CARBOHYDRATE RESERVES AND PLANT GROWTH SUBSTANCE SENSITIVITY IN PLASTIDS, STOMATA AND STATOCYTES DURING SHOOT DEVELOPMENT

Abstract

Starch grains are extremely rare in the plastids of dark-grown and greening cereals. Only plastids from very young dark-grown tissues (3–5 days from germination) or those cells which ultimately have a specialized function, such as statocytes, have prominent starch granules. Developing stomata are formed by dividing sub-epidermal cells which subsequently push into the epidermal layers. The plastids of such guard cells retain starch granules throughout this process. Studies using light-grown barley seedlings as a linear developmental model have shown that in a region close to the intercalary meristem above the earliest proplastids or eoplasts (which contain no starch grains) there is a distinctive amyloplast region which equally rapidly gives rise to a region above with starchless amoeboid plastids. Determination of patterns of carbohydrate content in light- grown, dark-grown and green cereal seedlings confirm that rapid starch accumulation and disappearance does appear to be an obligatory phase in the development of plastids in the light or in young, but not old, illuminated dark-grown tissue. In a preliminary study involving the use of various bioassays for plant growth regulator substances, regions in light-grown plants which rapidly form and mobilize starch grains have much higher contents of gibberellic acids and indol-3-yl-acetic acid than most other regions although the levels of abscisic acid are unchanged. It is concluded that a complex sequence of carbohydrate changes is involved in shoot development which is tissue specific and that only part of this synchronization is achieved by means of plant growth regulators. The sensitivity of such cells to growth substances changes as they mature.