Yielding ability of potato crops as influenced by temperature and daylength

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. It was easier to parameterize for simple models than complex models because the latter included more crop processes.To improve the model's general applicability, the key processes determining tuber dry matter production had to be identified. In all previous models, dry matter allocation was driven by a temperature-dependent development. This was an oversimplification for the purposes of this study and therefore a new relation describing dry matter allocation had to be developed. It was hypothesized that dry matter allocation is governed by a dominant tuber sink and that this dry matter allocation determines the earliness and thus the yielding ability of a potato crop. This hypothesis was confirmed in a series of experiments on early and late cultivars in the Netherlands (Chapter 3). The difference in dry matter allocation between cultivars however, was only partly explained by the differences in dry matter production. Leaf longevity also played a role in explaining earliness and yielding ability.To assess the effect of temperature, daylength and radiation on yield formation in potato, field trials were carried out in Rwanda (2 altitudes), Tunisia (Spring, Autumn and Winter) and the Netherlands. To study the interaction between environment and genotype, eight cultivars differing in earliness when grown in the Netherlands were planted. Total dry matter production and tuber yields were analysed in terms of light interception and light use efficiency. Variation in total and tuber dry matter production was mainly explained by the differences in light interception. Light interception was divided into average light intensity, length of the growing period and in maximum proportion of light intercepted. The light use efficiency was found to be inversely related to an increased radiation intensity. Variation in length of the growing season was the most important factor explaining the differences in total light interception. Shorter days at emergence and higher temperatures throughout the season resulted in a shorter growth cycle. The extent of the response differed between cultivars and was mainly explained in terms of duration of ground cover.To quantify the relation between tuber yield and the climatic factors; (temperature, daylength and radiation) the growth cycle of the potato crop was divided into three phases (Chapters 5). By relating the length of each phase to temperature, daylength and radiation, the influence of these factors on crop growth and development processes was determined. The variation in the length of all three phases contributed to the variation in the duration of the growth cycle and thus to the variation in tuber yield. Both higher temperatures and shorter daylengths hastened the development in the phase between emergence and tuber initiation, and the degree of the change depended on the cultivar. In the second phase, from start of tuber growth to end of leaf growth, temperature and daylength had similar effects but these were less than in phase 1. The last phase, from end of leaf growth to end of crop growth was shortened by high temperatures and high radiation. The shift in sink priorities between tubers and leaves in phase 1 affected phases 2 and 3, so part of the variation in these phases could be explained from the variation in phase 1. The information obtained in the experiments was combined with data from literature to obtain a complete overview of temperature and daylength reactions to the various growth and developmental processes. These relations were introduced in the simulation model developed in chapter 3. The model (LINTUL-POTATO described in Chapter 6) was used to explore different climate and temperature situations for a standard potato cultivar.Integrating the effects of temperature and daylength of the separate processes resulted in simulated tuber dry matter production levels that were agreeing with those reported in the literature. It is shown that as daylength increases, potato can tolerate a broader range of temperatures. In chapter 7 the model is verified against the set of experiments used to build the model and validated against independent data. The model explains the differences in observed tuber dry matter production between locations by incorporating the effects of temperature and daylength. The differences between cultivars were smaller than the differences between location and therefore explained less well. The values observed in experiments carried out in climates that were most suitable for potato production were simulated better than those from experiments carried out in less suitable conditions.Finally the yielding ability of a potato crop for a range of climates between the equator and 60°N was evaluated. The potential total dry matter production, potential tuber dry matter production and the ideotype were determined for each climate. The potential total dry matter was dependent on the length of the growing season. Tuber dry matter production was dependent on the length of the growing season and the length of the growth cycle: tuber yields were restricted when the growing season was too short to fit the growth cycle or when tuber initiation was too early and the conditions favoured tuber growth at the expense of leaf growth. In chapter 8 the advantages of the approach and the application of systems analysis in breeding and introduction of new cultivars are discussed. Combining observations at the process level with the explanatory capacity of the simulation model reveals how tuber dry matter production is affected by the climatic factors temperature, daylength and radiation. Temperature and daylength together affected the development before tuber initiation; further development was affected by temperature only. Radiation was found to influence growth rate only. Integrating the effects revealed that especially the period between emergence and tuber initiation were determinative for further growth and development of the crop. This insight can be used to design ideotypes in breeding, to improve the efficiency of selection procedures and to analyse yield gaps in potato production.