Abstract
The photosynthesis source–sink relationship in young Pinus canariensis seedlings was modified by stem girdling to investigate sprouting and cambial activity, feedback inhibition of photosynthesis, and stem and root hydraulic capacity. Removal of bark tissue showed a trade-off between sprouting and diameter growth. Above the girdle, growth was accelerated but the number of sprouts was almost negligible, whereas below the girdle the response was reversed. Girdling resulted in a sharp decrease in whole plant transpiration and root hydraulic conductance. The reduction of leaf area after girdling was strengthened by the high levels of abscisic acid found in buds which pointed to stronger bud dormancy, preventing a new needle flush. Accumulation of sugars in leaves led to a coordinated reduction in net photosynthesis (AN) and stomatal conductance (gS) in the short term, but later (gS below 0.07 mol m-2 s-1) AN decreased faster. The decrease in maximal efficiency of photosystem II (FV/FM) and the operating quantum efficiency of photosystem II (ΦPSII) in girdled plants could suggest photoprotection of leaves, as shown by the vigorous recovery of AN and ΦPSII after reconnection of the phloem. Stem girdling did not affect xylem embolism but increased stem hydraulic conductance above the girdle. This study shows that stem girdling affects not only the carbon balance, but also the water status of the plant.
Highlights
Photosynthesis products are modulated by source–sink equilibria within the plant (Jang and Sheen, 1994) and hydraulic constraints (Brodribb and Field, 2000) including the activity of root meristems
Profuse sprouting was associated with lower diameter growth (Figure 4)
abscisic acid (ABA)-GE was abundant in the needles of plants in all treatments and in buds in GW, where we found the highest content: 23.1 nmol g−1
Summary
Photosynthesis products are modulated by source–sink equilibria within the plant (Jang and Sheen, 1994) and hydraulic constraints (Brodribb and Field, 2000) including the activity of root meristems. Phloem serves as the long-distance transport pathway for photosynthate movement from source leaves to regions of active growth, storage structures, and other non-photosynthetic cells. Artificial girdling has traditionally been used to study apical control of branch growth (Münch, 1938) and the response dynamics of phloem transport (Parker, 1974), and to assess the effect of ion redistribution between phloem and xylem on xylem flow rate (Zwieniecki et al, 2004). The effect of stem girdling at the base of the stem differs from the effect of branch girdling commonly used in agricultural practices, because besides promoting carbohydrate accumulation above the girdle it prevents phloem transport to the roots. Studies about the physiological effects of complete stem girdling in forest trees are scarce compared to those carried out on branch girdling in horticulture
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