MANIPULATION OF ABIOTIC IN VITRO FACTORS TO IMPROVE THE PHYSIOLOGY AND SUBSEQUENT FIELD PERFORMANCE OF MICROPROPAGATED PLANTLETS

Field performance of in vitro cultivated plantlets will ultimately depend on their ability to withstand the transferring from in vitro to ex vitro conditions. Many factors of the in vitro environment may affect the ability of plantlets to withstand the ex vitro transfer thus being extremely difficult to define which abiotic factors can be manipulated to improve plantlet physiology and their subsequent performance in the field. Our research efforts have concentrated in two aspects of the physiology of micropropagated plantlets, namely stomatal functionality and photosynthetic capacity. These two physiological processes can be affected by various abiotic factors of the in vitro environment but there are three of them that are good candidates for manipulation with good possibilities of having effects on improved plantlet physiology. The air space of the culture vessel, the sugars added to the medium and the lighting conditions of growth rooms. Within the factors of the air space inside culture vessels, the high relative humidity that is normally present in standard culture vessels might create a situation where plants would be subjected to abnormally high water status and low leaf abscisic acid (ABA) concentrations that in turn, may affect their stomatal functionality. On the other hand, other abiotic factors such as light conditions in growth rooms and sugars in the medium are expected to affect the photosynthetic capacity of plantlets, important in the in vitro stage but instrumental when plants are transferred from a mixotrophic environment to a complete autotrophic situation when plants would be living in an ex vitro environment. Our results suggest that it is possible to improve plantlet physiology and subsequent field performance of micropropagated plants by manipulating some of these in vitro abiotic factors. Particularly promising for tropical plantlets might be the increase in light intensity and quality by growing plants in specially designed growth rooms, allowing plantlets to grow under natural light at the last stages of micropropagation processes. INTRODUCTION Two important bottlenecks for plant micropropagation industry are low survival during the in vitro to ex vitro transferring and limited growth and field performance once they are in the final plantation. It is extremely difficult to define which factors within the in vitro environment should be manipulated in order to improve these problems. Nevertheless, we have considered that two aspects of plantlet physiology should be good targets to improve transfer and survival and field performance, stomatal functionality and the development of an adequate photosynthetic machinery at the end of the in vitro stage. A broader review of some of these factors can be revised in Pospisilova et al. (1999) and also in the excellent Chapter by Dr. Pospisilova in this volume. Here, we will concentrate in revising our own efforts in two aspects of the physiology of micropropagated plantlets, namely stomatal funcionality and photosynthetic capacity. These two physiological processes can be affected by various factors within the in Proc. II IS on Acclim. and Establt. of Micropropagated Plants Eds.: J.M. Santamaria and Y. Desjardins Acta Hort. 748, ISHS 2007 vitro environment but there are three factors that were good candidates for manipulation with good possibilities of having positive effects on improved plantlet physiology. The gaseous environment of the culture vessel, the light intensity of culture rooms and some factors in the growth medium such as sugars. The gaseous environment within a standard culture vessel comprises two aspects, the gaseous composition (namely ethylene and CO2) and the high relative humidity (RH) that is commonly found in the air space of standard culture vessels. A high RH might create a situation where plants would be rarely subjected to water deficits situation that in turn it might affect stomatal functionality. On the other hand, abiotic factors such as light and sugars in the medium were expected to affect the photosynthetic capacity of plantlets, important in the in vitro stage but instrumental when plants are transferred from a mixotrophic environment to a complete autotrophic situation when plants would be living in an ex vitro environment. Regarding the light intensity in growth rooms, micropropagated plantlets normally grow with artifitial lights providing not only low light intensity but also limited light quality. Tube lamps normally provide a maximum of 60 μmol m s light intensity (PFFD) and an unbalanced amount of red light relative to blue light. These two factors are likely to affect both, the development of the photosynthetic apparatus as well as the rate of photosynthesis. Regarding the sugars, reports are available indicating that sucrose may affect photosynthesis of in vitro cultivated plantlets (Hdider and Desjardins, 1994; Desjardins, 1995; Synkova, 1997). At least in some species, the manipulation of some of the factors of the in vitro environment such as the gaseous and humidity environments, the lighting conditions or the sugars added to the medium, can result in an improvement of the physiological status of micropropagated plants.