Delayed greening in phosphorus-efficient Hakea prostrata (Proteaceae) is a photoprotective and nutrient-saving strategy

Life at the conservative end of the leaf economics spectrum: intergeneric variation in the allocation of phosphorus to biochemical fractions in species of Banksia (Proteaceae) and Hakea (Proteaceae)

In severely phosphorus (P)-impoverished environments, plants have evolved to use P very efficiently. Yet, it is unclear how P allocation in leaves contributes to their photosynthetic P-use efficiency (PPUE) and position along the leaf economics spectrum (LES). We address this question in 10 species of Banksia and Hakea, two highly P-efficient Proteaceae genera. We characterised traits in leaves of Banksia and Hakea associated with the LES: leaf mass per area, light-saturated photosynthetic rates, P and nitrogen concentrations, and PPUE. We also determined leaf P partitioning to five biochemical fractions (lipid, nucleic acid, metabolite, inorganic and residual P) and their possible association with the LES. For both genera, PPUE was negatively correlated with fractional allocation of P to lipids, but positively correlated with that to metabolites. For Banksia only, PPUE was negatively correlated with residual P, highlighting a strategy contrasting to that of Hakea. Phosphorus-allocation patterns significantly explained PPUE but were not linked to the resource acquisition vs resource conservation gradient defined by the LES. We conclude that distinct P-allocation patterns enable species from different genera to achieve high PPUE and discuss the implications of different P investments. We surmise that different LES axes representing different ecological strategies coexist in extremely P-impoverished environments.

Relatively high light inhibits reserves degradation in the Coptis chinensis rhizome during the leaf expansion by changing the source-sink relationship

The early spring is a seasonal high-light "window" for new leaf growth and photosynthetic carbon capture by the shade-tolerant evergreen understory plants. However, it remains unclear how light regulates the source-sink relationship between rhizome (RO), mature leaf (ML), and immature leaf (IL) during Coptis chinensis leaf expansion. Understanding this relationship is essential to reducing RO reserve degradation and ultimately promote RO biomass accumulation. The plants grew in an artificial climate chamber with low (50 μmol m-2 s-1) and relatively high (200 μmol m-2 s-1) light intensity treatments. Leaf fluorescence, foliar phosphorus (P) fractions, soluble sugars, starch, total P, and alkaloid concentrations in ILs, MLs, and RO were measured, and 13C labeling was used to indicate the direction of photosynthetic carbon flow between organs. The plants grown under high light intensity had higher levels of starch in RO and higher RO biomass at the end of the year compared to those grown under low light intensity. The photosystem II (PSII) operating efficiency [Y(II)], relative electron transport rate (rETR), and photochemical quenching (qP), as well as sucrose and glucose, in ILs and MLs under relatively high light, was higher than those under low light. The glucose and starch concentrations in ILs at 35 d was significantly higher than that at 15 d when plants were under 200 μmol m-2 s-1, while they were not significantly changed and remained low at 50 μmol m-2 s-1. The 13C was detected in the RO when plants were grown at 200 μmol m-2 s-1, regardless of ILs and MLs 13C labeling, while no 13C was detected in the RO when plants were under 50 μmol m-2 s-1. Additionally, the proportion of photosynthetic transport from ILs to MLs was significantly higher than that from MLs to ILs under the 50-μmol m-2 s-1 limit. Total P concentration in ILs was lower under relatively high light, but there was no difference in nucleic acid P concentration in ILs under the two light intensity treatments. The alkaloid concentration in RO was lower under 200 μmol m-2 s-1 than that under 50 μmol m-2 s-1. We propose that relatively high light reduces the need for carbohydrates and P stored in the RO to support IL growth by (1) accelerating the sink-to-source transition in ILs, which inhibits the use of reserves in the RO; (2) using energy from MLs to support IL growth, thereby reducing RO reserve consumption, and (3) reducing the demand for P by investing less in the development of photosynthetic machinery. Furthermore, under low light, MLs serve as a sink and rely on other organs for support, directly or indirectly exacerbating the reserves lost in the RO.

Characterizing the development of photosynthetic capacity in relation to chloroplast structure and mineral nutrition in leaves of three woody fruit species

Plants have evolved different developmental patterns of photosynthetic capacity to better adapt to changing environmental conditions. Natural variation in photosynthetic development offers great potential for improving crop productivity. In this study, leaf developmental patterns were characterized in three woody fruit tree species with distinct photosynthetic capacity and growth habits. Changes in the photosynthetic rate, photosystem II (PSII) efficiency, chloroplast ultrastructure, activities of photosynthetic enzymes, and contents of carbohydrates and mineral nutrients were examined at five developmental stages to explore the interspecific variation in photosynthetic development. Rapid development of photosynthetic machinery and high photosynthetic capacity were found in Indian jujube (Ziziphus mauritiana) and apple (Malus domestica), whose net CO2 assimilation rate (A) peaked at full leaf expansion (FLE). Litchi (Litchi chinensis), a delayed-greening species, showed slow development of photosynthetic competence, with A peaked after FLE. The low photosynthetic capacity of litchi during early leaf expansion was associated with its delayed chloroplast development, low accumulation of starch, and low activities of ribulose-1,5-bisphosphate carboxylase/oxygenase and NADP-glyceraldehyde-3-phosphate dehydrogenase. Correlations between mineral contents and A across leaf stages and species identified manganese as the rate-limiting nutrients in photosynthetic development in new leaves. Foliar spray of MnSO4 solution (1 g l-1) induced a short-term increase in photosynthesis in young leaves of litchi. These findings suggest that a better understanding of interspecific variation in photosynthetic development facilitates the development of new strategies for improving the photosynthetic efficiency of woody fruit trees.

Mechanisms for improving phosphorus utilization efficiency in plants

BACKGROUND

Limitation of plant productivity by phosphorus (P) supply is widespread and will probably increase in the future. Relatively large amounts of P fertilizer are applied to sustain crop growth and development and to achieve high yields. However, with increasing P application, plant P efficiency generally declines, which results in greater losses of P to the environment with detrimental consequences for ecosystems.

SCOPE

A strategy for reducing P input and environmental losses while maintaining or increasing plant performance is the development of crops that take up P effectively from the soil (P acquisition efficiency) or promote productivity per unit of P taken up (P utilization efficiency). In this review, we describe current research on P metabolism and transport and its relevance for improving P utilization efficiency.

CONCLUSIONS

Enhanced P utilization efficiency can be achieved by optimal partitioning of cellular P and distributing P effectively between tissues, allowing maximum growth and biomass of harvestable plant parts. Knowledge of the mechanisms involved could help design and breed crops with greater P utilization efficiency.