Search Results

Abstract

The rate and efficiency of photosynthesis (PS) is unquestionably one of the primary determinants of crop yield. Yet, as this symposium illustrates, many other factors contribute to plant productivity. Indeed, the exact relationship between PS rate and yield remains undefined. For several years we have now been interested in factors that affect PS rate in both C₃ and C₄ plants, not only within the plant, but also external to it. Internal factors affecting PS that will be discussed include leaf age, leaf canopy, photorespiration, stage of plant development (including both vegetative and reproductive phases), and the occurrence of little-recognized metabolic pathways such as polyol synthesis and transport. External factors that affect PS, in addition to obvious environmental determinants such as temperature, light, and water, include the seasonal growth cycle. While we still do not understand how all of these factors interact to affect yield, we are beginning to understand their singular effects. Control of these factors, once thought to be within reach even before the advent of biotechnology, remains likely, but distant.

Cranberry [Vaccinium macrocarpon (Ait.)] yield has been associated with photosynthate supply. However, the impact of temperature and radiation on photosynthesis of the cranberry plant is not well understood. The objective of this experiment was to characterize the photosynthetic response to radiation and temperature in order to develop a model for estimation of cranberry photosynthetic rates. Two cranberry cultivars, `Stevens' and `Ben Lear', were tested for photosynthetic response at air temperatures ranging from 15 to 35 °C and radiation intensities from 200 to 1200 μmol·m^-2·s^-1. Depending on temperature, maximum photosynthesis (P_max) was ≈10 or 12 μmol CO₂/m²/s (net photosynthesis) and the saturating radiation level was estimated to be 600 to 800 μmol·m^-2·s^-1. Cranberry quantum yield was estimated as 0.03 mol CO₂/mol photon. Both models; Blackman and the nonrectangular hyperbola with a Θ (angle of curvature) of 0.99 were a good fit for measured photosynthetic rates under controlled environment conditions. The disparity between modeled predicted values, and observed values in the field around midday, indicates a reduction in potential photosynthetic rates in a diurnal cycle that is consistent with the phenomenon of midday depression.