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Item Hybrid dahlia (Dahlia hybrida L.) seed production : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science in Plant Science (Seed Technology) at Massey University, New Zealand(Massey University, 1996) Han, HongyangThree experiments related to dahlia seed production were conducted at the Seed Technology Centre, Massey University, New Zealand in the 1995/1996 season, with the objectives being to: evaluate herbicide toxicity to dahlia; determine the effect of establishment method and plant density on dahlia seed yield and yield components; and determine the effect of sowing date on dahlia seed yield. Thirteen herbicides applied pre-emergence or post-emergence were evaluated in the herbicide experiment. EPTC (4.32 kg a.i/ha), oxyfluorfen (0.72 kg a.i/ha), oryzalin (4.5 kg a.i/ha), oxadiazon (1.52 kg a.i/ha), simazine (1.0 kg a.i/ha) and terbacil (0.96 kg a.i/ha) all caused some injury to direct sown and transplanted dahlias, and can not therefore be used safely on the dahlia crop. Of the pre-emergence herbicides, alachlor (1.92 kg a.i/ha), chlorpropham (3.2 kg a.i/ha), chlorthal-dimethyl (7.5 kg a.i/ha), pendimethalin (1.32 kg a.i/ha) and trifluralin (1.2 kg a.i/ha) did not injure either direct- sown or transplanted plants. Methabenzthiazuron (1.05 kg a.i/ha) did inhibit the early growth of direct sown dahlia, but plants recovered very quickly. All five herbicides could be used as pre-emergence herbicides for dahlia, but on a cost and weed control spectrum basis, alachlor or trifluralin are recommended. Terbacil (0.96 kg a.i/ha), while not affecting transplanted seedlings did damage direct sown dahlia, and should only be used as a pre-emergence herbicide in transplanted dahlia. From the post-emergence herbicides, chlorpropham (3.2 kg a.i/ha) did not injure either direct-sown or transplanted dahlia. Chlorthal-dimethyl (7.5 kg a.i/ha), haloxyfop (0.3 kg a.i/ha), and methabenzthiazuron (1.05 kg a.i/ha) caused some plant injury to early growth of direct-sown seedlings, but injured plants recovered quickly. Therefore, these herbicides can be recommended for both direct-sown and transplanted dahlia when applied post-emergence. Of these, the cheapest is methabenzthiazuron, and on this basis it is recommended for use in dahlia post-emergence. Method of establishment (transplanted seedlings or tubers) did not affect seed yield. Of five plant densities (0.8, 0.6, 0.4, 0.3 and 0.2 m square spacings), seed yield per square meter from a harvest when 80% of the seedheads had turned brown was greatest from the 0.4 m square spacing (12.3 g/m²). However, when seedheads were harvested as they ripened (i.e. over several weeks), the highest yield was at the 0.3 m square spacing (15.97 g/m²). Individual plant yield was highest (4.49 g/plant) at the lowest density (0.8 m square spacing), and the lowest (0.105 g/plant) at the highest density (0.2 m square spacing). Seedheads per plant contributed most to the differences, as more branches per plant at lower densities produced more seedheads per plant. Seed weight was slightly bigger at lower densities, while seed number per seedhead was greater at higher densities than at lower densities. The feasibility of direct seed sowing for dahlia seed production was confirmed in Palmerston North, New Zealand. Sowing dates from 7 November to 5 December did not produce any difference in seed yield and yield components. However, a 19 December sowing produced a significantly lower seed yield per plant, seedheads per plant and thousand seed weight. Later sowing delayed flowering time, shortened flowering duration and made seedheads ripen later. All sowing were harvested after frosts, and the seedheads in the latest sowing were very immature when harvested. In this environment, seed should not be sown later than 7 November.Item Seed production in tall fescue (Festuca arundinacea Schreb.) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Seed Technology at Massey University, Palmerston North, New Zealand(Massey University, 1992) Hare, Michael DavidThe Paced Auditory Serial Addition Task ( PASAT ) was lateralised for the purpose of investigation into hemispheric specialisation for temporal integration. A right ear advantage ( REA ), representing a left hemisphere ( LH ) advantage in normal participants, was predicted due to the sequential nature of the PASAT, the previous finding of a REA by Norman ( 1984 ), and Miller's ( 1996 ) theory on the LH specialisation for delayed axonal conduction. The REA was also expected given other abundant evidence on LH superiority for the processing of temporal information ( Bradshaw & Nettleton, 1981; Bryson, Mononen, & Yu, 1980; Elfgren & Risberg, 1998; Gordon, 1978; Leek & Brandt, 1983; Prior, Kinsella, & Giese, 1990; Robinson, & Solomon, 1974; Troyer, Moscovitch, Winocur, Alexander, & Stuss, 1998 ). However, no REA was found in the present research. Careful precision in randomising and counterbalancing arithmetic outcomes, removal of the digit seven because of its two-syllable pronunciation, and the randomisation