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The extent to which crops are adapted to specific environments greatly depends on how their development is affected by climatic factors. Development in bambara groundnut ( Vigna subterranea (L.) Verdc.) is known to be influenced by temperature and photoperiod. The objective of this study was to quantify the influence of these factors on reproductive development in selections from different origins.Models relating development rates to photoperiod and temperature with linear equations were made for different bambara groundnut selections on the basis of research in semi-controlled environments. The photoperiod and temperature responses could be explained very well by the photothermal conditions in the regions where the selections were obtained. Validation of the models with the results of glasshouse and field experiments showed good to reasonable agreement between observed and predicted times to flowering and podding.It is shown that the average photoperiod between flowering and podding determines the rate from flowering to podding, and that a gradual increase or decrease in photoperiod does not affect that rate. This means that photothermal models intended to predict bambara groundnut development in field situations with fluctuating photoperiods can be based on studies with constant photoperiods. It is also shown that growth and development in bambara groundnut are largely independent and that there are no strong direct photoperiod effects on dry matter partitioning.The usefulness of photothermal development models for identifying suitable selections for different locations and sowing dates is demonstrated in a simulation study for Botswana. It is concluded that the influence of photoperiod and temperature on bambara groundnut development can be quantified through descriptive linear models, using data from semi-controlled environment experiments with constant temperatures and photoperiods. These quantitative models, either on their own or incorporated into a crop grow

The study described in this thesis focuses on a quantitative understanding of the complex interactions in binary mixtures of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) under cutting. The first part of the study describes the dynamics of growth, production and the structural characteristics of contrasting grass and clover cultivars under field conditions. This basic information is used in the second part to quantify light absorption, C0 2 assimilation, radiation use efficiency (RUE) and light competition of the species using a modelling approach.Both species showed a seasonal pattern in growth of the dry matter (DM) and leaf area index (LAI) during the season so that the grass-dominated swards during spring shifted to clover dominance in summer. Without N fertilisation (-N), this seasonality was mainly controlled by the weather conditions. However, in fertilised mixtures (+N) grass was the dominant component of the mixture during the whole season and clover growth was always limited by light. The competitive ability and persistence of clover were determined by the structural characteristics of the cultivars. In the -N swards, both large and small-leaved clover cultivars had a higher proportion of their leaf area at the top canopy layers than their companion grass. In the +N mixtures, this was observed only in largeleaved clover, whereas the small-leaved cultivar was strongly overtopped. The difference between cultivars; was mainly due to the pattern of allocation of the DM into the supporting tissues.Experimental results showed two sources of vertical heterogeneity within the mixed grass-clover canopies: different patterns of LAI distribution and leaf dispersion. The validity of the canopy light partitioning model was considerably improved by introducing these sources of variation into the model. Using this model the RUE of species was calculated for different regrowth periods. Grass and clover had a different RUE in mixture and mon

As a consequence of the intensive use of herbicides during crop growth, many herbicideresistant biotypes have evolved. One of the first examples is the resistance of Chenopodium album against triazine-type herbicides. About ten years after this discovery, it was observed that triazine-resistant plants (R) have a lower rate of electron flow at the acceptor side of photosystem II. Later, the chloroplasts of the resistant biotype were described as having shadetype characteristics. In addition, the R biotype was found to have an altered lipid composition of the thylakoid membrane, to be more sensitive to high temperature and the thylakoid membrane has a low affinity for bicarbonate. In the presence of high irradiance, R is retarded in growth and development compared to the susceptible (S) biotype and has a higher sensitivity to photoinhibition (this thesis). The goal of this work was to contribute to a better understanding of the mechanistic relationship between light stress and photosynthetic yield, i.e. biomass production.In chapter 2 the effect of photoinhibition on electron transport and photophosphorylation in isolated chloroplasts is described. This research proved that photoinhibition causes a gradual uncoupling between electron transport and phosphorylation. This indicates that photoinhibition causes a proton leakiness of the thylakoid membrane. An important observation was that in vitro the two biotypes do not differ from each other in their sensitivity towards photoinhibition.Because R and S do not differ much in their sensitivity to photoinhibition in vitro, research was performed for the in vivo situation with intact leaves (Chapter 3). When plants have been grown at low light irradiance, little or no differences in dry matter productions appear to exist between the two biotypes. However, growth at high light irradiance causes a significantly lower production of the resistant biotype. Fluoresence studies indicated that the lower productivity of the resistant

