Claudio A. Chimenti

The effects of water stress and genotype on the dynamics of pollen-shedding and silking in maize

Abstract The effects of exposure to water stress at three developments stages (start of tasseling, start of pollen-shedding, and silking) on the viability of pollen and the dynamics of flowering in maize (Zea mays L.) were examined using a single genotype. Viability was not affected by stress at any stage, but alterations in the dynamics of flowering reduced the number of pollen grains available for pollination of the last ears to silk within the stressed populations. In a second experiment the effects of stress at the start of pollen-shedding on the availability of pollen during the presentation of silks was studied in a set of six cultivars. There was considerable variability between cultivars in the number of pollen grains produced per tassel, and water stress decreased the quantity of pollen grains harvested. There was a strong association between pollen availability and kernel number in the apical ear (as a proportion of control values). Two effects of stress on pollen availability were distinguished: a reduction in the duration of exposure of silks of the apical ear to pollen, and a fall in the rate of pollen production during the period in which receptive silks were present. The relative importance of these effects in determining pollen availability varied with cultivar.

Embryo-growth rate and duration in sunflower as affected by temperature

The thermal regime of the inflorescence has been shown to affect grain-filling dynamics and grain weight in several crop species, but temperature response functions for sunflower grain have not, as yet, been developed. We explored the effects of daily mean temperatures in the 12–40°C range during the embryo growth phase on embryo growth characteristics in cv. HA89 by growing plants in temperature-controlled glasshouse modules between the start of linear embryo growth and physiological maturity. Grain was harvested at regular intervals during the growth period and rates and durations of embryo growth derived from bi-phasic linear regression models fitted to the resulting embryo weight dynamics data. Embryo filling duration showed a curvilinear response to temperature with a minimum close to 34°C. In contrast, the curvilinear response of rate of embryo filling peaked at 25°C. The opposing effects of these two responses produced a continuously decreasing embryo size with temperature (P<0.0001) (average decrease 1.2% per °C) over much of the temperature range explored. A plot of the inverse of embryo growth duration against temperature served to estimate base and optimum temperatures of −1 and 34°C, respectively. The response functions and cardinal temperatures defined in this experiment are expected to improve the capacity of current crop simulation models to describe grain-filling in sunflower.

Responses to water stress of apoplastic water fraction and bulk modulus of elasticity in sunflower (Helianthus annuus L.) genotypes of contrasting capacity for osmotic adjustment

The responses to water stress of the bulk modulus of elasticity (ε) and the apoplastic water fraction were examined using six sunflower cultivars of differing capacity for osmotic adjustment (OA). Water stress did not affect the partitioning of water between apoplastic (ca. 20%) and symplastic fractions in leaves which expanded during the exposure to stress in any genotype. Hence, no genotype-linked effects on either the buffering of cell water status during stress or on the estimates of bulk leaf osmotic potential could be expected. Genotypes differed in the degree of change in ε (estimated from pressure/volume [P/V] curves) and OA (estimated using both ln RWC/ ln Ψo plots and P/V curves) induced by exposure to stress. In three genotypes ε increased significantly (p=0.05) as a consequence of stress, in another three change were small. OA was the only attribute of the three examined that could have contributed to turgor maintenance under stress. There was a strong negative association between leaf expansion and degree of OA across genotypes (r=−0.91) and a strong positive one between OA and ε (r=0.94). However all genotypes evidenced some degree of OA. These results are consistent with part of the genotype differences in OA being attributable to variations in leaf expansion during exposure to stress.

Grain number responses to temperature during floret differentiation in sunflower

Abstract The number of florets per head in sunflower is a function of the integral of the generative area of the inflorescence primordium during the phase of floret differentiation (FS5–FS8). To understand the effects of temperature during early reproductive stages on grain number, we quantified the effect of temperature on the duration of the FS5–FS8-period, on the maximum generative area, on floret number and viability, and on grain number. Plants of the sunflower (Helianthus annuus L.) inbred line HA89 were exposed to daily temperatures between 14 and 38°C for 14 days from FS4. Increasing temperature significantly reduced the integral of the generative area between floral stages 5 and 8, due to reductions in both the maximum size and the duration of the generative area. These variations in generative area integral translated into significant proportional variations in florets per head. However, the high floret numbers evoked by low temperatures did not result in a high number of filled grains per head. The latter variable exhibiting a convex response to temperature with an optimum at 25°C. Residual effects of early exposures to low and high temperatures significantly reduced the number of functional florets, filled grain, and increased unfilled grain (flats). The curvilinear response of grain numbers per head can be understood as the result of opposing effects of temperature on floret differentiation and functionality and on continuation of grain growth after flowering. Early exposure to low or high temperature also reduced leaf area and dry matter per plant at anthesis, which may have influenced the observed reduction in grain number. Caution is necessary in using the photothermal quotient as an estimator of grain number at low temperature.

Yield in water-stressed maize genotypes: Association with traits measured in seedlings and in flowering plants

Abstract Thirteen maize ( Zea mays L.) genotypes were evaluated for variability with respect to seedling traits reportedly associated with tolerance to water stress. Significant variability was found within this set of cultivars for chlorophyll loss under thermal stress, loss of intracellular electrolytes under thermal and desiccation stresses, and proline accumulation in response to desiccation. These attributes were not mutually correlated in the genotypes examined. A sub-set of seven genotypes was grown in a sand-nutrient solution culture in the field and subjected to stress at tasseling for about eight days. Yield was significantly reduced in all cultivars, but the magnitude of the reduction was variable. Relative kernel number of the uppermost ear (the major yield-determining factor in most cultivars) was strongly associated with duration of exposure to pollen of the same ear. Relative spikelet number of the uppermost ear was significantly reduced intwo cultivars by stress, but this response appeared to be independent of mean exposure to pollen. Relative spikelet number was associated with plant-leaf area: root weight ratio at tasseling. No association was found between seedling traits and relative yield, additional yield-related responses or other, presumably less complex, responses to stress such as loss of leaf area and fall in chlorophyll content.

Effects of ethephon on anatomical changes in sunflower (Helianthus annuus L.) stems associated with lodging

Abstract. Stem lodging causes significant losses in crops of cereals and oilseeds. The aim of the present study was to identify the anatomical causes that generate differences in response to stem lodging in sunflower. Two sunflower hybrids (Stay-Green, resistant to stem lodging; Zenit, susceptible to stem lodging) were grown at three crop population densities and artificially lodged at two advanced ontogeny stages (R7 and R8), which were preceded by ethephon application near the flower button stage (R1). Measurements included stem failure moment of force (Bs), thickness of primary and secondary structures in the stem lodging zone (t), diameter of the stem lodging zone (di), sclerenchyma packages area (sp), secondary xylem tissue area (xt) and yield. Stay-Green had significantly higher values for Bs, t, di, sp and xt. At higher crop densities and more advanced ontogeny stages these parameters were reduced, favouring stem lodging, although the effects were ameliorated by ethephon application through anatomical modifications. Zenit exhibited the greatest responses to ethephon application. The present study is the first field study identifying anatomical changes causing stem lodging and intraspecific variability in sunflowers. The information provided can be used by geneticists in selection programs for stem lodging tolerance in the context of increasing crop population densities to improve sunflower yield.