Harshi K. Gamage

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.

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.

Effects of light and fertilization on arbuscular mycorrhizal colonization and growth of tropical rain-forest Syzygium tree seedlings

This study investigated the effects of light and soil fertility, on arbuscular mycorrhizal fungi (AMF) colonization, and the growth responses (height and dry mass) of Syzygium seedlings. Seedlings of four Syzygium spp. were grown for 2 y in six different light treatments at the research station of the Sinharaja Forest, Sri Lanka. The light treatments exposed seedlings to: (1) 3%; (2) 16%; (3) 50%; (4) 100% of full sun (control); (5) short periods (2 h d−1) of direct sunlight; and (6) long periods (6 h d−1) of direct sunlight. In the 16% of full sun treatment five sets of fertilizer applications supplied: (1) magnesium; (2) potassium; (3) phosphorus; (4) all three nutrients combined; and (5) no fertilizer (control). The Syzygium species had the greatest mycorrhizal colonization in brighter treatments that provided direct light. Comparison across species revealed S. firmum to have moderate mycorrhizal colonization but high total dry mass. Syzygium operculatum had high percentages of mycorrhizal colonization while S. rubicundum had low percentages of mycorrhizal colonization especially in deep shade. Syzygium makul showed moderate levels of mycorrhizal colonization and dry mass, but low height growth. Among fertilizer applications, phosphorus enhanced seedling growth and mycorrhizal colonization for all species. However, species showed decreased growth with high amounts of potassium and combined fertilizer applications. Results suggest that AMF colonization will be highest, and Syzygium spp. growth greatest, beneath canopy openings large enough to receive direct sun in phosphorus-rich soils.

Key innovation or adaptive change? A test of leaf traits using Triodiinae in Australia

The evolution of novel traits (“key innovations”) allows some lineages to move into new environments or adapt to changing climates, whereas other lineages may track suitable habitat or go extinct. We test whether, and how, trait shifts are linked to environmental change using Triodiinae, C4 grasses that form the dominant understory over about 30% of Australia. Using phylogenetic and relaxed molecular clock estimates, we assess the Australian biogeographic origins of Triodiinae and reconstruct the evolution of stomatal and vascular bundle positioning. Triodiinae diversified from the mid-Miocene, coincident with the aridification of Australia. Subsequent niche shifts have been mostly from the Eremaean biome to the savannah, coincident with the expansion of the latter. Biome shifts are correlated with changes in leaf anatomy and radiations within Triodiinae are largely regional. Symplectrodia and Monodia are nested within Triodia. Rather than enabling biome shifts, convergent changes in leaf anatomy have probably occurred after taxa moved into the savannah biome—they are likely to have been subsequent adaptions rather than key innovations. Our study highlights the importance of testing the timing and origin of traits assumed to be phenotypic innovations that enabled ecological shifts.

Indigenous and modern biomaterials derived from Triodia ('spinifex') grasslands in Australia.

Plant-derived fibres and resins can provide biomaterials with environmental, health and financial benefits. Australian arid zone grasses have not been explored as sources of modern biomaterials including building materials. Triodia grasslands are a dominant vegetation type in the arid and semiarid regions of Australia covering a third of the continent. Of the 69 identified Triodia species, 26 produce resin from specialised cells in the outer leaf epidermis. In Aboriginal culture, Triodia biomass and resin were valued for their usefulness in cladding shelters and as a hafting agent. Since European settlement, Triodia grasslands have been used for cattle grazing and burning is a common occurrence to improve pasture value and prevent large-scale fires. Although Triodia grasslands are relatively stable to fires, more frequent and large-scale fires impact on other fire sensitive woody and herbaceous species associated with Triodia and invasion of exotic weeds resulting in localised changes in vegetation structure and composition. The extent and change occurring in Triodia grasslands as a result of altered land-use practices, fire regimes, and changing climate warrant careful consideration of their future management. Localised harvesting of Triodia grasslands could have environmental benefits and provide much needed biomaterials for desert living. Research is underway to evaluate the material properties of Triodia biomass and resin in the context of Indigenous and western scientific knowledge. Here, we review uses of Triodia and highlight research needs if sustainable harvesting is to be considered.

Comparative growth of four Syzygium species within simulated shade environments of a Sri Lankan rain forest

