Physiological and molecular responses to phosphorus (P) supply and mycorrhizal infection by Glomus intraradices were compared in European (River) and African (H511) maize (Zea mays) cultivars to examine the extent to which these responses differed between plants developed for use in high- and low-nutrient-input agricultural systems. Biomass, photosynthetic rates, nutrient and carbohydrate contents, mycorrhizal colonization and nutrient-responsive phosphate transporter gene expression were measured in nonmycorrhizal and mycorrhizal plants grown at different inorganic phosphorus (P(i)) supply rates. Nonmycorrhizal River plants grew poorly at low P(i) but were highly responsive to mycorrhizal infection; there were large increases in biomass, tissue P content and the rate of photosynthesis and a decline in the expression of phosphate transporter genes. Nonmycorrhizal H511 plants grew better than River plants at low P(i), and had a higher root : shoot ratio. However, the responses of H511 plants to higher P(i) supplies and mycorrhizal infection were much more limited than those of River plants. The adaptations that allowed nonmycorrhizal H511 plants to perform well in low-P soils limited their ability to respond to higher nutrient supply rates and mycorrhizal infection. The European variety had not lost the ability to respond to mycorrhizas and may have traits useful for low-nutrient agriculture where mycorrhizal symbioses are established.

Comparative analyses of aspects of the carbon (C) physiology and the expression of C transporter genes in birch (Betula pendula Roth.) colonized by the ectomycorrhizal fungus Paxillus involutus (Batsch) Fr. were performed using mycorrhizal (M) and non-mycorrhizal (NM) plants of similar foliar nutrient status. After six months of growth, the biomass of M plants was significantly lower than that of NM plants. Diurnal C budgets of both sets of plants revealed that M plants exhibited higher rates of photosynthesis and root respiration expressed per unit dry weight. However, the diurnal net C gain of M and NM plants remained similar. Ectomycorrhizal roots contained higher soluble carbohydrate pools and increased activity of cell wall invertase, suggesting that additional C was allocated to these roots and their ectomycorrhizal fungi consistent with an increased sink demand for C due to the presence of the mycobiont. In M roots, the expression of two hexose and one sucrose transporter genes of birch were reduced to less than one-third of the expression level observed in NM roots. Analysis using a probe against the birch ribosomal internal transcribed spacer region revealed that M roots contained 22% less plant RNA than NM roots. As the expression of birch hexose and sucrose transporter genes was reduced to a much greater extent, this suggests that these specific genes were down-regulated in response to alterations in C metabolism within M roots.

The influence of vesicular–arbuscular mycorrhizal (M) colonization on biomass production and photosynthesis of Trifolium repens L. was investigated in two experiments in which the foliar nitrogen and phosphorus contents of non‐mycorrhizal (NM) plants were manipulated to be no lower than that of M plants. Throughout both experiments there was a stimulation in the rate of CO 2 assimilation of the youngest, fully expanded leaf of M compared with NM plants. In addition, M plants exhibited a higher specific leaf area compared with NM plants, a response that maximized the area available for CO 2 assimilation per unit of carbon (C) invested. Despite the increased rate of photosynthesis in M plants there was no evidence that the additional C gained was converted to biomass production of M plants. It is suggested that this additional C gained by colonized plants was allocated to the mycorrhizal fungus and that it is the fungus, by acting as a sink for assimilates, that facilitated the stimulation in the rate of photosynthesis of the plant partner.

A comparative analysis of daily carbon (C) budgets and aspects of the C physiology of clover (Trifolium repens L.) colonized by vesicular‐arbuscular (VA) mycorrhizal fungi was carried out over a 70 d growth period under conditions designed to ensure that shoots of mycorrhizal (M) and non‐mycorrhizal (NM) plants were of similar nutrient status. C budgets did not differ on day 24 but by day 42 M plants had a significantly higher rate of photosynthesis than their NM counterparts when expressed on a whole shoot basis or unit dry weight basis. As both sets of plants were of the same size it was concluded that this greater C gain was the result of increased sink strength provided by the mycorrhizal fungus. By day 53 M plants had become larger than their uncolonized counterparts and a sink‐induced stimulation in the rate of photosynthesis was no longer apparent. M plants had higher root sucrose, glucose and fructose pools from day 24. Analyses suggested that these sugars were utilized for trehalose and lipid synthesis, for the production of the large extramatrical mycelium and for the support of the respiratory demands of the M root system. Increased C allocation to roots of M plants was associated with a stimulation of the activities of cell wall and cytoplasmic invertases and of sucrose synthase in roots colonized by VA fungi. Such increases in enzyme activity may provide the mechanism enabling increased partitioning of carbohydrate both to the M root system and the fungal symbiont.

