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Cotton aphid (Aphis gossypii Glover) is a worldwide pest of cotton (Gossypium hirsutum L.). High aphid densities visibly damage cotton leaves, reducing leaf photosynthesis and yield. In contrast, lower aphid densities usually do not visibly damage leaves but their effect on photosynthesis is unknown, complicating the decisions on the need for their control. The effect of cotton aphid at lower densities on gas exchange of cotton leaves was investigated under field conditions in the Namoi Valley, Australia. Photosynthesis, leaf area, stomatal opening, and aphid density were assessed on the third mainstem leaf below the terminal of aphid infested and uninfested control plants. Photosynthesis was reduced by a maximum of 27% at a density of 27.6 aphids cm−2 of leaf tissue. Negative linear relationships were found between aphid density and relative photosynthesis, relative conductance and relative transpiration. The temperature of leaves infested with >5 aphids cm−2 was higher than control leaves. Stomatal conductance was reduced faster than photosynthesis, and 87% of stomata on aphid infested leaves were closed, compared to only 34% on control leaves. Closure of stomata was implicated as a cause of reduced photosynthesis in aphid infested cotton leaves. Stomatal closure may be a direct response to cell damage from aphid feeding, as a plant defense or as a response to aphid induced stress. The results suggest that low levels of aphids (e.g., <7 aphids cm−2) on leaves in the upper canopy of cotton crops may be tolerated with low risk to photosynthesis. Core Ideas: Aphis gossypii feeding reduced the photosynthetic rate of cotton leaves which showed no visible damage.Photosynthesis, conductance, and transpiration declined as aphid density increased.Leaves fed on by aphids had a higher percentage of closed stomata than control leaves.Stomatal closure in response to aphid feeding is the likely cause of reduced photosynthesis.Low levels of aphids (e.g., <7 aphids cm−2) may be tolerated with low risk to photosynthesis. [ABSTRACT FROM AUTHOR]

Improving the quality of Australian cotton fiber is essential for maintaining industry viability. Two field experiments were conducted to assess the fiber quality and yarn performance of Australian bred cotton (five Gossypium hirsutum L. and one G. barbadense L.) genotypes. The work included the novel measurement of fiber maturity ratio, fiber linear density, and fiber diameter (ribbon width). The strongest yarns were produced using genotypes with the longest and finest fiber, for example, the strength of 20 tex yarns for the G. barbadense L. cultivar Sipima 280 (length = 36.6 mm, linear density = 143 mtex, ribbon width = 13.7 [micro]m) was 25.4 cN [tex.sup.-1] cf. the G. hirsutum L. cultivar Sicala 350B (length = 32.5 mm, linear density = 185 mtex, ribbon width = 14.5 [micro]m) yarn strength of 18.1 cN [tex.sup.-1]. Micronaire was an inferior indicator of yarn performance, for example, the G. hirsutum L. breeding lines CHQX12B and CHQX377 each had micronaire values of 4.4, but CHQX377 spun stronger yarns due to its finer and more mature fiber. Lint cleaning had the greatest influence on nep (fiber knot) generation for G. hirsutum L. genotypes, generating on average 104 neps [g.sup.-1] per lint cleaner passage. There was a negative association between fiber quality and yield, and a cost benefit analysis showed that fiber yield was the dominant economic factor compared to price premiums for better fiber quality. Alternative methods of determining fiber fineness will improve the value of Australian cotton. doi: 10.2135/cropsci2009.10.0600

Crop canopy temperature (Tc) is coupled with transpiration, which is a function of soil and atmospheric conditions and plant water status. Thus, Tc has been identified as a real-time, plant-based tool for crop water stress detection. Such plant-based methods theoretically integrate the water status of both the plant and its environment. However, previous studies have highlighted the limitations and difficulty of interpreting the Tc response to plant and soil water stress. This study investigates the links between cotton Tc, established measures of plant water relations and atmospheric vapour pressure deficit (VPDa). Concurrent measures of carbon assimilation (A), stomatal conductance (gs), leaf water potential (ψ1), soil water (fraction of transpirable soil water (FTSW)) and Tc were conducted in surface drip irrigated cotton over two growing seasons. Associations between A, gs, ψ1, FTSW and Tc are presented, which are significantly improved with the inclusion of VPDa. It was concluded that utilising the strong associations between ψ1, VPDa and Tc, an adjustment of 1.8°C for each unit of VPDa should be made to the critical Tc for irrigation. This will improve the precision of irrigation in Tc based irrigation scheduling protocols. Improved accuracy in water stress detection with Tc, and an understanding of the interaction the environment plays in this response, can potentially improve the efficiency of irrigation. [ABSTRACT FROM AUTHOR]

