative irradiance. Roots were affected more than shoots and hence the shoot: root ratio rose with increased shading. Water content varied with age and shading, the responses differing amongst different parts of the plant. Photosynthetic efficiency increased with moderate shading but decreased, as did relative growth rate, at 10% full daylight. The compensation point was estimated to be 7–8% full daylight (approximately 70 J cm‐2 day‐1).Nodule number and weight were reduced by shading. There was a highly significant linear relationship between root and nodule dry weights, I g root tissue supporting 50 mg nodule tissue. Acetylene‐reducing activity per plant was also proportional to log relative irradiance and this was largely accounted for by the smaller amounts of nodule tissue formed under shade. Activity per milligram fresh weight was only slightly reduced by moderate shading and this probably resulted from increased vacuolation of the infected cells. Only severe shading (10% full daylight) had a marked effect on activity per milligram fresh weight. Nodule senescence was not observed under shade, since the amount of nodule tissue formed was in equilibrium with growth of other parts of the plant. N content was higher in shaded plants after 1 week, but the difference diminished with time until at 5 weeks the N content was the same in shaded and control plants.Response to and recovery from moisture stress were similar to those reported for other species. Seedlings were grown on the fine sand taken from forests where L. arboreus is planted in New Zealand. In this sand the change from sufficient water to severe deficit occurs over a very small range of water contents (1.0–0.5% DW). Shaded plants responded to similar levels of stress as unshaded plants. Water potentials of leaves measured with a pressure bomb were correlated with acetylene‐reducing activity of nodules.