Seasonal differences in the capacity of perennial ryegrass to respond to gibberellin explained

Perennial ryegrass plants (Lolium perenne) were taken from an established field at two different stages in the season (mid-winter and again at mid-summer). They were then grown in a controlled environment to both “lock in” their contrasting developmental states and to look at the role of nitrogen supply, temperature, and developmental state separately to evaluate the potential of plants to respond to exogenous application of gibberellin. Responses to exogenous gibberellin (gibberellic acid, GA) were significant but were far smaller in summer-derived than winter-derived plants. The major difference in response to GA (compared with controls) between winter-derived and summerderived plants suggests that seasonal changes in plant developmental state have a major effect in the field on the capacity for the plants to respond to exogenous GA application. This effect is greater than that of temperature and N availability. This raises new prospects for making sustained increases in plant growth, but only if the fundamental mechanisms by which plants control their responses to environmental signals (e.g., temperature and soil N status) can be understood. The role of gibberellins (endogenous as well as externally applied) in changes in plant growth strategy presents a new challenge for forage plant science. Introduction Gibberellins have been proposed in New Zealand as a means to increase forage supply, and improve the match between forage supply and feed demand in pastoral systems (Beukes et al. 2012). Gibberellins (gibberellic acid, GA) are a family of naturally occurring plant hormones which regulate plant growth and influence various developmental processes including stem elongation, germination, dormancy, flowering, sex expression, enzyme induction, and leaf and fruit senescence (reviewed in Matthew et al. 2009). These highly bioactive hormones have been commercially available and widely used in plant physiological studies from the 1950s to the 1970s (Leben & Barton 1957). The bulk synthesis of gibberellins has made application increasingly affordable. Field-based research focusing on the use of GA on grassland species has shown in many cases (see Matthew et al. 2009) that GA application can increase overall forage dry matter (DM) production. There is, however, clear evidence that the stimulation is greatest early in the growing season, and that GA is less effective in mid-summer and early autumn (see Leben et al. 1959; Williams & Arnold 1964; Matthew et al. 2009). Possible causes for different seasonal responses might be: i. seasonal differences in the availability of nutrients (notably N) sufficient to support any extra growth. This has a bearing on fertiliser recommendations when using GA; ii. seasonal differences in temperature. Low temperature will clearly limit growth, though industry recommendations suggest GA is most effective at times when growth is limited by low temperature (NuFarm 2011); iii. major seasonal changes in the developmental state (e.g. cycle of vegetative growth/flowering) and associated growth strategy (Parsons et al. 2011) in our widely used perennial grasses; and iv. the possible effects of repeat applications of GA. To gather insights into each of these hypotheses we took plants from the field and grew them in a common controlled environment to “lock in” two contrasting developmental states. We then measured the roles of nitrogen supply, temperature, and developmental state, separately, on the potential of plants to respond to GA. Materials and Methods Plants and growing conditions Perennial ryegrass plants (cv. ‘Aberdart’) were collected from an established dairy pasture at two different stages in the growing season: i) June 2011; these being “winter-derived” and, ii) December 2011; these being “summer-derived” plants. Plants had all original soil washed from their roots and were separated into healthy, consistently sized plants consisting of approximately 184 183-188 (2012) Proceedings of the New Zealand Grassland Association 74: 10 tillers. These were transferred to plastic pots and planted in a 2:1 sand/clay medium with a known low nutrient retaining capacity. The plants were grown in a controlled environment (New Zealand Controlled Environment Laboratory, Plant and Food Research, Palmerston North). All plants were maintained on matting in drained plastic trays, which were flood subirrigated with tap water every second day to minimise any accumulation of nutrients within the pots. The plants were transferred to growth rooms (for environmental conditions see below) and were clipped to 6 cm above ground every 4 weeks for winter-derived plants and every 3 weeks for summer-derived plants (anticipating faster growth at the higher day length/light energy receipt). The plants were allowed to establish during at least two full defoliation cycles before GA treatments were applied and measurements were taken. Plants collected in winter (960 in total) were grown at 12.5°C and in short days (8 hours light) critical to sustain the plants in an induced but not initiated (Cooper 1952; Ryle & Langer 1963) winter developmental state. Radiation flux density was 620 μmols m-2 s-1. Plants were supplied every second day with either “high” (50% of the plant pots) or “low” (50% of the plant pots) mineral N solution, using half-strength Hoagland solution; 60 ml per plant/pot at 9 mM N or adjusted to 2.25 mM N to sustain contrasting N status (Rasmussen