David J. Connor

Physiology of yield expression in sunflower

Abstract This paper evaluates currently available information on the physiology of sunflower (Helianthus annuus L.) in the context of field growth responses and the determination of crop yield. It deals with phenology, water economy, mineral nutrition, carbon fixation and partitioning, yield formation, and seed quality. Attention is drawn to information on intraspecific variation of individual traits to highlight the possibilities that exist for cultivar adaptation and improvement. Properties commonly attributed to this crop such as high photosynthetic capacity, profligate use of water and low water-use efficiency are evaluated.

Evaluating physicochemical constraints of Calcarosols on wheat yield in the Victorian southern Mallee

Soil salinity, sodicity, and high extractable boron (B) are thought to reduce wheat yields on alkaline soils of south-eastern Australia; however, little quantitative information on yield penalties to edaphic constraints is available. The relationships between wheat yield of a B-tolerant cultivar and soil physicochemical conditions in the Victorian Mallee were explored using ridge regression analysis, using natural variation in the field. Wheat yields in the survey ranged from 1.3 to 6.1 Mg/ha, with low yields attributed to inadequate soil water supply during pre-anthesis growth. Crop sequences, fallow–wheat, and pulse–wheat left greatest soil water prior to sowing of the wheat crop, and lucerne–wheat the least. A descriptive model explained 54% of variation in wheat yield, with rainfall around anthesis, available soil water in the 0.10–0.40 m layer, nitrate in the 0–0.10 m layer at sowing and salinity, and sodicity in the 0.60–1.00 m layers being important factors. Subsoil salinity (ECe) and sodicity (ESP) appear to be effective surrogates for estimating the likelihood of water extraction in the deep subsoil. The analyses suggest that subsoils need to have an ECe <8 dS/m and ESP < 19% for crops to make use of water deep in the profile. Although soluble B ranged from 2 to 52 mg/kg in the 0.60–1.00 m layer of the alkaline soils considered, B appeared to have little correlation with root growth, water extraction, or yield of wheat, which has been attributed to B-tolerance of the cultivar tested and/or the overbearing effect of high Na+ in these soils.

Physiological basis of the response of harvest index to the fraction of water transpired after anthesis: A simple model to estimate harvest index for determinate species

Abstract The fraction of biomass partitioned to grain is critical to crop yield. Methods to predict this fraction are required for crop modelling. The relationship between harvest index ( hi ) and the fraction of water transpired after anthesis (θE) provides a simple basis for the estimation of hi for determinate species. In this paper, empirical and theoretical aspects of this relationship are examined and a method is developed to estimate hi . The model proposed is: hi pv = θ Ee /[1 − (a − bθ Ee )] Subscripts pv and E indicate that hi and θE are normalized by production value and vapour pressure deficit, respectively. The model also differentiates between source and sink restrictions to yield. The parameter a quantifies the ‘potential’ contribution of pre-anthesis assimilates to grain, and 1/(1−a + b) the ‘potential’ harvest index; both are genotype-dependent. The model accounted for 75% (P hi of wheat plants with hi ranging from 0 to 0.47 and θE from 0.05 to 0.47, and 81% (P hi of sunflower plants with hi from 0.13 to 0.46 and θE from 0.04 to 0.41.

Interception of photosynthetically active radiation and radiation-use efficiency of wheat, field pea and mustard in a semi-arid environment

Fractional ground cover, fractional PAR interception (f), canopy extinction coefficient (k) and radiation-use efficiency (RUE) for wheat, pea and mustard were examined in a field experiment conducted over 5 years in a semi-arid environment in Australia. Two crop sequences were compared: fallow‐wheat‐pea and mustard‐wheat‐pea. Significant periods of water stress occurred in some years. Complete ground cover was not achieved and f ranged to maxima of 0.77, 0.80 and 0.80 in wheat, pea and mustard, respectively. For wheat, fallow increased f under both average growing season and drought conditions, but pea PAR interception was not influenced by crop sequence. Estimates of k for wheat, pea and mustard were 0.82 (� 0.05), 0.76 (� 0.03) and 0.68 (� 0.12), respectively. Estimated RUE (aboveground biomass) of 1.81 (� 0.05), 1.52 (� 0.05) and 1.92 (� 0.12) g MJ � 1 intercepted PAR for wheat, pea and mustard, respectively, was measured over the vegetative phase. Seasonal conditions had minimal impact on k and RUE, and RUE was generally lower than the maxima reported in the literature, but they were similar to others reported in comparable environments and matched predicted unstressed RUE for C3 plants. This calls into question the application, in simulation models for crops that rarely reach full cover in such arid environments, of water supply or stress factors to RUE values established elsewhere on unstressed crops. Crown Copyright # 2003 Published by Elsevier B.V. All rights reserved.