Tropical rain forest trees spend their life in a heterogeneous light environment. During their life history, they may change their growth in relation to different levels of light availability. Some of their physiological processes (e.g. photosynthesis, carbon allocation, and meristern activity) change with light availability, and tune their development and morphology to the ambient light levels. The underlying physiological processes are not investigated in the present study. The focus is on the development and morphology of trees of canopy species in relation to the light availability in tropical rain forest. The possible consequences for survival, growth, and reproduction of trees are not assessed directly, but are discussed on a speculative basis.The relationships between the light environment, tree development, and morphology are investigated for trees of different size, ranging from small saplings to trees of adult stage. Trees of increasing size are compared in order to explore the changes in tree development and morphology, and their relation to the light environment, with ontogeny. Ontogeny is referred to as the overall growth and development pattern during tree life, both for individual trees and (in more general) for a given tree species.The field work for this thesis was carried out in French Guiana. This country in the north-east of South America has an area of 83.000 km 2and is covered by evergreen tropical rain forest. The field work was conducted at two biological stations. 'The Piste St. Elie' station is located 30 km from the coast, south of the town of Sinnamary, and the biological station 'Les Nouragues' is located 100 km from the coast, south of Cayenne. Two canopy tree species were selected for this study: Dicorynia guianensis Amshoff. (Caesalpiniaceae) and Vouacapoua americana Aubl. (Caesalpiniaceae). Both are common species in the forests of French Guiana, and are considered late successionals or shadetolerant species (Schulz 1960). In some ch

Accurate prediction of crop phenology is important not only for modelling purposes but also for crop improvement and management actions. The objective of this study was to develop a model for predicting phenological development to flowering in rice ( Oryza sativa L.).Data from the literature were analysed to evaluate a basic equation for crop development. The Beta function, commonly used as a skewed probability density function in statistics, was found to accurately describe responses of rice development rate to both temperature and photoperiod. Controlled-environment experiments were then conducted for detailed understanding of the physiological basis of photothermal responses of preflowering development in rice. Effects of day and night temperature on development to flowering were found to be different. The plants did not respond to photoperiod throughout the entire preflowering period, instead, the photoperiod- sensitive phase was sandwiched by two photoperiod-insensitive phases. It appeared that responses to both day and night temperature were stronger during the photoperiod-sensitive phase than during the two photoperiod-insensitive phases. The results also indicated that leaf appearance occurring simultaneously during the preflowering development had a different thermal response from that of the development per se .Based on the experimental results, a detailed model for photothermal responses of flowering in rice was developed, using the Beta function. The model, referred to as the three-stage Beta (3s-Beta) model, describes different photothermal responses during the photoperiod-sensitive phase and the photoperiod-insensitive phases of preflowering ontogeny. Using parameter values derived from controlled-environment experiments, the 3s-Beta model adequately predicted rice flowering dates observed in field conditions. It preformed better than several existing models over a wide range of environments.In order to assist new plant type design, the 3s-Beta model w

The optimization of light interception is essential for the production of highquality fruit. High light interception is required for high yields, whereas insufficient light exposure may lead to inferior flowering and a reduction of many characteristics of fruit quality. Orchard configuration is important for the manipulation of light utilization. In this context, the influence of planting density, arrangement, tree size and shape on light interception was quantified in a model approach. The results were evaluated with the use of datasets from experiments with apple.From the results of the calculation of light absorption and canopy photosynthesis, it could be inferred that early cultivars having a growing season of only four months achieve about the same potential production over a wide range of temperate regions. The production of cultivars with a longer season would, however, increase by at least one tonne fruit (fresh weight) per ha for every degree lower latitude. On the other hand, production at low latitudes may suffer from higher respiration costs.A model for the assessment of light transfer through trees was used to analyse the influence of density, planting system, and tree size and shape. It was concluded that trees with a conical shape have better light penetration into the lower tree parts than have those with parabolic or cylindrical shapes. Light interception and the amount of well-illuminated canopy increase with planting density particularly 9 leaf density of the crowns is not increased. Systems with a low ratio of between-to- within row distance (rectangularity) should intercept light very efficiently.In experiments, planting density was found to be the most important orchard factor for production per ha, which was actually due to leaf area. Systems at low rectangularities produced considerably more well-coloured fruit than did those at higher rectangularities. The product" of three-row bed systems on a triangular design lagged behind that of other s