In this study we tested the hypothesis that related tree species within the timber tree genus Syzygium differ in their shade-tolerance. We propose that difference in tolerance relate to the successional status and site affinities of each species found within the rain forests of southwest Sri Lanka. Seedlings of each of the four Syzygium species were grown for 24 months in replicated environmental treatments that simulated six different shade quality and quantities recorded from a Sri Lankan rain forest. Treatments were: (i) a deep uniform shade (DS) environment that comprised only 1% of photosynthetic photon flux density (PFD) as compared to that of the full open; (ii) a medium uniform shade (MS) environment receiving 14% of PFD as compared to the full open; (iii) a light uniform shade (LS) environment receiving 50% of PFD; (iv) the center environment of a small 200 m2 opening (SD), receiving 18% of PFD; (v) the center environment of a large 400 m2 canopy opening (LD), receiving 54% of PFD; and (vi) full sun (FS) receiving 100% of PFD. All species increased both above- and below-ground growth with increasing amounts of PFD. Seedling height, root collar diameter and dry mass gain were greatest in the brighter shade treatments with little discrimination shown among LD, LS, and FS. Significant differences in growth also occurred among the four species. Comparisons among species in the full sun (FS) treatment revealed S. rubicundum and S. operculatum to have greater height increments than S. makul and S. firmum. The low leaf mass ratio of S. operculatum, in particular, and S. rubicundum, suggests both to be prone to wilt during periods of desiccation. S. rubicundum also had greatest leaf and branch numbers and smallest leaves compared to the other three species. S. firmum in particular, but also S. makul, had larger, thicker leaves, with greater total dry mass in the FS treatment compared to the other two species. In the deep shade treatment (DS) S. firmum had greatest total dry mass and S. operculatum had the least. Taken together these findings reveal S. rubicundum and S. operculatum to be the most shade-intolerant of the four Syzygium species. Both appear prone to desiccation and water loss, though we speculate the small, numerous leaves and fine branches of S. rubicundum (characteristics of more drought-tolerant species) make this species less so. Both S. firmum and S. makul do best in the brighter shade treatments. Compared to the other two Syzygium spp., both are less susceptible to desiccation in high light environments because of their larger, thicker leaves and greater bulk. S. firmum appears to be the most shade-tolerant of the four Syzygium species. Findings have direct implications for forest management. To secure regeneration establishment and release or to create suitable planting environments Syzygium spp. require silvicultural treatments that account for species specific limitations of site (water availability) and shade (canopy opening size).

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

Heteroblastic species change their leaf morphology due to changes in light environment. However, growth and biomass allocation pattern do not contribute to their better survival relative to homoblastic congeners in low light. Thus, shade does not select for leaf heteroblasty.

Harvesting as an Alternative to Burning for Managing Spinifex Grasslands in Australia

Sustainable harvesting of grasslands can buffer large scale wildfires and the harvested biomass can be used for various products. Spinifex (Triodia spp.) grasslands cover ≈30% of the Australian continent and form the dominant vegetation in the driest regions. Harvesting near settlements is being considered as a means to reduce the occurrence and intensity of wildfires and to source biomaterials for sustainable desert living. However, it is unknown if harvesting spinifex grasslands can be done sustainably without loss of biodiversity and ecosystem function. We examined the trajectory of plant regeneration of burned and harvested spinifex grassland, floristic diversity, nutrient concentrations in soil and plants, and seed germination in controlled ex situ conditions. After two to three years of burning or harvesting in dry or wet seasons, species richness, diversity, and concentrations of most nutrients in soil and leaves of regenerating spinifex plants were overall similar in burned and harvested plots. Germination tests showed that 20% of species require fire-related cues to trigger germination, indicating that fire is essential for the regeneration of some species. Further experimentation should evaluate these findings and explore if harvesting and intervention, such as sowing of fire-cued seeds, allow sustainable, localised harvesting of spinifex grasslands.

Short Communication. A robust method for chromosome quantification and ploidy determination in woody species

Accurate determination of ploidy level of putative polyploid plants is essential for tree breeding and other applications. Methods for ploidy determination include quantification of chromosome numbers in root-tip cells via light microscopy and indirect assessment via anatomical and morphological traits. Flow cytometry is potentially a high-throughput method to quantify nuclear DNA content; however, it does not allow determining chromosome numbers and interfering compounds often prevent its use. Microscopy-based quantification of chromosomes in active root-tip cells remains the most unambiguous method for ploidy determination, although root tips are difficult to obtain from field-grown plants, and light microscopy can result in insufficient resolution in species with many and small chromosomes. Here, we present a robust technique that uses 2, 4-diamidino-2-phenylindole (DAPI) dye and 1000-fold magnification fluorescence microscopy for quantification of chromosomes in root and shoot tips of woody angiosperms and gymnosperms, and overcomes the reported difficulties. Rather than the conventional tip squashing, spreading tips on glass slides resulted in very good chromosome separation in diverse species, with up to 56 chromosomes and a chromosome size of 2–20 μm. Chromosome counts were performed in diploid Agathis robusta, Elaeocarpus angustifolius, Eucalyptus robusta, Paulownia tomentosa, Pongamia pinnata and Toona ciliata, and di- and tetraploid Acacia crassicarpa and Citrus species.

Leaf Serration in Seedlings of Heteroblastic Woody Species Enhance Plasticity and Performance in Gaps But Not in the Understory

Leaf heteroblasty refers to dramatic ontogenetic changes in leaf size and shape, in contrast to homoblasty that exhibits little change, between seedling and adult stages. This study examined whether the plasticity in leaf morphology of heteroblastic species would be an advantage for their survival and growth over homoblastic congeners to changes in light. Two congeneric pairs of homoblastic (Hoheria lyallii, Aristotelia serrata) and heteroblastic species (H. sexstylosa, A. fruticosa) were grown for 18 months in canopy gap and forest understory sites in a temperate rainforest in New Zealand. Heteroblastic species that initially had serrated leaves reduced leaf serration in the understory, but increased in the gaps. Heteroblastic species also produced thicker leaves and had higher stomatal pore area ( length), maximum photosynthetic rate, survival, and greater biomass allocation to shoots than homoblastic relatives in the gaps. Findings indicate that increased leaf serration in heteroblastic species is an advantage over homoblastic congeners in high light.