Submitted to: Tree Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 27, 2002
Publication Date: April 1, 2003
Citation: BASSETT, C.L., CALLAHAN, A.M. CHARACTERIZATION OF A TYPE II CHLOROPHYLL A/B-BINDING PROTEIN GENE(LHCB2*PP1) IN PEACH:II. MRNA ABUNDANCE IN DEVELOPING LEAVES EXPOSED TO SUNOR SHADE IN 'LORING'. TREE PHYSIOLOGY. Vol. 23, pgs. 473-480 April 2003. Interpretive Summary: In peach trees, vegetative growth (new shoots and leaves) often compete with reproductive growth (development of fruit) and can result in less fruit and/or fruit of smaller size. Pruning to control vegetative growth is one way orchardists address this problem, but it is a very labor-intensive method. Artificial shading of whole trees can also encourage development of larger fruit. However, this method is not economically practical. The use of photosynthetic inhibitors is another method to slow vegetative growth but is primarily limited by the number of chemical inhibitors available. Our study focused on examining peach vegetative growth on branches exposed to full sunlight or in deep canopy shade to see if there might be an effect of shading on leaf development and expression of an important photosynthetic gene (chlorophyll a/b binding protein abbreviated as cab). Shading affected the extent of leaf development and cab expression without altering the pattern of either. Although the number of fruit on each branch was lower in the shade, the rate at which the fruit naturally fell from the branch during the growing season was not affected by light. Our study supports previous observations of the effect of canopy shading on fruit drop and suggests that interfering with the expression or activity of the cab protein could represent another option for controlling vegetative growth in fruit trees.
Technical Abstract: Leaf development was followed in field grown, mature peach trees (Prunus persica [L.] Batsch, cv Loring) during the first half of the 1995 growing season on shoots exposed to full sunlight or shoots shaded by the canopy. The architecture and size of shaded shoots was significantly different from shoots exposed to full sun. Furthermore, the total number of leaves produced on shaded shoots was significantly smaller than that on shoots exposed to full sunlight throughout the season, although differences in leaf number between light conditions were greater as the season progressed. Interestingly, the overall pattern of leaf development along the shoot in sun or shade was qualitatively similar. The pattern of expression of the type II chlorophyll a/b-binding protein gene, Lhcb2*Ppl, determined by RNA abundance in leaves at different positions along the shoot was also similar between the two light conditions. The major difference between leaves under the different light conditions was in the lower levels of Lhcb2*Ppl RNA in mature, shaded leaves compared to those exposed to light. Although the number of fruit per shoot was significantly lower on shaded shoots than on those exposed to full sunlight, the rate of drop was not substantially different throughout the growing season, indicating that quantitative differences in leaf initiation and growth due to differences in light exposure did not adversely affect retention of fruits. However, based on comparison to a previous study of leaf development on non-fruiting trees, reproductive development did slow the rate of vegetative growth without affecting the overall pattern of leaf development along the shoots.