Today, potato is grown commercially in almost all climates of the world, except in tropical lowlands. The highest tuber yields are obtained in areas with temperate climates in North western Europe and the North-West of the United States. In tropical and subtropical climates tuber yields are lower and less stable. To determine if this is due to the lack of adaptation of potato to the conditions prevailing in the tropics, the effect of climate on potential yield was examined. The study presented in this thesis explored the potential and attainable yields of potato. It aimed to determine and quantify the effect of climate on potential tuber dry matter production and to assess the genetic variation in the sensitivity for this climate. Potential dry matter production in potato is governed by the genetic characteristics of a cultivar and the climatic factors, radiation, temperature and daylength. Hitherto, research into the effect of temperature and daylength on the potato crop has been done in two types of studies. The tuber yield was either correlated directly with these climate factors, or separate processes in the plant were related to temperature and daylength.The general introduction demonstrates that to understand the behaviour of the crop and the variation in tuber yields, the crop and its relations with the environment must be studied as an integrated system. The subsequent chapters describe a study in which experimental data are incorporated into a simulation model. In the experiments the relations of the individual crop characteristics to temperature and daylength were obtained and the model was used to calculate the consequences of variation in daylength and temperature for tuber dry matter production. First a series of models constructed in the Netherlands was tested with weather data from a series of years in Scotland (Chapter 2). There appeared to be no difference in the ability of the models to simulate tuber dry matter production after reparameterization.

The photothermal regulation of phenological development and growth in bambara groundnut ( Vigna subterranea ) wa s studied to elucidate the crop's potential and limitations in current or future cropping systems. Eight accessions from the germplasm collection of the International Institute of Tropical Agriculture (IITA) were used for a preliminary assessment of sensitivity to photoperiod. In some accessions, photoperiods of 14 h or longer delayed or inhibited the onset of two developmental stages, flowering and podding, in comparison with photoperiods of 11 h or less. Within an accession, podding was always affected more than flowering. Microscopic studies in the laboratory enabled the cause of the delay or absence of the onset of podding under long photoperiods (14 h or more) to be identified as a check in the growth of fertilized ovaries. The minimal inductive period for the onset of podding was determined for two accessions differing in sensitivity to photoperiod. In the comparatively photoperiod-sensitive accession, the time of pod induction in relation to pod position on the plant was studied too. Three accessions were used to test whether the rate of progress towards flowering and the rate of progress towards podding could be described as functions of mean diurnal temperature and/or photoperiod. Finally, earlier indications that photoperiod influences growth as well as development were verified. The results of the experiments demonstrate the flexibility in the development of bambara groundnut, particularly in relation to changes in photoperiod. This flexibility largely explains why the crop can produce itself under marginal conditions in rain-fed areas.

In this thesis the hypothesis that different molecular types of phytochrome have different physiological roles has been elaborated and an attempt has been made to assign functions to these phytochrome types. For this purpose a genetic approach was chosen and the following potential phytochrome mutants were used: the tomato aurea ( au ) mutant; the cucumber long-hypocotyl ( lh ) mutant and the Arabidopsis long-hypocotyl ( hy3 ) mutant. Comparing the response of the mutant affected in the abundance or function of a particular phytochrome type with that of its isogenic wild type shows directly the relevance of the phytochrome type concerned. The au mutant is proposed to lack the light-labile phytochrome type (phy A), while normal levels of the lightstable phytochrome type (phy B) are present. This results in a reduction of. (i) the inhibition of the hypocotyl growth; (ii) the anthocyanin synthesis; (iii) the chlorophyll content, but a normal end-of-day far-red (EOD FR) response. In the lh and hy3 mutants the opposite situation appears to be found: normal levels of phy A, but a lack of phy B. Both the lh and hy3 mutant do not show the EOD FR response, while the hypocotyl growth in continuous R and B is less inhibited when compared to the wild type. Therefore it is proposed that phy B plays a role in the EOD FR response, while both phy A and phy B are important for the inhibition of the hypocotyl growth. With the help of other mutants described in the literature a more detailed assignment of functions to the different phytochrome types is proposed.In addition, a potential phytochrome signal transduction mutant, the tomato high- pigment ( hp ) mutant, was characterized. This mutant exhibits exaggerated phytochrome responses, e.g. high anthocyanin synthesis and short hypocotyl length when compared to wild type. It. is proposed that the hp mutation affects an amplification step in the phytochrome transduction chain.

