Hemiparasite Phtheirospermum japonicum growth benefits from a second host and inflicts greater host damage with exogenous N supply.

While parasites are likely to connect to multiple host plants in nature, parasitism dynamics under multiple association conditions remain unclear and are difficult to separate from competitive effects. In this study, a five-compartment split root-box was constructed to allow a single facultative root hemiparasite, Phtheirospermum japonicum, to connect to zero, one or two Medicago sativa hosts while maintaining constant plant number and independently controlling nutrient supply. In the first experiment, we found that P. japonicum derived equal, additive benefits from attachment to a second host irrespective of parasite N status. In the second experiment, parasites were grown at four N levels in either parasitic or control conditions. Attachment caused a constant, absolute increase in parasite mass at all N levels, while host damage increased at higher parasite N levels despite an apparent decrease in host to parasite N transfer. Our findings suggest that host damage caused by P. japonicum may be strengthened by exogenous nitrogen supply to the parasite.

Effects of Light, N and Defoliation on Biomass Allocation in Poa annua.

Plants allocate biomass to above- and below-ground organs in response to environmental conditions. While the broad patterns are well-understood, the mechanisms by which plants allocate new growth remain unclear. Modeling approaches to biomass allocation broadly split into functional equilibrium type models and more mechanistically based transport resistance type models. We grew Poa annua plants in split root boxes under high and low light levels, high and low N supplies, with N supplied equally or unequally. Our data suggest that light level had the strongest effect on root mass, with N level being more important in controlling shoot mass. Allocation of growth within the root system was compatible with phloem partitioning models. The root mass fraction was affected by both light and N levels, although within light levels the changes were primarily due to changes in shoot growth, with root mass remaining relatively invariant. Under low light conditions, plants exhibited increased specific leaf area, presumably to compensate for low light levels. In a follow-up experiment, we showed that differential root growth could be suppressed by defoliation under low light conditions. Our data were more compatible with transport resistance type models.

Time Course of Root Axis Elongation and Lateral Root Formation in Perennial Ryegrass (Lolium perenne L.).

Grasses have a segmental morphology. Compared to leaf development, data on root development at the phytomer level are scarce. Leaf appearance interval was recorded over time to allow inference about the age of segmental sites that later form roots. Hydroponically grown Lolium perenne cv. Aberdart tillers were studied in both spring and autumn in increasing and decreasing day length conditions, respectively, and dissected to define the development status of roots of known age on successive phytomers basipetally on the tiller axis. Over a 90-day observation period spring and autumn tillers produced 10.4 and 18.1 root bearing phytomers (Pr), respectively. Four stages of root development were identified: (0) main axis elongation (~0-10 days), (1) primary branching (~10-18 days), (2) secondary branching (~18-25 days), and (3) tertiary and quaternary branching without further increase in root dry weight. The individual spring roots achieved significantly greater dry weight (35%) than autumn roots, and a mechanism for seasonal shift in substrate supply to roots is proposed. Our data define a root turnover pattern likely also occurring in field swards and provide insight for modelling the turnover of grass root systems for developing nutrient efficient or stress tolerant ryegrass swards.

A dynamic model of Rubisco turnover in cereal leaves.

A simple, improved model of Rubisco synthesis and degradation in cereal leaves is developed using data obtained over the leaf lifespan to return maximum likelihood values for Rubisco proteolysis and biosynthesis. It assumes that the time course of leaf Rubisco content can be described using a log-normal curve, and degradation of the Rubisco pool occurs exponentially. Curve parameters give an insight into how Rubisco dynamics differ among treatments or genotypes; also, statistical analyses can be performed more easily, requiring fewer data and allowing more flexibility in sampling than previous studies. Predicted patterns of synthesis correlate well with independent rbc gene transcript data. Rubisco degradation is a first-order decay process, and biosynthesis correlates with leaf elongation rates. As Rubisco degradation takes place according to first-order kinetics, control of leaf Rubisco concentration must be exerted by adjustment of biosynthetic, rather than proteolytic, rates.