Stubble retention and tillage in a semi-arid environment: 1. Soil water accumulation during fallow

Abstract A study was carried out over 4 years at two sites in semi-arid, north-west Victoria, Australia, to examine the effect of soil surface management during fallow on water and nitrogen accumulation and their effect on the growth and yield of the subsequent wheat crop. The fallow treatments comprised four combinations of stubble management and tillage in 18-month-long fallows of a fallow-wheat rotation together with a pea-wheat rotation which included an annual short (6 month) tilled summer fallow. The two sites are representative of the major soil types of the region; a chromic vertisol at Dooen and a calcic xerosol at Walpeup. This paper reports observations and analyses of soil water accumulation during the fallow phase. At Dooen, tillage without stubble increased soil water storage, at sowing, on average by 76 mm (range 24–122 mm) above the summer fallow. Stubble retention added a further 52 mm (range 36–65 mm). Zero tillage was beneficial in one year, and was enhanced by the presence of stubble. In contrast, at Walpeup, tillage without stubble provided an average of 37 mm more water at sowing than the summer fallow. Stubble retention without tillage increased water storage by 27 mm in only one year. At both sites the additional water stored due to stubble retention was located deep in the profile to 2.0 m. In general, zero tillage, primarily with stubble retention, offered large and consistent increases in soil water storage on heavy-textured clay soils in a 420-mm rainfall zone. On the lighter, sandy loam soil under the lower rainfall regime (343 mm), the advantage in soil water storage, through both stubble retention and zero tillage, was less frequent.

Contribution of pre-anthesis assimilates to grain-filling in irrigated and water-stressed sunflower crops I. Estimates using labelled carbon

Hall, A.J., Connor, D.J. and Whitfield, D.M., 1989. Contribution of pre-anthesis assimilates to grain-filling in irrigated and water-stressed sunflower crops. I. Estimates using labelled carbon. Field Crops Res., 20: 95-112. Labelled carbon was used to estimate pre-anthesis assimilate contributions to seed yield in sunflower crops irrigated throughout the season or stressed during grain-filling. At weekly intervals, commencing 23 days before anthesis and continuing through to maturity, previously unlabelled plots were exposed to labelled carbon dioxide. At maturity, the partitioning of label between the seed and the remainder of the crop (including roots) was determined. Estimates of pre-anthesis assimilate contributions to grain carbon were obtained by weighting these partitioning coefficients by seasonal crop gross photosynthesis data (Variant 1 ) and by crop dry-weight changes over the pre- and post-anthesis periods (Variant 2 ). Estimates of absolute contributions varied slightly between treatments and with estimation technique. Taking the synthesis costs of grain oil content into account, about 60 g C m- e (or about 22 or 27 % of the C content of seeds in irrigated and droughted crops, respectively ) originated in pre-anthesis assimilation. Estimates for stressed crops obtained using Variant 1 were lower than those obtained using Variant 2. The amount of water-soluble carbohydrate stored in stem, tap-root and receptacle at anthesis was equivalent to 86 g C m -2. During grain-filling, the losses from the non-seed portion of the crops in dry weight, in nitrogen-associated C, and in water-soluble carbohydrates were equivalent to 73, 14 and 60 g C m -s, respectively. Thus, labile C stored in the crop at anthesis considerably exceeded that required to sustain the estimated transfer to seeds, and observed losses from nonseed organs were similar in magnitude to these estimates.

Interrelationships between edaphic factors potentially limiting cereal growth on alkaline soils in north-western Victoria

Calcarosols of the Victorian southern Mallee comprise subsoils that are typically saline, sodic, and have high concentrations of soluble boron (B), which have the potential to restrict growth of rain-fed grain crops. This paper reports relationships between various soil factors, from 150 soil profiles over a survey area of 3600 km 2 , to determine if field texture, pH1:5, electrical conductivity (ECe), and Na + could be used to estimate exchangeable sodium percentage (ESP) and B. Assessment of soil profiles across 5 layers to 1 m (n = 750) showed that exchangeable Na + correlated well with both ESP (r = 0.96) and B (r = 0.88). High correlation also existed between ESP and ECe (r = 0.71) and between B and pH1:5 (r = 0.70). Using linear and asymptotic regression functions, ESP overall was defined by: ESP = 1.47 + 2.68 × Na + (r 2 = 93.9) or ESP = 26.53 – 29.84 × 0.84 ECe (r 2 = 75.5). Boron was described by: B = –0.34 + 3.93 × Na + (r 2 = 76.7) or B = 3.2 × 10 –6 × 6.11 pH 1:5 (r 2 = 68.5). Inclusion of multiple explanatory variates, using stepwise multiple regression, did not account for more variation; hence, prediction using several variables simultaneously appeared unnecessary. Rapidly determined Na + , by ion-specific electrode, could also accurately predict sodicity: ESP = 1.31 + 0.03 × Na + (r 2 = 95.1). Soils with a pH1:5 <8.1 were shown to have B levels not potentially toxic to cereal growth. Soil texture also provided valuable estimation of B. Soils in the sand to clay loam texture range did not have levels of B toxic to cereal growth, unlike soils in the light to heavy clay range, which frequently had levels of B potentially toxic to cereal growth. SR ro a Jt et

