Geneticists and breeders are positioned to breed plants with root traits that improve productivity under drought. However, a better understanding of root functional traits and how traits are related to whole plant strategies to increase crop productivity under different drought conditions is needed. Root traits associated with maintaining plant productivity under drought include small fine root diameters, long specific root length, and considerable root length density, especially at depths in soil with available water. In environments with late season water deficits, small xylem diameters in targeted seminal roots save soil water deep in the soil profile for use during crop maturation and result in improved yields. Capacity for deep root growth and large xylem diameters in deep roots may also improve root acquisition of water when ample water at depth is available. Xylem pit anatomy that makes xylem less "leaky" and prone to cavitation warrants further exploration holding promise that such traits may improve plant productivity in water-limited environments without negatively impacting yield under adequate water conditions. Rapid resumption of root growth following soil rewetting may improve plant productivity under episodic drought. Genetic control of many of these traits through breeding appears feasible. Several recent reviews have covered methods for screening root traits but an appreciation for the complexity of root systems (e.g., functional differences between fine and coarse roots) needs to be paired with these methods to successfully identify relevant traits for crop improvement. Screening of root traits at early stages in plant development can proxy traits at mature stages but verification is needed on a case by case basis that traits are linked to increased crop productivity under drought. Examples in lesquerella (Physaria) and rice (Oryza) show approaches to phenotyping of root traits and current understanding of root trait genetics for breeding.

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Root traits contributing to plant productivity under drought.

Global warming has led to increased temperature of the earth which is a major abiotic stress posing a serious threat to the plants. Photosynthesis is amongst the plant cell functions that is highly sensitive to high temperature stress and is often inhibited before other cell functions are impaired. The primary sites of targets of high temperature stress are Photosystem II (PSII), ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) while Cytochrome b559 (Cytb559) and plastoquinone (PQ) are also affected. As compared to PSII, PSI is stable at higher temperatures. ROS production, generation of heat shock proteins, production of secondary metabolites are some of the consequences of high temperature stress. In this review we have summarized the physiological, biochemical and molecular aspects of high temperature stress on the process of photosynthesis, as well as the tolerance and adaptive mechanisms involved.

Photosynthesis: Response to high temperature stress

Plants encounter various abiotic stresses due to their sessile nature which include heavy metals, salt, drought, nutrient deficiency, light intensity, pesticide contamination, as well as extreme temperatures. These stresses impose major constraints limiting crop production and food security worldwide. Abiotic stresses primarily reduce the photosynthetic efficiency of plants, due to their negative consequences on chlorophyll biosynthesis, performance of the photosystems, electron transport mechanisms, gas exchange parameters, and many others. A better understanding of the photochemistry of plants under these abiotic stresses can help in the development of pragmatic interventions for managing these stresses. Interestingly, in this review, we provide an overview of insight into different mechanisms affecting the photosynthetic ability of plants in relation to these abiotic factors. The present review describes how different abiotic stresses can pose deleterious impacts on plant photosynthetic machinery including cellular membranes, cell division and cell elongation, biosynthesis of photosynthetic pigments, as well as electron transport chain. It is important to understand the detrimental impacts of various abiotic stresses for better stress management because a comprehensive understanding of plant responses has pragmatic implication for remedies and management.

Photosynthetic Response of Plants Under Different Abiotic Stresses: A Review

Producing more food per unit of water has never been as important as it is at present, and the demand for water by economic sectors other than agriculture will necessarily put a great deal of pressure on a dwindling resource, leading to a call for increases in the productivity of water in agriculture. This topic has been given high priority in the research agenda for the last 30 years, but with the exception of a few specific cases, such as water-use-efficient wheat in Australia, breeding crops for water-use efficiency has yet to be accomplished. Here, we review the efforts to harness transpiration efficiency (TE); that is, the genetic component of water-use efficiency. As TE is difficult to measure, especially in the field, evaluations of TE have relied mostly on surrogate traits, although this has most likely resulted in over-dependence on the surrogates. A new lysimetric method for assessing TE gravimetrically throughout the entire cropping cycle has revealed high genetic variation in different cereals and legumes. Across species, water regimes, and a wide range of genotypes, this method has clearly established an absence of relationships between TE and total water use, which dismisses previous claims that high TE may lead to a lower production potential. More excitingly, a tight link has been found between these large differences in TE in several crops and attributes of plants that make them restrict water losses under high vapour-pressure deficits. This trait provides new insight into the genetics of TE, especially from the perspective of plant hydraulics, probably with close involvement of aquaporins, and opens new possibilities for achieving genetic gains via breeding focused on this trait. Last but not least, small amounts of water used in specific periods of the crop cycle, such as during grain filling, may be critical. We assessed the efficiency of water use at these critical stages.

Transpiration efficiency: new insights into an old story

http://oar.icrisat.org/7918/7/Root%20hydraulics%20-%20the%20forgotten%20side%20of%20root%20in%20drought%20adaptation.pdf

Root hydraulics: The forgotten side of roots in drought adaptation

Temperature effect on transpiration response of maize plants to vapour pressure deficit

While acrylic acid (AA)-based superabsorbent hydrogels (SAHs) have been widely used in multiple applications, the effects of counterion condensation and the polyelectrolyte effect at suppressing the effective degree of ionization in such systems limit their superabsorbency. Herein we describe the use, and investigate the mechanism, of sulfate comonomers containing different types of polymerizable functional groups for increasing the superabsorbency of acrylic acid-based SAHs. Specifically, acrylamido-2-methylpropanesulfonic acid (AMPS), 3-sulfopropyl acrylate (SPAK), and 3-sulfopropyl methacrylate (SPMK) sulfated monomers featuring similar distances between the polymer backbone and the sulfate group but different polymerizable groups were copolymerized with acrylic acid to fabricate SAHs. Measurements of the effective homopolymerization rate constants and copolymerization ratios associated with each copolymerization enabled the prediction of the relative chain distributions of monomers in each copolymer, as quantified by the blockiness parameter (i.e., the instantaneous or average number of consecutive monomers of each type polymerized). While all sulfated comonomers significantly enhanced swelling relative to an acrylic acid control, copolymers in which longer AA blocks are produced (AA-AMPS) resulted in lower swelling than copolymers in which shorter AA blocks are produced (AA-SPAK, AA-SPMK), a result attributed to the suppressed polyelectrolyte effect in copolymers with shorter AA blocks. The formation of limited length blocks of the sulfated monomer SPMK showed additional benefits for enhancing swelling, consistent with enhanced direct charge–charge repulsion between fixed charges in such copolymer systems. We anticipate this linkage between copolymerization kinetics and swelling properties offers the potential to enable improved rational design of superabsorbent hydrogels with higher sorbency. 

Maggie Zhu

Papers

Temperature effect on transpiration response of maize plants to vapour pressure deficit

Applying Copolymerization Kinetics to Understand and Optimize Swelling Responses in Superabsorbent Hydrogels