MORPHOLOGICAL AND ANATOMICAL EFFECTS OF SEVERE DROUGHT ON THE ROOTS OF LOLIUM PERENNE L.

Three experiments using soil-grown Lolium perenne plants were performed in order to investigate the effects of a gradually increasing drought stress on the death of root cells and the growth of lateral roots. Water potentials of -2 to -10 MPa caused death of the root cortex, but death of root tips occurred only at soil water potentials below -10 MPa. Low soil water potentials promoted lateral root initiation and elongation, the total length of lateral roots being between three and five times that of control plants. On rewetting severely droughted plants, root growth continued by elongation of existing, previously initiated, lateral roots.

THE DISTRIBUTION OF MYCORRHIZAS AMONG FAMILIES OF VASCULAR PLANTS.

From a literature search, information has been compiled on the mycorrhizal status under field conditions of 20 or more species in each of 25 families. The percentage of species which are mycorrhizal ranged from 100% in seven families to 8% in Cruciferae, many families having additional species that are sometimes mycorrhizal. No family in the list was consistently non-mycorrhizal. Apart from the Ericaceae, the families were either predominantly ectomycorrhizal or predominantly VA mycorrhizal. However, almost all families had at least one example of each of these mycorrhizal types.

EVIDENCE ON THE PATHWAYS OF PHOSPHORUS TRANSFER BETWEEN VESICULAR - ARBUSCULAR MYCORRHIZAL PLANTS.

Mycorrhizal hyphae can link one root to another, and it is possible that phosphorus passes from plant to plant by these links. We present evidence on whether this 'direct transfer pathway' is the major route between mycorrhizal plants, or whether most phosphorus passes from the roots of one plant to the soil before being taken up by the other plant's roots or its associated mycorrhizal hyphae (the 'soil pool pathway'). The time-course of loss of 32 P from Lolium perenne L. roots to solution was measured after 32 P had been fed to leaves. Another experiment indicated that the amount of 32 P lost to soil was not influenced by mycorrhizal infection. 32 P was applied to soil in which Plantago lanceolata L., mycorrhizal or non-mycorrhizal, was growing and the time-course of uptake determined. Using these results, two alternative models, the 'direct transfer model' and the 'soil pool model', were used to predict the time-course and amount of 32 P transfer from L. perenne to P. lanceolata. The predictions were then compared with measured transfers between these two species presented here and in a previous paper. The soil pool model's predictions of both amount and time-course of transfer gave the better fit to observation. The evidence thus suggests that direct hyphal links between roots are not important in phosphorus transfer between these plants.

Nutrient transport between ryegrass plants differeing in nutrient status.

It is known that nitrogen and phosphorus can be transferred from one living plant to another, but it is not known whether the amounts transferred are large enough to influence the growth of the species in the field. Two Lolium perenne plants were grown per pot of unfertilized soil. During 25 weeks one plant (the 'donor') in each pot was fed nutrients through leaves; donors in control pots received only water. The 'receiver' plants which shared a pot with the nutrient-fed donors increased significantly, compared with the controls, in leaf number and concentration of phosphorus, though not in concentration of nitrogen, potassium, calcium or magnesium. The rate of phosphorus transfer agreed well with previous results using 32P. The results are compared with rates of phosphorus uptake in the field. It is concluded that nutrient transfer could have an influence on the balance between coexisting species in the field, but probably the influence will be small.

Competition between grassland plants of different initial sizes.

Four species of grassland plant, Plantago lanceolata, Holcus lanatus, Lolium perenne and Rumex acetosa, were grown as monocultures and mixtures in pots of nutrient poor soil in a glasshouse for 8 months. There were four plants per pot and these were arranged in two competition modes: either root and shoot interactions were permitted, or only roots allowed to interact by using above-ground partitions. Time of introduction of seedlings was varied to give a range of plant size ratios at the start of the experiment. The factorial design catered for all combinations of species, competition modes and planting times, replicated in four blocks. The shoots were clipped at a fixed height at each of five harvests. Rumex grew badly and was mostly omitted from analysis of the data.By (i) following the change in the relationship of clip dry weights against planting time with successive harvests, (ii) plotting the change in the logarithm of the ratio of cumulative clip dry weights with time and (iii) the use of de Wit logarithmic ratio plots it was demonstrated that each monoculture and mixture combination's ratios of plant weights converged towards stable equilibrium values. Three hypotheses are put forward to explain why in monocultures a smaller plant was at a competitive advantage relative to a larger neighbour and was not suppressed in its growth by the latter. In mixtures this plant size effect was superimposed to different extents on the relative aggressiveness of the species considered. It was concluded that in a nutrient poor soil, when competition for light was low, root interactions can promote the co-existence of neighbouring plant species.

