6. Temperature and Aboveground Growth and Biomass Production
The effects of growth temperature on perennials are wide ranging, influencing many aspects of development, including leaf morphology, petiole length and stem thickness [56]. Early spring plants start their aboveground development under particular ecological conditions, i.e., comparatively low air and soil temperatures and high insolation. However, these parameters are not essential environmental factors for the completion of their short lifecycle in spring [1]. Once the leaf apex has emerged early in spring, the rapid growth of the whole plant starts. This could be guaranteed by the fact that the chloroplast structure is formed during underground growth in darkness, prior to the appearance of photosynthesis, which allows the plant to reach a maximum photosynthetic rate rapidly [57–59]. Relatively high temperatures (18/14 °C day/night temperature) during the growth period lead to faster growth in geophytes and earlier flowering. However, in this temperature range, the plants have smaller flowers and exhibit a shorter duration of vegetation. Moreover, a warmer spring always results in a lower bulb yield than for plants grown at lower temperature. In a cooler temperature (12/8 °C), bulb growth and leaf activity are maintained for a longer period [1,29,60–62]. Shorter growth time can also be explained by early activation of different sucrose-cleaving enzymes in the bulbs grown under warmer temperatures, which results in the faster shift from elongation to cell maturation and to the earlier cessation of starch accumulation [29,62]. Additionally, the rate of carbon assimilation at warmer temperatures rapidly becomes too high for the capacity to incorporate carbon into the bulb, leading to feedback inhibition of the photosynthetic rate. Since the size of the bulb is considered to be one of the major parameters that influence the quality of flowers, low temperature conditions are more favorable during the aboveground growth of early spring plants. This will promote the production of larger underground organs and flowers in the next spring.