Plant growth – resource or strategy limited: insights from responses to gibberellin

We used exogenous application of a phytohormone (gibberellin GA3, ‘GA’) to test the hypothesis that common perennial grasses may not be growing at all times to the limit of resource availability. Plants were taken from the field in winter, and again in summer and their responses to GA assessed under standard conditions, indoors, to reveal their ‘potential’ for growth at different times. Time of year, and associated developmental state, had a major impact on the capacity of plants to respond to exogenous GA, and less so their current growing conditions, temperature and N availability, during measurement. A major increase in dry matter (DM) production in winter-derived plants took place at both low and high N, with no evidence of a reduction in N content in tissues. That ryegrass plant growth can be stimulated, without externally adding resources, supports the hypothesis there is an element of internal control in how plants respond to ‘signals’ in their environment, that might be manipulated. This offers prospects for reducing environmental impacts (leaching, N2O) compared with obtaining the same yield increase by adding fertilizer N in early season. Responses to exogenous GA were detected (as significant) but far smaller in summer-derived plants. Molecular mechanisms of detection of N resource signals, developmental triggers and the role of endogenous gibberellin need to be unravelled to assess scope for breeding ryegrass germplasm to better match demands for increased production with greater resource-use efficiency.