Yield, yield components and source-sink relationships in water-stressed sunflower

Abstract Assimilate fluxes to/from various organs (J of sunflower (Helianthus annuus L.) were estimated in order to examine the following hypotheses: (1) plants with small storage capacity in stems as a result of short stature are able to maintain greater Jhead with consequently larger floret survival than taller plants when exposed to water stress before anthesis; and (2) plants with long stems (i.e. greater capacity to store assimilates) are able to maintain a greater Jseed than short-stature plants when exposed to water stress after anthesis. Assimilate fluxes were estimated using a budget accounting for labile carbohydrates, structural biomass and synthesis respiration, and maintenance respiration. Glasshouse-grown plants of three cultivars of different stature were compared under four water regimes. The reduction in floret number caused by water stress before anthesis was not related with Jhead. This, together with a greater concentration of labile carbohydrate in the heads of stressed plants relative to the well-watered controls, indicated that the survival of florets of water-stressed plants was probably not limited by assimilate availability. Seed filling depended strongly on stem assimilates as indicated by a significant negative association between Jseed and Jstem in the seed-filling period. The apparent contribution of stem assimilates to seed yield was cultivar dependent but was not strictly associated with plant height. The implications for selection of cultivars with improved mobilisation of stem assimilate to seeds are discussed.

Towards optimal designs for hedgerow olive orchards

An analysis of row height, row width, canopy slope, and alley width of hedgerow olive orchards was conducted to identify combinations that maximise productivity while allowing access to machinery for management. This objective requires maximising both the interception of incident solar radiation and its distribution over the canopy surface for productivity. For maximum productivity, all foliage must be illuminated above threshold values for the critical step in the shoot growth - floral initiation - flowering - fruit formation - fruit filling reproductive sequence. The key sites for adequate illumination are the bases of the canopy walls. For canopies with row height and alley width combinations that provide threshold illumination at the bases of the canopy walls, the area of productive foliage per unit orchard area is inversely related to row width because incident radiation received by broad canopy tops is better spread at smaller irradiance over more canopy wall. The analysis revealed that canopies of equal productivity can be formed with a range of combinations of height, slope, and alley width, and that optimal combinations are responsive to threshold values in the range 20-30% of incident radiation. Research is required to establish threshold values and the limits to row height and row width consistent with year-to-year production of reproductive shoots. The latter requires attention to cost and performance of machinery, the growth habit of the tree, and its response to pruning.

Stubble retention and tillage in a semi-arid environment: 3. Response of wheat

Abstract Wheat crops were studied over 4 years at two sites in north-west Victoria, Australia in response to fallow treatments. The four combinations of stubble management (with and without) and tillage (with and without) in 18-month-long fallows of a fallow-wheat rotation were compared with a tilled summer fallow prior to field pea and wheat crops in rotation. Grain yield responded diversely to fallow method. At Dooen, wheat yield from the tilled fallow without stubble increased over that in the field pea-wheat system by an average of 0.9 Mg ha−1 (from 2.7 to 3.6 Mg ha−1). Stubble retention further increased yield over the no-stubble treatments in three of the four years, averaging an additional 0.6 Mg ha−1. At Walpeup the tilled fallow without stubble increased yield above the field pea-wheat system in two years, by 0.7 Mg ha−1 from 2.6 to 3.2 Mg ha−1 in 1990 and by 0.6 Mg ha−1 from 1.7 to 2.3 Mg ha−1 in 1991. Stubble retention increased yield in only one year, 1988, by 0.5 Mg ha−1 and that was with tillage. Zero tillage had no independent effect on yield. Crop water use varied from 173 to 317 mm at Walpeup and from 247 to 435 mm at Dooen. At Walpeup, total crop water use was largely determined by its pre-anthesis component whereas at Dooen it was strongly related to water use during grain filling, indicative of the importance of water reserves deep in the profile, particularly under stubble retention with zero tillage. Total N uptake was strongly related to yield and to a lesser extent to soil mineral nitrogen at sowing. At Walpeup, high wheat grain [N] was observed in the field pea-wheat rotation (2.3 to 2.7% N) and, over both sites and all treatments, high grain [N] was loosely associated with low yield (Walpeup: R2 = 0.51, Dooen: R2 = 0.26). The response of wheat to conservation tillage was strongly driven by water availability and to a lesser extent N availability.