Achimenes is a herbaceous perennial of the Gesneriaceae family. Its origin is Central and South America. It has been cultivated as a pot plant since the Victorian era. Extensive hybridization has produced many attractive cultivars which have been Introduced in commercial floriculture both In the United States and Europe. Commercial Achimenes cultivars have short erect stems bearing flowers in the axils of the leaves and form rhizomes at the tips of underground stolons. At the end of the growing period, in late summer, the plants enter a period of senescence and rhizomes undergo a period of dormancy before they sprout for a new plant production in early spring . Thus cultivation of Achimenes is limited to one crop per year.The aim of the present study was to describe the effect of the environment, mainly light and temperature, on plant growth and development In order to realize optimum flower and / or rhizome production . Moreover the possibilities to manipulate plant development and release of rhizomes from dormancy by means of growth regulators are indicated.Vegetative growth and flower developmentGrowth and flowering were strongly affected by treatments involving duration, intensity and quality of light (Chapters 1 through 4). Longer photoperiods increase vegetative growth in most Achimenes cultivars. LD (16 h) increased plant height, shoot weight and number of flowers compared to SD (8 h). Increased duration of illumination, of the same light level, increased shoot dry weight and reduced plant height (Chapter 2). When the same daily light integral was maintained, long day i.e. low light Intensity favoured growth and flowering and increased relative growth rate (Chapter 3). This is due to increase in net assimilation rate but not in leaf area ratio suggesting a better light use efficiency when a low light intensity is combined with longer days. When daylength was extended with low intensity incandescent light (low R/ FR ratio) total leaf area , total dry weight of

One of the most important aspects of the life cycle of seed plants is the formation and development of seeds on the motherplant and the subsequent dispersal. An equally important element of the survival strategy is the ability of seeds to prevent germination in unfavorable conditions, such as the wrong season, low light irradiance, or an unfavorable soil composition. Seeds of many species may remain in the soil, either in a dry state or fully imbibed, for hundreds of years without losing their viability. Earlier investigations have shown that this state of dormancy may be broken and induced in a seasonal cycle. Evidently, temperature is an important regulating factor in these cycles. When dormancy is broken, seeds may germinate, providing the conditions are adequate. Hence, seeds must be able to 'sense' their environment. Again, temperature is important, but also two naturally occurring environmental factors, light and nitrate, are known to stimulate the germination of many (wild) species.Seeds of Sisymbrium officinale (hedge mustard) only germinate in the presence of light and nitrate (either endogenous or exogenous). Irradiation with far-red light (720 run) antagonized the stimulating action of red light (660 nm). Both wavelengths are part of the daylight spectrum. This is proof that the light-induced germination is mediated by the plant pigment phytochrome. The induction of germination by light and nitrate could also be inhibited by application of tetcyclacis, an inhibitor of the biosynthesis of gibberellins. Application of gibberellins 4 and 7 antagonized the inhibition. It was concluded that synthesis of gibberellins is part of the transduction chain that leads to germination. Comparison of the escape times for the antagonizing action of far-red light and the inhibition of tetcyclacis led to the conclusion that induction of germination occurred during the first 8 hours after irradiation with red light and application of nitrate, while the synthesis of gibberell

Study of photomorphogenesis is often complicated by the interaction of different photoreceptors regulating a given process or by the induction of multiple effects by a single photoreceptor. Mutants in which particular components of the morphogenetic pathways are eliminated provide the possibility of studying a more simplified form of photomorphogenesis. Three classes of photomorphogenetic mutants are proposed: photoreceptor, transduction chain and response mutants. In this study three mutants have been used: two have an elongated hypocotyl when grown in white light (the aurea ( au ) tomato mutant and the long hypocotyl ( lh ) cucumber mutant) and one with an enhanced pigment synthesis (the high pigment ( hp ) tomato mutant). The au mutant appears to be a photoreceptor mutant, lacking spectrophotometrically and immunochemically detectable labile phytochrome (lP). The lh mutant is proposed to lack stable phytochrome ( s P) or its function. These mutants enabled the role of s P, l P. and blue light (BL)/UV- photoreceptor(s) in several photophysiological processes to be studied. The results of these experiments indicate that l P plays a role in both hypocotyl elongation and anthocyanin synthesis in etiolated seedlings. This provides direct evidence that the 'bulk' l P is functional. In etiolated seedlings the au tomato mutant with its deficiency in l P is 'red-blind' and has a shift of fluence rate response curves for hypocotyl inhibition by BL and UV-A approximately l order of magnitude to higher fluence rates. In light-grown plants it is proposed that s P regulates the end-of-day far-red light (FR) response and the inhibition of hypocotyl elongation due to light perception by the cotyledons. Furthermore, these mutants with reduced phytochrome (P) content provide direct experimental evidence that the FR absorbing form of P (Pfr) is the active form indeed. If removal of the red light (RL) absorbing form of P (Pr) is the active photomorphogenetic process, instead of an i