Analysing the shoot and root response of wheat in a soil environment under variable water and nitrogen supply

Abstract

Increasing water variability and the cost and sustainability of nitrogen (N) fertiliser use are of growing concern globally. Soil water supply directly and indirectly regulates soil N availability. As a result, plants have to adapt their shoots and roots in order to optimise water and N uptake. This thesis seeks to investigate the interactive effects of water and N supply on important soil properties and the growth and physiology of two Australian wheat varieties, Gladius and Kukri, both possessing phenotypic traits and water/N use efficiencies. This research seeks to explore and discuss how soil moisture affects soil N dynamics, and subsequently how different root traits affect N and water acquisition in a complex soil environment. Overall, water and N supply affected root plasticity, shoot growth, soil N dynamics and microbial biomass carbon (C). Soil mineral N availability was strongly influenced by soil moisture, with the availability of ammonium and nitrate decreasing with low soil moisture. Changes to soil physiochemical properties were associated with changes in root architecture, C allocation to roots and shoots, and aboveground physiology. Moreover, the differing physiological responses of wheat varieties Kukri and Gladius to variable water and N supply have provided insights into the phenotypic responses that could potentially aid in enhancing water productivity, nutrient use efficiency and yields. The use of an automated gravimetric watering platform allowed the precise measurement of plant weight in real-time and irrigation of pots to a pre-programmed water level. This allowed three harvests to be conducted over a period of three months, with each harvest representing a different growth stage of wheat. The results highlighted that plants were more responsive to N, with low N negatively affecting plant growth. Additionally, moderate water encouraged plant growth with medium and high N, whereas plants were not well adapted to variable watering (wet/dry cycling). From this, we wanted to further investigate whether plants were capable of reusing nutrients in previously used soil. This led to 36 pots having undergone the same treatments as those in Experiment 1, left to dry down and the wheat heads were harvested. After three months, the pots were re-watered and re-planted with wheat (the old wheat root systems from the previous harvest remained in the pot). Results from both harvests showed a clear legacy effect, with wet/dry cycling producing biggest plants in the second crop season, with a flush of mineral N. The idea that frequency and quantity of watering would impact plant growth and soil nutrition differently led to cv. Gladius and Kukri being subjected to three water treatments and two N treatments. Results showed that water had a greater impact on plant growth than N, with frequency of water more detrimental to plant growth. However, plant recovery or adaptability was seen with the wet/dry cycles. Additionally, there was a phenotypic response difference between genotypes. Further investigation into root architectural response, soil N dynamics and N uptake in response to variable water and N treatments were important to test the hypothesis that under low N and water supply, plant C allocation changed. Under low N and low water, results showed that root properties, such as total root length and root tip number, increased, but root volume decreased. The average 15N uptake in roots was also measured by exposing excised roots to different forms of labelled N, with root uptake preference for nitrate-15N over ammonium-15N or glycine-15N. The results presented in this thesis highlight trade-offs between wheat shoots and roots in order to maintain growth. These trade-offs include increasing root growth under low N (trade-off: less shoot growth); and producing longer, thinner roots under low water and/or low N (trade-off: fewer roots). By understanding how these trade-offs affect water and N uptake, and ultimately growth efficiency, would help develop more precise water and nutrient application strategies and overall crop management strategies. These improvements would boost crop productivity, especially under abiotic stresses.