Effect of previous defoliation regime and mineral nitrogen on regrowth in white clover swards: photosynthesis, respiration, nitrogenase activity and growth

Small swards of white clover (Trifolium repens L.) cv. Haifa were grown in solution culture in a controlled environment at 24 °C day/18 °C night and receiving 500 μE m-2 S−1 PAR during a 14-h photoperiod. The swards were cut either frequently (10-d regrowth periods) or infrequently (40-d regrowth) over 40 d before being cut to 2 cm in height. Half of the swards received high levels of nitrate (2–6 mM N in solution every 2 d) after defoliation while the others received none. Changes in d. wt, leaf area and growing point numbers were recorded over the following 10 d. CO2 exchange was measured independently on shoots and roots and nitrogenase-linked respiration was estimated by measuring nodulated root respiration at 21% and 3% oxygen in the root atmosphere. There was a general pattern in all treatments consisting of an initial d. wt loss from roots and stubble and reallocation to new leaves, followed by a period of total d. wt gain and recovery, to a greater or lesser extent, of weight in non-photosynthetic parts. Frequently cut swards had a smaller proportion of their shoot d. wt. removed by cutting and had a greater shoot d. wt, growing point number and leaf area at the start of the regrowth period. As a result of these differences, and also because of differences in relative growth rates, frequently cut swards made more regrowth than infrequently cut. Initial photosynthetic rates were higher in frequently cut swards, although the lamina area index was very low, and it was concluded that stolons and cut petioles made a significant contribution to carbon uptake during the first few d. Infrequently cut swards continued to allocate carbon to new and thinner leaves at the expense of roots and stubble for longer than frequently cut swards and as a result achieved a similar lamina area index after 10 d. Nitrogenase-linked respiration was low in all treatments immediately after cutting: frequently cut swards receiving no nitrate maintained high nitrogenase activity, whereas recovery took at least 5 d in infrequently cut swards. Swards which received nitrate after cutting maintained only low rates of nitrogenase-linked respiration and their total nodulated root respiration over the period was lower than those receiving no nitrogen: greater regrowth in nitrate fed swards over the 10 d compared to N2-fixing swards was in proportion to this lower respiratory burden.