Fruiting Affects Photosynthesis and Senescence of Pecan Leaves

Abstract

Seasonal measurements of net photosynthesis (Pn), stomatal conductance (Cs), and endogenous senescence-related chemicals were made on pecan [ Carya illinoensis (Wang.) C. Koch] trees growing in the orchard environment. Leaves from terminal shoots of mature, 75-year-old ‘Moneymaker’ trees in the “on” alternately bearing phase maintained 10% to 40% higher Pn rates than did those of “off” phase trees. These “on” leaves also maintained higher levels of chlorophyll, protein, amino acids, and RNA, but less K. than “off” leaves. The presence of developing fruit suppressed the rate of late summer and autumnal leaf senescence, with “on” trees retaining their leaves in a higher state of vigor than those of “off” trees. Pn and Cs rates between leaflets of adjacent fruiting and nonfruiting shoots of 7-year-old ‘Desirable’ trees were also 10% to 15% higher for the fruiting shoots throughout the season. Pn and Cs rates were influenced such that gas exchange of leaves increased as sink demand increased. These data provide evidence that this sink-demand associated stimulus operates in a localized rather than a generalized manner and that it retards the rate of leaf senescence.