Project Leader: Warren Conaty

Key Researchers: Katie Broughton and Warren Conaty

Brief Summary of Project Objectives:

The aim of this project is twofold:1) Identify genetic diversity in the limiting transpiration trait and evaluate the suitability of this trait to cotton production environments/systems.2) Investigate the genetic diversity and impact of high temperature stress on floral reproduction.

Market/ end user:

Cotton growers through the Core Breeding Program is the primary end user of the research. 

Estimated year to uptake by end user:

Although this research has taken a more fundamental approach than the Abiotic Stress I project, the objective of this project is to deliver abiotic stress resistance traits through new varieties; however it is too early to estimate the delivery timeline.

Executive Summary

The aims of this project are (1) the identification of genetic diversity of water conservation traits, and (2) to investigate genetic diversity to high temperature stress on pollen viability. Although Abiotic Stress II took a more fundamental approach than Abiotic Stress I, the ultimate aim for this research remains the identification of abiotic stress resistance traits that may be applied into the CSIRO cotton breeding program.

Water conservation traits

This study has identified germplasm genetic diversity in water conservation traits: transpiration response to atmospheric water deficit (Vapour Pressure Deficit, VPD) and progressive soil water deficits (Fraction of Transpirable Soil Water, FTSW).

Experiments screening germplasm for the VPD induced limiting transpiration trait (TR_Lim VPD) confirmed genotypes the degree by which transpiration was limited by VPD varied across genotypes (ranging from 4.3 to 6.6 kPa).

Our modelling study indicated that the TR_Lim VPD trait does not necessarily reduce overall plant water use; greater transpiration rates in lower VPD environments offset water conservation at high VPD. While our field performance studies demonstrated that VPD and water availability during the peak flowering period accounted for the 66.7% of the variation in dryland cotton yield, yield performance between transpiration responses in high VPD environments were comparable.

Yield performance of the TR_Lim genotype was improved in some high VPD environments germplasm with the TR_Limtrait is unlikely to out-perform a genotype without the limiting transpiration trait with higher yield potential. In addition, while it may be possible for a TR_Lim VPD trait to improve cotton yield in projected future hotter, drier climatic conditions, overall crop water requirements are likely to be the same.

Experiments have also been conducted to screen cotton genotypes for a soil moisture induced limiting transpiration traits (TR_Lim FTSW). Although the range in FTSW thresholds were relitively small, genotypic variation in transpiration response to drying soil was evident. A trade-off between reduced photosynthetic activity and sustained physiological activity over prolonged water-deficit was found. Therefore, genotypes with low FTSW thresholds may provide improved performance in short water-deficit scenarios, owing to a higher level of productivity over a greater extent of the soil drying cycle. Further investigations are necessary before commercial varieties targeting this trait can be developed.

By their very nature, these complex physiological traits have strong interactions with the environment. Thus, their application not only requires genetic diversity, but also a reliable screening method. As such, this project has successfully undertaken preliminary studies focusing on metabolomic and transcriptomic responses to these environmental stressors. These ‘omics’ studies will provide an understanding of the response of gene and gene products to water deficits, with the hypothesis that they may provide an avenue to incorporate these complex physiological traits to the abiotic stress cotton breeding program. In addition, there is an expanding school of thought that pan’omic data streams may increase the genomic prediction capacities of breeding programs. Thus, these studies will be continued in the CBA 24: Rainfed Cotton Breeding project (Abiotic Stress III).

Pollen thermotolerance

Field studies have identified genetic variation in pollen viability, as determined via a triphenyl tetrazolium chloride based cellular viability assay. While this assay has a relatively high throughput, it would be difficult to deploy this screen widely across the breeding program. Thus, a detailed glasshouse study testing the effect of temperature on pollen viability and subsequent fruit retention and boll components was conducted.

Importantly, this study separated the effect of temperature on pollen viability and physiological heat stress on the maternal parent plant. The results of this study show that while pollen viability influences fruit retention, 80% retention can still be achieved with pollen viability as low as 30%; our field studies show that average pollen viability never observed below this 30% threshold. Therefore, due to the volume of pollen cotton plants produce, the indeterminate nature of cotton reproductive growth (i.e. compensatory ability following heat stress) and the patterns of heat waves in the Australian production regions, we conclude that the extent of genotype differences in pollen viability has little effect on an increase in fruit retention at high temperatures.

We conclude that high throughput pollen viability testing for reproductive heat tolerance are not necessary.

We hypothesise that the impact of high temperature on fruit retention is likely due to assimilate supply, not pollen viability.