Photosynthesis of sun and shade leaves of field-grown grapevine (<i>Vitis vinifera</i> L.) in relation to leaf age. Suitability of the plastochron concept for the expression of physiological age

Abstract

The relationship of photosynthesis to leaf age for sun and shade leaves was investigated in a field experiment over two years. The objective was to test the suitability of the plastochron index concept, proposed by ERICKSON and MICHELINI (1957), for the description of age-related changes in leaf photosynthetic activity in the field. The duration of a plastochron decreased strongly and similarly for sun and shade shoots with increasing temperature, whether temperature was expressed as a daily heat sum (degree days > 10-degrees-C.dd-1) or as the mean of daily minima and maxima. The responsiveness to temperature decreased during the season and the lower threshold for development increased. Plastochron duration was insensitive to the amount of light received in the canopy. Expressing time elapsed between leaf unfolding and attaining maximum rates of photosynthesis in degree days or days yielded changing relationships depending on when during the season leaves unfolded. In contrast, this relationship was constant if time was expressed in plastochrons. The results show that the plastochron is an adequate indicator of time as the plant experiences it, and is superior to the thermal time (degree day) and chronological time expressions commonly used to relate photosynthesis to development.