Rapid cycling of nitrogen and phosphorus from dying roots of Lolium perenne.

Previous experiments, using 32P pulse labelling, showed that when roots of Lolium perenne were detached from the shoot, a substantial proportion of the phosphorus in the roots could within a few weeks be released and be captured by another, living plant. This paper describes experiments designed to confirm and further investigate this rapid nutrient transfer. Roots from plants grown with ample N and P were detached and placed in litter bags in soil. They lost up to 60% of their initial N and up to 70% of their P in three weeks. Even when roots were grown with deficient P supply, resulting in C:P ratios of 300-400, they lost 20-30% of their initial P. Time-courses of 32P loss from roots suspended in solution gave results which agreed with these figures. The initially rapid rate of 32P loss had declined greatly within three weeks. In a pot experiment small L. perenne plants showed a marked increase in their N and P content during 30 days after a neighbouring large plant's shoot was removed, supporting rapid capture of nutrients lost from the detached roots. To investigate P loss from roots while attached to the shoot, L. perenne shoots were clipped every four days and 32P loss from the roots measured. After the third clip the rate of loss increased, eventually to more than four times that from the control plants.

Rapid loss of phosphorus from dying ryegrass roots: the chemical components involved.

Lolium perenne was grown in solution culture with either ample or deficient phosphate supply ('high-P' and 'low-P' plants). The concentration in the roots of phosphorus as water-soluble compounds, phospholipid and insoluble residue was measured. A supplementary experiment showed that the concentration of each component in the roots of low-P plants was similar to that in plants grown in P-deficient soil. The time-course of the decline of each P component was determined in roots detached from the shoot and left hanging in solution. During the three weeks residue P concentration in the roots declined little. In contrast, both types of root lost about three-quarters of their lipid P in the first week. Low-P roots lost little of their water-soluble P. High-P roots contained much more water-soluble P and lost much of it during the first two weeks. By the end of three weeks their water-soluble P content was levelling out at a value similar to that in low-P roots, suggesting a 'non-labile pool'. The rapid loss of lipid P from low-P roots comprised more than half of their total loss, and the possible ecological significance of this is discussed.

Interaction between Osmotic- and Pressure-induced Water Flow in Plant Roots.

When the pressure gradient across a root alters, there is often an apparent change in the permeability of the root to water. Fiscus (Plant Physiol. 1975. 55: 917-922) has suggested that this can be explained by a simple two-compartment model which takes into account rates of solute uptake into the xylem. A method of testing actual data against the Fiscus model is proposed; this shows that in some cases the apparent changes in permeability cannot be explained by the model. The model is not adequate to predict the amounts of solute reaching the xylem by passive drag: a three-compartment model would be more realistic.

Effects of decenylsuccinic Acid on the permeability and growth of bean roots.

Decenylsuccinic acid (DSA) at 10(-3)m has been reported to increase the permeability of bean root systems to water without seriously injuring the plants. We have confirmed the increase in permeability at 10(-3)m, but have found that 10(-4)m DSA reduces the permeability. Both concentrations cause leakage of salts from the roots and cessation of root pressure exudation. The roots of intact bean plants are killed by 1 hour's immersion in 10(-3)m DSA, but the plants may survive by producing new roots. Up to 4 hours in 10(-4)m DSA causes only temporary cessation of growth. Comparisons are made between the effects of DSA and some metabolic inhibitors. It is suggested that DSA is acting as a metabolic inhibitor, and that increase in water permeability is the result of injury to the roots. Experiments with 3 other species indicated variations in response to 10(-3)m DSA. These could be largely attributed to differences in susceptibility to injury.