Heliotropic movements of leaves radiative environment and gas exchanges through sunflower (Helianthus annuus L.) canopy profile as related to soil water status.

BACKGROUND. This paper is aimed at assessing the response of plant architecture of the sunflower crop to soil water deficit, to better understand its role in mitigating the detrimental effects of water stress and adapting plant gas exchange to the water conditions of the environment. METHODS. In summer 1996, a sunflower crop was tested under two irrigation regimes: a treatment irrigated only at sowing and a treatment irrigated on a regular basis by re-establishing 100% of water use. During the growing cycle the water and radiation balance were measured periodically. On two dates of the growing cycle, when plant water status was sufficiently differentiated, the following measurements were taken through the canopy profile: leaf and azimuth angle, incident photosynthetically active, transmitted and reflected radiation, LAI, water status, gas exchanges and dry matter of the top, middle and bottom layers of the crop. RESULTS AND CONCLUSIONS. The progressive decline of plant water status, by increasing leaf inclination, gradually modified the exposure of leaves to radiation, allowing them to intercept more light during the morning and afternoon, when environmental conditions are more favourable to assimilation processes, and to escape radiation in the sunniest hours of the day. The paraheliotropic movements not only adjusted the radiative load to the photosynthetic potential of leaves but also contributed to wards controlling the transpiration rate through the stomata. This allowed better modulation of the terms of the energy balance and, by optimizing gas exchanges of the leaves, certainly contributed to improved water use efficiency in the water stressed treatment. The greater light penetration through the profile of the stressed crop resulted in a higher photosynthesis rate and greater absorption of radiation in the middle and bottom layers of the canopy