of ear of presentation and stimulus presentation rate, possibly eliminated procedural artefacts that were present in Norman's ( 1984 ) study. Therefore, despite the sequential nature of the PASAT it now appears to lack efficacy for research into temporal integration. Other findings from the present research instead showed support for the hypotheses of the right hemisphere's ( RH ) specialisation for vigilance ( Coslett, Bowers, & Heilman, 1987; Dimond, 1979; Heilman & van den Abell, 1979, 1980; Howes & Boller, 1975; Ladavas, Del Pesce, Mangun, & Gazzaniga, 1994; Ladavas, Del Pesce, & Provinciali, 1989; Loring, Meador, & Lee, 1989; Pardo, Fox, & Raichle, 1991; Simos & Molfese, 1997; Whitehead, 1991; Wilkins, Shallice, & McCarthy, 1987; Yokohama et al., 1987 ). For normal participants a left ear advantage ( LEA ) was found and more clearly demonstrated with an unpredictable task condition and with the faster presentation rates ( 1.2 & 2.0 s ) of the lateralised PASAT. The LEA was shown with both the same ear stimulus pairings ( LL ) and the left then right pairings ( LR ). Inferior performance with the right then left ( RL ) stimulus pairings was found in the unpredictable task condition and another task condition in which the side of stimulus presentation was predictable. Clinical participants (i.e., participants with a history of schizophrenia ) did not demonstrate a LEA, and they showed no difference compared to normal controls in performance with the right ear ( LH ). However, the clinical participants did manifest a particular disadvantage when stimulus processing required a shift from left to right ear ( LR ), but not the reverse ( RL ). The clinical participants manifested the LR deficit in the unpredictable and predictable task conditions. This LR disadvantage bears some resemblance to a previous finding, using the Visual Reaction Time Task, of a lateralised deficit in disengaging attention from the left visual field. Some researchers ( Bustillo et al., 1997; Posner, Early, Reiman, Pardo, & Dhawan, 1988; Posner & Raichle, 1994; Wigal, Swanson, & Potkin, 1997 ) interpret this LR deficit as a problem of slowness of responding by the LH ( right visual field ) after having attention oriented to the opposite field. However, the findings in the present research of a LEA for normal controls (i.e., LL and LR ), a relative deficit with RL for normals, and no difference in right ear ( RR ) performance between controls and clinical participants, and notably with the unpredictable condition, suggests an alternative interpretation. The LR deficit in schizophrenia may instead be due to difficulty in disengaging from the left visual or auditory field rather than impaired performance with the right field. The LR deficit may be as a result of dysfunction of vigilance, which is normally attributed to greater proficiency of the right hemisphere. In addition, opposing trends were observed for the clinical participants compared to the normal participants at particularly slow presentation rates, and with the predictable task condition in which stimuli were presented singularly to each hemisphere. Clinical participants showed a worsening of performance with the task. Normal participants demonstrated better performance with this task compared to another task condition, much like the standard PASAT, in which both ears received a stimulus simultaneously. These trends reflect Hellige's ( 1987, 1993 ) model of cross-hemispheric integration in which it is hypothesised that for normal participants single hemisphere processing has some advantage with lighter tasks. These trends also reflect the possibility that people with schizophrenia, in slow stimulus conditions, have difficulty whereby they unnecessarily over engage both hemispheres, which wastes attentional resource that could otherwise be utilised for various other aspects of ongoing processing.Item Seed production in China aster (Callistephus chinensis (L.) Nees) : a thesis presented in partial fulfilment for the degree of Doctor of Philosophy in Seed Technology at Massey University(Massey University, 1992) Phetpradap, LuckanaSeed production of two cultivars (Powderpuff and Kurenai) of China aster (Callistephus chinensis (L.) Nees.), grown under field conditions, was assessed to determine plant responses to the effects of plant density, crop manipulation by hand pinching and the application of three plant growth regulating chemicals, and some selected herbicides. Plant density effects on vegetative plant growth, flowering pattern, seed development, seed yield and yield components were investigated in 1987/1988 using a radial spacing design which provided plant densities ranging from 4.2 to 44.7 plants m-2. Increasing plant densities increased branch numbers m-2 which led to increased leaf numbers, leaf area, plant dry matter and flowers and resulted in an increased number of potential seed production sites. The number of flower heads m-2 was the most important component determining final seed yield in both cultivars and was identified as an important plant characteristic to be manipulated