The photosynthetic capacity of stubble leaves and their contribution to growth of the lucerne plant after low level cutting

Abstract

The net carbon dioxide exchange rate (NCE) of lucerne (Medicago sativa cv. Hunter River) stubble leaves was found to be low (c. 55 ng CO₂ cm^-2 sec^-1) immediately after removing shoots 15 cm above the stem base. However, within several days the NCE rose to a maximum of 130 ng CO₂ cm^-2 sec^-1 on day 8. This peak rate was similar to that of recently expanded new leaves and held for about 3 days before declining. Leaves of similar age and position on uncut plants showed a steady decline in NCE. Leaf resistance, r'l, was low (c. 0.8 sec cm^-1) and did not change as the NCE increased during the first 8 days. Mesophyll resistance, r'm, to carbon dioxide diffusion was initially high and declined from c. 9 to 2 sec cm^-1 during this period. Comparison of dry weight changes of plant parts after stubble leaves were either retained or removed suggested that the tap-root benefited most by stubble leaf retention. The contribution to new shoots from stubble leaves appeared to be slight. However, when stubble leaves were exposed to ¹⁴CO₂ on day 11, photosynthate was mainly exported into the shoot arising from the axil of the exposed leaf. This suggests that stubble leaves substitute, in part or completely, for the supply of carbohydrate to stubble shoots normally derived from reserves mobilized in the tap-root. The likely causes of rejuvenation in NCE of stubble leaves after partial shoot removal are discussed along with the significance of this photosynthetic contribution to regrowth of the lucerne plant.