Pyridine nucleotide pools were measured in intact plastids from roots of barley (Hordeum vulgare L.) during the onset of NO2- assimilation and compared with the in vitro effect of the NADPH/NADP ratio on the activity of plastidic glucose-6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) from N-sufficient or N-starved roots. The NADPH/NADP ratio increased from 0.9 to 2.0 when 10 mM glucose-6-phosphate was supplied to intact plastids. The subsequent addition of 1 mM NaNO2 caused a rapid decline in this ratio to 1.5. In vitro, a ratio of 1.5 inactivated barley root plastid G6PDH by approximately 50%, suggesting that G6PDH could remain active during NO2- assimilation even at the high NADPH/NADP ratios that would favor a reduction of ferredoxin, the electron donor of NO2- reductase. Root plastid G6PDH was sensitive to reductive inhibition by dithiothreitol (DTT), but even at 50 mM DTT the enzyme remained more than 35% active. In root plastids from barley starved of N for 3 d, G6PDH had a substantially reduced specific activity, had a lower Km for NADP, and was less inhibited by DTT than the enzyme from N-sufficient root plastids, indicating that there was some effect of N starvation on the G6PDH activity in barley root plastids.

Alterations in the regulation of the Calvin cycle were studied in wheat leaves infected with powdery mildew (Erysiphe graminis f. sp. tritici ) during disease development. During infection the triazole fungicide hexaconazole (trade name ANVIL, Zeneca Agrochemicals) was applied to leaves to determine if removal of the fungal sink reversed any pathogen-induced alterations in host physiology. In mildewed and fungicide treated leaves the rate of photosynthesis per unit leaf area declined but was largely unchanged per unit chlorophyll. In addition, the proportion of excitation energy used photochemically (qP) decreased, the proportion dissipated non-photochemically (qN) increased and a loss of chlorophyll was observed in these leaves. By five days after inoculation triose-phosphates and fructose-1,6-bisphosphate had significantly increased while 3-phosphoglyceric acid, ribulose-1,5-bisphosphate and the maximum extractable activity of ribulose-1,5-bisphosphate carboxylase/oxygenase, NADP-glyceraldehyde-3-phosphate dehydrogenase and the stromal fructose-1,6-bisphosphatase had decreased in mildewed leaves. In fungicide treated leaves the amount of specific metabolites and the activity of enzymes was generally intermediate between those in a mildewed, and a healthy leaf. These data indicate a down-regulation of the Calvin cycle as a result of a decreased amount of Calvin cycle enzymes. It is suggested this caused an inhibition of the photosynthetic electron transfer chain and ultimately, the rate of photosynthesis in both mildewed and fungicide treated leaves.

The effects of powdery mildew (Erysiphe graminis f. sp. tritici ) on the carbohydrate metabolism of wheat leaves was studied during infection. During infection the triazole fungicide hexaconazole (trade name ANVIL, Zeneca Agrochemicals) was applied to leaves to determine if removal of the fungal sink reversed any pathogen-induced alterations in host physiology. Five days after inoculation there was a large accumulation of sucrose, glucose, fructose and hexose-phosphates and an increased partitioning of current photosynthate into starch rather than sucrose in both mildewed leaves and those treated with fungicide 2 and 3 days after inoculation. Within 48 h of inoculation, acid invertase activity bad doubled and this increase was maintained until 7 days after inoculation. Application of hexaconazole to a mildewed leaf caused a transient decline in invertase activity but over subsequent days activity increased again until it resembled that observed in a mildewed leaf. Hexaconazole severely inhibited fungal development and hence the size of the fungal sink. This suggests that the enhanced invertase activity was triggered by a signal from the fungus but was not directly related to the size of the fungal sink. These results suggest that (i) the stimulation of invertase activity in mildewed leaves altered the source-sink relationship of the leaf leading to an accumulation of soluble carbohydrates and a decrease in sucrose synthesis and (ii) that application of hexaconazole successfully prevented development of the fungus but that the metabolic alterations which occurred following the initial infection were not reversible within the time scale studied.

Powdery mildews are obligate pathogens. The fungus grows externally on leaf tissues and forms haustoria only in host epidermal cells. The haustorium is believed to be the absorptive organ through which the fungus acquires photosynthates for growth. The intimate interface established between fungus and host and subsequent alterations made to the region of the host plasmalemma in direct contact with the haustorium [1] may aid this function.