for improving seed yield. A period of 18 days was required to complete flowering within each individual flower head, since florets opened sequentially from the outside ring through to the centre. The flowering period lasted for 8 weeks. Each seedhead needed 30 or 39 days from first opening for seed to reach physiological maturity and seeds could remain on the seedhead for a further 9 or 12 days (cv. Kurenai and cv. Powderpuff respectively) before seed shedding started. Kurenai produced maximum seed yield at 27.8 plants m-2 (140 g m-2) and cv. Powderpuff at 17.4 plants m-2 (42 g m-2) but these yields did not differ significantly from those over a wide range of densities (between 12.7 to 44.7 plants m-2 in cv. Kurenai and 4.9 to 44.7 plants m-2 in cv. Powderpuff). Both cultivars exhibited a high ability for compensatory reproductive growth. Lodging and weeds were identified as constraints for seed production at this site and were studied in subsequent years. A series of experiments were conducted in 1987/1988 and 1988/1989 to evaluate herbicides which would provide good weed control without seed yield reduction, and which would not be phytotoxic to aster plants grown either as transplanted seedlings or when direct sown. For transplanted aster, a single application of oryzalin (3.75 kg a.i. ha-1 at 4 days after transplanting) provided excellent weed control and a tenfold increase in seed yield (to 568 kg ha-1). For direct sown aster, only trifluralin (2 kg a.i. ha-1) applied pre-sowing did not significantly reduce aster emergence, while oryzalin (4.5 kg a.i. ha-1 applied 10 days after sowing) provided the best aster seedling survival. However weed control from both chemicals was only partial and further work is required. In 1988/1989, hand pinching and the application of two different rates of three growth retardants, paclobutrazol (0.5 and 1.0 kg a.i. ha-1), daminozide (2.5 and 5.0 kg a.i. ha-1) and chlormequat chloride (1.5 and 3.0 kg a.i. ha-1) were carried out at two different growth stages (visible terminal bud and stem elongation stages) on cv. Powderpuff to investigate their retardation ability, any alterations in the partitioning of assimilate, and subsequent effects on seed yield and yield components. Powderpuff plant structure was altered by hand pinching only at the visible terminal bud stage. Neither pinching treatment increased seed yield, because, particularly for the earlier pinching time, fewer branches were produced from limited node numbers. The growth retarding effect of the three chemicals was transient and the differences in efficacy and the effective duration of each growth retardant treatment was recorded. The longevity of chlormequat activity in treated plants was short compared to paclobutrazol. Although paclobutrazol and daminozide decreased plant height at seed harvest, lodging was not prevented. None of the three chemicals increased flower head numbers or shortened the duration of flowering, and subsequently failed to increase seed yield. However, paclobutrazol showed enough promise for plant height reduction and seed yield improvement to warrant further investigation. Two experiments with paclobutrazol were conducted in 1989/1990. The first was on cv. Powderpuff, where two rates of paclobutrazol (0.5 and 1.0 kg a.i. ha-1) were applied at three growth stages (vegetative, terminal flower bud initiation and first visible terminal flower bud stages) to assess their effects on seed yield. The second was an investigation of cultivar/density responses, where the same two paclobutrazol rates were applied to two aster cultivars grown at two different plant densitites (16 and 36 plants m-2 for cv. Powderpuff and 25 and 49 plants m-2 for cv. Kurenai) at the terminal flower bud initiation stage. Paclobutrazol effects on China aster plant height were cultivar dependent. Both paclobutrazol rates effectively controlled plant height of cv. Kurenai but the results in cv. Powderpuff were inconsistent and the plant height reduction was insufficient to prevent lodging. Results from all the experiments showed that flowering was strongly influenced by environment (daylength and temperature), and since no growth retardant treatments shortened the duration of flowering, a high variation in seed maturity caused by sequential flowering and subsequent high losses of immature seeds during cleaning resulted in no significant seed yield increases. However, paclobutrazol significantly increased potential harvestable seed yield through increasing the number of seeds per plant when applied (i) at the vegetative stage at 1.0 kg a.i. ha-1 to cv. Powderpuff grown at 16 plants m-2 (56 % seed yield increase from 83.8 to 130 g m-2). (ii) at the terminal flower bud initiation stage at 0.5 kg a.i. ha-1 to late sown plants of cv. Powderpuff grown at 36 plants m-2 (48 % increase from 136 to 202 g m-2). and (iii) during flower bud initiation and early stem elongation at 0.5 and 1.0 kg a.i. ha-1 in cv. Kurenai grown at 49 plants m-2 (32 and 42 % increase from 178 to 236 and 253 g m-2 for the low and high rate respectively). Seed production problems and possibilities for the production of China aster seed under New Zealand and Thailand conditions are also discussed.Item Chemical manipulation of Lotus uliginosus Schk. cv. 