The radiation-balance components of a glasshouse rose crop were measured on five days representing successive stages of crop development from planting to a fully developed flowering canopy.
The glasshouse, a heated, aluminum structure glazed with diffusing glass and orientated N-S, transmitted between one-half and two-thirds of the incident global radiation with an unchanged near-infrared fraction. There was considerable spatial and diurnal variation in the fraction transmitted, most of which could be explained by reference to the angle of incidence to the roof.
20% of the solar radiation reaching the mature canopy was reflected, compared with 28% for individual leaves. Reflection in the near infrared range was twice as great for the canopy. 30% of the incident global radiation was transmitted to the floor of the glasshouse below the mature canopy, largely via the pathway between the rose beds. The measured absorptivity agreed to within 10% with the value calculated from canopy gap frequency assessed by hemispherical photographs and the calculations showed the importance of decreasing the diffuse component and the path space in increasing canopy absorption.
4% of the photosynthetically active radiation absorbed by the canopy was fixed in dry matter production.
The transpiration flux for the mature canopy, computed from measurements of water concentration difference and diffusive resistance of the leaves of the upper canopy, agreed with the values derived from soil water content measurements. The latent heat equivalent of the transpiration flux exceeded that of short wave absorption, the difference being satisfied by a convective heat flux independently demonstrated by measurements of a temperature inversion and downward flux of net terrestrial radiation during the day. Close agreement between the net radiation flux above the canopy and its transpiration flux suggests that the convective heat flux was derived from that fraction of the radiation flux not directly absorbed by the canopy.