The effect of nitrogen and drought stress on pea ( Pisum sativum) and lupin (Lupinus angustifolius) nodulation, dry matter partitioning and nitrogen content: A dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Horticultural Science with Honours

Abstract

To determine the effect of no nitrogen and water stress at different levels on pea (Pisum sativum) and lupin (Lupinus angustifolius) dry matter (DM) partitioning, nodulation and nitrogen content a trial was sown during the 1999 growing season at Lincoln University, Canterbury, New Zealand ( 45 °S) on 24 March.

To study the root growth of inoculated plants they were grown two to a pot. The pots were 15 cm diameter and 80 cm long, filled with river washed crusher sand containing no nutrients. Water return was based on the amount lost from 8 control pots kept at field capacity (100 % irrigation) and measured to return 100, 75, 50 and 25 % irrigation water returned to the fully irrigated treatment. Control pots were maintained at 100 % irrigation and with added nitrogen (N) at 25 days after sowing (100 kg/ha), 20 days after flowering (50 kg/ha) and 40 days after flowering (50 kg/ha). All nutrients were added in liquid form and mixed in the water supplied. Harvests began once all pea plants had at least one flower and continued for a further 3 harvests at 20 day intervals.

Water stress caused a decline in plant physiological status as the length and severity of drought stress increased. Root length was reduced by around 31 % at 25 % irrigation by 60 days after flowering, total plant biomass also declined by 56 % for peas (from 6.16 g) and 77 % for lupin (from 10 g) compared to treatments receiving 100 % irrigation. Physiological aging was also increased by high water stress and caused faster pod maturity in pea pod formation but lower total yields ( 48 % lower yields at 25% irrigation compared to 100, 75 and 50% at 3.42 g). The absence of N increased the speed of physiological aging and a 40 % reduction in DM partitioned within pea pods compared to treatments receiving 100 % irrigation with 0 N (3.6 g) at 60 days after flowering.

Pod production in peas caused a large shift in the shoot to root ratio and lead to increased partitioning of shoot DM over root dry weight (a ratio of 21.2 g shoot weight to lg root weight at 60 days after pea flowering). Lupins also had an upward slope but not at the same angle (4.9 g shoot weight:l g root weight at 60 days after pea flowering). Water stress had no significant effect on this ratio. Water stress and species had no effect on the percentage nitrogen contained in root tissue (2.7 % N). Species did affect the percentage nitrogen in above ground plant parts (lupins contained a 17 % higher % N than pea at 3.154 % ). However, because of its effects on DM accumulation, plants not subjected to water stress contained higher amounts of total N ( 141 % higher N content at 100% irrigation compared with 7 5. 7 mg for 25 % irrigation in peas and a 353 % increase in lupins from 87.0 mg). Nitrogen had a detrimental effect on total N content in peas (a reduction of 23.5 % from 182. mg). However, in lupins it tended to have a slight additive effect (an increase of 15A % 1, from 394.4 mg) but was not significantly higher. Increased water stress caused a reduction in nodulation. Pea nodulation tended to decline after flowering (a mean decline of 35 % from 211 nodules/plant). In the lupins nodulation was still increasing (a mean increase of 40 % from 170 nodules/plant) after flowering and was significantly higher than in peas (74 % higher than peas at 137 nodules/plant). Added N decreased nodulation in both peas (207 % from 260 nodules/plant) and lupins (143 % from 387 nodules/plant).

Stem length did not change in the semideterminate pea under water stress. However, in the indeterminate lupin the stem length increased 108 % and node number by 58 % with 100 % irrigation above the 25 % irrigation treatment at 51.4 cm and 32 nodes at 60 days after flowering. Intenode length in peas with N was decreased due to increased node numbers but not a corresponding increase in stem length. Lupin intenode length decreased 41 % only under sever stress conditions (25 and 50 % irrigation at 60 days after flowering) compared to 1.8 cm mean intenode length at 75 and 100 % irrigation.