Plants living under natural conditions are exposed to many adverse factors that interfere with the photosynthetic process, leading to declines in growth, development, and yield. The recent development of Chlorophyll a fluorescence (ChlF) represents a potentially valuable new approach to study the photochemical efficiency of leaves. Specifically, the analysis of fluorescence signals provides detailed information on the status and function of Photosystem II (PSII) reaction centers, light-harvesting antenna complexes, and both the donor and acceptor sides of PSII. Here, we review the results of fast ChlF analyses of photosynthetic responses to environmental stresses, and discuss the potential scientific and practical applications of this innovative methodology. The recent availability of portable devices has significantly expanded the potential utilization of ChlF techniques, especially for the purposes of crop phenotyping and monitoring.

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Chlorophyll a fluorescence as a tool to monitor physiological status of plants under abiotic stress conditions

Background
Agricultural production is often limited by low phosphorus (P) availability. In developing countries, which have limited access to P fertiliser, there is a need to develop plants that are more efficient at low soil P. In fertilised and intensive systems, P-efficient plants are required to minimise inefficient use of P-inputs and to reduce potential for loss of P to the environment.

Plant and microbial strategies to improve the phosphorus efficiency of agriculture

Limitation of grain crop productivity by phosphorus (P) is widespread and will probably increase in the future. Enhanced P efficiency can be achieved by improved uptake of phosphate from soil (P-acquisition efficiency) and by improved productivity per unit P taken up (P-use efficiency). This review focuses on improved P-use efficiency, which can be achieved by plants that have overall lower P concentrations, and by optimal distribution and redistribution of P in the plant allowing maximum growth and biomass allocation to harvestable plant parts. Significant decreases in plant P pools may be possible, for example, through reductions of superfluous ribosomal RNA and replacement of phospholipids by sulfolipids and galactolipids. Improvements in P distribution within the plant may be possible by increased remobilization from tissues that no longer need it (e.g. senescing leaves) and reduced partitioning of P to developing grains. Such changes would prolong and enhance the productive use of P in photosynthesis and have nutritional and environmental benefits. Research considering physiological, metabolic, molecular biological, genetic and phylogenetic aspects of P-use efficiency is urgently needed to allow significant progress to be made in our understanding of this complex trait.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.2012.04190.x

Opportunities for improving phosphorus-use efficiency in crop plants.

Publisher Summary This chapter focuses on the metabolite levels in specific cells and the subcellular compartments of plant leaves. Leaf material is difficult to fractionate because a typical plant cell contains several different, mechanically fragile subcellular organelles and is surrounded by a mechanically strong plant cell wall. Rather than attempting to isolate whole organelles or cells, leaves are frozen in liquid N 2 and then broken to small fragments that are enriched in material from a given compartment. These fragments are physically separated under conditions when the metabolic activity or redistribution of metabolites is prevented and subsequently their metabolism is quenched. The exclusion of water, or the use of extremely low temperatures, prevents metabolic activity during the fractionation procedures. The chapter describes the silicone oil centrifugation that allows chloroplasts to be separated from the remainder of the protoplast and to be quenched within 2–3 sec of disrupting the protoplast.

Metabolite levels in specific cells and subcellular compartments of plant leaves

INTRODUCTION .. . . . . .... ......... . . . . . . . . ....... ........ . ... .. ......... 421 LIGHT DYNAMICS IN CANOPIES 422 Theory . . . . . . . . . .. . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . ... . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422 Empirical Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . ....... . . . . . . . 424 PHOTOSYNTHESIS IN TRANSIENT LIGHT .. . ..... . . . . . . .. ........ .. . . . . . . . ...... .. . . ...... 425 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425 Measurement of Transient Photosynthesis ...... . . . . . . ......... .. . .. . . . . . . . . . . . . . . . 427 Dynamics of CO2 Assimilation during Sunj1ecks . . . ... . .. ... . . .. . . . . . . . . .. .. ... 429 Limitation of Sunj1eck Use by the P hotosynthetic Induction Requirement ........ ..... 436 Modulation of Induction State in Natural Light Regimes . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . 440 ENVIRONMENTAL CONSTRAINTS ON SUNFLECK UTILIZATION . . . . . . ........ . . . 442 Temperature and Water Stress ..... .. . . . . . . . . . . . ..... . . ........ . . . . . . . . .... .... ... . .. 442 Ph otoinhibition during Sunflecks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. . ........ .... . . . . .. 443 CONCLUDING REMARKS 444

