Photosynthesis, Leaf Resistances, and Ribulose-1,5-Bisphosphate Carboxylase Degradation in Senescing Barley Leaves

Abstract

The relationship between loss of ribulose-1,5-bisphosphate carboxylase (RuBPCase) and the decline in photosynthesis during the senescence of barley primary leaves was assessed. Loss of RuBPCase accounted for about 85% of the decrease in soluble protein. The senescence of leaves is usually associated with loss of soluble protein, predominantly RuBPCase2 (5,12,13,21, 25, 26).Protein degradation and remobilization provide an important source of N and S for other parts of the developing plant (4, 21). The relationship between the function of RuBPCase as a storage protein (7, 12) and its catalytic function has not been well defined. Loss of RuBPCase during leaf senescence is usually accompanied by a loss in in vitro RuBPCase activity and a decline in CER (5,12,13,21,22, 25, 26). It is generally assumed that RuBPCase breakdown results in an increase in mesophyll resistance and a decline in gross photosynthesis, a component of CER. However, the concentration of RuBPCase catalytic sites in leaves can be as high as 3 mm (G. Lorimer, personal communication). Thus, RuBPCase concentration may not always limit in vivo RuBPCase activity and photosynthesis. The decline in CER is probably due not only to a decrease in gross photosynthesis but also to an increase in photorespiration (22). In addition, the decrease in CER may not be entirely a result of an increase in mesophyll resistance, as previously reported (26) in stomatal resistance (22, 24). In fact, stomatal aperture has been recently suggested as one of the main controlling factors in senescence (17). To date, no comprehensive study has been reported of the relationship between RuBPCase degradation and alterations in gross photosynthesis (as opposed to CER) and stomatal and mesophyll resistance. In the present study, all of these parameters were measured and were related to changes in RuBPCase activity, soluble protein, protease activity, total free amino acids, Chl, and carbohydrates. These measurements were made on intact leaves of barley (Hordeum vulgare L.). Although detached leaves are commonly used to study senescence, recent reports (9, 19) have demonstrated that detached leaves do not necessarily senesce in the same manner as intact leaves.MATERIALS AND METHODS Plant Culture. One hundred barley (H. vulgare L., var. Numar) seeds were planted at a depth of 2 cm in plastic pots (13.5 cm in diameter x 15.0 cm tall) containing Vermiculite. Each pot received 800 ml of nutrient solution at planting. Additional nutrient solution (100 ml/day) and distilled H20 were supplied via cotton wicks extending from the bottom of each pot into a 1-liter glass jar. The nutrient solution contained, in mmol/l: Ca(NO3)2, 5; KNO3, 1; K2SO4, 2; MgSO4, 4; NH4H2PO4, 2; and in ,tmol/l: MnSO4, 18.3; H3BO3, 8.0; ZnSO4, 3.8; CuSO4, 1.5; (NH4)6Mo7024, 0.1; NaCl, 28.2; and Fe as Fe-ethylenediamine di-(O-hydroxyphenylacetate), 110.4. Li'ht intensity at pot height in the growth chamber was 550 ,uE m s-as provided by a mixture of incandescent and metal halide lamps. Air temperature during t...

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Developmental and age-related processes that influence the longevity and senescence of photosynthetic tissues in arabidopsis.

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