Kg Rickert

Radiation use efficiency increases when the diffuse component of incident radiation is enhanced under shade

Theoretical analyses have shown the radiation use efficiency of maize, soybean, and peanut to increase with a decrease in the level of incident radiation and an increase in the proportion of diffuse radiation. This study compared the growth and radiation use efficiency of Panicum maximum cv. Petrie (green panic) and Bothriochloa insculpta cv. Bisset (creeping bluegrass) beneath shading treatments (birdguard and solarweave shadecloths) with that in full sunlight. A level of incident radiation reduced by 25% under birdguard shadecloth decreased final yield and final leaf area index, but increased canopy leaf nitrogen concentration and radiation use efficiency (19-14%) (compared with the full sun treatment). A similar level of reduced incident radiation under solarweave shadecloth (which provided an increased proportion of diffuse radiation), increased final yield and radiation use efficiency (46-50%). An understanding of the effects of composition of incident radiation on radiation use efficiency of tropical grasses enables more accurate estimation of potential pasture growth in shaded environments. It also has impact upon crop production in glasshouses and greenhouses.

Temperature and photoperiod sensitivity of development in five cultivars of maize (Zea mays L.) from emergence to tassel initiation

The ability to predict phenological development is crucial to the successful use of crop simulation in crop adaptation studies. Previous studies have shown existing predictive algorithms to be inadequate when applied to a broad range of cultivars and environments. The primary objective of the study was to quantify the temperature and photoperiod responses of the rates of development during emergence to tassel initiation (ETI) for a diverse set of maize cultivars. Five cultivars of maize, differing in maturity and adaptation, were sown on seven dates from 1 October 1993 to 29 March 1994 and grown under natural and extended (16.5 h) photoperiods at Gatton, South East Queensland, under non - limiting conditions of water and plant nutrient supplies. Timing of emergence and tassel initiation were observed for all treatments. The base, optimum, and maximum temperatures, and photoperiod sensitivity of each cultivar were determined using an iterative optimisation procedure. The critical photoperiod (12.5 h) was adopted from literature sources, as there was inadequate range in short photoperiods in the present study to determine it with confidence. Photoperiod extension increased the duration of ETI and increased the number of leaves on all cultivars, the largest increases occurring in a tropically adapted cultivar (Barker), in five of the seven sowings. No response to photoperiod extension occurred in the crops sown on 24 th February and 29 th March 1994. The temperature response was the same in all cultivars, and was best described by a three-stage broken-stick linear function. Photoperiod sensitivity was linear at photoperiods in excess of 12.5 h. The optimised base, optimum and maximum temperatures were 8, 34 and 40 degrees C respectively. Photoperiod sensitivity, expressed as the increase in number of leaves produced per hour of photoperiod in excess of 12.5 h, ranged from 0.3 to 1.5 leaves h-1. When expressed as the increase in thermal duration of the photoperiod sensitive interval prior to tassel initiation, it was 5.0 to 27.3 oCd h-1, (using the optimised base, optimum and maximum temperatures). The fitted values for the real time duration of ETI were in close agreement (RMSD = 2.1 days) with the 60 observed values, which spanned a range of 13 - 34 days. The optimised values for temperature and photoperiod responses should be used for improved prediction of tassel initiation in maize crop simulation models that include detailed prediction of crop ontogeny.

Modelling leaf production and crop development in maize (Zea mays L.) after tassel initiation under diverse conditions of temperature and photoperiod

Prediction of the initiation, appearance and emergence of leaves is critically important to the success of simulation models of crop canopy development and some aspects of crop ontogeny. Data on leaf number and crop ontogeny were collected on five cultivars of maize differing widely in maturity and genetic background grown under natural and extended photoperiods, and planted on seven sowing dates from October 1993 to March 1994 at Gatton, South-east Queensland. The same temperature coefficients were established for crop ontogeny before silking, and the rates of leaf initiation, leaf tip appearance and full leaf expansion, the base, optimum and maximum temperatures for each being 8, 34 and 40 degrees C. After silking, the base temperature for ontogeny was 0 degrees C, but the optimum and maximum temperatures remained unchanged. The rates of leaf initiation, appearance of leaf tips and full leaf expansion varied in a relatively narrow range across sowing times and photoperiod treatments, with average values of 0.040 leaves (degrees Cd)-1, 0.021 leaves (degrees Cd)-1, and 0.019 leaves (degrees Cd)-1, respectively. The relationships developed in this study provided satisfactory predictions of leaf number and crop ontogeny (tassel initiation to silking, emergence to silking and silking to physiological maturity) when assessed using independent data from Gatton (South eastern Queensland), Katherine and Douglas Daly (Northern Territory), Walkamin (North Queensland) and Kununurra (Western Australia).

