Response of electron transport rate of water stress-affected grapevines : Influence of leaf age

The present study examined the electron transport rate of photosynthesis in response to light for leaves of different age following the imposition of water stress in Vitis vinifera during the growth period when leaf carbon gain is largely used for sugar accumulation into the berries. Vines grown in pots were gradually dehydrated to soil moisture levels of100, 60, 40 and 25 % of field capacity and at each soil moisture level the photosynthetic light response of selected leaves was recorded using chlorophyll fluorescence analysis. Light response curves (LRC) were established by fitting a model of the form of a non-rectangular hyperbola to the data. Changes in quantum yield (Φ), the convexity of LRC (Θ) and the maximum electron transport rate (JTsat), following adjustment to water stress and as a result of leaf aging, were analysed and their contribution towards the formation of LRCs was assessed. F varied little due to leaf aging and as a result of falling soil moisture content, whereas JTsat declined strongly following soil dehydration and the decline was exacerbated by leaf aging. Such behaviour suggests that biochemical/stomatal limitations played a more important role in determining the water stress-related decline of photosynthesis than biophysical processes of energy transfer and electron transport. Θ of LRCs, under well-watered conditions was highest for basal leaves with a leaf plastochron index (LPI) of about 16.6 but only small changes were obvious during adjustment to water stress. The most important adjustment in total photosynthetic electron flow following the onset of water deficits resulted from a depression in JTsat and was most obvious in leaves of a basal (LPI 16.6) and medium (LPI 6.9) position on the shoot. Results suggest that young, fully mature leaves are an important source of carbohydrate assimilation during berry ripening.