'Grasslands Maku' grown for seed production : a thesis presented in partial fulfilment of the requirements for the degree Doctor of Philosophy in Agricultural Science with specialization in Seed Technology at Massey University, Palmerston North, New Zealand(Massey University, 1991) Tabora, Rogelio SThis thesis reports results of experiments conducted at Massey University, Palmerston North, New Zealand on seed production of 'Grasslands Maku' lotus (Lotus uliginosus Schk. syn. Lotus pedunculatus Cav.). Roth field (2 years) and single plant (1 year) studies were conducted, and plant responses to the effects of three plant growth regulating chemicals applied at different rates and times, and the effects of hand cutting, were recorded. Chapter 3 records an analysis of growth of individual Maku lotus plants and outlines the morphological characteristics which may limit seed yield potential. One major factor influencing seed yield, in addition to the crop's long indeterminate flowering and pod shattering characteristics, was the competition which developed between vegetative and reproductive growth for photosynthetic assimilate. The plant continuously produced new main shoots which, along with the growth and development of lateral shoots, cumulatively added to the dense vegetative 'bulk' during the early summer under increasing conditions of daylength and temperature. Intense lateral branching occurred at the same sites as inflorescences also appeared in November. Intensive vegetative and reproductive growth developed simultaneously during the flowering months (December and January). As a result, physiological compensation became important and was expressed in floret bud, floret and pod abortion which decreased potential seed yield. Reproductive abortion was more prevalent in the early and late season flowers. Main shoots originating from September to December contributed most to final seed yield. Chapter 4 deals with results of a two-year field experiment (1988-90) investigating the effects of foliar-applied plant growth regulators (paclobutrazol and cycocel for two years, Alar for one year), at different rates and times of application; and hand cutting (for one year) at the same times as chemical application. Paclobutrazol significantly altered vegetative shoot development by reducing shoot internode length and total dry matter yield, and by inhibiting apical dominance thus inducing lateral branching which subsequently increased reproductive sites. Paclobutrazol application rates of 0.5 or 1.0 kg ai/ha in early October (50 days before the onset of flowering) significantly increased seed yield by 53.7 and 75.9% respectively in 1988-89 and at 54.8 and 111.9% respectively in 1989-90. These yield increases were due to increased numbers of flowers in the first year and an increase in numbers of pods per umbel and seeds per pod in the second year. Paclobutrazol (1.0 kg ai/ha) applied in November or December also significantly increased seed yield by 63.3 and 53.7% respectively in the first year and by 68.6 and 67.2% respectively in the second year. Residual effects of paclobutrazol on the succeeding crop of Maku lotus also significantly increased seed yield by about 79.4% compared to the untreated plants but analysis of yield components did not clearly explain why. Cycocel application did not retard internode length or increase flowering. However, application at 2.5 kg a.i./ha in early October (50 days before flowering) significantly increased seed yield in both years (89.6 and 92.2%, respectively). Cycocel applied in November and December and at a lower rate (1.25 kg ai/ha) also increased seed yield but to a lesser extent and was less consistent in its effect. These yield increases resulted from an improvement in seed yield components, particularly enhanced pods per umbel and seeds per pod. Although cycocel did not reduce the dense vegetative 'bulk' of the crop, its cost-effectiveness makes it a cheaper alternative (about 94% cost saving) than paclobutrazol. Alar application and hand cutting which were used only during the first year, did not affect main shoot internode length, seed yield components or germination percentage. Alar applied in October at two rates (2.0 and 4.0 kg ai/ha) increased seed yield by 76.6 and 86.1% as a result of increased main and lateral shoot production which consequently increased the number of flowers per unit area. Later application (November and December) did not produce this effect. Hand cutting, especially in December, delayed flowering and depressed seed yield due to the removal of earlier-formed main shoots (September, October and November) which are major contributors to seed yield. The plant responded positively to the applications of paclobutrazol and cycocel during the mid-season flowering by decreasing reproductive abortion as shown by the increased number of pods per umbel and seeds per pod. The early and