Nitrate paradigm does not hold up for sugarcane.

Modern agriculture is based on the notion that nitrate is the main source of nitrogen (N) for crops, but nitrate is also the most mobile form of N and easily lost from soil. Efficient acquisition of nitrate by crops is therefore a prerequisite for avoiding off-site N pollution. Sugarcane is considered the most suitable tropical crop for biofuel production, but surprisingly high N fertilizer applications in main producer countries raise doubt about the sustainability of production and are at odds with a carbon-based crop. Examining reasons for the inefficient use of N fertilizer, we hypothesized that sugarcane resembles other giant tropical grasses which inhibit the production of nitrate in soil and differ from related grain crops with a confirmed ability to use nitrate. The results of our study support the hypothesis that N-replete sugarcane and ancestral species in the Andropogoneae supertribe strongly prefer ammonium over nitrate. Sugarcane differs from grain crops, sorghum and maize, which acquired both N sources equally well, while giant grass, Erianthus, displayed an intermediate ability to use nitrate. We conclude that discrimination against nitrate and a low capacity to store nitrate in shoots prevents commercial sugarcane varieties from taking advantage of the high nitrate concentrations in fertilized soils in the first three months of the growing season, leaving nitrate vulnerable to loss. Our study addresses a major caveat of sugarcane production and affords a strong basis for improvement through breeding cultivars with enhanced capacity to use nitrate as well as through agronomic measures that reduce nitrification in soil.

Phenotypic variation in heteroblastic woody species does not contribute to shade survival.

BACKGROUND AND AIMS: Leaf heteroblasty involves dramatic phenotypic differences between adult and seedling leaves while leaves of homoblastic plants display only small differences. This study tested whether, in low-light environments, the marked difference in the morphology of seedling leaves that characterizes heteroblastic species confers advantages for seedling survival and growth compared with homoblastic congeners. METHODOLOGY: Four pairs of heteroblastic and homoblastic species in genera Hoheria, Aristotelia, Pseudopanax and Melicope were grown in simulated full sunlight (100 % of light, red:far red ratio (R:FR) = 1.25) or in simulated forest understorey shade (5 % of full sunlight, R:FR ratio = 0.25) in a glasshouse. PRINCIPAL RESULTS: After 9 months, 100 % of seedlings of both homoblastic and heteroblastic species survived in full sun while in the understorey treatment there were 25 % fewer heteroblastic survivors than homoblastic congeners. Compared with homoblastic congeners, all heteroblastic species except for Pseudopanax crassifolius produced more and smaller leaves and branches, but grew more slowly in height, root collar diameter and total biomass both in full sun and in forest understorey treatments. CONCLUSIONS: Homoblastic species survive and grow better in the forest understorey light treatment, suggesting that heteroblastic seedling leaf morphology does not give an advantage over homoblastic congeners under low light intensities.

Plants can use protein as a nitrogen source without assistance from other organisms.

Nitrogen is quantitatively the most important nutrient that plants acquire from the soil. It is well established that plant roots take up nitrogen compounds of low molecular mass, including ammonium, nitrate, and amino acids. However, in the soil of natural ecosystems, nitrogen occurs predominantly as proteins. This complex organic form of nitrogen is considered to be not directly available to plants. We examined the long-held view that plants depend on specialized symbioses with fungi (mycorrhizas) to access soil protein and studied the woody heathland plant Hakea actites and the herbaceous model plant Arabidopsis thaliana, which do not form mycorrhizas. We show that both species can use protein as a nitrogen source for growth without assistance from other organisms. We identified two mechanisms by which roots access protein. Roots exude proteolytic enzymes that digest protein at the root surface and possibly in the apoplast of the root cortex. Intact protein also was taken up into root cells most likely via endocytosis. These findings change our view of the spectrum of nitrogen sources that plants can access and challenge the current paradigm that plants rely on microbes and soil fauna for the breakdown of organic matter.