Incoming energy management and photoinactivation as a function of light interception in peach leaves

Abstract

Orchard yields are a function of the light intercepted, although this linear relation is less tight when greater than 50% of available light is intercepted. Above this level, light exceeds photosynthetic requirements and can expose leaves to photoinhibition. The "asymmetric" peach orchard was designed and planted to obtain in the field six different light intensities at any time of the day and three daily light interception profiles: low (C), middle (E) and high (W). This paper reports on how incoming light energy is managed in attached leaves subjected to several radiative regimes along the day. Combined measurements of gas exchange and chlorophyll fluorescence (quenching analysis) were performed during the day to quantify the absorbed energy used for net photosynthesis and that dissipated by the photoprotective mechanisms. In addition, the amount of inactive PSII after one day of irradiation was calculated via chlorophyll fluorescence determination. Net photosynthesis was linearly related to irradiance up to a saturating point of 1000-1200 μmol photon m-2 s-1. Non-photochemical quenching (NPQ) played the most important role in photoprotection, but its activity was reduced at low irradiance, possibly due to a sub-optimal trans-thylakoid ΔpH. The non-net carboxylative mechanisms (NC) were the main photoprotective mechanisms at middle-low irradiance levels and probably facilitated the establishment of the trans-thylakoid ΔpH needed for full activation of NPQ. At the end of the photosynthetic period the amount of inactive PSII was the highest in W followed by E and C samples. In addition, daily carbon accumulation did not reflect the light interception ranking, with E having the highest, and W the lowest. These findings support the conclusion that under irradiances exceeding the photosynthetic requirements, photoprotection and photoinactivation biochemistries divert energy in the form of carbohydrates and reducing power from tree growth and productivity. The amounts of these losses can be significant.