Potential for breeding sweet pepper adapted to cooler growing conditions : a physiological and genetic analysis of growth traits in Capsicum

Sweet pepper (Capsicum annuum L.) is an important greenhouse crop in the<st1:place w:st="on"><st1:country-region w:st="on">Netherlands</st1:country-region></st1:place>. In 2005, the production area of sweet pepper in the<st1:place w:st="on"><st1:country-region w:st="on">Netherlands</st1:country-region></st1:place>exceeded 1230 ha. Capsicum plants generally require relatively high temperatures to grow and produce fruit. As a consequence, the energy input needed to grow sweet pepper is high, approximately 42 m ³ natural gas per m ² of greenhouse area per year. Social and political demands cause the horticultural sector to search for ways to improve the energy efficiency in horticultural production.<o:p></o:p></span>Scientific research has contributed in several ways to the increase of energy efficiency in Dutch horticulture: technical measures and dynamic climate control - using the ability of a crop to compensate for temperature changes within a limited period - have been shown to lead to a significant energy reduction compared to standard climate conditions.<o:p></o:p></span>Another approach to save energy is to lower the greenhouse temperature. To do so cultivars are needed that tolerate lowered temperatures without loss of yield. In this thesis the possibility to breed sweet pepper cultivars for adaptation to cooler growing conditions is studied. To this end, the physiology of growth and development of pepper and related Capsicum species were observed on young plants, and the inheritance of growth related traits was analysed. Selection for rapid growth in the vegetative growth phase of Capsicum is most likely an efficient method for the selection of plants with good growth and dry mass production in the generative phase (Chapter 6).<o:p></o:p></span>To be able to perform repeated non-destructive measurements of leaf area on single plants without harvesting and sacrificing them (Chapter 5), a method was developed to estimate leaf area from non-destructive measurements in C. annuum (Chapter 2). Leaf area could be predicted from the product of leaf length and width (<span class=SpellE>α•L•W</span>); by the addition of both linear and quadratic leaf width terms the model (Area = 0.61906•L•W + 0.2060•W ² - 0.5142•W) became independent of plant age and accession and could be used for leaf area estimation during the whole vegetative phase as well as at the start of the generative growth phase of Capsicum . It was demonstrated that this model predicts total plant leaf area accurately, even when the length and width of only 25% of the leaves were measured. The value of this method was shown in Chapter 5 where a large number of genetically unique F ₂ and F ₃ plants could be measured several times.<o:p></o:p></span>In Chapter 3 the influence of lowered temperature on relative growth rate (RGR; increase in dry mass per unit biomass per unit of time) and its underlying physiological and morphological traits are described in a group of wild and cultivated Capsicum accessions, using a growth analysis at two temperature regimes (21.1°/18.7°C and 17.3°/14.7°C; day/night). The growth related traits were net assimilation rate (NAR; increase in dry mass per unit leaf area per unit time), the leaf area ratio (LAR; the leaf area per unit of total plant dry mass), specific leaf area (SLA; leaf area per unit leaf dry mass) and leaf mass fraction (LMF; the fraction of total plant dry mass allocated to the leaves). The relative difference in RGR between plants grown at different temperature conditions was used as a measure for tolerance to lowered temperatures.<o:p></o:p></span>Based on this difference in RGR, some Capsicum accessions were shown to be better adapted to lowered temperatures than others, indicating an opportunity to breed for lowered temperature tolerance in Capsicum. Under both temperature regimes NAR proved to be the most important factor affecting the variation in RGR among the Capsicum accessions. NAR and LAR were negatively correlated in a group of ten Capsicum accessions. Some accessions combined a relatively high NAR and a relatively high LAR under lowered temperatures resulting in a limited effect of temperature on RGR. Thus, as we argued in Chapter 6, the trade-off between NAR and LAR in many accessions does not exclude the possibility that high LAR and high NAR can be combined, resulting in a single genotype with an increased RGR.<o:p></o:p></span>Subsequently, we tried to identify which traits can be used to simplify the selection for RGR in breeding programmes (Chapter 4). RGR provides important information about general plant growth and development but is difficult to assess in breeding programs.<span class=GramE>By identifying physiological and morphological traits that are related with RGR but are easier to obtain than RGR itself, indirect selection for RGR may become feasible in practical breeding.</span>The relationship between RGR and a number of physiological and morphological plant traits (relative leaf growth rate (RLGR), leaf, stem and total fresh and dry mass, plant height, leaf area, number of leaves) was studied in broader collection of Capsicum accessions as well as in a group of sweet pepper cultivars. The relations between RGR and leaf, stem and total fresh and dry mass, plant height, leaf area, number of leaves found in the collection of Capsicum accessions was not found in the group of sweet peppers, limiting the practical use of these traits as a measure for RGR. RLGR, on the other hand, was strongly correlated with RGR. Also, RLGR can be calculated from non-destructive leaf area measurements using the model described in Chapter 2. This makes RLGR a valuable, easy-to-measure trait that can be used as an indirect measure for RGR (Chapter 6).<o:p></o:p></span>In Chapter 4, we also made a comparison between the variation in the ability to adapt to lowered temperatures in cultivated sweet pepper varieties on the one hand and the variation found in the broader collection of Capsicum accessions studied in Chapter 3 on the other hand; within the group of sweet peppers, the variation for lower temperature tolerance was only limited. We concluded that further exploitation of the genetic variation for lowered temperature tolerance in wild relative Capsicum species is recommendable in order to breed cultivars with increased energy efficiency.<o:p></o:p></span>The possibility to use this physiological and morphological information on Capsicum in breeding programmes for lowered temperature tolerance is described in Chapter 5. The inheritance of RLGR and several other physiological and morphological plants traits were examined in an intraspecific C. annuum F ₂ population. The most important agronomic trait for which a QTL could be identified in the F ₂ population and validated in a group of 25 selected F ₃ lines was RLGR. In addition, 14 more<span class=SpellE>QTLs</span>related to plant growth and development were identified; the effect of six of them (e.g. leaf mass fraction (LMF), dry mass, number of leaves) could be validated in a set of F ₃ lines. In the sweet pepper cultivars studied in Chapter 4 traits of these morphological<span class=SpellE>QTLs</span>were not correlated to RGR and are therefore most likely of limited value with respect to plant growth, although they might be more valuable with respect to plant development. Most important, this study shows that marker assisted selection can be used to breed for growth and development in Capsicum .<o:p></o:p></span>In short, this thesis presents new knowledge and tools for breeding lowered temperature tolerant sweet pepper cultivars, thus enabling a reduction in energy use and CO2 emission