Effects of polyacrylamide co-polymer on tomato (Lycopersicon esculentum Mill.) grown in different container media transplanted into soil : a dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Horticultural Science with Honours at Lincoln University

Abstract

Container grown plants transplanted into open ground have to rely on the water supply retained in the root ball, before its roots extend into the surrounding soil, and exploit the available water source. Under increasing water stress, the transplant may wilt and die. Polyacrylamide Co-polymer (PAM), a synthetic, cross-linked water absorbing polymer, has been widely used in improving water balance, survival and growth of plants. An experiment at Lincoln University Aluminex glasshouse was set up to determine the efficacy of PAM as a transplant aid in transplanting container grown tomato (Lycopersicon esulentum Mill.) into soil. Tomato plants grown in PB2 bags with either Templeton silt loam, 80 % composted bark 20 % sand, 80 % composted bark 20 % perlite, 50 % composted bark 50 % perlite, or 50 % composted bark 50 % sand, were transplanted into 10-litre pots of soil amended with PAM to simulate open ground condition. PAM was applied into the transplanting holes at 0 g, 0.6 g, and 1.2 g dry PAM pre-expanded in tap water. Applying PAM into the transplanting holes did not delay the number of days for the transplants to reach permanent wilting point (PWP), nor increased the water content over time at field capacity (FC) or PWP. Increasing PAM levels increased total water lost at PWP by 2.6 % V/V (for 6 g dry PAM) and 7.3 % V/V (for 1.2 g dry PAM) respectively, and the highest rate of 1.2 g produced the most dry matter at 15.28 g per plant. Container media of 80 % composted bark 20 % sand increased water content at FC by 3.5 % V/V with 0.6 g dry PAM, and remained constant when PAM was increased to 1.2 g. 50 % composted bark 50 % perlite increased water content from 31.6 % to 34.7 % with increasing PAM levels. This increase in water content at FC by these 2 types of container media with increasing PAM level was doubtful due to the application of PAM at the bottom of the transplanting holes. The water content at PWP determined the amount of total available water to the transplants but, was dependent on the types of media the transplants were grown in, due to the varying water holding capacity at FC and pore sizes for holding water. Media with higher composted bark levels (80 %) mixed with either perlite or sand, produced greater water content of contained media at both PWP (9 % V /V) and FC (8 %). Media consisting of sand mixed with composted bark had most water molecules held at low tension and thus lost more water (11.48 % to 13.40 % V/V) per kPa based on moisture release characteristics derived from laboratory studies. However, the total available water from the laboratory studies (33 % to 37 %) was higher than the simulated transplanting experiment in the glasshouse (15 % to 22 % V/V). It was concluded that, applying PAM at the bottom of the transplanting holes would contribute to the dry matter production per plant and the total water lost, but would not increase the water content in the contained media at both FC and PWP, and the water content over time. Using Moisture release characteristics derived from the laboratory were not adequate for relating moisture retention of media in actual ground bed situations. In addition, using days to reach PWP as a measure of plant moisture stress was not practical.