The Influence of Mineral Nutrition on the Root Development of Trees III. PLASTICITY OF ROOT GROWTH IN RESPONSE TO CHANGES IN THE NUTRIENT ENVIRONMENT

Lodgepole pine seedlings were grown in solution culture with their root systems divided be tween two contrasting nutrient regimes, viz. a high regime, which provided near optimum conditions for growth, and a low regime in which nutrients were limiting. The high nutrient regime stimulated root growth whereas the roots receiving a low level of nutrients grew com paratively slowly and in certain instances stopped elongating and became brown. After dif ferential root growth had been induced certain plants were transferred to a uniform high nutrient environment and the previously deprived parts of the root system responded by renewed growth and their growth rate increased in comparison with roots which remained in low nutrient conditions. The results show that initial nutritional differences in the environment do not lead to domi nance in certain roots ; the root system remains remarkably plastic. INTRODUCTION A common characteristic of tree root systems is the markedly uneven development of neighbouring lateral roots, a feature which has been attributed to large differences in their rates of diameter growth (Brown, 1971). A localized stimulation of root growth has been observed by the application of mineral nutrients to specific regions of a root system in various species, e.g. Sitka spruce (Picea sitchensis (Bong.) Carr. ) (Coutts and Philipson, 1976) and in certain cereals (McClure, 1972; Drew, Saker, and Ashley, 1973). When roots develop in response to variations in their environ ment, it is not known whether an already vigorously growing part of the root system might dominate a weaker portion or whether the weaker root may be stimulated into new activity, once uniform conditions are restored. In this paper we examine the responses of roots of Lodgepole pine (Pinus contorta Douglas ex Loudon) to an uneven nutrient environment and examine the problem of the possible dominance of the vigorously growing part of the root system. MATERIALS AND METHODS Plants Pinus contorta seeds of a South Coastal origin were germinated in horticultural peat and grown in growth chambers with 18 h days at 20 °C. Fourteen days after germination the seedlings were This content downloaded from 157.55. 39.129 on Thu, 28 Jul 2016 04:16:25 UTC All use subject to http://about.jstor. org/terms 1072 Goutts and Philipson—Plasticity of Boot Growth transferred to solution culture in a glasshouse at a minimum temperaure of 15 °C and with the day length extended to 18 h by Hg vapour lamps. The plants were grown for one month during which the roots were periodically trimmed to yield two even portions on each plant. Seventy trees with even root systems were selected for the 133 d experiment which was carried out from March to July. Sampling times in days refer to the times since the uneven nutrient conditions were initiated. Solution culture apparatus A complete mineral nutrient solution adjusted to pH 5 was used (Hocking, 1971) at two concentrations. Our unpublished work on a range of nutrient concentrations showed that 0*25 strength Hocking's solution produced near-optimum growth and this concentration was used during the preparatory stage, for a high nutrient treatment in contrasting nutrient regimes, and for a uniform nutrient environment. A 0-01-strength solution produced shoot deficiency symp toms and minimal root growth ; this strength was used for the low nutrient treatment in con trasting nutrient regimes. The uniform nutrient environment consisted of 5 1 containers of solution, both roots being placed in the same solution. There were two plants per container between days 56 and 84, and one per container between days 84 and 133. The contrasting nutrient environments were twin 850 ml containers of solution with the plants supported between them with one root in each ; one container held 0-25-strength nutrients, the other 0-01. There were two plants in each twin container betwen days 0 and 56 and one per container between days 56 and 84 In both types of apparatus even aeration was applied by introducing air through hypodermic needles which pierced the plastic walls of the containers near the base. The rate of aeration was approximately 400 ml min-1 and 1000 ml min-1 in the 850 ml and 5 1 containers respectively. All nutrient solutions were changed weekly.