Experiments were carried out under controlled conditions to investigate the physiological bases for species differences in yield and nutrient responses to variations in phosphorus supply. Buffel grass (Cenchrus ciliaris), and to a less extent Mitchell grass (Astrebla elymoides), showed a much larger yield response to increasing phosphorus supply than mulga grass (Thyridolepis mitchelliana). Mitchell and mulga grasses had much lower relative growth rates than buffel grass. Mulga grass required a lower external phosphorus concentration for optimal growth than Mitchell and buffel grasses; this was attributed to its superior system for absorbing and transporting phosphate from low concentrations, but was not associated with any yield advantage, yield being related more to the photosynthetic than to the nutritional characteristics of the plants. Differences between species in their external phosphorus requirements for growth and their distribution in semiarid Queensland are discussed.
The export of (14)C from leaves of Lycopersion esculentum (Mill.), Capsicum frutescens (L.) and Amaranthus caudatus (L.) was followed by in vivo counting after exposure of the leaf to a 5 min pulse of (14)CO2. In all instances the time course of export showed two or more exponential phases. There was an initial rapid period of export which was followed by a slower phase after about 2 h. About 12-14 h after exposure to (14)CO2 this second phase was superseded by an even slower phase of export which continued for more than 24 h. In tomatoes the initial phase was most rapid in plants bearing fruit which had been heated to 30°C instead of the standard 15-20°C; it was slowest when the fruit were removed. In Amaranthus the rate of the initial phase was shown to be positively correlated with photosynthesis and when the latter was prevented by either darkness or the absence of CO2 the rate of loss of (14)C was reduced. The data were used to test a model of carbon movement from a leaf which postulated the presence of two carbon pools which turned-over at different rates. The photosynthetic carbon entered the pool with the faster rate of turn-over-the 'labile' pool-and exchanged with the other, 'storage', pool. Export from the leaf was from the 'labile' pool. The results suggested that a third, longer term, storage pool should be included in the model and that the exchange between the pools should be non-linear.
This paper investigates the effects of uptake of nitrate and the availability of internal N reserves on growth rate in times of restricted supply, and examines the extent to which the response is mediated by the different pools of N (nitrate N, organic N and total N) in the plant. Hydroponic experiments were carried out with young lettuce plants (Lactuca sativa L.) to compare responses to either an interruption in external N supply or the imposition of different relative N addition rate (RAR) treatments. The resulting relationships between whole plant relative growth rate (RGR) and N concentration varied between linear and curvilinear (or possibly bi-linear) forms depending on the treatment conditions. The relationship was curvilinear when the external N supply was interrupted, but linear when N was supplied by either RAR methods or as a supra-optimal external N supply. These differences resulted from the ability of the plant to use external sources of N more readily than their internal N reserves. These results show that when sub-optimal sources of external N were available, RGR was maintained at a rate which was dependent on the rate of nitrate uptake by the roots. Newly acquired N was channelled directly to the sites of highest demand, where it was assimilated rapidly. As a result, nitrate only tended to accumulate in plant tissues when its supply was essentially adequate. By comparison, plants forced to rely solely on their internal reserves were never able to mobilize and redistribute N between tissues quickly enough to prevent reductions in growth rate as their tissue N reserves declined. Evidence is presented to show that the rate of remobilization of N depends on the size and type of the N pools within the plant, and that changes in their rates of remobilization and/or transfer between pools are the main factors influencing the form of the relationship between RGR and N concentration.
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