An experiment was conducted to determine if negative associations between yield and fiber quality in cotton (Gossypium hirsutum L.) were due to leaf CO2 exchange rate (CER). A 3-year study evaluated six high-quality genotypes, with two high-yielding commercial control cultivars and a G. barbadense L.cultivar, for CER and stomatal conductance to water vapor (g) across contrasting seasons. Yield, fiber quality, and harvest index (HI) were compared with CER and g measured on the uppermost fully expanded leaf on two occasions between mid-flower and cutout. The results show no significant associations between CER and fiber-quality parameters (p<0.05). There was no association between CER and yield, HI, or stem weight. It was concluded that highquality lines did not photosynthesize less than the high-yielding commercial check. There was a positive association between HI and yield (p<0.01). An inefficient growth habit as measured by a low HI in at least one high-fiber quality genotype was a possible reason for lower lint yield. [ABSTRACT FROM AUTHOR]

Exploiting genetic variation through inter-specific breeding has improved cotton yield, fibre properties and adaptability. The objectives of this study were to examine heritability and predicted selection response of yield components and fibre properties in a recombinant inbred line (RIL) population from an inter-specific cross between Gossypium hirsutum (Gh) variety Guazuncho 2, and G. barbadense (Gb) line VH8-4602. A population of 93 and 82 RILs was tested in two seasons, with two parents and local controls, Sicot 75 (Gh) and Sipima 280 (Gb) in field experiments. Seed cotton samples hand harvested before and after defoliation were used to measure lint percent, boll weight, 100 seed weight and the lint to measure fibre length, uniformity, short fibre index (SFI), elongation, strength, micronaire, maturity ratio (MR), percent of maturity (PM) and fineness. There was large phenotypic variation for individual traits and transgressive segregation occurred in lint percent, lint weight/seed, fibre no./seed, uniformity, SFI, elongation, MR and PM. Narrow sense heritabilities were moderate for yield components (34.3-41.2%) and for key fibre properties, length, strength, micronaire and fineness (38.3-42.1%), which led to a predicted selection response of 6.7-24.0% for yield components and 3.9-10.9% for key fibre properties under a selection intensity of 10%. Favourable associations were found between key fibre properties, but an adverse association between lint percent and each of these fibre properties. Only five RILs were identified with desirable combinations. The results demonstrated the value of exploiting inter-specific variation to develop cotton germplasm and how breeding strategies can be improved. [ABSTRACT FROM AUTHOR]

Differences in micronaire of cotton fiber can affect grower returns, and influence textile quality. Therefore quantifying those effects that influence micronaire are important in developing management practices to optimise micronaire. This study proposes a method for predicting seasonal crop micronaire. The aim was to quantify the response of micronaire to temperature during boll filling and assess this information's ability to predict micronaire on an independent dataset. Utilising existing data from sowing time experiments in Australia that spanned three decades, linear responses of micronaire to both daily average and minimum temperatures were developed (r² =0.68 for both). These responses coupled with an estimate of temperature during the boll filling period when the majority of bolls were undergoing fiber thickening were able to successfully predict the micronaire on an independent dataset (r²=0.42) despite no adjustment for other climate and management factors that may influence crop micronaire. The ability to predict temperature effects on micronaire will be useful to assess reasons for seasonal and regional differences in micronaire and assess opportunities to modify micronaire with changes in management practices that influence the timing of boll development. [ABSTRACT FROM AUTHOR]

Cotton fibers are trichome cells composed primarily of cellulose. Mature fibers have more cellulose and a greater degree of cell wall thickening, and perform better than less mature fibers during textile processing. An automated polarized light microscope instrument called Siro- Mat that measures cotton fiber cell wall thickening was employed to assess the maturity of developing fibers from single cotton fruit. Fruit were taken from the first fruiting branch and position on glasshouse grown Gossypium hirsutum L. (Upland) and G. barbadense L. (Pima) plants, sequentially harvested from 24 days postanthesis (dpa) at approximately four-day intervals up until approximately 50 dpa. The instrument assessed an average of 13,000 fiber snippets per fruit. Upland fibers matured at a slower rate than Pima fibers up to 35 dpa. However, after 45 dpa Upland fibers had achieved a higher average maturity (i.e. 0.99 birefringence maturity index (BMI), cf. 0.79 for Pima). For both species the uniformity of fiber maturity increased as fibers matured up until 35 dpa for Upland and 29 dpa for Pima (i.e. the BMI coefficient of variation decreased as BMI increased during fruit development). It is envisaged that SiroMat will be a useful tool in helping to understand and manage fiber maturity by characterizing the maturation dynamics of cultivars with different inherent fiber properties, and for cultivars subjected to different environmental and agronomic conditions. [ABSTRACT FROM AUTHOR]

