Crop Management in Greenhouses: Adapting the Growth Conditions to the Plant Needs or Adapting the Plant to the Growth Conditions?

Strategies for improving greenhouse crop production should target both developing advanced technological systems and designing improved plants. Based on greenhouse experiments, crop models and biotechnological tools, this paper will discuss the physiology of plant-greenhouse interactions. It is discussed how these interactions can be applied to control the production process at Northern and Mediterranean climatic conditions. Absorption of light by the leaves is important for maximum crop photosynthesis. For this, it is important to have plants that develop as fast as possible a sufficient leaf area index. The question is: what leaf area index is needed for optimal crop performance? Most of the light is absorbed by the upper part of the canopy. Can we improve the light distribution in the canopy and, moreover, does this increases yield or quality? Virtual plant models may help to address this question. In some cases removal of older leaves can improve yield, while in other cases removal of young leaves may accomplish the same objective. In summer time the light transmission of the greenhouse is often reduced by growers to avoid plant stress. However, in several cases this stress is only an indirect effect of light, because other growth factors (e.g. temperature, humidity) tend to be suboptimal. In Northern countries CO2 supply is commonly used. The introduction of semi-closed greenhouses allows to maintain high CO2 concentrations all year round. In Mediterranean countries, a large yield increase is still feasible by CO2 supply. Optimum growth conditions means that there is a good balance among different climate conditions. The source/sink ratio of a crop (ratio between production and demand of assimilates) often reflects whether these conditions are balanced. Variation in the source/sink balance affects formation and abortion of organs, product quality and production fluctuations. Some examples are shown on temperature control based on the source/sink balance of a crop. Drought and salinity may limit production especially in the Mediterranean. Morphological and metabolic traits, with known genetic bases, can be functionally altered to test current hypotheses on plant-environment interactions and eventually design a greenhouse plant. Reasonably, such a plant should have specific shoot vs. root developmental patterns, efficient water and nutrient uptake systems as well as other specific features that have not been sufficiently explored. Elucidation of the complex plant-greenhouse interactions would establish a physiological basis to improve both product quality and resource use efficiency in greenhouse. INTRODUCTION Greenhouse production allows growers to improve growth conditions for maximizing crop production, product quality and resource use efficiency. Strategies for these improvements should target both developing advanced technological systems and a [email protected] Proc. IS on Prot. Cult. Mild Winter Climate Eds.: Y. Tüzel et al. Acta Hort. 807, ISHS 2009 164 designing improved plants. The development of advanced systems should aim at controlling growth conditions such that they meet the demand of the plant. At the same time, the design of the plants should aim at a plant that is better suited to cope with the growth conditions in the greenhouse. Physiological crop models are powerful tools to identify the desired growth conditions, to explore effects of growth conditions related to the introduction of new technologies as well as to identify the target traits of a crop that are particularly important for a specific environment. Biotechnology provides tools to generate plants with improved traits as well as to explore the potentials of crop improvement. Based on greenhouse experiments, crop models and biotechnological tools, this paper will discuss the physiology of plant-greenhouse interactions. It is discussed how these interactions can be applied to control the production process at Northern and Mediterranean climatic conditions. RESULTS AND DISCUSSION Light Light forms the basis for growth of plants, as it is the driving force for photosynthesis. Besides, light quality and length of photoperiod may affect developmental processes in the plant.. For most crops a 1% light increment results in 0.5 to 1% increase in harvestable product (Marcelis et al., 2006). This is an average value, which depends on several factors. For instance, the relative effect of light on growth is greater at lower light levels, at higher CO2 concentrations and at higher temperatures. Consequently, the relative effect is larger in winter than in summertime and the effect is larger in Northern than in Mediterranean regions. The effect of light on growth also depends on the duration and moment that the light level is changed. Besides a positive effect on yield quantity, light usually has a positive effect on quality as well. Light should not be considered as a separate growth factor in greenhouse horticulture, as it forms an integral part of the total farm management. Many growers, for instance, choose a higher temperature, a lower plant density and different cultivar when the light level is increased. Photosynthesis shows a saturating response to light. At low light levels photosynthesis increases rapidly, but at higher levels effects of light diminish. The level at which photosynthesis saturates is not a constant, but may depend on amongst others CO2 concentration, nutrient (especially N) concentrations or the season (most likely the main factor is light level during the previous weeks). A common mistake made by many authors is neglecting the difference between response curves at the leaf level and the crop level. Many authors measure photosynthesis of single leaves (or a few cm of a leaf) and apply this curve to predict the response of the whole canopy. This, however, can only be done when a simulation model is used that accounts for light penetration in a canopy. The top leaves may saturate at 500-1000 μmol PAR m s (depending on growth conditions), but the leaves below the top leaves are not saturated and can use additional light. For instance when leaf photosynthesis saturates at about 600 μmol PAR m s, a crop with LAI of 6 only saturates at 1100 μmol PAR m s (Fig. 1). The higher the LAI the higher the light intensity at which saturation occurs (Fig. 1). In Northern countries lamps are used to improve growth and quality under poor light conditions. However, even in these Northern countries in summer often screens or white wash is used to prevent too high radiation levels in the greenhouse. In Mediteranean countries a large fraction of light is prevented to enter greenhouse by shading nets, screens or white wash. Considering the photosynthesis response of canopies far too much shading is applied. In fact large amounts of light which could drive growth, are unused. Nevertheless, if less shading was applied production of high quality produce in the present systems would reduce. In most cases the growth impairment at high radiation levels is not the result of a too high light intensity, but rather because of a too high heat load or too high vapour pressure deficit of the air. In addition most shading measures increase the diffuseness of the light. If we can control temperature and air humidity by