#74 - Jan 2024 - Garden Stuff - © Sandy Lang - slang@xtra.co.nz FRUITS ’n’ HORMONES Jan/Feb: Mid/late summer. Water woes. Lawns brown off (dormant) but recover. Save limited water for plants that might die. Fruits: The Angiosperms are the most recent group (clade) to emerge (~300 mya). They bear flowers that morph into fruits that ‘contain’ (Gk angeion) ‘seeds’ (Gk sperma). Ignoring little, hard fruits, what about the big, soft fruits we eat? You see: Fruiting starts with a flower that opens, lasts 1 or 2 days, then the petals fall, then the remains of the flower falls. But after petal fall a few (~5%) now petal-less ones stay on. You can see a tiny green fruitlet at the tip of the flower stalk. Fruit set is this transition from flower to fruitlet. You don’t see: In the base of each flower is a tiny ovary, in this are some unfertilised egg cells. When the flower opens, a pollen grain lands on the sticky stigma, where it germinates, sends a pollen tube several mm down through the style to the ovary, where it finds an egg cell, which it fertilises to produce an embryo, which grows to be a mini plant, inside a seed, inside a fruit. Action: From the moment the egg cell is fertilised, the embryo moves fast or it’s a gonner. If flower opening is day 1, pollination is ~day 2, fertilisation ~day 3, petals drop off ~day 4, the remains (stalk, ovary, egg cells) drops off ~day 6. Hormones: How does the plant know not to shed the flower? The moment the embryo is formed it starts sending out hormones. These tell the plant not to shed the flower. Next, they tell the flower stalk to produce vascular tissue (xylem, phloem) to deliver water, sugar and minerals to the embryo so it can grow to form a seed. They also tell the ovary wall to grow to form a tiny fruitlet around the seed(s). All this happens within ~7 days. Big seeds = more hormones: In a one-seeded fruit (plum) the bigger the seed, the more hormones, the bigger the fruit. More seeds = more hormones: In apples (max 10-seeds) often not all egg cells are fertilised, so fewer seeds - but fewer seeds = less hormones = smaller fruit. True too for grapes (max 4 seeds), for tomatoes (max 150-300 seeds) and for kiwifruit (max 1000-2500 seeds). Lopseeded: If seeds aren’t uniformly distributed, hormones are asymmetrical, and fruit is lopsided.___________________________________