• Yield progress of 16.1 kg lint ha−1 y−1 was observed between 1968 & 2012. • Selection has improved biomass, harvest index, lint percent & photosynthesis; yield gains have been driven by increasing lint percent. • Selection has not altered reproductive allocation or the amount/efficiency of light capture. • Gains in leaf-level carbon assimilation are only weakly associated with lint yield. • Current breeding efforts should focus on direct selection for yield, future efforts on enhancing early season growth & carbon assimilation. The factors responsible for yield progress can be analysed through yield determinant frameworks. These conceptual models consider factors such as crop growth dynamics, partitioning of vegetative and reproductive biomass and yield components to provide insights into the factors responsible for observed genetic gains and opportunities for future gains. The aim of this study was to use direct cultivar comparison to assess the rate of genetic gain in the CSIRO (Australia) cotton breeding program, and to understand how factors within a conceptual yield determinant framework relate to yield performance. Using field experimentation, yield progress of 16.1 kg lint ha−1 y−1 was observed in ten cultivars released between 1968 and 2012. This study identified that selection pressure has resulted in improvements in total dry matter (TDM), harvest index (HI), lint percentage and carbon assimilation. While gains have been made in these four parameters, improvements in lint yield have largely been driven by altering HI through increasing lint percentage. Although improvements have been made in TDM, the reproductive allocation of total biomass and the amount and efficiency of light capture has not been altered in modern cultivars. Future gains in lint yield will require the concurrent maintenance of harvest index while producing larger plants with more fruiting branches that capture more incident radiation with increased efficiency. As the collection of phenotype data such as biomass, boll number, boll size and radiation use efficiency at the scale required in a commercial breeding program is largely aspirational, we conclude in the short term improvements may be achieved through direct selection for yield. Future efforts should be placed in increasing early season growth rates, and in the longer term enhancing carbon assimilation rates. Importantly, due to trait associations and the effects of trade-offs between functional components, factors within a conceptual framework must not be considered in isolation. [ABSTRACT FROM AUTHOR]

• Leaf Na concentration at early flowering is a reliable measure for identifying genotypes with low leaf Na trait in cotton. • Low leaf Na trait is associated with higher Na accumulation in stems and roots. • K and P use efficiency is higher in the line with low leaf Na trait. • Incorporating low leaf Na trait in an elite cultivar background does not compromise yield and fibre quality. The ability of a plant to maintain low Na concentration is a critical component for its tolerance to salt affected soil stresses. Leaf Na concentration was investigated to understand its usefulness in screening of cotton for tolerance to soil sodicity and to assess its effect on agronomic performance. A set of family lines and high yield lines with low and high leaf Na concentration were tested in field experiments with sodic soils. The youngest fully expanded leaves at early flowering and whole plant biomass samples at the cut-out stage were collected for nutrient analysis. Plots were harvested for yield and fibre quality. Leaf Na concentration was found to be a reliable measure for differentiating test lines with low and high leaf Na trait. Leaf K/Na and P/Na ratio acted similarly. Low leaf Na concentration was due to higher Na accumulation in plant stems and roots. It was associated with higher K/Na and P/Na ratio in leaves and bolls, and responsible for improved K and P use efficiency. High performing lines with the low leaf Na trait had yield and fibre properties competitive with the control. It is concluded that the low leaf Na trait at flowering provided a reliable phenotypic indicator for cotton tolerance to soil sodicity. Breeding for the trait can improve sodicity tolerance, K and P use efficiency, while maintaining agronomic performance. [ABSTRACT FROM AUTHOR]

Highlights • High fibre strength needs to be combined with high yield to sustain grower profitability. • Additive gene effects primarily contributed to fibre strength. • Lint yield was moderately but negatively correlated with fibre strength. • Crossing high strength with high yield cottons locally adapted improves both fibre strength and yield. Abstract There is a consistent demand to improve cotton fibre quality from both the producer and the end user. However, this is a major hurdle for breeders due to its negative association with yield. This study investigated the breeding values of different cotton fibre strength sources for improvement without compromising lint yield. A North Carolina mating design II was performed between five high yield locally bred female parents (>2100 kg/ha) and four high fibre strength male parents (>31 g/tex) to obtain 20 crosses. Their F 1 's were grown in the glasshouse and the F 2 's and F 3 's in the field, for two years, to determine general and specific combining ability of lint yield and fibre quality traits. The general combining ability effect was much larger than specific combining ability effect in determining significant variability among the crosses, implying that all traits were controlled primarily by additive gene action. Heritability estimates were moderate to high (0.36 ∼0.80) for lint percentage and fibre quality traits except fibre maturity. Lint yield had a moderate negative association with fibre strength. Early generation tests in the glasshouse and field identified the same good combiners. Acala Ultima and Sicala 40 were good general combiners for high fibre strength but poor for lint yield. However, two new locally bred high strength parents had positive general combining ability for both yield and fibre strength. Therefore, adapted high strength parents must be chosen first in crossing to achieve improvements in not only strength but also other quality traits and yield, demonstrating the importance of adaptation in parent selection in breeding for both yield and quality. [ABSTRACT FROM AUTHOR]