late season flowers did not respond to the effects of these chemicals. Chapter 5 reports single plant studies on the effects of paclobutrazol and cycocel applied at 1.0 or 1.25 kg a.i./ha, respectively on shoot growth and reproductive development. Both chemicals significantly altered shoot growth in favour of reproductive growth. Paclobutrazol applied in early October (48 days prior to flowering) significantly reduced the number of main shoots following treatment, but increased the number of lateral shoots and altered the plant structure by inducing earlier branching at lower nodes on main shoots. Cycocel applied at the same plant growth stage did not alter the shoot numbers but, like paclobutrazol, induced the earlier formation of lateral shoots prior to flowering. Both chemicals effectively altered the time of vegetative and reproductive growth, reduced competition for assimilates in favour of seed yield, and increased pods per umbel and seeds per pod. The same PGRs applied later (November and December) did not alter plant structure or time of occurrence of vegetative and reproductive growth. The implications of crop manipulation for improving seed yield in Lotus uliginosus are discussed. Chapter 6 highlights the major conclusions and recommendations from this study, and suggests areas for future research.Item An evaluation of antioxidant and hydration treatments for the improvement of the storability of Soybean (Glycine max (L.) Merr.) seeds : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Seed Technology at Massey University(Massey University, 1992) Sakunnarak, NitAntioxidant and hydration treatments were evaluated for their potential to improve the storability of soybean cvs. Amsoy and Davis under different ageing conditions (accelerated ageing at 40°C, 36-100% RH; controlled deterioration at 40°C, 20% seed moisture; or slow ageing at 35°C, 9 or 12% seed moisture). Despite previous reports in the literature, no protective effects of treatment were found using 1% α-tocopherol, 0.1% butylated hyroxytoluene (BHT) in acetone solution for 16 h, (β-mercaptoethanol 0.52-52 ppm in aqueous solution or 0.1-1% iodine in calcium carbonate. High concentrations of BHT (2.2%) or iodine (1%) caused toxic effects to germination performance (as determined by normal germination, viability, fresh and dry weights of normal seedling axes). Acetone, used as a carrier for antioxidant treatments, was toxic to soybean seeds. Initial seed moisture contents, levels of mechanical damage, treatment duration and seed lot variation were key factors affecting susceptibility to acetone toxicity. Tetrazolium staining showed that acetone did not cause damage to a specific tissue but rather increased the area of dead tissue which had been mechanically damaged. Hydration-dehydration pre-storage treatments of soybean also showed damaging effects. Soaking treatments caused injury to low vigour lots immediately and reduced ageing resistance in high vigour material. Moisture equilibration (ME) for 24-48 h had no effects on seed performance, but an extended moisture equilibration period up to 72 h increased rates of germination loss during subsequent ageing. However, post-storage hydration treatments showed some capacity to repair damaged seeds. Increased conductivity of seed leachate was always significantly correlated with loss of germination performance, suggesting that membrane damage was related to seed deterioration. Changes in lipid and membranes were assessed in seeds aged at 40°C, ~100% RH or 35°C, 9% constant seed moisture. No changes in total lipid content due to ageing or treatments suggested that changes in storage lipid were not related to germination performance. Significant losses of phospholipid (PL) from cotyledons occurred 4 days after accelerated ageing. These losses were correlated with loss of seed performance, but no changes in PL contents from axes were detected throughout the ageing period. By itself, this result suggests that cotyledon damage may be an important contributing factor to seed deterioration, but the transmission electron microscopy (TEM) study indicated that damage occurring in axes due to accelerated ageing was more severe than in cotyledons. Acetone or water soaking pretreatments increased rates of loss of germination performance which were associated with PL losses or ultrastructural abnormalities in both axes and cotyledons. In addition, ME treatment (72 h) applied to slowly aged seeds accelerated PL loss in axes but, this was unrelated to seed performance. These data therefore indicate that PL losses per se are not a fundamental cause of seed deterioration and may not be the first event in membrane damage. Also germinating seeds seem to be able to repair some damage of this type. No changes in proportions of polyunsaturated fatty acids were observed in either total lipid from seed tissues or microsome fractions, indicating that lipid peroxidation was not involved in seed deterioration due to ageing or treatments. This, of course, explains the ineffectiveness of the antioxidant treatments in this study and