Predicting broccoli development: II. Comparison and validation of thermal time models.

Models predicting broccoli ontogeny and maturity should ideally be precise and readily adopted by farmers and researchers. The objective of this study was to compare the predictive accuracy of thermal time models for three broccoli (Brassica oleracea L. var. italica Plenck) cultivars (‘Fiesta’, ‘Greenbelt’ and ‘Marathon’) from emergence to harvest maturity (Model 1), from emergence to floral initiation (Model 2), and from floral initiation to harvest maturity (Model 3). Comparisons were also made between Model 1 and Model 4 (Models 2 and 3 combined). Model 1 is useful when the timing of floral initiation is not known. When Model 1 was tested using independent data from 1983 to 1984 sowings of three cultivars (‘Premium Crop’, ‘Selection 160’ and ‘Selection 165A’), it predicted harvest maturity well. Prediction of floral initiation using Model 2 is useful for timing cultural practices, frost and heat avoidance. Where timing of floral initiation was recorded, predictions of harvest maturity were most precise using Model 3, since the variation which occurred from emergence to floral initiation was removed. The good predictions for Model 4 suggests that it would best predict the chronological duration from emergence to harvest maturity. # 2000 Published by Elsevier Science B.V.

Environmental control of potential yield of sunflower in the subtropics

A simple framework was used to analyse the determinants of potential yield of sunflower (Helianthus annuus L.) in a subtropical environment. The aim was to investigate the stability of the determinants crop duration, canopy light interception, radiation use efficiency (RUE), and harvest index (HI) at 2 sowing times and with 3 genotypes differing in crop maturity and stature. Crop growth, phenology, light interception, yield, prevailing temperature, and radiation were recorded and measured throughout the crop cycle. Significant differences in grain yield were found between the 2 sowings, but not among genotypes within each sowing. Mean yields (0% moisture) were 6 . 02 and 2 . 17 t/ha for the first sowing, on 13 September (S1), and the second sowing, on 5 March (S2), respectively. Exceptionally high yields in S1 were due to high biomass assimilation associated with the high radiation environment, high light interception owing to a greater leaf area index, and high RUE (1 . 47-1 . 62 g/MJ) across genotypes. It is proposed that the high RUE was caused by high levels of available nitrogen maintained during crop growth by frequent applications of fertiliser and sewage effluent as irrigation. In addition to differences in the radiation environment, the assimilate partitioned to grain was reduced in S2 associated with a reduction in the duration of grain-filling. Harvest index was 0 . 40 in S1 and 0 . 25 in S2. It is hypothesised that low minimum temperatures experienced in S2 reduced assimilate production and partitioning, causing premature maturation.

Freeze-induced reduction of broccoli yield and quality

Summary. Sub-zero temperatures can result in freezing injury of broccoli (Brassica oleracea L. var. italica Plenck) plants and thereby reduce head yield and quality. In order to predict effects of frosts, it is desirable to know the stages of development at which broccoli plants are most susceptible to freezing injury. In this study, the effect of a range of sub-zero temperatures for a short period at different stages of crop development were assessed and quantified in terms of mortality, yield and quality of broccoli. Whole plants in pots or in the field were subjected to sub-zero temperature regimes from –1 to –19°C. Extracellular ice formation was achieved by reducing temperatures slowly, at –2°C per hour. The floral initiation stage was most sensitive to freezing injury, as yields (fresh and dry head weights) were significantly reduced at –1 and –3°C, and the shoot apices were killed at –5°C. There was no significant yield reduction when the inflorescence buttoning stage was treated at –1 and –3°C. Although shoot apices survived the –5°C treatment at buttoning, very poor quality heads of uneven bud size were produced as a result of arrested development. The lethal temperature for pot-grown broccoli was between –3 and –5°C, whereas the lethal temperature for field-grown broccoli was between –7 and –9°C. The difference was presumably due to variation in cold acclimation. Freezing injury can reduce broccoli head yield and quality and retard plant growth. With regard to yield and maturity prediction, crop development models based only on simple thermal time without restrictions will not apply if broccoli crops are frost damaged.