Sunflecks and photosynthesis in plant canopies

Photosynthesis, assimilate partitioning and intracellular distribution of orthophosphate (Pi) in barly (Hordeum vulgare L.) leaves were measured in plants grown with either 25, 1 or 0 mmol· 1−1 nutrient phosphate supply. Phosphate deficiency resulted in a significant decrease in the leaf Pi, diminished rates of photosynthesis and a decrease in the sucrose/starch ratio in the leaves. Changes in the cytoplasmic Pi content were relatively small in comparison with the large variations in vacuolar Pi. The cytoplasmic Pi concentration was slightly higher in the leaves of plants grown at 25 mmol·l−1 Pi than in those grown at 1 mmol·l−1 Pi and was decreased in the phosphate-deficient plants in which photosynthesis was inhibited. With barley plants grown in phosphate-deficient media, very little, if any, Pi was present in the vacuole. All of the cellular Pi was in the cytoplasm. Barley, spinach (Spinacia oleracea L.) and soya (Glycine max L.) plants were grown to a comparative stage of phosphate deficiency as measured by leaf Pi levels. These species showed a uniform response to phosphate deficiency by increasing starch synthesis relative to sucrose but the accompanying limitation on photosynthetic capacity varied considerably among the species. Interspecific differences in assimilate partitioning between starch and sucrose were maintained over a wide range of Pi supply.

The relationship between phosphate status and photosynthesis in leaves : Effects on intracellular orthophosphate distribution, photosynthesis and assimilate partitioning.

Metabolite levels have been compared in the dark and during photosynthesis in leaves and protoplasts from spinach, pea, wheat and barley. In protoplasts the subcellular distribution was also studied. The levels of triose phosphates and sugar bisphosphates were high in the light and low in the dark. The hexose phosphates and 3-phosphoglycerate levels in the dark were very variable depending on the plant material. In most conditions, hexose phosphates and triose phosphates were mainly in the extrachloroplast compartment, while 3-phosphoglycerate and the sugar bisphosphates were mainly in the chloroplast compartment. Leaves always had a very low triose phosphate: 3-phosphoglycerate ratio in the dark, but in protoplasts this ratio was higher. Detailed studies with spinach showed that metabolite levels were very dependent on the availability of carbohydrate in the leaf, particularly starch. Starch mobilisation is not controlled just by the availability of inorganic phosphate and accumulation of phosphorylated intermediates. Hydrolysis of starch may provide precursors for sucrose synthesis while phosphorolysis leads to provision of substrates for respiration. Starch breakdown generates high enough levels of hexose phosphate to support substantial rates of sucrose synthesis in the dark. Respiration is not greatly increased when metabolite levels are high during starch mobilisation. Higher levels of metabolites shorten the length of the induction phase of photosynthesis.

A comparative study of metabolite levels in plant leaf material in the dark.

Individual pools of intracellular inorganic phosphate (Pi) can be observed in the dark in intact cells, protoplasts and chloroplasts from photosynthetic tissue by using 31P nuclear magnetic resonance (n.m.r.). Estimates for the pH of vacuolar and extravacuolar compartments are reported although it is shown that intracellular pH is determined by the pH of the suspending medium. Mannose treatment of asparagus (Asparagus officinalis) cells and spinach (Spinacia oleracea) protoplasts results in the inhibition of photosynthesis. The mechanism of mannose phosphate sequestration of free Pi is supported by the 31P n.m.r. spectra of mannose-treated tissue. There is a fundamental difference in 31 P n.m.r. spectra of mannose-treated spinach protoplasts and asparagus cells, reflecting a difference in the availability of vacuolar Pi for cellular metabolism in these species. The 31P n.m.r. spectrum of intact spinach chloroplasts is reported.

Observations on the phosphate status and intracellular pH of intact cells, protoplasts and chloroplasts from photosynthetic tissue using phosphorus-31 nuclear magnetic resonance.

C Spencer

Papers

The relationship between phosphate status and photosynthesis in leaves : Effects on intracellular orthophosphate distribution, photosynthesis and assimilate partitioning.

A comparative study of metabolite levels in plant leaf material in the dark.

Observations on the phosphate status and intracellular pH of intact cells, protoplasts and chloroplasts from photosynthetic tissue using phosphorus-31 nuclear magnetic resonance.