suggests that hydrolytic damage may be involved in PL loss and seed deterioration. Possible reasons for the differences between these results and the small amount of other work published on seed treatments in soybean are discussed, but it was clear that chemical or hydration treatments of soybean are unlikely to be a commercially viable option for maintenance of soybean quality where poor storage conditions are inevitable. Nevertheless. this study indicates some interesting areas for future research into the critical events involved in soybean deterioration.Item Physical changes in maize (Zea mays L.) grains during postharvest drying : a thesis presented in partial fulfillment of the requirements for degree of Doctor of Philosophy in Seed Technology at Massey University(Massey University, 2000) Kim, Tae HoonStress cracking due to high temperature drying has been of concern to the maize industry because it can lead to increases in broken grain and fine material during subsequent handling. In this study, several factors affecting physical characteristics of maize grain, particularly those related to stress cracking, were investigated. In the first year (1995-1996), the effects of several preharvest factors; hybrid, nitrogen, harvest grain moisture content, and postharvest drying factors including drying temperature and relative cooling rate on physical attributes and stress cracking in grain were investigated. Grain hardness (hard to soft endosperm ratio (H/S ratio)) was significantly affected by the interaction between hybrid and nitrogen. The effect of drying temperature and harvest moisture on drying time was dominant, while drying rate was significantly affected by hybrid and drying temperature. The effect of cooling rate on stress cracking and stress crack index (SCI) stood out among the main effects. At the lowest cooling rate of 0.23 (°C/°C/min.)•10-2, checked stress cracking (checking) was minimal, and SCI was less than 100. However, at higher cooling rates from 0.55 to 1.11 (°C/°C/min.)•10-2, grains had more than 25% multiple stress cracking, regardless of the levels of hybrid, nitrogen, harvest moisture and drying temperature. The predicted SCI for the three hybrids reached a maximum around at 0.75 (°C/°C/min.)•10-2, cooling rate, irrespective of levels of nitrogen and drying temperature. In the second experiment (1996-1997), the effects of grain hardness and morphological factors (grain size and shape) at a single grain drying rate and the development stress cracking over time were investigated. The re-parameterized Morgan-Mercer-Flodin (MMF) model successfully predicted the increasing rate (κ) and the maximum value (α) of percentage checking in various sizes, shapes and hardness of grains time after drying. From the data analysis, the maximum value of checking (α) showed a significant correlation with grain length (r = -0.707), thickness (r = 0.620), roundness (r = 0.703) and the shortest diffusion pathway (SDP; r = 0.627). While, the increasing rate (κ) of percentage checking with time after drying was significantly correlated with grain bulk density (r = -0.564), hardness ratio (r = -0.611) and drying rate (r = 0.551), and to a lesser extent (r > 0.35), with the grain size parameters including hundred-grain weight, grain length, and width. Based on this result, it was suggested that removing small and rounded grains could reduce checked stress cracking by up to 40 to 50% in some dent maize hybrids. In addition, the standardized multiple regression for single grain drying rate according to H/S ratio and grain weight accounted for from 65 to 74% of the variation. Tempering grain at high temperatures reduced stress-cracked grains significantly. However, the effect of tempering on stress cracking in the hard grain hybrid was small. In the 1997-1998 experiment, a breakage tester (HT-I drop tester) was developed and single grain breakage at various grain temperatures and times after drying was determined. Both hard and soft maize hybrids had minimal breakage at high grain temperatures (78 to 110°C), while decreasing grain temperature increased breakage exponentially. This indicated that grain temperature should be considered as a co-factor for measuring grain breakage. After drying at both 60°C and 120°C, the percentage breakage measured at ambient temperature increased rapidly during cooling in air at an ambient temperature of 20°C and a relative humidity around 65-70%. Breakage reached a maximum after about 10 minutes from the start of cooling. A Mitscherlich function was used to describe the chronological development of percent grain breakage and the analysis of the function parameters for the extent (maximum) and rate of breakage indicated that there was a significant interaction between hybrid and drying temperature for the development of grain breakage after drying. In conclusion, the MMF and Mitscherlich models described stress cracking and grain breakage during drying and cooling of maize grain. These studies provide valuable information to grain industries to assist with minimizing grain damage during drying.