Sap

Osmosis is what moves it.  Plants lose moisture through pores in their leaves and so moisture taken in by the roots moves up through the plant.  The less watery cells above are diluted by the more watery ones below.  The plant moves nutrients and hormones around by the same mechanism, in a similar way to our blood.

Spring sown plants grow faster, so there is more soft new growth than an autumn sown broad bean that has a bit more of a struggle on its hands.

Temperature is the usual factor.  Things in plants carry on year round, but in winter everything slows to a crawl.  Some plants become completely dormant and die back to tubers or roots (like potatoes).  Some just sit and wait it out and grow a bit now and then when they can.

Have a look at this (meant for kids but it explains it pretty well): https://www.science-sparks.com/transport-in-plants/

Think of xylem and phloem as highway lanes, moving sap in different directions.  Xylem transports water and minerals from the roots to the leaves. Phloem moves food substances (made by photosynthesis) from leaves to the rest of the plant.

At times, sap rising in a plant can be nutrient-rich (think of sap rising in a maple tree in early spring, carrying all that sugar).

still trying to get my head around the downward sap factor what cause the downward pressure

So I'll jump in & then someone will be along in a minute to word it better (!) but it's all caused by  osmosis & if you understand that, then you're on the way towards understanding many natural things.

You know how a solution wants to be even all the way over - say you  pour two different strengths of sugar solution into each end of the same bowl they will even out, you don't stay with areas of stronger and weaker solution.

Now presume you have a bowl with a membrane down the middle. The membrane has minute holes in it so water can go through but sugar can't (water has very small molecules, sugar has much bigger molecules). This is called a semi-permeable membrane.  Pour your two sugar solutions into either side of this & they want to do the same as before - to even out but the sugar can't move, so water does. Water moves from the weak solution to the strong one until they are the same strength. So in your bowl the side that started with the stronger solution will get more full.

Living things are made up of minute cells that act like your bowl. The cell walls are all semi-permeable membranes.

Osmotic pressure is where water comes into a cell by osmosis, so that pushes outwards (or in our case downwards). As long as photosynthesis keeps making more sugar in the plant's leaves, that will attract water in & osmotic pressure will push the sap away.