Functional Significance of Shade‐Induced Leaf Senescence in Dense Canopies: An Experimental Test Using Transgenic Tobacco

Abstract

Canopy photosynthesis models have predicted an optimal leaf area index (LAI; leaf area per unit surface area) and leaf nitrogen distribution at which whole-plant carbon gain per unit N is maximized. In this study we experimentally tested these models, using transgenic P SAG12 -IPT tobacco (SAG; Nicotiana tabacum L.) plants with delayed leaf senescence and therefore a greater LAI and more uniform N distribution than the wild type (WT). In a competition experiment, the increased density of surrounding WT plants caused a greater reduction in dry mass of mature SAG target plants than in that of WT target plants, indicating negative effects of delayed leaf senescence on performance at high canopy density. Vegetative SAG plants achieved a lower calculated daily carbon gain than competing WT plants because the former retained leaves with a negative carbon gain in the shaded, lower part of the canopy. Sensitivity analyses showed that the carbon gain of SAG plants would increase if these lower leaves were shed and the N reallocated from these

Photosynthetic Productivity: Can Plants do Better?

“…The P SAG12 :IPT plants retained their leaves longer than WT plants as expected based upon previous observations (Boonman et al, 2006;Jordi et al, 2000;Gan & Amasino, 1995). The difference between the total number of nodes on the stem, a measure of all the leaves that had ever been produced, and the live leaves present at the time of the harvest is a measure of the number of leaves that had been lost in the canopy due to senescence and abscission ( Table 2).…”

“…Assuming that light compensation point changes little with leaf age and/or canopy position, it would appear that the lower leaves on the P SAG12 :IPT stems were a net carbon drain on the plant as a result of self-shading. Boonman et al (2006) made similar observations for P SAG12 :IPT and WT tobacco plants grown at high densities where lower leaves were subject to both intra-canopy and inter-canopy shading. Boonman et al found that growth rates were indistinguishable between the two genotypes at these high planting densities even though the P SAG12 :IPT plants had substantially more photosynthetic tissue than the WT plants.…”

“…Boonman et al found that growth rates were indistinguishable between the two genotypes at these high planting densities even though the P SAG12 :IPT plants had substantially more photosynthetic tissue than the WT plants. This uncoupling of total leaf area from plant growth was attributed to the older, deeply-shaded leaves in the P SAG12 :IPT plants acting as net respiratory tissues even during the day (Boonman et al, 2006). Unexpectedly, in the present study, light response curves for upper-canopy leaves indicated that the Pmax rates of the new, upper-canopy leaves tended to be lower (p=0.07; t-test) in the P SAG12 :IPT than in the WT plants (Fig.…”

Optimisation of photosynthetic carbon gain and within-canopy gradients of associated foliar traits for Amazon forest trees

“…We assume that the affected understorey trees have a relatively low photosynthetic capacity of A * 0 =5 µmol m −2 s −1 with L=1.0 and with k P =0.15. We emphasise that this is only a very rough estimate, designed merely to give an indication of the likely importance of the effect, also noting that it overlooks the importance of leaf production and vertical positioning as well as the dynamics of leaf production in relation to the optimisation of L (Hikosaka, 2003;Boonman et al, 2006).…”

Suplementação luminosa no tomateiro cultivado em diferentes sistemas de condução em ambiente protegido

“…Two stem plants had higher productivity of commercial fruits, as well as it proved to be the most effective strategy in the use of water and nutrients. (BOONMAN et al, 2006;XU et al, 1997).…”