Effects of lighting and air movement on temperatures in reproductive organs of plants in a closed plant growth facility

Effects of lighting and air movement on temperatures in reproductive organs of plants in a closed plant growth facility

Temperature increases in plant reproductive organs such as anthers and stigmas could cause fertility impediments and thus produce sterile seeds under artificial lighting conditions without adequately controlled environments in closed plant growth facilities. There is a possibility such a situation could occur in Bioregenerative Life Support Systems under microgravity conditions in space because there will be little natural convective or thermal mixing. This study was conducted to determine the temperature of the plant reproductive organs as affected by illumination and air movement under normal gravitational forces on the earth and to make an estimation of the temperature increase in reproductive organs in closed plant growth facilities under microgravity in space. Thermal images of reproductive organs of rice and strawberry were captured using infrared thermography at air temperatures of ten to eleven degrees Celsius. Compared to the air temperature, temperatures of petals, stigmas and anthers of strawberry increased by twenty-four, twenty-two and fourteen degrees Celsius, respectively, after five minutes of lighting at an irradiance of one hundred sixty watts per square meter from incandescent lamps. Temperatures of reproductive organs and leaves of strawberry were significantly higher than those of rice. The temperatures of petals, stigmas, anthers and leaves of strawberry decreased by thirteen, twelve, thirteen and fourteen degrees Celsius, respectively, when the air velocity was increased from zero point one to one point zero meters per second. These results show that air movement is necessary to reduce the temperatures of plant reproductive organs in plant growth facilities.

One. Introduction

Plant growth and reproduction in space have been of increasing concern as the possibility increases for long-term manned space flights. The feasibility of maintaining long-term manned space missions is dependent on growing crops in Bioregenerative Life Support Systems or space farms that will provide food, carbon dioxide oxygen conversion and water purification. For space farming, the movement of heat and gases between plant surfaces and their environment is affected by the absence of buoyancy dependent convective transport, limited ventilation and high ethylene concentrations in plant growth facilities causing poor development of plant reproductive organs and poor yields.

in space. They also discussed heat and mass transfer in space farming. Yamashita et al. discussed heat and mass transfer in Martian greenhouse dome under the low atmospheric pressure and low gravity.

In space farming, the utilization of closed plant culture facilities is anticipated. In such facilities without adequate air circulation systems, there may be adverse effects on the crops. Insufficient air movement around plants increases the resistance to gas diffusion in the leaf boundary layer and thus limits photosynthesis and transpiration of plants, resulting in suppression of plant growth and development. Therefore, the enhancement of gas exchange in leaves and resulting plant growth would be dependent on appropriate control of air movement in controlled environment facilities.

On earth, convection occurs with uneven temperature distribution. Air movements are induced by convection even in a closed chamber with no forced ventilation system. There is, however, no natural convective or thermal mixing under microgravity conditions in space because buoyancy is considered to be negligible under microgravity conditions. The limited convection would reduce plant growth by limiting heat and gas exchange on plant leaves. Effective air movement in a closed plant production system is thus essential to enhance the heat and gas exchange between plants and the ambient air, and consequently promote growth of plants in space.

Several space flight experiments with plants have observed poor seed production and genetic aberrations. Space-specific stress caused chromosomal anomalies in cells of plants grown in space. Bubenheim et al. reported that pollen from super-dwarf wheat grown on the Mir space station contained only one nucleus due to a high level of atmospheric ethylene, while normal viable pollen is tri-nucleate. Carbon dioxide enrichment and air exchange in plant growth chambers allowed pollen and ovule development to occur normally, and absence of convective air movement caused disorder of pollen development under space flight conditions.

Kitaya et al. reported that leaf temperatures increase in microgravity because sensible and latent heat exchanges between leaves and the ambient air are retarded compared to normal gravity. The temperature increase under decreasing gravity levels was most significant at the narrow region of the leaf blade. Poor seed production and genetic aberrations are possibly due to excessive temperature increase in plant reproductive organs such as anthers and stigmas under artificial lighting in closed plant growth facilities under microgravity conditions in space.

Terrestrial research has been initiated to study the thermal environment of plant reproductive organs to estimate the temperature increase in these organs in plant growth facilities in space. In the present study, effects of illumination and air movement on the temperature increase in glumes, petals, anthers and stigmas of plant reproductive organs were investigated under normal gravity conditions.